Dissertations / Theses on the topic 'Seagrass ecosystems'

Water motion has been shown to influence almost every aspect of the ecology of seagrass communities; seagrass communities have likewise been shown to significantly influence water movement around them. This thesis examines the important role of water motion on seagrass ecosystems by integrating field and laboratory studies of several aspects of seagrass ecology influenced by water motion. To facilitate the study of hydrodynamics of seagrass ecosystems, a solid state electronic current meter was designed and developed, using thermistors as flow sensing devices. Important characteristics of the meters include: no moving parts, compact size, high sensitivity (resolution better than ± 0.5 cm s-1), and high sampling rate (greater than 0.2 Hz). Deployment of the meters in field measurements provided reliable and meaningful results of flow conditions through seagrass canopies, and they show great potential for use in many studies of marine ecology. Field studies of water velocity profiles revealed significant differences between the shapes of profiles of different seagrass species, particularly between species of Posidonia and Amphibolis. Of particular note is the observation of a region of high water velocity beneath the leafy canopy of Amphibolis, which was not present in the Posidonia plants. Water velocity profile measurements, sediment grain size analyses and standing stock measurements were conducted across an exposure gradient in a Posidonia sinuosa meadow. These studies revealed that, while the exposed location experienced a higher ambient water velocity than the sheltered site, the baffling influence of the seagrass canopy reduced the water velocity to approximately the same at both sites, within the meadow, although the effects varied seasonally. It was also observed that the seagrass meadow produced apparent skimming flow under the low flow conditions measured at the sheltered location; this phenomenon reflects the capacity for flow redirection over the canopy, and has important implications for the sub-canopy ecosystem and the protective role of seagrasses on the seabed. Field and laboratory studies on the role of seagrass density on the hydrodynamic nature of seagrass ecosystems revealed that water velocity profiles through meadows of reduced densities, and different shoot arrangements, were markedly different to “natural” profiles, implying the existence of a “critical density” (approximately 25 % of natural meadow density) with regard to canopy hydraulics. The role of water motion at an individual leaf scale was investigated with a series of laboratory flume studies of photosynthetic rates of seagrass and algae. The results show that the response of photosynthetic rate to water velocity depends very much on the plant species, with the algae markedly more productive (on a unit chlorophyll basis) than the seagrasses tested. Increases in photosynthetic rate were observed at water velocities above approximately 2.5 cm s-1; negligible photosynthetic activity was observed below this velocity. Calculation of P v. I curves indicated that the Posidonia species had high Ik values at low velocities (1360 :mol quanta m-2 s-1 for P. australis and 250.8 :mol quanta m-2 s-1 for P. sinuosa at 1.58 cm s-1), which decreased with increasing water velocity (to 138.9 and 24.77 :mol quanta m-2 s-1 for P. australis and P. sinuosa respectively), while the algal species had relatively constant values of Ik across all water velocities (85.42 to 312.7 :mol quanta m-2 s-1 for Ulva lactuca and 169.7 to 573.9 :mol quanta m-2 s-1 for Laurencia cruciata). Dye visualization studies showed that the algae remained quite rigid at all the velocities tested, while the seagrass leaves compressed as velocity increased. This resulted in an increased rate of turbulence creation by the algae, which is believed to enhance photosynthetic rates, through improved nutrient exchange rates across the boundary layer adjacent to the thallus. Further dye visualization studies revealed the significance of blade morphology on the creation of microscale turbulence at the surface of seagrass leaves. Epiphytic growth on seagrass leaves was observed to play an important role in breaking up water flow across the leaf surface, thereby enhancing the creation of microscale turbulence. From these studies, it is clear that water motion influences all aspects of the functioning of all components of seagrass communities, playing a role in nutrient supply, reproduction, physical stability, temperature and metabolic functions. The influence of seagrass meadows on coastal hydrodynamics is also apparent, with potential impacts on sediment stability, recruitment of benthic species and coastal erosion. This thesis has clearly demonstrated that water motion is an important parameter in seagrass ecology, and requires serious consideration in seagrass research, conservation and rehabilitation programmes.

Water motion has been shown to influence almost every aspect of the ecology of seagrass communities; seagrass communities have likewise been shown to significantly influence water movement around them. This thesis examines the important role of water motion on seagrass ecosystems by integrating field and laboratory studies of several aspects of seagrass ecology influenced by water motion. To facilitate the study of hydrodynamics of seagrass ecosystems, a solid state electronic current meter was designed and developed, using thermistors as flow sensing devices. Important characteristics of the meters include: no moving parts, compact size, high sensitivity (resolution better than plus-minus 0.5 cm s-1), and high sampling rate (greater than 0.2 Hz). Deployment of the meters in field measurements provided reliable and meaningful results of flow conditions through seagrass canopies, and they show great potential for use in many studies of marine ecology. Field studies of water velocity profiles revealed significant differences between the shapes of profiles of different seagrass species, particularly between species of Posidonia and Amphibolis. Of particular note is the observation of a region of high water velocity beneath the leafy canopy of Amphibolis, which was not present in the Posidonia plants. Water velocity profile measurements, sediment grain size analyses and standing stock measurements were conducted across an exposure gradient in a Posidonia sinuosa meadow. These studies revealed that, while the exposed location experienced a higher ambient water velocity than the sheltered site, the baffling influence of the seagrass canopy reduced the water velocity to approximately the same at both sites, within the meadow, although the effects varied seasonally. It was also observed that the seagrass meadow produced apparent skimming flow under the low flow conditions measured at the sheltered location; this phenomenon reflects the capacity for flow redirection over the canopy, and has important implications for the sub-canopy ecosystem and the protective role of seagrasses on the seabed. Field and laboratory studies on the role of seagrass density on the hydrodynamic nature of seagrass ecosystems revealed that water velocity profiles through meadows of reduced densities, and different shoot arrangements, were markedly different to 'natural' profiles, implying the existence of a 'critical density' (approximately 25 % of natural meadow density) with regard to canopy hydraulics. The role of water motion at an individual leaf scale was investigated with a series of laboratory flume studies of photosynthetic rates of seagrass and algae. The results show that the response of photosynthetic rate to water velocity depends very much on the plant species, with the algae markedly more productive (on a unit chlorophyll basis) than the seagrasses tested. Increases in photosynthetic rate were observed at water velocities above approximately 2.5 cm s-1; negligible photosynthetic activity was observed below this velocity. Calculation of P v. I curves indicated that the Posidonia species had high Ik values at low velocities (1360 :mol quanta m-2 s-1 for P. australis and 250.8 :mol quanta m-2 s-1 for P. sinuosa at 1.58 cm s-1), which decreased with increasing water velocity (to 138.9 and 24.77 :mol quanta m-2 s-1 for P. australis and P. sinuosa respectively), while the algal species had relatively constant values of Ik across all water velocities (85.42 to 312.7 :mol quanta m-2 s-1 for Ulva lactuca and 169.7 to 573.9 :mol quanta m-2 s-1 for Laurencia cruciata). Dye visualization studies showed that the algae remained quite rigid at all the velocities tested, while the seagrass leaves compressed as velocity increased. This resulted in an increased rate of turbulence creation by the algae, which is believed to enhance photosynthetic rates, through improved nutrient exchange rates across the boundary layer adjacent to the thallus. Further dye visualization studies revealed the significance of blade morphology on the creation of microscale turbulence at the surface of seagrass leaves. Epiphytic growth on seagrass leaves was observed to play an important role in breaking up water flow across the leaf surface, thereby enhancing the creation of microscale turbulence. From these studies, it is clear that water motion influences all aspects of the functioning of all components of seagrass communities, playing a role in nutrient supply, reproduction, physical stability, temperature and metabolic functions. The influence of seagrass meadows on coastal hydrodynamics is also apparent, with potential impacts on sediment stability, recruitment of benthic species and coastal erosion. This thesis has clearly demonstrated that water motion is an important parameter in seagrass ecology, and requires serious consideration in seagrass research, conservation and rehabilitation programmes.

Water motion has been shown to influence almost every aspect of the ecology of seagrass communities; seagrass communities have likewise been shown to significantly influence water movement around them. This thesis examines the important role of water motion on seagrass ecosystems by integrating field and laboratory studies of several aspects of seagrass ecology influenced by water motion. To facilitate the study of hydrodynamics of seagrass ecosystems, a solid state electronic current meter was designed and developed, using thermistors as flow sensing devices. Important characteristics of the meters include: no moving parts, compact size, high sensitivity (resolution better than plus-minus 0.5 cm s-1), and high sampling rate (greater than 0.2 Hz). Deployment of the meters in field measurements provided reliable and meaningful results of flow conditions through seagrass canopies, and they show great potential for use in many studies of marine ecology. Field studies of water velocity profiles revealed significant differences between the shapes of profiles of different seagrass species, particularly between species of Posidonia and Amphibolis. Of particular note is the observation of a region of high water velocity beneath the leafy canopy of Amphibolis, which was not present in the Posidonia plants. Water velocity profile measurements, sediment grain size analyses and standing stock measurements were conducted across an exposure gradient in a Posidonia sinuosa meadow. These studies revealed that, while the exposed location experienced a higher ambient water velocity than the sheltered site, the baffling influence of the seagrass canopy reduced the water velocity to approximately the same at both sites, within the meadow, although the effects varied seasonally. It was also observed that the seagrass meadow produced apparent skimming flow under the low flow conditions measured at the sheltered location; this phenomenon reflects the capacity for flow redirection over the canopy, and has important implications for the sub-canopy ecosystem and the protective role of seagrasses on the seabed. Field and laboratory studies on the role of seagrass density on the hydrodynamic nature of seagrass ecosystems revealed that water velocity profiles through meadows of reduced densities, and different shoot arrangements, were markedly different to 'natural' profiles, implying the existence of a 'critical density' (approximately 25 % of natural meadow density) with regard to canopy hydraulics. The role of water motion at an individual leaf scale was investigated with a series of laboratory flume studies of photosynthetic rates of seagrass and algae. The results show that the response of photosynthetic rate to water velocity depends very much on the plant species, with the algae markedly more productive (on a unit chlorophyll basis) than the seagrasses tested. Increases in photosynthetic rate were observed at water velocities above approximately 2.5 cm s-1; negligible photosynthetic activity was observed below this velocity. Calculation of P v. I curves indicated that the Posidonia species had high Ik values at low velocities (1360 :mol quanta m-2 s-1 for P. australis and 250.8 :mol quanta m-2 s-1 for P. sinuosa at 1.58 cm s-1), which decreased with increasing water velocity (to 138.9 and 24.77 :mol quanta m-2 s-1 for P. australis and P. sinuosa respectively), while the algal species had relatively constant values of Ik across all water velocities (85.42 to 312.7 :mol quanta m-2 s-1 for Ulva lactuca and 169.7 to 573.9 :mol quanta m-2 s-1 for Laurencia cruciata). Dye visualization studies showed that the algae remained quite rigid at all the velocities tested, while the seagrass leaves compressed as velocity increased. This resulted in an increased rate of turbulence creation by the algae, which is believed to enhance photosynthetic rates, through improved nutrient exchange rates across the boundary layer adjacent to the thallus. Further dye visualization studies revealed the significance of blade morphology on the creation of microscale turbulence at the surface of seagrass leaves. Epiphytic growth on seagrass leaves was observed to play an important role in breaking up water flow across the leaf surface, thereby enhancing the creation of microscale turbulence. From these studies, it is clear that water motion influences all aspects of the functioning of all components of seagrass communities, playing a role in nutrient supply, reproduction, physical stability, temperature and metabolic functions. The influence of seagrass meadows on coastal hydrodynamics is also apparent, with potential impacts on sediment stability, recruitment of benthic species and coastal erosion. This thesis has clearly demonstrated that water motion is an important parameter in seagrass ecology, and requires serious consideration in seagrass research, conservation and rehabilitation programmes.

Our world is subjected to a panoply of drivers of change. In this context, the understanding on how our biosphere resists, absorbs or is altered by the changes, appears as a hot question in ecology. In this respect, two ecological concepts appear as essential, resilience and biotic interactions. Resilience is related to how ecosystems persist under stress or suffering disturbances. Interactions among species are to a large part responsible for the delivery of ecosystem functions, and form the architecture of biodiversity. Moreover, a substantial part of ecosystem resilience is founded on species interactions. This thesis is an attempt to shed some light on these issues through the deep exploration of specific case studies in seagrass ecosystems, in particular how seagrasses respond to external drivers (or how resilient they are), how these responses affect species interactions and which mechanisms allow coexistence of species linked by positive and negative interactions. Our approach is based upon field observations and field manipulative experiments. Chapter 1 shows how an increase of organic matter in sediment weakens the mutualism between the bivalve Loripes lucinalis and the seagrass Cymodocea nodosa. The mechanism implied is the effect of this increase (and, probably, the resulting anoxia) on seagrass root morphology (plant trait), which results in a lower provision of habitat for the bivalves, whose abundance decreases. The weakening of the mutualism can potentially decrease the resilience of these ecosystems to eutrophication and, therefore, compromise their persistence. Chapter 2 describes a facilitative cascade in which the seagrass C. nodosa favors the abundance of the pen shell Pinna nobilis, which positively affects the sea urchin Paracentrotus lividus, which in turn consumes the seagrass. We suggest that the persistence of this three-species assemblage rests on the very local impact of sea urchins on the seagrass, likely driven by behavioural and denso-dependent processes. Chapter 3 and 4 show that fast-growing species such as C. nodosa are highly resilient to stress or disturbances when affecting only the aboveground parts, recovering fast (within two weeks) from a single event of disturbance. C. nodosa shows several mechanisms of tolerance, such as compensatory growth, reallocation of internal resources and enhancement of the formation of new modules, when coping to repeated defoliation simulating herbivory. However, when the belowground parts are lost by disturbances, recovery is highly delayed up to two years and is dependent on the characteristics of the disturbance such as size and timing. Overall, this research has contributed to increase our understanding on how ecosystems respond to changes and how species interactions are maintained and disrupted. We have shown that environmental changes can alter the functioning of seagrass ecosystems at least in two directions. Firstly, by altering fundamental biological interactions, such as the seagrass-lucinid mutualism and, secondly, by affecting the resilience of ecosystems dominated by a foundation species, which promote species coexistence. Advances in the two complementary and interlinked directions will be crucial to better manage and preserve ecosystems and prevent their potential collapse under the increasing human-induced change the world is submitted to.
El nostre món està sotmès a un ampli ventall de forces que tendeixen a provocar canvis. En aquest context, entendre com la biosfera resisteix, absorbeix o és alterada per aquestes forces resulta una qüestió candent, especialment per l'ecologia. Al respecte, dos conceptes ecològics esdevenen essencials: la resiliència i les interaccions biològiques. La resiliència és la capacitat de persistència o recuperació que tenen els ecosistemes sotmesos a estrès o pertorbacions. Les interaccions entre espècies (efectes de l'existència d'una espècie sobre la fitness d'una altra) contribueixen al manteniment de les funcions ecosistèmiques i, en un cert sentit, constitueixen l'arquitectura de la biodiversitat. A més, la resiliència dels ecosistemes depèn , en gran part, d’aquestes interaccions. Aquesta tesi és un intent d’aprofundir en els aspectes esmentats a través d'una sèrie de casos d’estudi en ecosistemes d’angiospermes marines. Concretament, el que fem és estudiar com els ecosistemes d’angiospermes marines responen a les forces causants de canvis, com aquestes respostes vénen mitjançades per canvis en la interacció entre espècies, i provar d'esbrinar els mecanismes que permeten la coexistència d’espècies que es troben vinculades per interaccions positives i negatives. La nostra aproximació es basa tant en observacions com en experiments en el camp. El Capítol 1 mostra com un increment de matèria orgànica en el sediment debilita el mutualisme entre el bivalve Loripes lucinalis i l’angiosperma marina Cymodocea nodosa. El mecanisme implicat que es proposa per explicar-ho està relacionat amb la plasticitat morfològica de la planta. Així, un increment en la matèria orgànica del sediment (i, probablement, l’anòxia que se'n segueix), fa que la planta modifiqui la morfologia de les seves arrels, que esdevenen molt menys ramificades i fan disminuir per tant la disponibilitat d'hàbitat per als bivalves. Una debilitació del mutualisme pot, potencialment, disminuir la resiliència d’aquests ecosistemes a l’eutrofització i, per tant, comprometre la seva persistència. El Capítol 2 descriu una cascada de facilitació en la qual l’angiosperma marina C. nodosa afavoreix l’abundància del gran bivalve Pinna nobilis, que ajuda a incrementar l'abundància de la garota Paracentrotus lividus, que al seu torn consumeix l’angiosperma. Suggerim que la persistència d’aquest sistema de tres espècies, aparentment inestable (tres interaccions concatenades circularment, dues de positives i una de negativa) es basa en què la interacció negativa (l’efecte de les garotes sobre l’angiosperma) té un abast molt limitat, probablement degut tant al seu comportament alimentari com a les defenses de la planta enfront de l'herbivorisme. Els Capítols 3 i 4 mostren que les espècies de creixement ràpid, com ara C. nodosa, són altament resilients a l'estrès o a les pertorbacions quan aquestes afecten només les parts aèries de les plantes (defoliació parcial o total), recuperant-se ràpidament (dues setmanes) després d'una pertorbació puntual en el temps. C. nodosa mostra diversos mecanismes de tolerància a la defoliació, com ara el creixement compensatori, la reassignació de recursos interns i l’increment en la taxa de formació de nous mòduls. Tanmateix, quan les pertorbacions provoquen la pèrdua de les parts subterrànies (rizomes i arrels), la recuperació és molt més lenta, i triga fins a dos anys. A més, aquesta recuperació depèn de les característiques de la pertorbació com ara la mida de l'àrea afectada i l’època de l'any en què es produeix. En general, aquesta tesi ha contribuït a comprendre millor les respostes dels ecosistemes als canvis. Hem pogut documentar alguns processos que permeten la coexistència entre espècies, així com mecanismes de resiliència específics que esdevenen ecosistèmics quan es manifesten en espècies fundadores d'hàbitat. També hem demostrat com els canvis, més enllà d'afectar espècies individuals més o menys emblemàtiques, poden provocar alteracions de formes més subtils, com ara erosionant la seva resiliència mitjançant la modificació d’interaccions biològiques. Els avenços en totes aquestes direccions complementàries i interrelacionades són crucials per a gestionar i preservar els ecosistemes i evitar el seu possible col·lapse.

Seagrasses constitute the basis for diverse and productive ecosystems worldwide. In East Africa, they provide important ecosystem services (e.g. fisheries) but are potentially threatened by increasing resource use and lack of enforced management regulations. The major aim of this PhD thesis was to investigate effects of anthropogenic distur-bances, primarily seaweed farming and coastal fishery, in East African seagrass beds. Seaweed farming, often depicted as a sustainable form of aquaculture, had short- and long-term effects on seagrass growth and abundance that cascaded up through the food web to the level of fishery catches. The coastal fishery, a major subsistence activity in the region, can by removing urchin predators indirectly increase densities of the sea urchin Tripneustes gratilla, which has overgrazed seagrasses in several areas. A study using simulated grazing showed that high magnitude leaf removal – typical of grazing urchins – affected seagrasses more than low magnitude removal, typical of fish grazing. Different responses in two co-occurring seagrass species furthermore indicate that high seagrass diversity in tropical seagrass beds could buffer overgrazing effects in the long run. Finally, a literature synthesis suggests that anthropogenic disturbances could drive shifts in seagrass ecosystems to an array of alternative regimes dominated by other or-ganisms (macroalgae, bivalves, burrowing shrimp, polychaetes, etc.). The formation of novel feedback mechanisms makes these regimes resilient to disturbances like seagrass recovery and transplantation projects. Overall, this suggests that resource use activities linked to seagrasses can have large-scale implications if the scale exceeds critical levels. This emphasizes the need for holistic and adaptive management at the seascape level, specifically involving improved techniques for seaweed farming and fisheries, protection of keystone species, and ecosystem-based management approaches.

Anthropogenic disturbances are ubiquitous in coastal marine ecosystems. As such, more intensive monitoring efforts are necessary to conserve these valuable habitats. Bioindicators, organisms that predictably respond to changes in environmental variables, may be utilized in monitoring efforts to assess ecosystem functioning. To incorporate organisms into monitoring programs as bioindicators managers need to first understand the difference between the natural phenology of the focal organisms and their responses to different forms of anthropogenic disturbance. To determine if gammaridean amphipods could be used as indicators of changes in environmental quality in sub-tropical seagrass ecosystems, I conducted spatial and temporal surveys of amphipod communities in south Florida. Amphipod community structure varied significantly across sites and seasons. Variation in community structure was largely driven by macrophyte biomass, food availability, seasonally variable factors (epiphyte abundance, dissolved oxygen, salinity, and temperature), water-column nitrogen concentration, and factors related to freshwater input, including low Thalassia testudinum and high Halodule wrightii densities, and salinity. Amphipods are also susceptible to mechanical damage in seagrass habitats and could be used as indicators of ecological functioning of a region. A major source of mechanical damage in seagrass ecosystems is caused by boat propellers. I simulated propeller scars in continuous seagrass beds to investigate the effects of scarring on seagrass ecosystem functioning. Seagrasses located adjacent to propeller scars experienced a shift in the limiting resource from light to phosphorus. Amphipod community structure, however, was not impacted by scarring, but amphipod density was reduced in fragmented patches. To determine if plant-herbivore interactions were impacted by propeller scarring, we removed amphipods from half of the experimental plots and measured epiphyte biomass and community composition. Top-down control on epiphyte biomass or community composition by amphipods was not affected by fragmentation, despite reduced amphipod densities. My dissertation research demonstrates that amphipods could be incorporated into existing management programs in sub-tropical seagrass ecosystems as environmental indicators. Reduced amphipod densities in fragmented seagrass beds suggests that amphipods could also be used as ecological indicators, but more research is needed to determine the extent of the impacts of fragmentation on higher trophic levels.

Coastal marine sediments have recently been identified as globally important stocks of organic carbon (Corg) that, if compromised, could significantly exacerbate global greenhouse gas emissions. While resource managers and policy makers are eager to incorporate this ecosystem service into seagrass ecosystem valuation frameworks, similar to those already in existence for terrestrial forests, there has been insufficient information regarding how environmental conditions and seagrass ecology control carbon storage. These include the influence of the seagrass to the production and preservation of soil organic matter, the fate of stored carbon following conversion of coastal wetlands, and the interactions between organic and inorganic carbon cycling. This dissertation intends to to understand the drivers of Corg storage and preservation to better prioritize and evaluate the worth of seagrasses to large scale carbon cycles and greenhouse gas mitigation planning. Long-term experiments and thorough field surveys reveal that seagrasses are not categorically necessary nor sufficient for long-term Corg storage. Soil Corg stocks as well as their recalcitrance and breakdown rates are all correlated with sediment grain size, where muddy, fine sediments have higher Corg stocks that are less likely to breakdown. Sediment grain size can be influenced by the presence of seagrasses at some sites, likely where the leaf canopy can modify local hydrology enough to create a depositional environmental that wouldn’t otherwise exist. However, similar depositional environments that collect and store Corg can be obtained through local geomorphological features and natural hydrology, independent of benthic flora. This distinction has important implications on how soil C is managed to continue its preservation. The relation between seagrass Corg and CO2 can be blurred by calcification and carbonate dissolution processes that occur concurrently, and have direct but antagonistic effects on CO2. Carbonate processes are dependent on local environmental factors, though augmented by biological processes, thus the ability of carbonate processes to interfere with seagrass Corg storage and loss is limited to geographic areas where processes can occur. Warm, shallow waters, like those in Florida Bay, encourage calcification, though the magnitude of soil inorganic and organic carbon interaction can vary locally as well. Seagrasses are declining globally thus additional ecosystem value via greenhouse gas mitigation could greatly benefit conservation efforts. To make conservation efforts worthwhile to greenhouse gas mitigation, these findings help to consider and prioritize sites where risk and impact of Corg lost is more severe.

The role of consumer-resource relationship is the basis of the structure and functioning of ecological communities. The traditional lens for studying such interactions have tended to omit the importance of the species behaviour, particularly in the case of plant-herbivore interactions. However, they way the herbivore exploits the plant and the way the plant responds to that pressure is crucial in determining the stability of the interaction. Thus, both herbivore and plant performances, contextualised in the environment where the interaction takes place, can act as drivers of the resilience of the system. This thesis focuses on Posidonia oceanica ecosystems and specifically on the interaction between this seagrass and its two main herbivores – the fish Sarpa salpa and the sea urchin Paracentrotus lividus –. I assess the importance of herbivore behaviour (the aggregation patterns and feeding behaviour of S. salpa and the canopy cover dependence of P. lividus) and the resilience provided feedback mechanisms (direct responses of the plant and indirect ecosystem processes) under changing conditions resulting from global change. Disentangling these components of the interaction allows us to assess its sensitivity to each of the components and to test the response and resilience of the system under different conditions. Chapter 1 shows the importance of body size of Sarpa salpa individuals in their individual feeding activity, shoaling aggregation patterns and shoal feeding strategies, as well as the potential consequences of this on the seagrass P. oceanica. On the one hand, the larger the individuals are, the more feeding activity they show. On the other hand, S. salpa individuals tend to aggregate with conspecifics of the same body size and group size is positively related to the size of the individuals in the group. In addition, feeding strategies increase in complexity with group size and tend to be focused on very specific spots within the meadow. Thus, as individuals grow, they increase their potential impact on the seagrass, both in terms of their consumption capacity and the formation of large shoals capable of concentrating their consumption in very specific areas of the meadow. This distribution of herbivory can lead to spatial heterogeneity with consequences for the functioning of the ecosystem dominated by P. oceanica. In chapter 2, a number of regulatory mechanisms arising from an episode of intense herbivory are found to provide resilience to Posidonia oceanica system once its canopy height is reduced. Four of the mechanisms evaluated function as feedback mechanisms, one of them being actively deployed by the plant (compensatory growth) while the other three (preference for an alternative resource, increased risk of predation and reduced urchin numbers due to competition for the resource and loss of cover) are triggered indirectly and their effectiveness is based on inducing changes in the behaviour of the herbivore Paracentrotus lividus. The results obtained show how P. oceanica is able to invest efforts in recovering part of the lost leaf biomass while the system itself is able to regulate herbivory pressure as long as the environmental conditions are appropriate to the occurrence of these mechanisms. The role of these mechanisms is key to avoid the potential collapse of P. oceanica meadows under the stress caused by an episode of intense herbivory. In chapter 3, the effect of global warming on the Posidonia oceanica – Sarpa salpa interaction is assessed through a combination of gradient approaches in the field and manipulative laboratory experiments. On the one hand, the results show that increasing water temperature significantly increases S. salpa growth rates during its larval stage, reducing its larval period (fewer days in the water column) and limiting its dispersal, while showing no effect on feeding behaviour during its adult phase. On the other hand, warming negatively affects the growth rates of P. oceanica and makes it more palatable towards S. salpa according to the results from the preference experiment. Our study shows that S. salpa could develop faster in warmer environments during its most vulnerable stage, increasing its survival but decreasing its dispersal capacity. At the same time, it could increase its preference for P. oceanica in its adult stage, which, together with the reduction of seagrass growth, could considerably intensify the strength of the interaction under warming conditions. All in all, the results of this study have contributed to confirm the relevance of herbivores’ behaviour in the way they exploit P.oceanica, mainly the feeding strategies of S. salpa, and how feedback mechanisms, provide resilience and allow the ecosystem to be maintained in an vegetated state. All of this gets relevant when contextualised within the process of global change, with a probable strengthening of the plant-herbivore interaction. Understanding the sensitivity of the interaction equilibria to each of its components is crucial to decide where to invest conservation efforts in these ecosystems and to be able to anticipate how changes in the contextual conditions may alter the final balance of the interaction.
El papel de la relación consumidor-recurso es la base de la estructura y el funcionamiento de las comunidades ecológicas. La óptica tradicional usada en el estudio de estas interacciones ha tendido a omitir la importancia del comportamiento de las especies, sobre todo en el caso de las interacciones planta-herbívoro. Sin embargo, tanto la forma en la que el herbívoro explota a la planta y como la manera en que ésta responde a esa presión son cruciales para determinar la estabilidad de la interacción. Por lo tanto, las actuaciones del herbívoro y la de la planta, contextualizadas en el entorno en el que tiene lugar la interacción, pueden actuar como mediadores de la resiliencia del sistema. Esta tesis se centra en los ecosistemas de Posidonia oceanica y, concretamente, en la interacción entre esta fanerógama con sus dos principales herbívoros -el pez Sarpa salpa y el erizo de mar Paracentrotus lividus-. En esta tesis se evlaúa la importancia del comportamiento de los herbívoros (los patrones de agregación y las estrategias de alimentación de S. salpa y la dependencia de la cobertura foliar de P. lividus) y la resiliencia proporcionada por los mecanismos de retroalimentación (respuestas directas de la planta y procesos indirectos del ecosistema) bajo condiciones cambiantes resultantes del cambio global. Desentrañar estos componentes de la interacción nos permite evaluar su sensibilidad a cada uno de los dichos componentes y comprobar la respuesta y la resiliencia del sistema bajo diferentes condiciones. En el capítulo 1 se muestra la importancia del tamaño del cuerpo de los individuos de Sarpa salpa en sus tasas de consumo individual, en sus patrones de agregación y en las estrategias alimenticias de grupos, además de las potenciales consecuencias de estos procesos sobre la fanerógama Posidonia oceanica. Por un lado, cuanto mayores son los individuos, mayores tasas de consumo muestran. Por otro, los individuos de S.salpa tienden a agregarse con conspecíficos de la misma talla, mientras que el tamaño de los grupos se relaciona positivamente con la talla de los individuos que lo forman. Además, las estrategias de alimentación aumentan en complejidad con el tamaño del grupo y tienden a focalizarse en puntos muy concretos de la pradera. Así, a medida que los individuos crecen, aumentan su potencial impacto en la fanerógama, tanto por su capacidad de consumo como la formación de grandes bancos capaces de concentrar su herbivoría en áreas muy concretas de las praderas. Esta distribución de la herbivoría puede provocar una heterogeneidad espacial con consecuencias sobre el funcionamiento del ecosistema dominado por P. oceanica En el capítulo 2 se comprueba la existencia de una serie de mecanismos reguladores surgidos de un episodio de herbivoría intensa que aportan resiliencia al sistema de Posidonia oceanica una vez disminuida su bóveda foliar. Cuatro de los mecanismos evaluados funcionan como mecanismos de retroalimentación, siendo uno de ellos desplegado activamente por la planta (crecimiento compensatorio) mientras que los tres restantes (preferencia por un recurso alternativo, incremento del riesgo de depredación y disminución del número de erizos por competencia por el recurso y pérdida de cobertura) se desencadenan de forma indirecta y su eficacia se basa en inducir cambios en el comportamiento del herbívoro Paracentrotus lividus. Los resultados obtenidos muestran como P. oceanica es capaz de invertir esfuerzos en recuperar parte de la biomasa foliar perdida, mientras que el mismo sistema es capaz de regular la presión de herbivoría siempre que las condiciones del entorno sean propicias para la aparición de dichos mecanismos. El papel resiliente de estos mecanismos es clave para evitar el potencial colapso de las praderas de P. oceanica bajo el estrés provocado por un episodio de herbivoría intensa. En el capítulo 3 se evalúa el efecto del calentamiento global en la interacción Posidonia oceanica – Sarpa salpa a través de una combinación de aproximaciones de gradiente en campo con experimentos manipulativos de laboratorio. Por un lado, los resultados muestran que el aumento de la temperatura del agua incrementa significativamente las tasas de crecimiento de S.salpa durante su etapa larval, acorta su período en dicha etapa (menos días en la columna de agua) y limita su dispersión, mientras que no muestra ningún efecto en la actividad herbívora durante su fase adulta. Por otro lado, el calentamiento afecta negativamente las tasas de crecimiento de P. oceanica y la vuelve más palatable frente a S. salpa de acuerdo con los resultados en el experimento de preferencia. Nuestro estudio muestra que S. salpa podría desarrollarse más rápido en su etapa más vulnerable, aumentando su supervivencia, pero disminuyendo su capacidad de dispersión, mientras que podría incrementar su preferencia por P. oceanica en su etapa adulta, lo que, junto con la reducción del crecimiento de la fanerógama, podría intensificar considerablemente la fuerza de la interacción. En definitiva, los resultados de esta tesis han servido para constatar la relevancia del comportamiento de los herbívoros en su forma de explotar el recurso, principalmente las estrategias de alimentación de S. salpa, y como los mecanismos de retroalimentación aportan resiliencia y permiten al ecosistema mantenerse en un estado óptimo. Todo ello cobra más importancia al contextualizarlo dentro del proceso del cambio global, dado el probable fortalecimiento de la interacción planta-herbívoro. Conocer la sensibilidad de la interacción a cada uno de los componentes es crucial para decidir donde hay que invertir los esfuerzos de conservación en estos ecosistemas y poder así anticiparnos a como los cambios en las condiciones de contorno pueden alterar el equilibrio final de la interacción.

Coastal ecosystems, especially the vegetated areas, are among the most threatened ecosystems in the world, undergoing a fast and constant decline. Their losses are of serious concern due to their elevated production, providing many ecosystem services essential to the well-being of our societies. Behind the regressive trends of the coastal ecosystems, there is a plethora of adverse human pressures, going from local and regional impacts, including anthropogenic activities in and outside the coastal regions, to large-scale drivers of change, such as the global warming. Nevertheless, there is a critical lack of long-term information about the vegetated coastal ecosystems, information that can provide baseline ecological data of their natural dynamics and vulnerability. Seagrasses are marine plants, engineering species that form underwater meadows, which, among many other services, provide essential habitat for many other organisms. Seagrasses meadows are experiencing a widespread decline since the early 20th century. This regression is accelerated for the Mediterranean endemic seagrass species Posidonia oceanica. Long-term studies are of particular interest in P. oceanica meadows because this species is a large-slow growing and long-lived seagrass, which substantial changes and responses manifest over time scales of decades to centuries. A deeper understanding of seagrass long-term dynamics can help managers to apply meadow-specific actions and act at the appropriate temporal scales. The discipline of paleoecology allows the study of long-term ecosystem dynamics on time scales of centuries to millennia, and it can be used in seagrass meadows thanks to the organic deposits accumulated below P. oceanica meadows. Paleoreconstructions using seagrass deposits are still scarce and have mainly focused on allogenic (externally controlled) processes. In this dissertation, a paleoecological approach at a regional spatial-scale was used to explore the long-term dynamics of the autogenic and biotic ecological components of Mediterranean seagrass meadows, mainly P. oceanica meadows. Initially, we investigated the usefulness of several biogeochemical proxies and a technique (FTIR-ATR spectroscopy) so far unexplored in seagrass deposits, as well as which were the main biogeochemical processes recorded by them. We described the long-term dynamics of the seagrass ecosystem, the main drivers of change, and their relative importance. The results indicated that seagrass long-term dynamics are oscillating. Even though most meadows showed regressive trends during the last 150 years, seagrass trends varied spatially, with the main spatial differences occurring at the inter-regional level. Differences in long-term dynamics between local sites seemed mostly dependent on the environmental background of each site, which also affected seagrass long- term resilience. The major factors responsible for long-term variability of seagrass ecosystem dynamics were multiple and at both, local and large spatial scales. However, the balance between the contribution of local and large-scale drivers varied spatially. The influence of climate seemed especially crucial in meadows surrounded by more turbid waters, under the influence of higher fluvial discharges. These meadows showed lower long-term ecosystemic resilience. In summary, this research showed that seagrass long-term dynamics can be studied through their paleoecological record, providing a valuable frame of reference for evaluating the magnitude of current changes and consequences of combined diverse impacts on these marine ecosystems. The results of this thesis indicated that despite some spatial variability of the long-term dynamics, the major changes occurred over the last century, predominating trends of seagrass decline or community compositional changes. Moreover, our results point to a more acute negative impact of present climate change in meadows where light availability is compromised due to local factors. The overall spatial variability regarding seagrass long-term dynamics highlights the need for meadow-specific local management with background information, information that can be obtained from paleoecological studies.
Los ecosistemas costeros, especialmente aquellos dominados por macrófitos sumergidos o semi-sumergidos, se encuentran entre los ecosistemas más amenazados del mundo, sufriendo un rápido y constante declive. Sus pérdidas son motivo de grave preocupación debido a su elevada producción, y a que proporcionan muchos servicios ecosistémicos esenciales para el bienestar de nuestras sociedades. Detrás de las tendencias regresivas de los ecosistemas costeros, se encuentra una plétora de presiones humanas adversas, que van desde impactos locales y regionales, incluidas actividades antropogénicas dentro y fuera de las regiones costeras, hasta agentes de cambio a gran escala, como el calentamiento global. Sin embargo, existe una falta crítica sobre el cambio a largo plazo de los ecosistemas costeros vegetados, información que puede proporcionar datos ecológicos de referencia sobre sus dinámicas naturales y vulnerabilidad. Las fanerógamas marinas son plantas superiores creadoras de estructuras tridimensionales complejas que, entre otros muchos servicios, dan lugar a un hábitat de elevada biodiversidad. Estos macrófitos están experimentando un declive generalizado desde principios del siglo XX, regresión especialmente acelerada para la especie endémica mediterránea, Posidonia oceanica. Los estudios a largo plazo son de particular interés en las praderas de P. oceánica, pues al ser una planta marina de gran tamaño, de crecimiento lento y de vida larga, sus cambios y respuestas sustanciales se manifiestan en escalas de tiempo de décadas a siglos. Una comprensión más profunda de la dinámica a largo plazo de las fanerógamas marinas puede ayudar a los gestores a aplicar acciones específicas y actuar en las escalas temporales adecuadas. La disciplina de la paleoecología permite el estudio de la dinámica de los ecosistemas a largo plazo en escalas de tiempo de siglos a milenios, y se puede aplicar en praderas de fanerógamas de P. oceanica gracias a los depósitos orgánicos acumulados bajo ellas. Las reconstrucciones paleoecológicas que usan suelos de praderas marinas aún son escasas y las que hay se han centrado principalmente en procesos alogénicos del ecosistema (controlados externamente). En esta tesis se ha hecho uso de reconstrucciones paleoecológicas en praderas de fanerógamas mediterráneas, principalmente de P. oceánica, a una escala espacial regional con el objetivo de explorar la dinámica a largo plazo de los componentes ecológicos autogénicos y bióticos. Inicialmente, se investigó la utilidad de varios proxies (indicadores) biogeoquímicos y de una técnica (espectroscopía FTIR-ATR) hasta ahora inexplorados en depósitos de praderas marinas, así como cuáles eran los principales procesos biogeoquímicos registrados por estos depósitos. Los resultados obtenidos permitieron describir la dinámica a largo plazo de las praderas marinas, así como sus principales impulsores del cambio a largo plazo y su importancia relativa. Se observó que la dinámica a largo plazo es oscilante, y que la mayoría de las praderas mostraban tendencias regresivas durante los últimos 150 años. Sin embargo, estas tendencias de declive variaban espacialmente, ocurriendo las principales diferencias a escala interregional. Las diferencias en la dinámica a largo plazo entre localidades parecían depender principalmente del contexto ambiental de cada sitio, lo que también afectaba a la resiliencia a largo plazo de las praderas. Los resultados revelan que los principales factores responsables de la variabilidad a largo plazo son múltiples, incluyendo factores de carácter local pero también regional y global. Sin embargo, la contribución relativa entre los factores de influencia local y de gran escala varia espacialmente. La influencia del clima parece especialmente crucial en praderas creciendo en aguas más turbias, bajo la influencia de descargas fluviales más abundantes. Estas praderas mostraron una menor resiliencia ecosistémica a largo plazo. En resumen, esta investigación ha demostrado que las dinámicas a largo plazo de las fanerógamas marinas se pueden estudiar a través de su registro paleoecológico, proporcionando un valioso marco de referencia para evaluar la magnitud de cambios actuales y las consecuencias de diversos impactos combinados en estos ecosistemas marinos. Los resultados de esta tesis revelan que, a pesar de cierta variabilidad espacial de las dinámicas a largo plazo, los cambios más importantes han ocurrido durante el último siglo, predominando las tendencias de declive de la fanerógama o cambios en la composición de las comunidades que alberga. Además, nuestros resultados apuntan a un impacto negativo más agudo del actual cambio climático en aquellas praderas donde la disponibilidad de luz se ve comprometida debido a causas locales. La variabilidad espacial general de las dinámicas a largo plazo de las praderas marinas destaca la necesidad de una gestión local específica a cada pradera, con información previa contextual, información que se puede obtener a partir de estudios paleoecológicos.

Coastal marine ecosystems are among the most impacted globally, attributable to individual and cumulative effects of human disturbance. Anthropogenic nutrient loading is one stressor that commonly affects nearshore ecosystems, including seagrass beds, and has positive and negative effects on the structure and function of coastal systems. An additional, previously unexplored mechanistic pathway through which nutrients may indirectly influence nearshore systems is by driving blooms of benthic jellyfish. My dissertation research, conducted on Abaco Island, Bahamas, focused on elucidating the role that benthic jellyfish have in structuring systems in which they are common (i.e., seagrass beds), and explored mechanistic processes that may drive blooms of this taxa. To establish that human disturbances (e.g., elevated nutrient availability) may drive increased abundance and size of benthic jellyfish, Cassiopea spp., I conducted surveys in human-impacted and unimpacted coastal sites. Jellyfish were more abundant (and larger) from human-impacted areas, positively correlated to elevated nutrient availability. In order to elucidate mechanisms linking Cassiopea spp. with elevated nutrients, I evaluated whether zooxanthellae from Cassiopea were higher from human-disturbed systems, and whether Cassiopea exhibited increased size following nutrient input. I demonstrated that zooxanthellae population densities were elevated in human-impacted sites, and that nutrients led to positive jellyfish growth. As heightened densities of Cassiopea jellyfish may exert top-down and bottom-up controls on flora and fauna in impacted seagrass beds, I sought to examine ecological responses to Cassiopea. I evaluated whether there was a relationship between high Cassiopea densities and lower benthic fauna abundance and diversity in shallow seagrass beds. I found that Cassiopea have subtle effects on benthic fauna. However, through an experiment conducted in a seagrass bed in which nutrients and Cassiopea were added, I demonstrated that Cassiopea can result in seagrass habitat modification, with negative consequences for benthic fauna. My dissertation research demonstrates that increased human-driven benthic jellyfish densities may have indirect and direct effects on flora and fauna of coastal marine systems. This knowledge will advance our understanding of how human disturbances shift species interactions in coastal ecosystems, and will be critical for effective management of jellyfish blooms.

Aquaculture is of growing importance in the global seafood production. The environmental impact of aquaculture will largely depend on the type of environment in which the aquaculture system is placed. Sometimes, due to the abiotic or biotic conditions of the seascape, certain aquaculture systems tend to be placed within or near specific ecosystems, a phenomenon that in this thesis is referred to as aquaculture system - ecosystem links. The exposed ecosystems can be more or less sensitive to the system specific impacts. Some links are known to be widespread and especially hazardous for the subjected ecosystem such as the one between the shrimp aquaculture and the mangrove forest ecosystem. The aim of this thesis was to identify and investigate causes and consequences of other spatial links between aquaculture and ecosystems in the tropical seascape. Two different aquaculture system - ecosystem links were identified by using high resolution satellite maps and coastal habitat maps; the link between sea cage aquaculture and coral reefs, and the one between seaweed farms and seagrass beds. This was followed by interviews with the sea cage- and seaweed farmers to find the drivers behind the farm site selection. Many seaweed farmers actively choose to establish their farms on sea grass beds but sea cage farmers did not consider coral reefs when choosing location for their farms. The investigated environmental consequences of the spatial link between sea cage aquaculture and coral reefs were considerable both on the local coral reef structure, and coral associated bacterial community. Furthermore, coral reef associated fish are used as seedlings and feed on the farms, which likely alter the coral food web and lower the ecosystem resilience. Unregulated use of last resort antibiotics in both fish- and lobster farms were also found to be a wide spread practice within the sea cage aquaculture system, suggesting a high risk for development of antibiotic resistant bacteria. The effects of seaweed farms on seagrass beds were not studied in this thesis but have earlier been shown to be rather substantial within the borders of the farm but less so outside the farm. Further, a nomenclature is presented to facilitate the discussion about production system - ecosystem links, which may also be used to be able to incorporate the landscape level within eco-certifying schemes or environmental risk assessments. Finally - increased awareness of the mechanisms that link specific aquaculture to specific habitats, would improve management practices and increase sustainability of an important and still growing food producing sector - the marine aquaculture.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

Coastal vegetation plays an essential role in regulating climate change and water quality, especially seagrass meadows that cover up to 1.6 million km2 worldwide and have been identified as globally significant filters and sinks of biogeochemical elements, including organic carbon (Corg) and nitrogen (N). Over the past century, the accelerating intensity and duration of natural and anthropogenic disturbances have caused severe regime shifts in estuarine and other coastal ecosystem dynamics, causing seagrass losses and affecting their valuable ecosystem services. Although knowledge of long-term seagrass ecosystem dynamics is scarce, including baseline conditions prior to major disturbances, the limited available information has been crucial to assess the human and natural impacts on coastal ecosystems. For example, it has provided answers to key questions related to seagrass ecology, climate change and management, including the estimation of the ecological significance and the monetary value of the biogeochemical sinks associated with seagrass ecosystems. This thesis aims to use seagrasses biogeochemical sinks as archives to reconstruct and understand : 1) baseline conditions in Australian estuaries, and spatial variability in environmental change, including perturbations that triggered the loss of seagrass meadows; 2) the significance of seagrasses in global biogeochemical cycles, as well as the processes that support their potential to store carbon and nitrogen; and 3) the losses of biogeochemical sinks as a consequence of seagrass meadow losses through human-induced impacts and extreme weather events. In this thesis, seagrass sedimentary archives encompassing the last few centuries have been studied to decipher centennial-scale environmental change in temperate estuaries, and the impacts of marine heatwaves and eutrophication on the Corg and N biogeochemical sinks associated to seagrass ecosystems. In the first data chapter of this thesis, the impact of land-use change on the ecological dynamics of temperate estuaries in Australia is reconstructed based on the analysis of multiple proxies in seagrass sedimentary archives encompassing the last 500 years. This palaeoecological study revealed the effect of land-use change following European settlement in the 1800s in Australia on the ecological condition of the estuaries, highlighting the deterioration of seagrass meadows following increased coastal development and agriculture activity after World War II. The second data chapter examined the effect of seagrass loss due to eutrophication on seagrass soil Corg stocks and fluxes, and provides pioneering estimates of CO2 emissions following disturbance of seagrass ecosystems that can be used to support the development of seagrass blue carbon projects (conservation and restoration) to mitigate climate change. The results showed that seagrass loss alone does not necessarily drive erosion of soil Corg, but when combined with sufficient hydrodynamic energy at the sediment surface ( > 0.20 m/s in this case), significant losses occurred (88–95% of soil Corg stocks). The study provided first-order estimates of potential CO2 emissions from eutrophication-induced seagrass loss since the 1950s in Australia, with 161,150 hectares of seagrass habitat loss that likely resulted in the release of 11–21 Tg CO2 (equivalent to a 2% increase in annual CO2 emissions from land-use change). These data will be crucial to inform the implementation of seagrass blue carbon into the Australian climate change mitigation policy. The third data chapter assessed soil N stocks and accumulation rates in Australian seagrass meadows, and provides pioneering estimates of soil N depletion following disturbance of seagrass meadows due to eutrophication and marine heatwaves, and identified the main drivers and potential ecological consequences of those losses. The results showed that Australian seagrasses capture 216–910 Gg N yr-1, equivalent to 96–105% of N runoff from Australian catchments. On the other hand, Australian-wide seagrass losses since the 1950s likely resulted in the loss of 435-720 Gg N from their soils, which likely enhanced eutrophication processes and resulted in adverse ecological consequences. This thesis provides novel and key information on the role of seagrasses as biogeochemical sinks and sources. This information can inform management practices of estuarine and other coastal ecosystems and highlights the value of seagrass sedimentary archives for determining baseline cycles and to reconstruct the time-course of ecological change in response to natural and anthropogenic disturbances. This thesis also highlights the need to conserve and restore seagrass meadows due to their value as natural archives and biogeochemical sinks, demonstrating their potential as a Natural-based Solution for contributing to climate change mitigation.

The tropical seascape is herein defined as a landscape made up of five ecosystems: coastal terrestrial forests, mangrove forests, seagrass beds, coral reefs and the deep sea. Previous studies have shown that men and women use the tropical seascape in different ways. If the seascape was to change as a result of anthropogenic climate change, then men and women could potentially be affected differently by those changes. The seascape is particularly vulnerable to the predicted rise in sea-level and ocean warming, but the coastal terrestrial forests and mangrove forests are in addition vulnerable to the increased storms and hurricanes a warmer climate is predicted to lead to. While men and women utilizes these ecosystems in many different ways, this study found, based on the literature reviewed, that in a worst-case scenario all parts of the seascape could potentially be destroyed and none of the activities performed there today could be performed in the future. The deep sea would not be destroyed, but the fish living there would move to higher latitudes and deeper waters, effectively ending the fishing practices in the tropical waters. To save the tropical seascape anthropogenic climate change would have to stop on a global scale, since the problem cannot be solved on a local or regional level.

Seagrass meadows, like coral reefs, are in decline globally but are often neglected in marine policy and conservation despite their equally critical ecosystem services. Both habitats can be heavily impacted by wave surges, rainfall-induced earth movement and flooding, changes to water temperature, salinity, and acidity, and increased levels of turbidity—all occurring at increased rates due to a changing global climate. We demonstrate that multispectral satellite imagery, geospatial tools, and classification techniques can be used to inform management by identifying and quantifying changes in seagrass distribution and the presence of sediment-related threats. Results from Dominica indicate near-shore seagrass habitat area increased by 195.7 hectares between 2016 and 2019, suggesting a continued expansion of Halophila stipulacea. Further analysis showed 22.4 hectares of accreted coastal sediment and 1362.2 hectares of suspended sediment captured, placing 424.4 hectares of sensitive reef area at risk of experiencing tissue abrasion or reduced photosynthetic activity. Our methods can be used by marine resource managers and policy makers to inform decisions relating to fisheries production, emissions trading, disaster risk mitigation, and invasive species monitoring, facilitating sustainable growth in the blue economy.

During the last decade, the academic interest for Earth’s natural carbon sinks and their role concerning climate change has increased. Today, many scientists around the world are trying to calculate different ecosystem’s potential to sequester and store carbon dioxide from the atmosphere. As a newcomer to the scientific arena, the term ‘blue carbon’ has been well received by scientists in the field. ‘Blue carbon’ highlights the carbon captured and stored by productive ecosystems along the world’s coasts. The term refers to coastal wetlands – such as mangrove forests, salt marshes and seagrass meadows – and it came to life as the scientific community recognized these ecosystems’ significant potential as effective carbon sinks. New research indicates that these ecosystems’ complex and vertical root systems can store much larger amounts of carbon in the soil than any other terrestrial ecosystem. By studying this subject, scientists are trying to understand how these ecosystems can help us in the quest of removing excessive carbon dioxide from the atmosphere. The goal of this thesis is to conduct a literature review, aiming to analyse and compile the new research on ‘blue carbon’ that has been published during the last 10 years. The paper aims to investigate whether the ecosystem’s potential as carbon sinks differ from each other, and what threats they will face in the future. It will additionally review if scientists have been able to unite around any predictions about what the future for ‘blue carbon’ – and its role in mitigating climate change – will look like.
Under det senaste decenniet har intresset kring naturliga kolsänkors potential och roll i att mildra klimatförändringar ökat. Idag är det många forskare som arbetar med att beräkna mängden kol som olika ekosystem runt om världen kan lagra i sin biomassa och i jorden under dess rötter. Som en nykomling på den vetenskapliga arenan, har termen ’blue carbon’ blivit väl mottaget av forskare inom området. ’Blue carbon’ syftar på det kol som fixeras och lagras av de produktiva ekosystemen längs världens kuster. Termen refererar till kustbelägna våtmarker – så som mangroveskogar, saltträsk och sjögräsbäddar – och introducerades efter att den vetenskapliga världen erkänt deras imponerande potential som kolsänkor. Ny forskning tyder på att deras avancerade och vertikala rotsystem kan lagra mer koldioxid i marken än vad vanliga terrestra skogar kan. Genom att studera detta ämne försöker forskare att förstå hur dessa ekosystem kan hjälpa oss att avlägsna överskottet av koldioxid från atmosfären. Målet med denna uppsats är att utföra en litteraturstudie och analysera, samt sammanställa den nya forskningen om ’blue carbon’ som publicerats de senaste 10 åren. Uppsatsen kommer undersöka hur stor skillnad det är mellan de olika ekosystemen och vilka hot de står inför i framtiden. Dessutom kommer den undersöka ifall forskare kommit närmre i att enas kring förutsägelser om framtiden för ’blue carbon’, och hur dess roll i att mildra klimatförändringarna kommer se ut.

Rising carbon dioxide (CO2) concentrations in the atmosphere will increase the average pCO2 level in the world oceans, which will have a knock-on effect on the marine ecosystem. Coastal seagrass communities one of the most productive marine ecosystems are predicted to benefit from the increase in CO2 levels, but long-term effects of elevated CO2 on seagrass communities are less understood. Population reconstruction techniques was used to investigate the population dynamics of Cymodocea nodosa meadows, exposed to long term elevated CO2 at volcanic seeps off Greece and Italy. Effect of elevated CO2 was noticed on the growth, morphometry, density, biomass and age structure at CO2 seeps. Above to below ground biomass ratio of C. nodosa were higher at CO2 seeps than at reference sites. The plastochrome interval were similar at all CO2 seeps. The shoot age and shoot longevity of plants were lower at seeps than reference sites. The present recruitment (sampled year) of the seagrass were higher than long-term average recruitment of the communities near the seeps. Carbon to nitrogen ratios (%DW) of C. nodosa were higher in leaves at seeps. Annual leaf production was higher near the seeps. This study suggests increased production of C. nodosa under elevated CO2 levels, but other co-factors such as nutrients, trace metal toxicity must also be taken into consideration while predicting effects of future CO2 concentrations. Volcanic CO2 seeps are now being used as natural analogues for ocean acidification studies although these areas can be affected by trace element input and may alter ecosystem responses to gradient in carbonate chemistry. Here Fe and a range of trace elements (Cd, Co, Cu, Hg, Mn, Pb, Ni and Zn) were analysed from sediments and from the roots, rhizomes and leaves of seagrass at six CO2 seeps and reference sites off Greece and Italy. There were higher metal levels in sediment and seagrasses at all CO2 seeps than reference sites. Sediment Quality Guideline Quotient, a commonly used pollution index, indicated that some of the metals (Cd, Cu, Hg, Ni) were in high enough concentrations to have adverse biological effects, such as Cu at Ischia site and Hg at Vulcano. Higher accumulation of elements from sediments in roots and leaves at CO2 seeps were found from Bio Sediment Accumulation Factor index. There were higher levels of Cu, Fe, Mn and Zn in leaves and rhizomes for P. oceanica and higher levels of Cd, Co, Cu, Fe and Zn in C. nodosa compartments at CO2 seeps. Fe and Mn were found with positive correlation within sediment-roots and sediment-rhizomes, whereas Cd, Co and Pb were found with positive correlation in compartments of C. nodosa. In P. oceanica positive correlation were only observed for Cd within sediment-roots and plant compartments. Low pH and ocean acidification increased the concentration of elements at CO2 seeps than reference sites. Thus, caution is needed, when using volcanic seep systems as analogue for the effects of rising CO2, as metals can reach levels that are toxic to seagrass, masking any potential benefits of increased levels of carbon dioxide for seagrass productivity. Net community production (NCP) and community respiration (CR) were measured under air exposed and CO2 enriched conditions for intertidal Z. noltei meadows and unvegetated sediment communities during emersion in summer and winter seasons. Community production and respiration were measured in-situ using benthic chambers. CO2 flux under air and CO2 enriched conditions were measured over a series of short term incubations (30min) using an infra-red gas analyser. Incident photosynthetic active radiation (PAR) was recorded during the incubations covering the daily and seasonal variation. Linear regression model was used to test the effects of irradiance on net community production. NCP of Z. noltei community were higher under CO2 enriched conditions than air exposed conditions in both summer and winter seasons. There was no effect of CO2 on the CR rate of Z. noltei community in summer season. NCP of sediment community were higher in summer season and winter season under CO2 enriched conditions. Sediment CR rates were higher in winter than summer season. The light compensation point of Z. noltei and sediment community were lower in both seasons under CO2 enriched conditions. Seasonal budget of community production was higher in Z. noltei than sediment communities. A clear effect of PAR was noticed on the net community production of both communities. Higher PAR intensities resulted in higher NCP under CO2 enriched conditions for both communities. CO2 enrichment will have a positive effect on the intertidal communities during emersion.

Shallow seagrass ecosystems frequently experience physical disturbance from vessel groundings. Specific restoration methods that modify physical, chemical, and biological aspects of disturbances are used to accelerate recovery. This study evaluated loss and recovery of ecosystem structure in disturbed seagrass meadows through plant and soil properties used as proxies for primary and secondary production, habitat quality, benthic metabolism, remineralization, and nutrient storage and exchange. The efficacy of common seagrass restoration techniques in accelerating recovery was also assessed. Beyond removal of macrophyte biomass, disturbance to seagrass sediments resulted in loss of organic matter and stored nutrients, and altered microbial and infaunal communities. Evidence of the effectiveness of restoration actions was variable. Fill placement prevented additional erosion, but the resulting sediment matrix had different physical properties, low organic matter content and nutrient pools, reduced benthic metabolism, and less primary and secondary production relative to the undisturbed ecosystem. Fertilization was effective in increasing nitrogen and phosphorus availability in the sediments, but concurrent enhancement of seagrass production was not detected. Seagrass herbivores removed substantial seagrass biomass via direct grazing, suggesting that leaf loss to seagrass herbivores is a spatially variable but critically important determinant of seagrass transplanting success. Convergence of plant and sediment response variables with levels in undisturbed seagrass meadows was not detected via natural recovery of disturbed sites, or through filling and fertilizing restoration sites. However, several indicators of ecosystem development related to primary production and nutrient accumulation suggest that early stages of ecosystem development have begun at these sites. This research suggests that vessel grounding disturbances in seagrass ecosystems create more complex and persistent resource losses than previously understood by resource managers. While the mechanics of implementing common seagrass restoration actions have been successfully developed by the restoration community, expectations of consistent or rapid recovery trajectories following restoration remain elusive.

The loss of large-bodied herbivores and/or top predators has been associated with large-scale changes in terrestrial, freshwater, and marine ecosystems around the world. Understanding the consequences of these declines has been hampered by a lack of studies in relatively pristine systems. To fill this gap, I investigated the dynamics of the relatively pristine seagrass ecosystem of Shark Bay, Australia. I began by examining the seagrass species distributions, stoichiometry, and patterns of nutrient limitation across the whole of Shark Bay. Large areas were N-limited, P-limited, or limited by factors other than nutrients. Phosphorus-limitation was centered in areas of restricted water exchange with the ocean. Nutrient content of seagrasses varied seasonally, but the strength of seasonal responses were species-specific. Using a cafeteria-style experiment, I found that fast-growing seagrass species, which had higher nutrient content experienced higher rates of herbivory than slow-growing species that are dominant in the bay but have low nutrient content. Although removal rates correlated well with nutrient content at a broad scale, within fast-growing species removal rates were not closely tied to N or P content. Using a combination of stable isotope analysis and animal borne video, I found that green turtles (Chelonia mydas) – one of the most abundant large-bodied herbivores in Shark Bay – appear to assimilate little energy from seagrasses at the population level. There was, however, evidence of individual specialization in turtle diets with some individuals foraging largely on seagrasses and others feeding primarily on macroalgae and gelatinous macroplankton. Finally, I used exclusion cages, to examine whether predation-sensitive habitat shifts by megagrazers (green turtles, dugongs) transmitted a behavior-mediated trophic cascade (BMTC) between sharks and seagrasses. In general, data were consistent with predictions of a behavior-mediated trophic cascade. Megaherbivore impacts on seagrasses were large only in the microhabitat where megaherbivores congregate to reduce predation risk. My study highlights the importance of large herbivores in structuring seagrass communities and, more generally, suggests that roving top predators likely are important in structuring communities - and possibly ecosystems - through non-consumptive pathways.

Coastline communities have experienced a marked increase in human populations over the last few decades. This increase in population places disproportionate pressure on coastal ecosystems to provide economic services to support local economies. At the same time, overuse of these services can aid in the destruction of the ecosystems responsible for them. Seagrass ecosystems are mainly found near coastlines, and are typically a chief provider of some of these economic goods and services. Many previous studies have documented the ecological functions of this seagrasses. Unfortunately, our increasing knowledge of seagrass structure and function has not been fully incorporated into economic models estimating their value. In this dissertation, I focus on the seagrass ecosystem in southern Biscayne Bay, and simultaneously study the ecological dynamics of the seagrass beds, and estimate its economic value. This value is based on recent ecological models in the literature as well as data I collected from the system. I focused on Biscayne Bay due to, 1) the relevance that this question had to the relationship between Biscayne Bay and the Miami metropolis, and 2) the lack of existing reliable models that explore this relationship in this area. More specifically, I became very interested in this question while working for Biscayne National Park, where such a model would have improved seagrass restoration work taking place there. I found that southern Biscayne Bay is dominated by Thalassia testudinum, with other seagrasses following a spatial pattern primarily determined by salinity and water column nutrient distribution. Syringodium filiforme was mostly found east of the islands, Halodule wrightii was mostly found near the shoreline, and Halophila engelmenii was spotted at only two of the 190 sites visited. T. testudinum distribution was largely unaffected by nutrient enrichment at all sites, but it appeared to induce severe herbivory further from the coastline. For the calendar year 2004, we deduced using a Total Ecosystems Valuation (TEV) model that seagrass ecosystems potentially contributed over $198 million US dollars to the local economy. We argue that a simultaneous understanding and use of both ecological and economic models is important for future conservation efforts of seagrass ecosystems.

Trophic downgrading of ecosystems necessitates a functional understanding of trophic cascades. Identifying the presence of cascades, and the mechanisms through which they occur, is particularly important for seagrass meadows, which are among the most threatened ecosystems on Earth. Shark Bay, Western Australia provides a model system to investigate the potential importance of top-down effects in a relatively pristine seagrass ecosystem. The role of megagrazers in the Shark Bay system has been previously investigated, but the role of macrograzers (i.e., teleosts), and their importance relative to megagrazers, remains unknown. The objective of my dissertation was to elucidate the importance of teleost macrograzers in transmitting top-down effects in seagrass ecosystems. Seagrasses and macroalgae were the main food of the abundant teleost Pelates octolineatus, but stable isotopic values suggested that algae may contribute a larger portion of assimilated food than suggested by gut contents. Pelates octolineatus is at risk from numerous predators, with pied cormorants (Phalacrocorax varius) taking the majority of tethered P. octolineatus. Using a combination of fish trapping and unbaited underwater video surveillance, I found that the relative abundance of P. octolineatus was greater in interior areas of seagrass banks during the cold season, and that the mean length of P. octolineatus was greater in these areas compared to along edges of banks. Finally, I used seagrass transplants and exclosure experiments to determine the relative effect of megagrazers and macrograzers on the establishment and persistence of three species of seagrasses in interior microhabitats. Teleost grazing had the largest impact on seagrass species with the highest nutrient content, and these impacts were primarily observed during the warm season. My findings are consistent with predictions of a behaviorally-mediated trophic cascade initiated by tiger sharks (Galeocerdo cuvier) and transmitted through herbivorous fishes and their predators.

As climate change continues, climactic extremes are predicted to become more frequent and intense, in some cases resulting in dramatic changes to ecosystems. The effects of climate change on ecosystems will be mediated, in part, by biotic interactions in those ecosystems. However, there is still considerable uncertainty about where and how such biotic interactions will be important in the context of ecosystem disturbance and climactic extremes. Here, I review the role of consumers in seagrass ecosystems and investigate the ecological impacts of an extreme climactic event (marine heat wave) and subsequent widespread seagrass die-off in Shark Bay, Western Australia. Specifically, I compare seagrass cover, shark catch rates, and encounter rates of air breathing fauna in multiple habitat types before and after the seagrass die-off to describe post-disturbance dynamics of the seagrass community, shifts in consumer abundances, and changes in risk-sensitive habitat use patterns by a variety of mesoconsumers at risk of predation from tiger sharks (Galeocerdo cuvier). Finally, I conducted a 16 month field experiment to assess whether xi loss of top predators, and predicted shifts in dugong foraging, could destabilize remaining seagrass. I found that the previously dominant temperate seagrass Amphibolis antarctica is stable, but not increasing. Conversely, an early-successional tropical seagrass, Halodule uninervis, is expanding. Following the die-off, the densities of several consumer species (cormorants, green turtles, sea snakes, and dugongs) declined, while others (Indo-Pacific bottlenose dolphins, loggerhead sea turtles, tiger sharks) remained stable. Stable tiger shark abundances following the seagrass die-off suggest that the seascape of fear remains intact in this system. However, several consumers (dolphins, cormorants) began to use dangerous but profitable seagrass banks more often following seagrass decline, suggesting a relaxation of anti-predator behavior. Experimental results suggest that a loss of tiger sharks would result in a behaviorally mediated trophic cascade (BMTC) in degraded seagrass beds, further destabilizing them and potentially resulting in a phase shift. My work shows that climactic extremes can have strong but variable impacts on ecosystems mediated in part by species identity, and that maintenance of top predator populations may by important to ecological resilience in the face of climate change.

Coastal seagrass ecosystems are complex habitats that are increasingly influenced by human perturbations. Disturbances that affect the strength of bottom-up (i.e. resource availability) and top-down (i.e. consumer) controls may also influence biomass distribution between trophic levels, sediment biogeochemistry, and seagrass ecosystem metabolism. Here, I experimentally tested how top-down and bottom-up perturbations interact with community structure (diversity, food chain length of epibenthic consumers) to alter sediment biogeochemistry and ecosystem metabolism in an experimental eelgrass (Zostera marina ) system. My data indicated that resource availability influenced SOM composition and ecosystem metabolism. Light availability tended to be a stronger determinant of SOM composition while nutrient enrichment affected secondary production of invertebrate grazers more strongly than primary producers or SOM. Top-down predator effects on SOM composition and ecosystem flux rates tended to be weak. However, the strength of the trophic cascade may partly be a function of grazer community composition and grazer susceptibility to predation. Finally, my results indicated that grazer species identity and community composition strongly influenced SOM composition. In addition to the main effects of light, nutrients, predators, and grazers there were a variety of interactive effects between resources and food web composition. Consequently, the effects of resource availability and food web composition on seagrass ecosystem functioning should not be considered in isolation.

The work of this Master of Science thesis project is an analysis of salinity effects on nearshore epifauna along the western shore of Biscayne Bay in southeast Florida, USA. Field collection surveys have found a high probability of occurrence of bigeye mojarra (Eucinostomus havana) in salinities near 25 ppt. In a salinity gradient observation experiment test subjects of the same species and size class were also observed more frequently at 24 ppt. In this analysis presence and abundance patterns found in field surveys were compared with behavioral results obtained in the observation tank. This apparatus provided insight into distribution patterns of the bigeye mojarra (Eucinostomus havana) and possible changes in distribution that may result from habitat changes in the future. Historically, the western shore of Biscayne Bay was more freshwater marsh than the mangrove dominated marine environment that prevails today. Changes to fresh water inputs into the Bay are planned through projects of the Comprehensive Everglades Restoration Plan (CERP). CERP is a joint Florida state and U.S. federal effort to redesign surface water flow through the canal system of South Florida, replenish the Everglades ecosystem, and restore a more natural quantity, timing, and distribution of flow into Biscayne and Florida Bays. Approved by the U.S. Congress as part of the Water Resources Development Act of 2000, CERP will be implemented by the South Florida Water Management District (SFWMD) and the U.S. Army Corps of Engineers (USACE). This plan is designed to restore the ecosystem from its freshwater core to the coastal wetlands recreating a condition close to that existing before the current system of flood control drainage canals was begun in 1903 and continued by the federal Central and Southern Florida Project in 1948 (www.evergladesplan.org). Changes are planned to divert a portion of canal flows to Biscayne Bay into coastal wetlands as sheet flow and surface runoff. Planned changes to freshwater delivery may change the habitat along the shoreline and thus the distribution of prey organisms living in this habitat. This may in turn affect predator fish important to local recreational and commercial fisheries as well as other predators such as wading birds. The analysis and prediction provided in this thesis work is important for better understanding the effects of restoration efforts on the Bay nearshore habitat and its condition as essential fish habitat, which is federally regulated by the Magnuson-Stevens Fishery Conservation and Management Act (Magnuson-Stevens, 1996). The distribution of small fish and invertebrate inhabitants of the nearshore environment and habitat environmental qualities have been recorded over the past five years from throw-trap surveys of the western shoreline of Biscayne Bay as part of a CERP-sponsored monitoring program. This pattern is correlated with salinity, but there may be other factors affecting the distribution of this species.

Small-scale fisheries employ many millions of people around the world, and are particularly important in developing countries, where the dependency on marine resources is high and livelihood diversification options are scarce. In many areas of the world however, small-scale fisheries are at risk which threatens the food security and wellbeing of coastal people. Small-scale fisheries management has in many cases been insufficient and new comprehensive approaches are recommended to achieve social-ecological sustainability in the long-term. The aim of this thesis is to analyze empirically how social-ecological elements of seagrass-associated small-scale fisheries in the Western Indian Ocean region can be addressed for a transformation from the current mostly degraded state to more sustainable social-ecological systems and secure future livelihoods. The main method used was semi-structured interviews with local fishers. The main findings show the crucial contributions seagrass-associated small-scale fisheries make to food security and income generation and highlight the need to acknowledge the social-ecological importance of seagrasses in the seascape (Paper I). A discrepancy between low societal gains of the fishing of sea urchin predator fish species and their crucial importance in the food web (in controlling sea urchin populations and the associated grazing pressure on seagrasses) was identified (Paper II). These results suggest catch-and-release practice of sea urchin predator fish species, which could contribute to more balanced predator – sea urchin – seagrass food webs in the long run. The use of illegal dragnets was identified as a major threat to local seagrass meadows (Paper IV). Institutional elements influencing the use of such destructive dragnet were identified to be normative, cultural-cognitive and economic, which constitutes an institutional misfit to the current emphasis on regulative elements in a hierarchical manner (Paper III). Concerning future co-management initiatives, gear restrictions and education were the favoured management measures among all fishers (Paper IV). A majority of fishers were willing to participate in monitoring and controls, and most fishers thought they themselves and their communities would benefit most from seagrass-specific management. These findings highlight the need for actions on multiple scales, being the local-, management-, policy- and governance levels. The suggested actions include: education and exchange of ecological and scientific knowledge, gear management including the cessation of dragnet fishing, strengthening of local institutions, an active participation of fishers in enforcement of existing rules and regulations and an introduction of adequate alternative livelihood options.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 4: Manuscript.

The design of effective species management and recovery plans for sea turtle populations requires targets that are informed by an understanding of knowledge gained at the level of individuals/populations in the context of the wider goal of protecting an ecosystem's structural and functional attributes. In this thesis I present the first detailed investigation of the multiple levels at which sea turtles, particularly green turtles (Chelonia mydas), interact with ecosystems. I begin by developing a framework for an age-structured population-level assessment of food consumption for hawksbill (Eretmochelys imbricata), loggerhead (Caretta caretta), and green turtles. This entailed construction of species-specific growth models for the western Atlantic, and subsequent integration of results with morphometric, survival, abundance, and food conversion efficiency to derive consumption estimates. At the ecosystem level, I developed models for the Caribbean and Hawai’í, where green turtles are present at very low abundances and reaching carrying capacity, respectively. In the Caribbean, results showed that green turtle grazing of seagrass substantially altered habitat complexity, reducing the refuge role of seagrass to reef fish and invertebrates, and leading to potentially dramatic changes in species biomass and composition. In Hawai‘i, by feeding on algae, green turtles were found to contribute to the resilience of reefs in the face of disturbance, a functional role that needs to be explicitly included in future studies of reef dynamics. Taken together, these findings highlight the need to consider trophic and indirect interactions in the evaluation of sea turtles’ role within ecosystems. Accurate and reliable estimates of foraging habitat extent are essential to inform realistic and sustainable turtle recovery targets, particularly given the current degraded state of coastal ecosystems. Using Landsat satellite imagery, I present a novel mapping approach for seagrass habitats at large scales. Such regional initiatives are also urgently needed if the international community is to meet aims to conserve 10 – 30 % of specific habitats. A comparison between reef extent, determined using remote sensing and existing data, showed that discrepancies ranged from + 1,316 % to - 64 %, underlining our limited ability to ascertain progress towards adopted global marine conservation targets based on current data.

Many coastal ecosystems around the world are dominated by submerged aquatic vegetation (SAV) habitats. These SAV habitats are known to provide many highly valuable ecosystem services such as habitat for commercial important species and increased water clarity. Water flow is an environmental variable which can have measurable effects on the ecosystem services provided by SAV, but is often not considered in studies assessing these services. This dissertation sought to investigate the links between SAV, primarily seagrasses, and hydrodynamics, paying special attention to the effects on sediments and fauna. Three main areas are discussed: (1) the effects of SAV on flow, (2) the effects of SAV and flow on deposition in SAV beds, and (3) the effects of SAV and flow on faunal communities in SAV beds. Seagrasses and other SAV reduce currents, attenuate waves, and dampen turbulence within their vegetative canopies, which in turn can enhance deposition and reduce the resuspension of sediment, organic matter, and passively settling larvae. The ability of SAV to retard flow may be further enhanced by increases in vegetated structure, such as shoot density, biomass, or canopy height, which can promote increased abundance and diversity of in- and epifauna within SAV beds. Ultimately, it is clear that hydrodynamics is an important factor that shapes SAV communities both physically (e.g. deposition, sediment structure, etc.) and biologically (e.g. faunal community composition, predation pressure, food availability, etc.).

Seagrasses are marine foundation species that form ecologically important habitats in coastal areas around the world. They provide a range of ecosystem services, including coastal protection and the recently recognised large contribution to global carbon sequestration and storage. To date, the majority of published studies on the aforementioned ecosystem services is limited to specific geographic regions and seagrass species. This PhD study attempted to explore and provide the first evidence, to the best of our knowledge, on the role of Scottishseagrasses as carbon sinks and sediment stabilisers. In 2013, shoot dynamics of Zostera noltii plots were monitored biweekly and seasonally in the Forth estuary and digital images of the surveyed plots were taken for the development of a remote sensing technique which would accurately estimate the vegetation cover. In 2014, sediment samples from vegetated and unvegetated plots within beds of Z. marina and Z. noltii were collected from all the major estuaries along the east coast of Scotland, from the Firth of Forth in the south to Dornoch Firth in the north. Samples were analysed for organic matter, organic carbon, radionuclides 210Pb, 137Cs and 241Am, and δ13C in order to determine the organic matter and organic carbon density, longevity and sources of carbon respectively. To explore the role of seagrass in sediment deposition and stability, surface elevation was measured monthly in seagrass plots and bare sediment in the Forth estuary over two years. The results and main mechanisms underlying these findings are reported and discussed in detail in each chapter. In short, the proposed method based on digital images provided estimates of seagrass coverage that are more accurate than observers' estimates, with some constraints when macroalge and/or extreme light are present. Intertidal seagrass meadows in Scotland showed significantly enhanced carbon storage compared with bare sediment. Seagrass plots contained variable quantities of carbon in their sediments with species composition having a significant effect on carbon stocks, whereas depth and seagrass abundance had no effect on carbon stores. Despite their small above-ground biomass Scottish seagrass plots had a strong influence on sediment deposition and prevented erosion. Further research is needed to understand what factors drive large carbon sequestration and storage at some sites, thus contributing policy-relevant information on the prediction of the seagrass carbon hot-spots. Also, long-term datasets on surface elevation change are important in order to understand the effect of all the processes involved on sediment deposition in seagrass beds.

Located 100 km west of Key West, Florida, Dry Tortugas National Park (DRTO) is a largely untouched subtropical marine ecosystem that serves as an important developmental habitat, nesting ground, and foraging area for several species of sea turtles, including green turtles. The Park supports a recovering population of green turtles comprised of resident juveniles, subadults, and adults of both sexes; nesting females include residents and migrating females that only return to nest. Stable isotope analysis has been applied widely to describe the trophic ecology of green turtles, from urbanized bays with significant anthropogenic input, to relatively pristine ecosystems with healthy populations at carrying capacity. However, there is a paucity of published literature about the trophic ecology of green turtles in DRTO. This study describes the trophic ecology occupied by two distinct size groups (61 green turtles < 60 cm (SCL) and 98 green turtles > 60 cm (SCL)). Flipper tissue and plasma were analyzed for stable isotopic composition of C and N. Flipper tissue values for δ15N (3.41‰ to 9.69‰) and δ13C (-22.43‰ to -5.38‰) fall within literature values for green turtles, and the wide range of values indicated they could potentially feed at multiple trophic levels. Understanding the trophic ecology of this population of green sea turtles is instrumental to effective management and habitat preservation strategies in DRTO.

Waterfowl are known to be capable of influencing wetland ecology in a number of ways, sometimes to the detriment of other species that also inhabit this type of environment. Western Golden Bay including Farewell Spit is one of the largest areas of intertidal sand flat habitat in New Zealand and supports a wide array of species including internationally important populations of bar-tailed godwits (Limosa lapponica) and red knot (Calidris canutus). These species, particularly red knot, have declined in number over the last the 25 years at this site. Another numerous species at this site, the black swan (Cygnus atratus), has been suggested as a possible contributor to the observed decline in wader numbers through their impact on the habitat. This thesis presents the findings of a research project on the role of black swans in the tidal seagrass (Zostera muelleri) ecosystem in western Golden Bay carried out between October 2007 and October 2008. In an effort create a clear picture of what role the black swans play in this environment the project focused on four major aspects of swan-ecosystem interactions. The first of these looked at the activity patterns of black swan. This showed the swans’ activity is largely dictated by the tidal cycle with foraging occurring during the intertidal period when the seagrass is accessible while roosting is mostly confined to around high and low tides. The second part of the project explored the influence black swans have on the tidal seagrass landscape through their foraging habits. This showed that while swan foraging occurs across the tide flats it is concentrated on denser patches, on both small (meters) and large (hectares) scales. Experimental grubbings showed that the grubbing activity of swans is capable of forming and expanding bare sand patches within seagrass beds and that these bare patches can persist for at least two months. The third part of the project focused on the direct impacts of swan foraging on the seagrass and associated benthic invertebrates. Exclusion plots showed that at some sites swan foraging can significantly reduce Zostera biomass and invertebrate biodiversity. The final aspect examined was the role of swan in biomass and nutrient cycling. A faecal deposition survey showed swans consume 23.40 g DW ha-1 day-1 of Zostera. The average intake rate was 27.25 g DW ha-1 day-1. Nutrient analysis of seagrass 4 showed that shoot material has significantly higher N, P, Ca and fibre than rhizome and that rhizome has significantly more soluble carbohydrates than shoots. On the basis of the swans’ direct and/or indirect influences on Zostera muelleri beds and the associated invertebrate fauna, swans could arguably be considered to be a major ecosystem engineer in the intertidal sandflats of Golden Bay.

Seagrasses are submerged marine plants that provide essential ecosystem functions, but are declining in abundance worldwide. As angiosperms, seagrasses are capable of sexual reproduction, but also propagate asexually through clonal rhizome growth. Clonal growth was traditionally considered the primary means for seagrass propagation. Recent developments in genetic techniques and an increasing number of studies examining seagrass population genetics, however, indicate that sexual reproduction is important for bed establishment and maintenance. Few studies have investigated the reproductive biology and ecology of sub-tropical seagrass species, although this information is necessary for effective management and restoration. This work investigates the influence of pore-water nutrients on flowering, water flow on seed dispersal, consumption on seed survival, and describes the reproductive phenology in Texas for the two dominant seagrass species in the Gulf of Mexico: turtle grass (Thalassia testudinum) and shoal grass (Halodule wrightii). These species exhibit distinctive reproductive seasons that span summertime months, but reproductive output varies spatially and temporally. Results of an in situ nutrient enrichment experiment indicate that turtle grass produces fewer flowers (but more somatic tissue) when exposed to high pore-water ammonium than when exposed to low pore-water ammonium, suggesting that nutrient loading has the potential to reduce seagrass reproductive output. Seed consumption may also limit reproduction and recruitment in some areas, as laboratory feeding experiments show that several local crustaceans consume shoal grass and turtle grass seeds and seedlings, which do not survive consumption. Dispersal experiments indicate that seed movement along the substrate depends on local water flow conditions, is greater for turtle grass than shoal grass, and is related to seed morphology. Under normal water flow conditions in Texas, turtle grass secondary seedling dispersal is relatively minimal (< 2.1 m d⁻¹) compared to primary dispersal, which can be on the order of kilometers, and shoal grass secondary seed dispersal can be up to 1.1 m d⁻¹, but seeds are likely retained in the parent meadow. Results from this work can be used when developing seagrass management, conservation and restoration actions and provide necessary information concerning a life history stage whose importance was historically under-recognized.
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碩士
國立臺灣海洋大學
海洋環境化學與生態研究所
102
In this study, the carbonate parameters around the Dongsha Atoll (DA), including dissolved inorganic carbon (DIC), total alkalinity (TA), pH, partial pressure of CO2 (pCO2) and saturation state of aragonite (a), were thoroughly investigated in June (summer) and December (winter) 2013. The results show that the seagrass-dominant areas, including the small lagoon of the Dongsha Island (DI), and the north and west coasts of the DI, were sinks of atmospheric CO2 in both summer and winter. On the contrary, the coral reef-dominant area, i.e. the DA lagoon, was a source of atmospheric CO2.This difference can be attributed to that photosynthesis is the controlling process in the seagrass-dominant area, which can consume CO2 and thereby make the DI lagoon a sink of atmospheric CO2; meanwhile calcification is the predominant biogeochemical process in the coral reef-dominant area, which can release CO2 and thereby make the DA lagoon a source of atmospheric CO2. The spatial variations of the carbonate parameters around the DI lagoon and the DA lagoon were largely controlled by the coverage of seagrass and coral-reef, respectively. For the DI lagoon, when the seagrass coverage is higher, pCO2 and DIC are lower and pH and a are higher, and vice versa. For the DA lagoon, when coral reef coverage is higher, TA, pH and a are lower, and vice versa. Furthermore, the annual air-sea CO2 exchange flux in the seagrass-dominant areas was estimated to be -8.01±4.18 mmolC m-2 d-1. If the estimated annual CO2 flux is extrapolated to the entire DA seagrass bed, it could take up 288 ton C yr-1 from the atmosphere. This result is lower than the previous estimate of 788 ton C yr-1 based on seagrass productivity measurement. We suggest that this discrepancy may arise from the fact that a fraction of carbon produced by the seagrass were released back into the water column when detritus was decomposed, and thus cause an overestimate for atmospheric CO2 uptake. Finally, we suggest that if high a water in the seagrass-dominant area can flow into the coral reef-dominant area, it may be beneficial for the DA to cope with ocean acidification.

碩士
國立中興大學
生命科學系所
106
Since the Industrial Revolution, the concentration of carbon dioxide (CO2) in the atmosphere has risen rapidly, resulting in ocean acidification (OA) and warming, which has threatened coastal ecosystems. Highly productive seagrasses usually live adjacent to or among coral reefs and can absorb CO2 in seawater, regulate the pH and the aragonite saturation state (ΩAr). The purpose of this study was to use coral reefs mesocosms to examine the effects of seagrasses on coral reef ecosystems under OA and warming. The net ecosystem production (NEP) was used to determine the functioning of the mesocosms. Three mesocosms without seagrass and three mesocosms with seagrass were designed to conduct four stages of OA and warming experiments. A non-acidified CO2 partial pressure of 400 ppm was used as control and 800 ppm was used as the OA scenario. The first stage was unacidified and 25˚C; the second stage was OA and 25˚C; the third stage was OA and warmed up to 28˚C; the fourth stage was OA and the temperature was raised to 31˚C. The results showed that under the OA scenario, the coral calcification rates with seagrasses were higher in OA (25˚C) and OA (28˚C) than without seagrasses. In addition, the OA scenario proliferated the macroalgae, and the growth rate of macroalgae without seagrasses was higher than that with seagrasses. Therefore, OA (25˚C) significantly increased the NEP of the mesocosms without seagrasses, but OA had no effect on the NEP of the mesocosms with seagrasses. Ecosystem respiration (ER) increased gradually with increasing temperature under the scenario of OA, but gross primary production (GPP) in OA (28˚C) reached the maximum, then dropped in OA (31˚C). Due to the high temperature environment of 31 ̊C, the growth rate of macroalgae reduced and the mortality increased. The mesocosms without seagrasses dropped sharply at OA (31˚C) by 0.93±0.22 g O2 m-2 d-1, while the mesocosms with seagrasses decreased moderately by 0.68±0.13 g O2 m-2 d-1. Under the condition of OA and warming in the future, seagrasses can maintain the calcification rate of corals. Seagrasses can inhibit the growth of macroalgae, caused the magnitude of NEP declined less and maintain the stability of coral reef ecosystems at a high temperature of 31˚C.

Research Doctorate - Doctor of Philosophy (PhD)
Seagrasses are generally known for their significant role in marine and estuarine ecosystems. The growth in human population along the coastal regions, where the seagrass live, makes them very vulnerable to the human-induced disturbances. Large-scale seagrass decline has been reported worldwide due to this problem. There is an evident need to monitor seagrass population to predict future changes and to protect coastal ecosystems from further degradation. The decline of seagrass beds results in their fragmentation and appearance of smaller patches of seagrass isolated from each other, The first goal of this thesis is to study how fragmentation of seagrass beds influence their role in the ecosystem. This study focused on how fragment size and its distance from the main bed influence abundance of mobile epifauna associated with seagrass. Artificial seagrass units were constructed to mimic the seagrass fragmentation at a small scale. The result from this experiment suggested neither fragment size nor on-patch location (edge vs middle) adequately account for variation in the abundance of seagrass-associated epifauna. The distance from large beds of seagrass was important, however. Fragments placed far away from the natural seagrass were colonized to a grater degree than the fragments placed near seagrass beds. Large fragments were also colonized more than the small ones at the furthest distance from natural beds. Thus, fragmentation does not necessary lead to decease in epifaunal abundances. The small isolated patches may serve as refuge sites of the marine organisms. The second part of the thesis specifically deals with the effect of heavy metal contamination on seagrass and associated fauna. Despite the well-publicised issue of metal contamination of highly urbanised estuaries and its effect on seagrasses, this is the first study that assessed the contamination effect on the seagrass community using a range of bioindicators and biomarkers in order to obtain an integral picture of the contamination effect. It was found the seagrass, Zostera capricorni accumulated high concentration of heavy metals and provide a good correlation between the concentration in their tissues and in the sediment. Moreover, there was no evidence of impact on the seagrass biomass, shoot and leaf density. Thus, this seagrass can be used as a good bioindicator because of the above reasons, and also they are sedentary and abundant in the polluted site, which makes them easily available for sampling. The abundance of epifaunal organisms associated with the seagrass was used as a bioindicator at the community level. Only gastropods decreased in abundance in the contaminated site in spring (when the number of seagrass fauna generally higher), and this might potentially be a good bioindicator in this system. Shell dimension and fluctuating asymmetry were used as biomarkers for this purpose. It was found that a bivalve associated with seedgrass, the ark cockle Anadara trapezia, in the polluted location (Cockle Baby) showed distinct morphological characters compared to the ones in unpolluted locations. The cockles were bigger, heavier and had bigger shell-height/shell-length ratios, but appeared much less abundant, which is contradicted with historically high abundance of this species in this location (Cockle Bay). Moreover, the cockles exhibited higher shell asymmetry compared to the ones in unpolluted locations. Leaf dimensions and leaf asymetry of seagrasses, and Halophila ovalis, were also employed as biomarkers. The results revealed that there was no significant difference in leaf asymmetry of seagrass from polluted and unpolluted locations. The effect of heavy metal might have been surpassed by other factors such as nutrients, which were also high in the polluted area. Interestingly, the leaves of seagrass in the polluted location compared to unpolluted ones were longer and wider indicating that there was a possibility that the detrimental effect of heavy metals on seagrass was compensated by favourable effects of elevated nutrients. Laboratory studies were conducted to determine whether heavy metals themselves affected seagrasses. The results revealed that heavy metals exposure affected the growth of seagrass, and lead to increase in fluctuating asymmetry. Thus, leaf dimensions, shell dimensions and fluctuating asymmetry of seagrass and seagrass associated fauna were sensitive to metal pollution and they might be suitable to indicated contamination in seagrass system.

碩士
國立臺灣海洋大學
海洋環境與生態研究所
106
Seagrass meadow ecosystems occupy only ~0.2% of the ocean, however they play a disproportional role in the ocean carbon cycles. Seagrass meadows may contribute ~10% of the burial of organic carbon in the ocean. Meanwhile, the high primary productivity is beneficial for increasing pH and carbonate saturation(ΩAr). Hence seagrass ecosystem is recognized as being helpful against ocean acidification(OA) and increase of atmospheric carbon dioxide. To clarify these abilities whether exist in seagrass meadow at Dongsha Isaland, this study conducted a thorough diurnal investigation on the carbon chemistry in January(winter) and August(summer) 2016 at two seagrass meadows around the Dongsha Island: one is in the inner lagoon seagrass ecosystem and the other is on the north shore seagrass ecosystem. In the inner lagoon, the results demonstrated that DIC was lower in winter than summer (1741±94 < 2137±203 μmol/kg); TA was higher in winter than summer (2567±50 > 2517±169 μmol/kg); pH was higher in winter than summer (8.63±0.09 > 8.22±0.16); pCO2 was lower in winter than summer (64±19 < 379±149 μatm) and ΩAr was higher in winter than summer (8.8±0.8 > 4.8±1.1). On the north shore, DIC was lower in winter than summer (1962±218 < 2003±96 μmol/kg); TA was higher in winter than summer (2365±63 > 2271±74 μmol/kg); pH was higher in winter than summer (8.18±0.28 > 8.04±0.19); pCO2 was lower in winter than summer (378±355 < 627±313 μatm) and ΩAr was higher in winter than summer (4.5±1.8 < 3.2±1.1). These results indicated that inner lagoon seagrass bed in winter had the strongest ability against OA, because of the highest value of pH and ΩAr. Besides, the extremely low seawater pCO2 render the inner lagooon to be a strong sink of atmospheric carbon dioxide. According to the stoichiometric analysis of DIC and TA plot, these abilities were derived from the restricted hydrodynamic environments and different dominant biogeochemical process. Due to the restricted hydrodynamic environments, the effect of biogeochemical process was more evident in the inner lagoon than the north shore. In the winter, the dominant biogeochemical process was photosynthesis, that resulted in decreasing DIC and increasing TA, which leading to the increasing pH and ΩAr and the decreasing pCO2. In the summer, the dominant biogeochemical process was sulfate reduction, that resulted in elevated DIC and TA, which led to the less increasing pH and ΩAr.

Long and short effects of increased CO2 on seagrass carbon metabolism were investigated. Cymodocea nodosa meadows, exposed to long term elevated CO2 at shallow volcanic CO2 seeps off Greece and Italy were investigated using population reconstruction techniques. Growth, morphometry, density, biomass and age structure was affected by high CO2. Above to below ground biomass ratio of C. nodosa were higher at CO2 seeps. C. nodosa population grew faster but longevity of plants was lower at the seeps. The present recruitment (sampled year) of the seagrass was higher near the seeps. Carbon to nitrogen ratios (%DW) in leaves and annual leaf production of C. nodosa were higher at seeps. Elements (Fe and Cd, Cu, Co, Hg, Pb, Mn, Ni and Zn) were analysed from sediments and P. oceanica and C. nodosa compartments from CO2 seeps off Greece and Italy. Sediment Quality Guidelines Quotient pollution index indicated higher element level seeps than reference sites with possibility of moderate to adverse biological impacts. Element concentration in the sediments and seagrass compartments were higher at seeps. Higher accumulation of elements in the seagrass compartments were observed. Net community production (NCP) and community respiration (CR) of intertidal Z. noltii and unvegetated sediment communities were measured under air exposed and CO2 enriched conditions seasonally. Increase in NCP of Z. noltii and sediment community were observed with elevated CO2 concentrations than natural conditions within similar light range. NCP of Z. noltii was higher than sediment community in both summer and winter seasons under CO2 enriched conditions. CR of both communities were less affected under CO2 enriched conditions. Light compensation point of Z. noltii were lower than sediment communities in summer season with elevated CO2 levels. Seasonal community production of Z. noltii were higher than sediment communities. Significant effect of light on NCP was observed from linear regression model.
Foram investigados efeitos a curto e longo prazo do CO2 no metabolismo do carbono das ervas marinhas. As pradarias de Cymodocea nodosa, expostas durante um longo período a CO2 elevado, no vento vulcânico de CO2 ao largo da Grécia e da Itália, foram investigadas utilizando técnicas de reconstrução populacional. O Crescimento, a morfometria, a densidade, a biomassa e a estrutura etária foram afetados pelo alto teor de CO2. A razão entre a biomassa da parte aérea e parte subterrânea de C. nodosa foi superior nos ventos de CO2. A população de C. nodosa cresceu mais rapidamente, mas a longevidade das plantas foi menor nos ventos de CO2. O recrutamento atual (ano amostrado) da erva marinha foi maior perto das infiltrações. As taxas de carbono - azoto (% PS) nas folhas e a produção foliar anual de C. nodosa foram maiores nos ventos de CO2. Os elementos (Fe e Cd, Cu, Co, Hg, Pb, Mn, Ni e Zn) foram analisados a partir de sedimentos e das várias partes de P. oceanica e C. nodosa dos ventos de CO2 ao largo da Grécia e da Itália. O Índice de Poluição do Quociente das Diretivas de Qualidade do Sedimento indicou que o nível de elemento é mais alto nos sítios de CO2 do que nos de referência com possibilidade de impactos biológicos moderados a adversos. A concentração de elementos nos sedimentos e nas várias partes das plantas foi maior nos sítios de CO2. Foi observada uma maior acumulação de elementos nas das ervas marinhas. A produção liquida da comunidade (NCP) e a respiração da comunidade (CR) das populações intertidais de Z. noltei e de zonas não sedimentadas foram medidas sob condições expostas ao ar e enriquecidas com CO2, sazonalmente. O NCP de Z. noltei e dos sedimentos comunidade foi mais elevado em concentrações elevadas de CO2 do que as condições naturais, para um intervalo de luz semelhante. O NCP de Z. noltei foi maior do que no sedimento da comunidade nas estações de verão e inverno, em condições enriquecidas com CO2. A CR de ambas as comunidades foi menos afetada em condições enriquecidas com CO2. O ponto de compensação para a luz de Z. noltii foi menor do que o dos sedimentos das comunidades na estação do verão com elevados níveis de CO2. A produção comunitária sazonal de Z. noltii foi maior do que no sedimento das comunidades. O efeito significativo da luz no NCP foi observado a partir do modelo de regressão linear.

This study examined aspects of the interactions between dugongs, green turtles and their tropical seagrass food. In order to examine the effects of herbivory on the community structure, productivity, and nutritional composition of seagrass, experiments simulating intensive and light dugong grazing (uprooting whole plants) and intensive turtle cropping (removal of aboveground biomass) were carried out in intertidal seagrass beds at Cardwell (18°14'S, 146°E) and Ellie Point (16 ° 53 S, 145 ° 46 ' E) on the northeast Queensland coast. Grazing experiments at Cardwell and Ellie Point were monitored monthly for a year before the seagrass samples were harvested. An additional short-term experiment was also carried out at Cardwell only, wherein samples were harvested one month and two months after cropping, while those from the grazing plots were harvested after four months. Seagrasses were harvested opportunistically from eight sites and from four depths at one site to investigate specific and spatial variation in nutrient composition. The effect of artificial nitrogen and phosphorus fertiliser treatments on seagrass nutrients was investigated experimentally at Shelley Beach (19°19'S, 146°50'E). Determinants of the nutritional composition of tropical seagrasses and the nutritional basis of the observed feeding preference of these herbivores were also considered. Two techniques were used in seagrass ecology for the first time: (1) Video recording was used for monitoring temporal changes in the species composition and abundance in tropical seagrass communities; (2) Near infra-red reflectance spectroscopy (NIRS) was used to measure the concentrations of the following: nitrogen, organic matter, neutral detergent fibre, acid detergent fibre, lignin, water soluble carbohydrate, and starch and in vitro digestibility of dry matter. The development of the NIRS technique involved the collection of 10 species of seagrasses: Halophila ovalis, H minor, H spinulosa, H decipiens, H. trichostata, Halodule uninervis, Cymodocea serrulata, C. rotundata, Syringodium isoetifolium, and Zostera capricorni (with H uninervis and Z capricorni exhibiting two varieties). From this collection, a seagrass database consisting of 1,165 samples of leaves (n = 556), roots/rhizomes (n = 552), whole plant (n = 11), seeds (n = 3), and detrital matter (n= 43), including the samples from the grazing experiments, was developed. Then, using NIRS, the spectra of all samples were collected. From this spectral population, some 200 spectra representative of the whole population were selected, using a computer algorithm package (NIRS 3) as the calibration set and prediction equations (multivariate models) developed for the above seagrass nutritional components. The nature and extent of the effects of grazing and cropping were related to: (1) the intensity of the grazing impact; and (2) the nature of the seagrass community, including its species composition and location. In a mixed-species bed at Ellie Point, intensive grazing altered the species composition by promoting the growth of a more opportunistic (short-lived) species, Halophila ovalis in the spaces created by the grazing disturbance at the expense of a long-lived species, Zostera capricorni. Grazing also reduced the amounts of detrital matter. The species composition of a monospecific bed of Halodule uninervis was not affected by grazing. Both light and intensive grazing, and cropping increased the net above-ground biomass productivity of H. ovalis and Halodule uninervis. Recovery times varied from months for H. ovalis and Zostera/Cymodocea at Ellie Point to more than one year for H uninervis at Cardwell. In both cases, grazing improved the seagrass bed as grazing habitat for dugongs and green turtles. Simulated dugong grazing improved the nutritional composition (nitrogen and water soluble carbohydrate) of H. ovalis and H uninervis. This improvement was detectable 10 to 12 months later. In short-term experiments, both grazing and cropping increased the leaf nitrogen concentration of H uninervis. The digestibility (in vitro) of dry matter of H uninervis moderately increased after grazing and cropping. Grazing and cropping had variable effects on the fibre and lignin contents of H. uninervis depending on the plant part, nature and intensity of herbivory and duration of the recovery. Enhanced nutrients in the sediments increased the concentrations of nitrogen, starch, and fibre of H minor and H uninervis. The nutritional composition of seagrasses also varied between plant parts, among species, between varieties, among depths, and among locations (sites). Halophila species, together with Syringodium isoetifolium, were more digestible than Z. capricorni, C. serrulata, and C. rotundata, while Halodule uninervis had the highest nitrogen and starch concentrations of any of the species. Dugongs and green turtles appear to optimise their diet by selecting food species that maximise digestible nutrients. This is achieved by selecting seagrass species that are more digestible and have higher nutrients (e.g. nitrogen and carbohydrates/starch) and/or species which can compensate for grazing. Changes in feeding habitats due to herbivory by dugongs and green turtles affect the functional dynamics of tropical seagrass ecosystems through the alteration of resource availability and sediment redox conditions resulting from grazing disturbance. Consequently, mosaics of patches of varying species and nutritional compositions are produced at a local scale. A major and long-term reduction in the number of dugongs and green turtles in some areas may lead to an irreversible degradation of their habitats as preferred food species are replaced by less-preferred species. In other areas, other forms of natural disturbance and environmental constraints probably maintain the community at a low seral stage.

Abigail Scott studied how herbivore grazing can modify seagrass meadows in the Great Barrier Reef. She found that herbivores act as ecosystem engineers in these seagrass meadows, particularly green turtles and dugong. Abigail's results will be used to inform seagrass monitoring on the Great Barrier Reef.

碩士
國立臺灣海洋大學
海洋環境與生態研究所
106
Since the beginning of industrial revolution, more than one quarter of anthropogenic CO2 emissions have been absorbed into the oceans, driving down ocean pH and aragonite saturation state (ΩAragonite), a process known as ocean acidification (OA). The coral reef is a sensitive ecosystem to OA, and previous studies show that OA has a negative impact on the calcification rate for most of coral species. These earlier studies have mainly focused on individual species and/or community-level responses to OA. However, to better predict OA impact on ecosystem level, it needs to consider the interactions among the biological communities. Moreover, recent studies also suggest that seagrass meadows could have potential in serving as chemical “refugia” for the associated coral reefs facing future OA. This potential derives from their ability to modify seawater carbon chemistry through high levels of primary productivity. Therefore, the purpose of this study is to examine if seagrass could be beneficail for coral reef calcification at present and future OA scenarios using mesocosm approach. In this study, we monitored the variations of carbon chemistry parameters (DIC, TA, and pH) in 6 tanks of mesocosm under present (stage 1) and OA (stage 2) conditions, in which 3 tanks with seagrasses (experimental group), and the other three without seagrass (controlled group). Results show that net ecosystem production (NEP) under the present condition in the controlled and experimental groups is 4.76 and 5.26 μmolC kg-1 day-1, respectively, and there is no statistically significant difference between the two groups, suggesting that NEP from algae would increase in the controlled group due to the lack of competition from seagrass. The average net ecosystem calcification (NEC) in the first and second stages is 5.23 and -1.12 μmolC kg-1 day-1, respectively, the fomer is statistically significant higher than the latter. This suggests that OA has a negative impact on the NEC of the coral reef ecosystem no matter with segrass or not. Fianlly, by compiling NEC and ΩAragonite data of the present and previous studies, we suggest that the threshold value of ΩAragonite is approximately 1.82, at which NEC will switch from positive to negative. Assuming this threshold value of ΩAragonite, the simulated result shows that coral reef in tropical will begin to decline at a atmopsheric pCO2 level of approximatively 882 μatm.

Bahia de la Ascension (BA) is a pristine, shallow, karst bay located in the Mexican Caribbean, a region experiencing rapid population growth stimulated by intense tourism development. The overall objective of this study was to address the natural hydrographic variability of this inherently vulnerable ecosystem and assess its influence on a key habitat, the seagrass. The chapters follow the three-branched nature of the study which tackled the connected ecosystem issues of coastal hydrology, physical dynamics of flow and circulation, and the ecological dynamics of the seagrass species Thalassia testudinum in BA. Freshwater input to BA is primarily by submerged groundwater discharges and surface runoff; both sources are derived from fissures in the aquifer but feature distinct water quality due to the interaction with adjacent wetlands. Hurricanes explain 36 percent of the interannual precipitation variability in the region. The water balance indicates a persistent net outflow from BA to the adjacent shelf, suggesting an intense exchange across inlets. Both diurnal and semidiurnal tidal frequencies are attenuated in the inner bay, where a meteorologically-induced subtidal water level increase may occur during four-day southeasterly winds. A clear SW-NE salinity gradient was established during dry and rainy seasons, with a strong tidally-driven marine influence throughout the central basin, and a perennial mesohaline ambient in the southwestern-most bay, where hydrodynamics are primarily controlled by wind stress. Thalassia testudinum is the dominant seagrass species in BA, occupying ~90 percent of the substrate, including the freshwater-influenced inner bay. High nutrient inputs, including phosphorus which might have limiting effects in karst environments, along with the wind-driven circulation controlling water residence times are associated with the successful development of T. testudinum (up to 1,461.23 g DW m-2) within the SW bay. Farthest into the central basin, Thalassia consistently exhibited an inverse correlation between abundance and density of shoots. This pattern was enhanced under exceptional precipitation and inputs of denuded organic matter resulting from hurricanes making landfall on this region. The relationship between nutrient distribution and the above/belowground ratio suggested that Thalassia growing in BA favors the development of the aerial component as nutrients availability increases. This study provides a basic understanding of the most important processes molding the patterns of variability exhibited by T. testudinum in Bahia de la Ascension. The salinity gradient and external nutrient supply, along with the hydrodynamic component, define the spatial scale at which the connectivity between the adjacent wetland, the bay, and the shelf may occur.

Most coastal ecosystems are dominated by marine macrophytes that deliver a range of ecologically and economically important services such as carbon and nitrogen cycling and storage, and habitat provision to a range of associated species. The relative contribution of these services among different vegetated habitats, however, and their alteration due to anthropogenic stressors is little known. In this thesis, I first examined the within and between ecosystem structure and services of eelgrass (Zostera marina) and rockweed (Ascophyllum nodosum) beds in Atlantic Canada. Both habitats significantly enhanced the overall abundance and diversity of associated species, whereas differences in the spe-cies assemblages were attributed to differences in canopy structure within and between habitats. Changes in the canopy structure of the foundation species will affect associated food webs and ecosystem services. Next, I used large-scale field surveys to examine the effects of eutrophication on the structure and services of eelgrass beds. As eutrophication increased, plant dominance shifted from eelgrass to macroalgae and phytoplankton at a regional scale. The faunal community showed increases in filter feeders, detritivores and some herbivores, while sensitive species declined. These faunal changes can be linked to enhanced food availability and predation refuge offered by increased phytoplankton and opportunistic macroalgae. However, the loss of eelgrass and sensitive species highlight the negative consequences of eutrophication on eelgrass ecosystems and the services they provide. I also reviewed the global carbon and nitrogen storage and habitat services of mangroves, salt marshes, seagrass meadows and macroalgal beds. Despite only occupying 0.7% of the ocean area, together these ecosystems make up 12% of the oceanic carbon stock thereby playing an important role in global carbon and nitrogen storage. Moreover, these macrophyte habitats enhanced species richness and abundance of associated fauna and juvenile fishes. Overall, my findings indicate that each macrophyte habitat has its strengths yet all are essential in providing the full range of ecosystem services. Increasing human impacts along the coasts, however, are threatening macrophyte ecosystems worldwide, and their further decline will impair the provision of important services and human well-being. Lastly, I discuss the implications of my work for management and conservation.

Informing marine planning and the management of species at ecosystem-scales is difficult because data are generally lacking at that scale. Collecting empirical information on the distribution and/or abundance of species across broad spatial scales is expensive and logistically difficult. Accurate and efficient monitoring programmes that assess the response of species to management actions often cannot be conducted at ecosystem-scales due to time, expertise and cost constraints. The Great Barrier Reef World Heritage Area (GBRWHA) of Queensland, Australia, is the world’s largest World Heritage Area (approximately 348,000 km2) and second largest marine protected area (MPA). The region supports a variety of habitats and species including coastal seagrasses and globally significant populations of the dugong (Dugong dugon), a threatened marine mammal. Seagrasses, dugongs and their habitats are exposed to multiple anthropogenic threats along much of the 2,300 km coastline of the GBRWHA. Assessing the effectiveness of the current management arrangements for seagrasses and dugongs and informing the design of new regimes is challenging due to the difficulties associated with data collection and monitoring at the scale of the coastal GBRWHA. My thesis goal was to overcome the difficulties associated with informing the management of coastal seagrasses and dugongs in the GBRWHA by using spatial models and risk assessments in geographical information systems (GIS). My objectives were to: (1) develop spatial models of seagrasses and dugongs at the scale of the coastal GBRWHA; and, (2) use these models to estimate the risk to coastal seagrasses and dugongs from their anthropogenic threats. This approach allowed me to compare and rank the threats to identify the most severe risks, and to locate specific sites that require conservation actions. I used spatial information on the distribution of coastal seagrasses and predictor variables along with ecological theory and expert knowledge to inform the design of a Bayesian belief network, and to develop a predictive seagrass habitat model. The Bayesian belief network quantified the relationship (dependencies) between seagrass habitats and eight environmental drivers: relative wave exposure, bathymetry, spatial extent of flood plumes, season, substrate, region, tidal range and sea surface temperature. The outputs of the modelling exercise were probabilistic GIS-surfaces of seagrass habitat suitability for the entire GBRWHA coast in both the wet and dry seasons at a planning unit of 2 km * 2 km. Quantitative information on the relative impact of the anthropogenic threats to coastal seagrasses is incomplete or unavailable, and the cumulative impact of multiple threats is difficult to measure and predict. In the light of this uncertainty, I used expert knowledge to evaluate the relative risk of coastal seagrass habitats to their hazards. Vulnerability scores derived from expert opinion, spatial information on the distribution of threats and the probabilistic GIS-surfaces of seagrass habitat suitability were used to delineate areas of low, medium and high relative risk to coastal seagrass habitats. I found that whilst most planning units in the remote Cape York region of the GBRWHA are classified as low risk, almost two thirds of coastal seagrass habitats along the urban coast are at high or medium risk from multiple anthropogenic activities. Reducing the risk to coastal seagrass habitats in 13 sites identified for conservation action would require: (1) improving the quality of terrestrial water that enters the GBRWHA; (2) mitigating the impacts of urban and port infrastructure development and dredging; and, (3) addressing the hazards of shipping accidents and recreational boat damage. I derived a spatially explicit dugong population model from spatial information on the abundance and distribution of dugongs collected by a 20 year time-series of aerial surveys. Data from the aerial surveys were corrected for differences in sampling intensity and area sampled between surveys prior to the development of the model. I interpolated the corrected data to the spatial extent of the aerial surveys using the geostatistical estimation method of universal kriging. The model estimated the relative density of dugongs across the GBRWHA at the scale of 2 km * 2 km dugong planning units (the same spatial scale as the seagrass habitat model). I classified each dugong planning unit as of low, medium, or high conservation value on the basis of the relative density of dugongs estimated from the model and a frequency analysis. I compared the spatially explicit dugong population model with information on the distribution of commercial gill-netting activities to estimate the risk of dugong bycatch in the GBRWHA. I found that new management arrangements introduced in the GBRWHA in 2004 appreciably reduced the risk of dugong bycatch by reducing the total area where commercial netting is permitted. Restructuring of the industry further reduced the total area where netting is conducted. Netting is currently prohibited in 67% of dugong planning units of high conservation value, a 56% improvement over the former management arrangements. I identified four sites where netting is still conducted in dugong planning units of high and medium conservation value. Conservation actions including area closures or modified fishing practices should be considered for these regions. In addition to commercial gill-netting, dugongs are threatened by Indigenous hunting, trawling, vessel traffic, and poor quality terrestrial runoff. I developed a rapid approach to assess the risk to dugongs from multiple anthropogenic threats in the GBRWHA, and evaluated options to ameliorate that risk. Expert opinion and a Delphi technique were used to identify and rank anthropogenic threats with the potential to adversely impact dugongs and their habitats. I quantified and compared the distribution of these threats with the spatially explicit model of dugong distribution and found that almost all dugong planning units of high (96%) and medium (93%) conservation value in the GBRWHA are at low risk from human activities. Decreasing the risk to dugongs from anthropogenic threats in four sites that I identified for conservation action would require netting or Indigenous hunting to be banned in the remote Cape York region, and the impacts of vessel traffic, terrestrial runoff and commercial netting to be reduced in urban areas. The approach I developed in this thesis was able to overcome the difficulties associated with informing marine planning and management at ecosystem-scales by using spatial models and risk assessments in GIS to: (1) quantify the spatial distribution of species; and, (2) assess the risk to species and identify sites for conservation action. I was able to achieve this outcome in a data-inadequate environment by combining qualitative assessments on the relative impact of multiple anthropogenic threats with spatial models of species and threat distributions. Implementing conservation actions at the sites that I identified for management will provide the greatest positive result for coastal seagrasses and dugongs at the scale of the GBRWHA. Future research should be directed at understanding the constraints and opportunities for management in the region to ensure that effective implementation of conservation actions can be achieved.