Rozprawy doktorskie na temat „Saltmarsh ecology”

Accelerated sea-level rise poses a significant threat to coastal habitats. Salt marshes are critical coastal ecosystems, providing a host of services such as storm protection, food production, and carbon storage. Persistence of salt marshes in the face of rising sea levels relies, in part, on vertical accretion. Current ecogeomorphic models and empirical studies emphasize the importance of the positive relationship between plant production and vertical accretion via sediment trapping by stems aboveground and belowground organic matter production. Thus, changes in plant production influence salt marsh persistence with sea-level rise. However, studies and models of marsh accretion do not consider the effects of animal-mediated changes in plant production. Here, I tested how two co-occurring marsh crustaceans, Uca pugnax (marsh fiddler crab) and Sesarma reticulatum (purple marsh crab), which have contrasting effects on smooth cordgrass (Spartina alterniflora) production, indirectly influence sediment deposition and belowground organic matter contribution, through observational surveys and field manipulation. S. reticulatum feeds directly on S. alterniflora, while U. pugnax facilitates S. alterniflora production through burrowing and biodeposits. I found that U. pugnax facilitated S. alterniflora biomass in some marshes, but not others. However, this facilitation of S. alterniflora biomass did not enhance sediment deposition. U. pugnax had no effect on belowground components of vertical accretion (i.e. root production and decomposition). These results suggest that in isolation, U. pugnax has little impact on saltmarsh geomorphic processes. S. reticulatum reduced S. alterniflora above- and belowground biomass; however, sediment deposition increased as S. alterniflora biomass decreased, contrary to models of ecogeomorphology. This trend was likely due to sediment being resuspended by crab bioturbation, as U. pugnax abundances were higher in S. reticulatum-grazed areas than in non-grazed areas. When U. pugnax occurred in areas of low S. reticulatum grazing, S. alterniflora biomass and sedimentation was similar to areas with only U. pugnax. I suggest that the negative impacts of S. reticulatum are exaggerated when intense grazing results in completely unvegetated areas and subsequent increases in U. pugnax density, where bioturbation erodes sediments. Thus, while S. reticulatum can increase the susceptibility of marsh sediments to physical erosion by removing vegetation, it may also do so by facilitating U. pugnax bioturbation. However, when S. reticulatum grazing intensity is low, facilitation of S. alterniflora growth by U. pugnax can mitigate the negative effect of grazing, which suggests that the net effect of these species may depend on their relative abundance. This study demonstrates that ecological interactions, in addition to physical processes, have significant effects on marsh persistence as sea level rises, and merit incorporation into ecogeomorphic models and empirical studies of marsh accretion.

Saltmarsh is an important coastal habitat located in the littoral zone of estuaries. Australian saltmarsh area is decreasing due to agricultural and urban development and invasion by mangrove. The aim of the study was to assess the contribution made by saltmarsh as a habitat and a source of food items for fish. Three saltmarsh sites were studied, with Towra Point chosen as a site for detailed ecological study. When corrected for water volume, fish densities were found to be higher within the saltmarsh compared to the adjacent mangrove. Although the fish assemblages in saltmarshes differed significantly from mangroves the overall ratio between commercially and ecologically valuable species in these habitats are similar, a result suggesting the importance of temperate saltmarsh as habitat for economically important fish. Significant export of crab larva from saltmarsh (average crab larval abundance 2124.63 m-3 outgoing water) is a positive contribution to the estuarine food chain supplementing the nutritional requirements of estuarine fish. While the diet of the crabs producing this larvae seems dependant on the saltmarsh environment (given the contrasting isotopic signatures of Sesarma erythrodactyla in saltmarsh and mangrove, and the similarity of isotopic signatures in the saltmarsh for Sesarma erythrodactyla and Helograpsus haswellianus), the crabs do not seem to be dependent on any of the common species of saltmarsh plant, but rather depend on particulate organic matter (POM) derived from local and other sources. Crab larva are a prey item for many estuarine fish, including commercially important species, as evidenced by gut content analysis of fish visiting the saltmarsh flats during spring tides. The results strongly suggest that emphasis be given to ecosystembased management for an estuary rather than component (e.g., vegetation) based managed as defined by the Fisheries Management Act (1994) and the State Environmental Planning Policy 14.

Runnelling is a popular method of physical habitat modification employed on saltmarshes to control pest mosquito populations. The runnelling method involves linking the tidal source to isolated mosquito breeding pools via shallow channels that enable slow water movement of low amplitude tides. Increased tidal flushing inhibits mosquito development. The range of organisms which inhabit saltmarsh are likely to be influenced by altered tidal conditions as they exhibit specific physicochemical requirements for feeding, burrowing or growth. The dynamic nature of saltmarsh may mean that changes to the tidal frequency of a particular region of the saltmarsh promotes extension of marine-like conditions. Because runnels increase the frequency of flooding tidal events in specific regions of the saltmarsh this study predicted that resulting changes would be evident in the physical conditions of saltmarsh substrate, in the transport of buoyant vegetative propagules, in the population characteristics of surface grazing snails and in the density and aperture of crab burrows after flooding and non-flooding tidal events. The physical impacts of runnelling were determined at three marshes which appeared similar in terms of topography, substrate and tidal conditions. Soil water content and consolidation were measured using two sampling protocols: a) comparisons between modified and unmodified shores; and, b) comparisons with increasing lateral distance across the shore from the runnel edge. At one marsh, moisture levels were significantly higher at runnelled than at unrunnelled sites when tides filled the runnels, but this pattern was not found at the other marshes. Soil consolidation was greater at higher shore heights, but was not different between runnelled and unrunnelled shores. Measurements at different lateral distances from runnels demonstrated higher moisture levels and lower consolidation up to 5 m from the edge but not further away. Groups of marked Avicennia marina propagules were released at the three runnelled saltmarshes during flooding and non-flooding tidal events. Groups of propagules released within 10 m of a runnel were always transported significantly further from the starting position and further up the saltmarsh shore after both flooding and non-flooding tides than any other groups. In addition, the pattern of stranding on saltmarsh for significantly different groups was closely associated with the path of runnel construction so that propagules were located either in the runnel or in depressions linked to the runnel that had been isolated mosquito-breeding pools prior to runnelling. It is likely that altered physical soil conditions significantly affected the distribution and size structures of Salinator solida and Ophicardelus spp. snails recorded at the three saltmarshes. The interaction of tidal period and the presence of a runnel contributed to patterns with significant differences between runnelled and unrunnelled regions of the marsh. Generally, the runnel population of snails exhibited flood-like features even during non-flood periods. The distribution and size classes of snails did not differ with lateral distance from runnels. The burrow characteristics of the crab Helograpsus haswellianus were compared to increase the accuracy of estimating abundance from burrow counts. Including only those burrows which were obviously maintained by resident crabs significantly increased the confidence limits of estimating crab abundance using only burrow density counts. This method was applied to runnelled and unrunnelled sites to assess any changes in the density of burrows associated with the presence of runnels. Again, it is likely that physical soil conditions resulting from increased tidal frequency at the runnel did influence crab burrowing with fewer small burrows being found at the runnelled site, low on the shore. In addition, mid- and large-sized burrows tended to dominate close to the runnel edge. Site-specific soil characteristics may help to explain the lack of continuity in patterns associated with runnel effects on non-target saltmarsh resources. While the runnel may increase the soil water content of clayey substrates at some sites it could also result in de-watering of porous sandy soils at other shores. This was evident in the structure of the snail population and distribution of crab burrows which appeared to reflect altered soil physical characteristics associated with the runnel. Runnelling does affect non-target organisms in saltmarsh. However, the scale of impact was usually locally restricted (< 10 m from the runnel edge). The fact that patterns were not recorded at all sites suggests that the influence of runnels is variable and limited by substrate and some biological conditions. Given the efficiency and popularity of runnelling as a physical control method for reducing pest vector mosquito habitat, this study found no evidence to suggest that its use should be discontinued on any ecological basis measured.

Runnelling is a popular method of physical habitat modification employed on saltmarshes to control pest mosquito populations. The runnelling method involves linking the tidal source to isolated mosquito breeding pools via shallow channels that enable slow water movement of low amplitude tides. Increased tidal flushing inhibits mosquito development. The range of organisms which inhabit saltmarsh are likely to be influenced by altered tidal conditions as they exhibit specific physicochemical requirements for feeding, burrowing or growth. The dynamic nature of saltmarsh may mean that changes to the tidal frequency of a particular region of the saltmarsh promotes extension of marine-like conditions. Because runnels increase the frequency of flooding tidal events in specific regions of the saltmarsh this study predicted that resulting changes would be evident in the physical conditions of saltmarsh substrate, in the transport of buoyant vegetative propagules, in the population characteristics of surface grazing snails and in the density and aperture of crab burrows after flooding and non-flooding tidal events. The physical impacts of runnelling were determined at three marshes which appeared similar in terms of topography, substrate and tidal conditions. Soil water content and consolidation were measured using two sampling protocols: a) comparisons between modified and unmodified shores; and, b) comparisons with increasing lateral distance across the shore from the runnel edge. At one marsh, moisture levels were significantly higher at runnelled than at unrunnelled sites when tides filled the runnels, but this pattern was not found at the other marshes. Soil consolidation was greater at higher shore heights, but was not different between runnelled and unrunnelled shores. Measurements at different lateral distances from runnels demonstrated higher moisture levels and lower consolidation up to 5 m from the edge but not further away. Groups of marked Avicennia marina propagules were released at the three runnelled saltmarshes during flooding and non-flooding tidal events. Groups of propagules released within 10 m of a runnel were always transported significantly further from the starting position and further up the saltmarsh shore after both flooding and non-flooding tides than any other groups. In addition, the pattern of stranding on saltmarsh for significantly different groups was closely associated with the path of runnel construction so that propagules were located either in the runnel or in depressions linked to the runnel that had been isolated mosquito-breeding pools prior to runnelling. It is likely that altered physical soil conditions significantly affected the distribution and size structures of Salinator solida and Ophicardelus spp. snails recorded at the three saltmarshes. The interaction of tidal period and the presence of a runnel contributed to patterns with significant differences between runnelled and unrunnelled regions of the marsh. Generally, the runnel population of snails exhibited flood-like features even during non-flood periods. The distribution and size classes of snails did not differ with lateral distance from runnels. The burrow characteristics of the crab Helograpsus haswellianus were compared to increase the accuracy of estimating abundance from burrow counts. Including only those burrows which were obviously maintained by resident crabs significantly increased the confidence limits of estimating crab abundance using only burrow density counts. This method was applied to runnelled and unrunnelled sites to assess any changes in the density of burrows associated with the presence of runnels. Again, it is likely that physical soil conditions resulting from increased tidal frequency at the runnel did influence crab burrowing with fewer small burrows being found at the runnelled site, low on the shore. In addition, mid- and large-sized burrows tended to dominate close to the runnel edge. Site-specific soil characteristics may help to explain the lack of continuity in patterns associated with runnel effects on non-target saltmarsh resources. While the runnel may increase the soil water content of clayey substrates at some sites it could also result in de-watering of porous sandy soils at other shores. This was evident in the structure of the snail population and distribution of crab burrows which appeared to reflect altered soil physical characteristics associated with the runnel. Runnelling does affect non-target organisms in saltmarsh. However, the scale of impact was usually locally restricted (< 10 m from the runnel edge). The fact that patterns were not recorded at all sites suggests that the influence of runnels is variable and limited by substrate and some biological conditions. Given the efficiency and popularity of runnelling as a physical control method for reducing pest vector mosquito habitat, this study found no evidence to suggest that its use should be discontinued on any ecological basis measured.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Australian School of Environmental Studies
Full Text

Heterotrophic bacterial activity and nitrogen fixation are fundamental to nutrient regeneration and nitrogen cycling in saltmarsh ecosystems. Ecological and physiological aspects of bacterial production and nitrogenase activity in marine sediments and water were examined in Langebaan Lagoon, a temperate saltmarsh ecosystem. Emphasis was placed on factors modulating rates and patterns of nitrogen fixation. Nitrogen fixation appeared to be dominated by heterotrophic bacteria. Rates of nitrogen fixation (estimated by the acetylene reduction technique), and bacterial production (estimated by tritiated thymidine incorporation, Tri) were higher in fine, muddy sediments near the head of the lagoon (Geelbek) than in coarser, sandy sediments near the mouth of the lagoon (Oesterwal). These comparisons (between sites) reflected the higher bacterial abundance and organic content of sediments from Geelbek. Examinations of five sedimentary microhabitats at each site (including those associated with beds of the seagrass Zostera capensis, burrows of the sandprawn Callianassa kraussi at Oesterwal, and burrows of the mudprawn Upogebia africana at Geelbek) showed that bacterial activity was higher in surface sediments than in subsurface sediments. Highest rates of nitrogen fixation (annual mean, 0.28 + 0.07 nmol C2H4 g-1 dry sediment h-1) were measured in Zostera bed sediments at Geelbek. Thymidine incorporation activity and nitrogenase activity were higher in burrow linings than in adjacent subsurface sediments, suggesting that burrow linings provided an improved subsurface environment for bacterial activity. Burrow linings also had a higher organic content than subsurface sediments away from burrows. Nitrogenase activity was not detected in lagoon water.

Mangroves and saltmarshes are ecologically important coastal ecosystems; unfortunately, these low-lying coastal ecosystems are vulnerable to global climate change. As sea-levels rise, mangroves are expected to shift their distribution landward towards higher elevation sites that are occupied by other plants, including saltmarsh taxa. Therefore, mangrove recruits at the leading edge of expansion may interact with diverse assemblages of saltmarsh plants, and these interactions could influence the success of mangrove encroachment into higher tidal-elevation areas. The purpose of the research presented here was to investigate empirically the ecological interactions that may influence the recruitment of the black mangrove, Avicennia germinans, into saltmarsh habitats. Saltmarsh plants frequently occurred at the landward boundary of mangrove forests at two sites selected for field studies along the west coast of Florida: Cannon Island and Upper Tampa Bay Park. On Cannon Island, two different field tests investigated mangrove propagule entrapment and dispersal within saltmarsh vegetation. In the first experiment, the entrapment of mangrove propagules within saltmarsh plants, exhibiting different growth forms, was examined during seasonal high tide events. Natural polyculture plots retained a mean (±;SE) 59.3% (±;11.0) of emplaced propagules. Monocultures varied in their propagule retention capacities with plots of S. virginicus retaining on average 65.7% (±;11.5) of transplanted propagules compared to 7.2% (±;1.8) by B. maritima and 5.0% (±;1.9) by S. portulacastrum. Monocultures of the salt marsh grass, Sporobolus virginicus, and natural saltmarsh polycultures containing S. virginicus retained significantly more propagules than either of two succulent plants (i.e., Batis maritima and Sesuvium portulacastrum). Using digital images, saltmarsh plant structure was quantified; the number of entrapped mangrove propagules displayed a significant and positive correlation (r2 = 0.6253, p = 0.00001) with the amount of structure provided by saltmarsh plants. Therefore, the first field study identified structural and functional differences between saltmarsh plants. A second field study employed marked propagules in order to further examine the dispersal patterns of propagules at saltmarsh boundaries comprised of plants with different growth forms (i.e., grass vs. succulent) during seasonal high tides. Saltmarsh plant boundaries erected by taxa with distinct growth forms differentially influenced the proportion of propagules that dispersed seaward and the distance propagules moved seaward. In fact, nearly twice as many propagules dispersed seaward at boundaries erected by succulent plants compared to boundaries composed of grass. The results of this field study support my previous findings that propagule dispersal is comparatively lower in saltmarsh grass than in succulent saltmarsh plants. The findings from these two field studies suggest that the permeability of boundaries formed by saltmarsh plants may modulate landward dispersal of A. germinans propagules. The final field study was conducted at Upper Tampa Bay Park, where a second species of saltmarsh grass, Monanthochloe littoralis, co-occurred with the grass, S. virginicus, and succulent saltmarsh plants to form a mosaic landscape of saltmarsh plant patches. Patches were weeded to create 3 saltmarsh treatments: 1) M. littoralis monoculture; 2) S. virginicus monoculture; and 3) polycultures containing both grasses and at least one other saltmarsh taxa. Propagules of A. germinans were emplaced into saltmarsh patches and followed for 11 weeks. On the last sampling date, the greatest number of A. germinans (n = 51) had successfully established as seedlings within the M. littoralis monoculture plots. In contrast, only 20 (22% of the propagules initially emplaced) A. germinans seedlings established in S. virginicus monoculture plots. These findings suggest that among grass taxa, species identity influences mangrove establishment success, which builds upon our previous findings that demonstrated that saltmarsh growth form (i.e., grass vs. succulent) influenced mangrove propagule dispersal. Combined the findings from these field studies indicate that interactions among the early life history stages of black mangroves and neighboring plants influence mangrove recruitment. Specifically, these field studies provide empirical evidence that the species composition of saltmarsh plants influences mangrove propagule dispersal and seedling establishment. The work presented here has implications for understanding the suite of ecological interactions that may influence mangrove encroachment into saltmarsh habitats at higher tidal elevations as sea-levels rise.

Previous research into bottom-up processes on saltmarshes has mainly focused on the influence of plant succession on herbivores. This study will present original research exploring the influence of bottom-up processes in a saltmarsh ecosystem between three trophic levels: Orchestia, redshanks, and sparrowhawks. Density dependence, may be the dominant top-down effect when higher numbers of sparrowhawks and redshanks are present, and may mask top-down and bottom-up trait effects which are constant. Bottom-up effects begin to emerge when cold conditions force redshanks from muddy creeks onto the saltmarsh to forage for Orchestia, because their primary prey, Corophium become less available. Larger flocks form and feeding on Orchestia requires them to balance a need to profit from the best available feeding patches and to be vigilant to sparrowhawk attack. Redshank vulnerability is compounded, because Orchestia hide in cold temperatures, so probing in the soil with their heads down makes them more vulnerable to sparrowhawk attack. Larger flocks may be able to exploit areas closer to sparrowhawk-concealing cover at the terrestrial boundary because they feel safer in greater numbers. Warmer temperatures make Orchestia more active which attracts redshanks, which can simultaneously feed and be vigilant because they peck and catch crawling and jumping Orchestia with their heads up. Consequently, increased flock size may temporarily depress Orchestia abundance, so that redshanks become spaced, leaving isolated individuals more vulnerable to attack. Therefore, it is a temperature-dependent bottom-up process which impacts upon both Orchestia and redshank behaviour, which then may influence the hunting success of sparrowhawks. Whether the characteristics of this saltmarsh ecosystem and the trophic dynamics can be compared to other examples is questionable. Saltmarshes probably differ in their topography and the way in which environmental conditions affect them that then defines which species are present and how these species interact.

Increasing human pressure combined with sea level rise and increased storminess is threatening coastal ecosystems around the world. Among these ecosystems, saltmarshes are particularly endangered due to their position in temperate areas with low wave action where human density is often high (e.g. estuaries). Around the UK, centuries of land reclamation have led to a substantial decrease of the area of saltmarsh. Over the past decades, restoration schemes have been implemented in numerous coastal locations in an attempt to counteract this loss. Such schemes involve allowing sea water to inundate a previously embanked area and letting the vegetation develop naturally, thereby reverting to saltmarsh through natural colonisation. However, surveys of restored areas that have looked at the recovery of plant species diversity or functional characteristics often show that restored saltmarshes do not reach the state of a natural saltmarsh ecosystem. While there is much data at the species level, recovery of plant intra-specific diversity (genetic diversity) has not been assessed in restored saltmarsh although this component of biodiversity is receiving increasing attention for its effect on ecosystem function. This thesis represents the first attempt to (1) characterize the nation-wide genetic structure of two important north-west European saltmarsh plant species, the common saltmarsh grass (Puccinellia maritima) and the sea arrowgrass (Triglochin maritima) and (2) compare levels of genetic diversity and structure between restored and natural ecosystems. Microsatellite molecular markers were developed for both species. Using innovative methods to analyse the genetic data obtained for these two polyploid species, this thesis highlights that genetic diversity at the national scale is organised regionally for both species, although gene-flow is still restricted between populations within the same region. Gene-flow between populations is determined by different processes depending on the species. While coastal processes mainly influence gene dispersal in P. maritima, overland routes of dispersal are involved for T. maritima. These differences are believed to be due to differences in dispersal ecology between the two species. Although gene-flow exists between distant saltmarshes, the genetic analysis of P. maritima and T. maritima colonists arriving on restored sites highlighted their local origin and reaffirmed that it is preferable to restore saltmarsh where a nearby natural saltmarsh can act as a source of colonists. A multiple paired-site comparison identified similar genetic diversity between restored and natural saltmarshes indicating that restoration of local genetic diversity is rapid for both species. A single site comparison at Skinflats in the Forth estuary compared fine-scale spatial genetic structure between the restored and natural saltmarsh. Interestingly, no structure was detected for T. maritima either in restored or natural saltmarsh. In contrast, a strong genetic structure organised along the elevation gradient was observed in the natural saltmarsh for P. maritima but was absent in the restored saltmarsh. The origin of this structure is not clear but could be due to restricted gene-flow between individuals from different elevations due to strong post-zygotic selection, as suggested in previous work. In any case, this lack of structure in the restored saltmarsh indicates that genetic recovery is incomplete in this respect for P. maritima. This thesis introduces the growing field of restoration genetics to saltmarsh ecology and identifies the principal population genetic trends in two of the species dominating the vegetation of north-west European saltmarshes community. The information given here will be useful for restoration practitioners and provides a strong foundation for future work characterizing the importance of genetic diversity for saltmarsh function.

Conservation of intertidal habitats in the UK is vital in order to continue to support nationally and internationally important populations of non-breeding waterbirds. Historic reclamation for agriculture and industry has resulted in the loss and degradation of large areas of these intertidal habitats in estuaries and they continue to be threatened by sea-level rise. Managed realignment is one method which is increasingly being used to restore intertidal habitats. As managed realignment is a relatively new restoration technique, the extent to which knowledge of the biology of estuaries is applicable to managed realignment sites is unclear. Habitat restoration is often unsuccessful or incomplete, so a detailed knowledge of both the natural system and the characteristics of restored systems will usually be necessary to recreate fully-functional estuarine habitats. This thesis focuses on Nigg Bay Managed Realignment Site (Nigg Bay MRS), the first managed realignment site in Scotland, and follows the first four years of ecological development to gain an understanding of how breached realignment can be used to restore intertidal habitats to support non-breeding waterbirds. This thesis has six major aims: (i) to describe the development of saltmarsh, (ii) to describe the development of intertidal flat, (iii) to describe the colonisation by non-breeding waterbirds (iv) to determine how tidal cycle and weather affect patterns of waterbird use, (v) to determine which factors affect the spatial distribution of waders and finally (vi) to determine the patterns of use by individual birds. Four summers after the re-establishment of tidal conditions, almost all of the saltmarsh species recorded on the nearby saltmarsh had colonised Nigg Bay MRS, although recognisable communities had yet to establish. Three winters after the re- establishment of tidal conditions in Nigg Bay MRS, the sediments had a significantly smaller particle size and higher organic matter content compared to the fine sands of the adjacent intertidal flats. The intertidal invertebrate community also differed from the adjacent intertidal flats. Nigg Bay MRS attracted large numbers of non-breeding waterbirds and supported each of the most common wader and wildfowl species present in the wider estuary. Nigg Bay MRS performs a number of important functions for non-breeding waterbirds by: (i) providing a foraging and resting habitat when the tide is absent and intertidal sediments in Nigg Bay are exposed; (ii) providing a foraging resource as the tide passes over the intertidal sediments within the site once the intertidal flats in Nigg Bay are inundated; and (iii) providing a high tide roosting site. On days with low temperatures and high wind speeds, more waterbirds use Nigg Bay MRS, suggesting that it is likely to be providing sheltering benefits. Nigg Bay MRS also provides top-up feeding habitat. The factors that often influence the spatial distributions of waders in estuaries appear to be operating within Nigg Bay MRS. Wader densities are greater on the intertidal flats when they are accessible than on the saltmarsh. Wader densities are also greatest close to creeks and drainage channels, possibly due to higher invertebrate densities, more accessible prey or sheltering benefits. Colour-ringing and radio-tracking of Common Redshank established that Nigg Bay MRS has a subset of regular users, including both adults and juveniles, and the wader assemblage at night may differ from the assemblage during the day. These findings are discussed in terms of the implications for locating, designing and managing future managed realignment projects.

Natural systems face pressures exerted by natural physical-chemical forcings and a myriad of co-occurring human stressors that may interact to cause larger than expected effects, thereby presenting a challenge to ecosystem management. This thesis aimed to develop new information that can contribute to reduce the existing knowledge gaps hampering the holistic management of multiple stressors. I undertook a review of the state-of-the-art methods to detect, quantify and predict stressor interactions, identifying techniques that could be applied in this thesis research. Then, I conducted a systematic review of saltmarsh multiple stressor studies in conjunction with a multiple stressor mapping exercise for the study system in order to infer potential important synergistic stressor interactions. This analysis identified key stressors that are affecting the study system, but also pointed to data gaps in terms of driver and pressure data and raised issues for potentially overlooked stressors. Using field mesocosms, I explored how a local stressor (nutrient availability) affects the responses of saltmarsh vegetation to a global stressor (increased inundation) in different soil types. Results indicate that saltmarsh vegetation would be more drastically affected by increased inundation in low than in medium organic matter soils, and especially in estuaries already under high nutrient availability. In another field experiment, I examined the challenges of managing co-occurring and potentially interacting local stressors on saltmarsh vegetation: recreational trampling and smothering by deposition of excess macroalgal wrack due to high nutrient loads. Trampling and wrack prevention had interacting effects, causing non-linear responses of the vegetation to simulated management of these stressors, such that vegetation recovered only in those treatments simulating the combined prevention of both stressors. During this research I detected, using molecular genetic methods, a widespread presence of S. anglica (and to a lesser extent S. townsendii), two previously unrecorded non-native Spartinas in the study areas.

Tese de doutoramento, Geografia (Geografia Física), Universidade de Lisboa, Instituto de Geografia e Ordenamento do Território, 2016
The main objective of this thesis is to study the impacts of morphology, structure and flora transformations on the former reclaimed saltmarshes of Alvor and Arade (Algarve, Portugal) and to understand how these changes can be connected with ecological restoration. The methodological approach used to assess the effects of land-use changes on saltmarshes (Chapter II) relied on combining vegetation surveys and the spatial analysis of historic maps (c. 1800) and aerial photographs (1958-2010), which were analysed to map saltmarsh ecosystems and quantify land-use changes. Additionally, vegetation surveys contributed to the identification of saltmarsh typologies: tidally restored saltmarshes (TRS) and enclosed mixed marshes (EMM). The third Chapter focus on comparing the characteristics of natural saltmarshes with TRS and EMM towards sediment composition and accretion dynamics, linking these with possible consequences for vegetation communities. Accretion rates vary between 0.2 mm/year and 15.53 mm/year. Naturally recovering marshes show lower accretion and higher variability in grain size distribution. TRS present a floristic structure that is similar to natural saltmarshes, while the topographic changes of EMM originate grain size differences but floristic similarities. This is linked with the disturbance gradient that influences floristic diversity. Aiming to improve the ecosystem services provided by saltmarshes, a coastal defence index was developed to respond to missing institutional-periodical data and demanding software, as well as to be of international application (Chapter IV). Landscape metrics were selected according to shape, complexity, and connectivity parameters, and added to average elevation and distance to the coast, for 1972 and 2010. An equation that measures coastal protection was developed, taking into account the results of PCA and the percentage of explained variation of each component. By using this index, target coastal defence parameters can be outlined, strategies for their conservation designed, and ecological restoration considered. Floristic composition and diversity developing in secondary marshes result from passive recovery of former reclaimed marshes. To assess the state of these marshes and to evaluate the feasibility of ecological recovery projects, a combined statistical analysis was applied to understand differences in secondary marshes (TRS and EMM) and international examples of managed realignment. A similarity index was calculated to support these comparisons (Chapter V). Large differences in the floristic composition of Atlantic and Mediterranean saltmarshes hinder the application of managed realignment projects on unmanaged saltmarsh development. Saltmarsh vegetation changes derived from passive recovery develop higher similarity in terms of floristic composition and structure (low, medium and high salt marsh) than active recovery works. Despite the emergence of ubiquitous species, unmanaged saltmarshes are with the local species pool (natural saltmarshes).