Dissertations / Theses on the topic 'Mangrove'

Mangrove ecosystems thrive in (sub)tropical, intertidal areas where adaptations

like vivipary and the hydrochorous dispersal of propagules become an absolute

necessity. As propagule dispersal and early growth allow for the replenishment of

existing stands and colonization of new habitats, many authors recognize the

importance of these stages in structuring mangrove populations and communities.

However, when it comes to the actual propagule dispersal and recruitment

mechanisms, there is an apparent lacuna in the current understanding of

mangrove ecology. The period between the mature propagule falling from the

parental mangrove tree and the early growth of the established seedling, under

various possible circumstances, remains in the dark. In this study we focus on this

particular period by investigating both the places where these propagules end up

as the pathways their dispersal units follow. And we go one step further.

Mangrove forests are being destroyed worldwide at a threatening pace despite

their tremendous asset to coastal human communities and associated biological

species. The effect of human-induced (cutting and mangrove conversion to

aquaculture ponds) as well as indirectly and/or ‘naturally’ evolving disturbances

(sea level rise) on propagule hydrochory occupies an important place in this study.

Dispersal of water-buoyant propagules of the family Rhizophoraceae and

Acanthaceae (now including the Avicenniaceae) was studied in Gazi Bay (Kenya),

Galle and the Pambala-Chilaw Lagoon Complex (Sri Lanka). The study sites

differ both in tidal regime and vegetation structure, covering an interesting variety

of ecological settings to examine propagule dispersal. Field data and experiments

ranging from micro/ mesotopographical measurements and successive propagule

counts to hydrodynamic and propagule dispersal experiments were collected or

executed in situ.

Two main methodological approaches were employed. Firstly, the question on

mechanisms of propagule recruitment was addressed by statistically investigating

the effect of microtopography, top soil texture and above-ground-root complexes on

the stranding and self-planting of propagules (Chapter 2&3). Afterwards,

suitability maps were created using Geographical Information Systems (GIS) to

assess whether a particular mangrove stand has the ability to succesfully

rejuvenate. Furthermore, the effect of degradation (tree cutting) (Chapter 2&3),

sea level rise (Chapter 2&4) and microtopography-altering burrowing activities of

the mangrove mud lobster Thalassina anomala (Chapter 3), was incoporated in the

GIS-analyses. Secondly, the combined set-up of hydrodynamic modelling and

ecological dispersal modelling was developed to simulate propagule dispersal

pathways influenced by dispersal vectors (tidal flow, fresh water discharge, wind),

trapping agents (retention by vegetation or aerial root complexes) and seed

characteristics (buoyancy, obligated dispersal period) (Chapter 5&6). This type of

approach provided the possibility to explore propagule dispersal within its

ecological context, but was also applied to an implication of shrimp pond area

restoration (Pambala-Chilaw Lagoon Complex, Sri Lanka) (Chapter 5) and to

evaluate changes in propagule dispersal when sea level rises (Gazi Bay, Kenya)

(Chapter 6).

The main findings regarding propagule recruitment indicate that propagules are

not distributed equally or randomly within a mangrove stand, yet species-specific

distribution for anchorage occurs. Characteristics of the environment

(microtopography, top soil texture and above-ground root complex) influence

propagule recruitment in a way that complex root systems (e.g. pencil roots and

prop roots) facilitate the entanglement of dispersal units and a more compact soil

texture (like clay and silt) and a predominant flat topography creates suitable

areas for stranding and self-planting of propagules. This combines effects of

existing vegetation and abiotic factors on mangrove propagule establishment.

Since propagule dispersal is not solely determined by species-specific propagule

characteristics (e.g. buoyancy, longevity, etc.), I emphasize that propagule sorting

by hydrochory has to be viewed within its ecological context. Propagule retention

by vegetation and wind as a dispersal vector, deserve a prominent role in studies

on propagule dispersal. The significance of dense vegetation obstructing long

distance dispersal (LDD in its definition of this work), mainly in inner mangrove

zones, supports our main finding that propagule dispersal is largely a short

distance phenomenon. ‘Largely’ is here understood as quantitatively, not

excluding epic colonization events of rare but important nature.

In accordance with the Tidal Sorting Hypothesis (TSH) of Rabinowitz (1978a),

smaller, oval-shaped propagules were found to disperse over larger distances than

bigger, torpedo-shaped propagules. We can however not fully support the TSH

because (1) these differences are no longer valid when comparing between torpedoshaped

propagules of different sizes and (2) propagule dispersal is not always

directed towards areas more inland, but can be strongly concentrated towards the

edges of lagoons and channels

Anthropogenic pressure on mangrove ecosystems, more specifically clear-felling or

mangrove conversion to aquaculture ponds, imposes limitations on propagule

recruitment due to reduced propagule availability and a decrease in suitable

stranding areas where the architecture of certain root complexes, like prop roots

and pencil roots, function as propagule traps. These types of pressure appear to

have more severe consequences on propagule dispersal than the effect of sea level

rise on mangroves. Mangrove forests, which are not situated in an obviously

vulnerable setting, can be resilient to a relative rise in sea level if a landward shift

of vegetation assemblages and successful early colonization is not obstructed by

human-induced pressures. Also, and this renders mangrove forests vulnerable in

spite of their intrinsic resilience, when the ‘capital’ of forest is severely reduced or

impoverished as happens extensively worldwide, the ‘interest’ on this capital,

understood as propagule availability, delivery and trapping, will not allow them to

efficiently cope with sea level rise, putting sustainability of mangrove ecosystem

services and goods at risk.

In a larger framework of mangrove vegetation dynamics, knowledge on propagule

dispersal will benefit management strategies for the conservation of mangroves

worldwide, besides its fundamental interest to fully fathom the ecology of this

particular marine-terrestrial ecotone formation.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Les mattes de procaryotes constituent une niche écologique diversifiée. peu étudié!<..en mangrove et jamais décrjte dans les Antilles. L'objectif de cette thèse est de décrire les procaryotes filamenteux reposant sur le sédiment marin et de caractériser les interactions entre la matte, milieu et entre les différents micro-organismes. Nous avons d'abord caractérisé les micro-organismes majoritaires de la matte par le séquençage de l'ADNr 16S et des hybridations moléculaires (FISH). Il a ainsi été possible de décrire deux nouvelles espèces de Beggiatoaceae dont la structure a été étudiée par microscopie électronique. De plus, afin de caractériser l'environnement chimique de ces bactéries, des capteurs potentiométriques ont été utilisés. Ils ont permis de démontrer que les Beggiaotaceae oxydaient les sulfures en prélevant l'oxygène du milieu, tandis que des analyses de microscopie couplées à de l'EDX ont mis en évidence des réserves de soufre intracellulaires dans ces bactéries. Le séquençage de l'ADNr 16S des cyanobactéries filamenteuses a mis en évidence quatre nouvelles espèces de l'ord~e des Oscillatoriales. Deux d'entre elles, du genre Planktothricoides ont révélé une toxicité positive sur les Artemia sp:, les deux autres ont été identifiées comme proches d'Oscillatoria spongeliae, une espèce symbiotique d'éponge, bien que les coupes uItrafines d'une de ces souches aient montré une morphologie interne proche des Tychonema. L'analyse des données a permis d'étoffer les connaissances sur les communautés microbiennes de mangrove et constitue une base intéressante de travail sur la contribution des micro-organismes au fonctionnement de l'écosystème mangrove dans son ensemble
Procaryotic mats are ecological niches, occuring in many ecosystems. Those structures are little studied in marine mangrove sediment and have never been described in West Indies. The purpose ofthis thesis is to characterize the interactions between the mat and the environment, and between the different filamentous micro-organisms living into the mat. In a first place, we characterized the mainly micro-organisrns of the mat by sequencing 16S rDNA. We confirmed these sequences by molecular hybridization (FISH) with specific probes designed from sequences obtained. Thus, it was possible to describe two new Beggiatoaceae strains, whose structure has been studied by electronic microscopy. Furthemore, we used potentiometric captors in situ and in laboratory, by creating a mesocosm, in order to define the chemical environment where involved these bacteria. These experiences proved that the Beggiatoaceae of the mat oxidized the sulfur taking the oxygen from the medium, while the microscopic analysis coupled with EDX showed intracellular sulfur granules into the bacteria. The study of the filamentous cyanobacteria of the mat highlighted four new cyanobacteria strains, all studied by electronic rmcroscopy. Two of them belongs to Planktothrlcoides genus and showed positive toxicity on Artemia salina. The others species are close to Oscillatorla spongeliae, a sponge symbiont, nevertheless, one ofthem showed an intemal morphology close to Tychonema. The analysis of the different results adduces sorne knowledge about mangrove microbial communities. Furthermore, this study can be used as an interesting base to study the micro-organisms contribution in functioning mangrove ecosystem

In South Africa mangrove forests are located in estuaries from Kosi Bay in KwaZulu-Natal (KZN) to Nahoon Estuary in the Eastern Cape. The aims of this study were to determine the present state of mangroves in KwaZulu-Natal, by assessing the current population structure, the changes in cover over time and associated anthropogenic pressures. A second objective of this study was to determine the effect of harvesting on the population structure and sediment characteristics in the Mngazana mangrove forest. To determine if harvesting was sustainable at Mngazana Estuary; the growth and mortality rates and associated growth conditions were measured. Finally by using population modelling sustainable harvesting limits were determined by predicting the change in population structure over time. The study focussed on the KwaZulu-Natal province as a fairly recent study addressed mangrove distribution and status in the Eastern Cape Province. A historical assessment of all mangroves forests in KwaZulu-Natal (KZN) revealed that the potential threats to mangroves in South Africa include; wood harvesting, altered water flow patterns coupled with salinity changes, prolonged closed-mouth conditions and subsequent changes to the intertidal habitat. As a result mangroves were completely lost from eleven estuaries in KZN between 1982 and 1999 and a further two estuaries by 2006. Mangroves only occurred in those estuaries where the mouth was open for more than 56 percent of the time with the exception of St Lucia, where the mouth has been closed for longer but the mangrove communities have persisted because the roots of the trees were not submerged. All mangrove forests in KZN were regenerating in terms of population structure as they had reverse J-shaped population curves as well as high adult: seedling ratios. Kosi Bay and Mhlathuze Estuary were two of the larger forests that showed signs of harvesting (presence of tree or branch stumps), but the greatest threat to smaller estuaries seems to be altered water flow patterns due to freshwater abstraction in the catchments and the change of land use from natural vegetation to sugar-cane plantations. These threats affect the hydrology of estuaries and the sediment characteristics (particle size, redox, pH, salinity, temperature) of the mangrove forests. The environmental conditions under which the mangrove forests currently exist were determined for five species. Lumnitzera racemosa and Ceriops tagal exhibited a narrow range of conditions as these species are only found at Kosi Bay, while Avicennia marina, Bruguiera gymnorrhiza and Rhizophora mucronata were found to exist under a wider range of conditions. The growth rate and response to environmental conditions of the three dominant species were important to determine as these species are impacted by harvesting. Mangrove growth rates were measured at Mngazana Estuary in the Eastern Cape, the third largest mangrove forest in South Africa. Areas of this estuary where mangroves harvesting has occurred, show significant differences in sediment characteristics as well as changes in population structure in harvested compared to non harvested sites. The growth rate (in terms of height) of Avicennia marina individuals increased from seedlings (0.31 cm month-1) to adults (1.2 cm month-1), while the growth of Bruguiera gymnorrhiza stabilised from a height of 150 cm at 0.65 cm month-1. The growth of Rhizophora mucronata peaked at 0.72 cm month-1 (height 151-250 cm) and then decreased to 0.4 cm month-1 for taller individuals. Increases in diameter at breast height (DBH) ranged between 0.7 and 2.3 mm month-1 for all species. Some environmental variables were found to be important drivers of growth and mortality of individuals less then 150 cm. A decrease in sediment pH significantly increased the mortality of Avicennia marina seedlings (0-50 cm) (r = - 0.71, p<0.05) and significantly decreased the growth of Rhizophora mucronata and Bruguiera gymnorrhiza seedlings (r = -0.8, r = 0.52 – p < 0.05 respectively). At Mngazana Estuary, mortality of this species showed a positive correlation with sediment moisture content indicating that this species prefers drier conditions. The density of Rhizophora mucronata was significantly correlated to porewater temperature in Northern KZN as was the growth of adult (>300 cm) Rhizophora trees at Mngazana Estuary. Mortality of Avicennia marina individuals (51-150 cm) was related to tree density indicating intraspecific competition and self thinning. Selective harvesting of particular size classes of Rhizophora mucronata was recorded when comparing length of harvested poles (~301 cm) and the size class distribution of individuals. Taking into account the differences in growth rate for each size class for this species it will take approximately 13 years to attain a height of 390 cm which is the height at which trees are selected for harvesting at this estuary. This is 2.6 times slower than those individuals growing in Kenya. The feasibility of harvesting is dependent on the growth rate of younger size classes to replace harvested trees as well as the rate of natural recruitment feeding into the population. Different harvesting intensity scenarios tested within a matrix model framework showed that limits should be set at 5 percent trees ha-1 year-1 to maintain seedling density at > 5 000 ha-1 for R. mucronata. However harvesting of Bruguiera gymnorrhiza should be stopped due to the low density of this species at Mngazana Estuary. Harvesting of the tallest trees of Avicennia marina can be maintained at levels less than 10 percent ha-1 year-1. Effective management of mangrove forests in South African is important to maintain the current state, function and diversity of these ecosystems. Management recommendations should begin with determining the freshwater requirements of the estuaries to maintain the mouth dynamics and biotic communities and deter the harvesting of (whole) adult trees particularly those species that do not coppice. Further management is needed to ensure that forests are cleared of pollutants (plastic and industrial), and any further developments near the mangroves should be minimized.

Les forêts de mangrove contribuent de manière significative au flux d'énergie, au cycle des nutriments et du carbone dans l'océan côtier, étant un puits pour le CO2 atmosphérique. Les forêts de mangroves sont très productives et stockent une quantité élevée de carbone à la fois dans leurs sols et dans leur biomasse.Au cours de la décomposition de la litière, les nutriments et le carbone peuvent être recyclés ou exportés vers les écosystèmes adjacents par l'action des marées. La mangrove de Can Gio (Ho Chi Minh Ville, Vietnam), dégradée par l'épandage de défoliants pendant la guerre du Vietnam, a pu être restaurée grâce à la replantation et à la régénération naturelle. À ce jour, la forêt de mangrove de Can Gio est la plus grande forêt de mangrove contiguë au Vietnam, et est devenue la première réserve biosphère UNESCO dans ce pays. L'objectif principal de cette thèse était de caractériser le cycle du carbone dans la forêt de mangrove tropicale.Les résultats de cette thèse ont permis de:- Développer des équations allométriques permettant d’estimer la biomasse aérienne de la forêt plantée de mangroves de l’espèceRhizophora apiculatadans le sud du Vietnam;- Calculer les stocks de carbone totaux dans différents peuplements de mangrove se développant sous le climat tropical du sud du Vietnam;- Caractériser les taux de décomposition de la litière et évaluer la dynamique des nutriments et des métaux traces au cours des processus de dégradation de la litière, ainsi que l'évolution de δ13C pendant la décomposition;- Déterminer la variabilité saisonnière des flux de CO2 à différentes interfaces: sol-air, eau-air et tronc-air, et caractériser les profils de concentration en CO2 dans la canopée
Mangrove forests significantly contribute to energy flow, nutrient and carbon cycling in the coastal ocean, being a sink for atmospheric CO2. Mangroves forests are highly productive and store high amount of carbon both in their soils and in their biomass. During leaf litter decomposition, nutrients and carbon can be recycled or exported to adjacent ecosystems by the tidal action. Can Gio mangrove, degraded by the spraying of defoliants during the Vietnam War, successfully recovered through replantation and natural regeneration after 40 years. To date, the Can Gio mangrove forest is the largest contiguous mangrove forest in Vietnam, and became the first Mangrove Biosphere Reserve in this country. The main objective of this PhD thesis was to characterize carbon cycling within the Can Gio mangrove forest, which is a tropical one.The results of this PhD thesis allowed to: - Develop allometric equations and to estimate the aboveground biomass of Rhizophora apiculata Blume planted mangroves forest in Southern Vietnam; - Calculate the total carbon stocks in different mangrove stands developing under the tropical climate of Southern Vietnam; - Characterize the leaf litter decomposition rates, and assess nutrients and trace metals dynamics during litter decay processes, as well as the evolution of δ13C during decay; - Determine the seasonal variability CO2 fluxes at different interfaces: soil-air, water-air and trunk-air, and to characterize CO2 concentrations profiles in the canopy

A study on gender roles in mangroves management was conducted in Trieu Phuoc and Trieu Do communes of Trieu Phong district, Quang Tri province to gain a better understanding of gender roles in mangrove management. Research showed that local people are mainly dependent on fishing and aquaculture around the mangroves. Women have a good understanding about the role of mangroves and they are associated with mangroves not less than men, but so far their role has been overlooked. Mangrove management process seems to exclude women. This reduces the common management capacity of community. In addition, communities do not have a common regulation on the management and protection of mangrove forest resources and environment. A number of solutions are recommended such as strengthening the participation of civil society and women in mangrove forest management, developing a mangrove protection strategy and community-based regulations including gender.
Nghiên cứu về vai trò giới trong quản lý rừng ngập mặn (RNM) được thực hiện trên địa bàn 2 xã Triệu Phước và Triệu Độ của huyện Triệu Phong, tỉnh Quảng Trị nhằm có được sự hiểu biết hơn về vai trò giới trong quản lý RNM. Nghiên cứu đã chỉ ra rằng người dân ở đây chủ yếu sống dựa vào việc đánh bắt và nuôi trồng thủy sản tại khu vực RNM. Phụ nữ có hiểu biết khá tốt về vai trò crò RNM và họ gắn liền với RNM không kém nam giới, nhưng cho đến nay vai trò của họ gần như không được nhìn nhận. Công tác quản lý RNM còn hạn chế nữ giới tham gia. Điều này làm giảm năng lực quản lý chung của cộng đồng. Ngoài ra các cộng đồng vẫn chưa có quy ước chung về quản lý, bảo vệ tài nguyên và môi trường RNM. Từ đó nghiên cứu đề xuất một số giải pháp như tăng cường sự tham gia của các tổ chức dân sự và phụ nữ trong quản lý RNM, xây dựng chiến lược bảo tổn RNM và các quy ước quản lý RNM dựa vào cộng đồng bao gồm giới.

Increasing urbanisation of coastal areas is placing unprecedented pressure on estuarine environments. Impacts associated with urbanisation can alter the health of estuarine ecosystems, reducing their capacity to provide valuable ecosystem services. In order protect these environments, relationships between estuarine health and urbanisation should be understood and applied to sustainable urban planning in coastal areas. Responses measured in estuarine biota offer much potential for providing pragmatic and sensitive means for the detection of urban impacts in estuaries. I assessed the responses of estuarine biota to increasing levels of urbanisation in the rapidly developing coastal region of southeast Queensland, Australia. I measured a suite of biotic indicators, namely: the δ15N of mangroves and crabs as a tracer for urban N sources, sediment chlorophyll a concentration (Chl a) as an estimate of the biomass of microphytobenthos (MPB), and a condition index in crabs, measured as the hepatosomatic index (HI). Urbanisation was estimated as the percentage of catchment covered by impervious surfaces (impervious cover, IC). Increases in impervious surfaces such as roads, footpaths and car parks are closely linked to urbanisation. These surfaces concentrate and convey pollutants generated from urban activities into coastal waterways and estuaries, thus making IC a useful indicator of urban impact.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

Interactions between species are dynamic and are likely to shift with changes in species ranges due to climate change. With the expansion of new species into incumbent ecosystems a variety of abiotic and biotic factors shape the rate, pattern, and method of invasion. This dissertation utilizes one such boundary of transition, the salt marsh-mangrove ecotone, located in the northern Gulf of Mexico. This dynamic coastal wetland has recently seen the expansion of sub-tropical mangrove species into a previously salt marsh dominated system. This collection of works provides pointed case studies seeking to understand local and patch scale dispersal dynamics, expansion, recruitment, growth, and survival rates along an elevational gradient, and understanding interactions both above- and belowground between mature mangroves and the surround salt marsh. We find that dispersal is overwhelmingly dominated by propagule export, that black mangrove and smooth cordgrass differ in their ability to provide structural provisioning in the short term after restoration, and that mature mangroves have much greater belowground extent than aboveground. By better understanding species-specific interactions at the salt marsh-mangrove ecotone, a greater understanding of future expansion rates can be gained.

A significant challenge of the 21st century is the growing health threat stemming from drug-resistant infectious diseases. There is an undeniable need to discover new, safe and effective drugs with novel mechanisms of action to combat this threat. A study of drugs currently on the market showed that natural products account for approximately 75% of new anti-infective drugs, either as new agents or analogs based upon their structure. Unfortunately, major pharmaceutical companies have cut back tremendously in natural products research in part due to the frustrating obstacle of frequent rediscovery of compounds. Fungi in particular are difficult to work with in that they do not always produce the same variety and quantities of secondary metabolites under laboratory conditions. One of the groundbreaking discoveries evolving from genomics research is the observation that many fungi possess more gene clusters encoding for the production of secondary metabolites than the reported number of natural products isolated from those organisms. Simple epigenetic modifications such as DNA methlytransferase or histone deacetylase inhibition can activate silenced genes leading to the genesis of novel chemistry from the fungus. The work presented herein is a study of the isolation and characterization of anti-infective compounds from Floridian mangrove endophytes. In addition, epigenetic modifications were explored in order to increase the production of secondary metabolites as well as for the purpose of generating new analogs not found in the controls. Finally, structure activity relationship studies were performed in order to maximize the anti-malarial and antibiotic activity of cytosporone E.

Investigations on the biodiversity in relation to conservative and non-conservative parameters in the Mangalavanam mangrove ecosystem, located in the northern fringes of Cochin City have been carried out from January to June 2002. The mangrove ecosystem is regularly under tidal influence and hence submergence and emergence of land takes place depending on the tidal amplitude. The average dissolved oxygen of the water was found to be 3.5ml/l despite the fact, phytoplankton was abundant in the ecosystem. It reveals that respiratory demand of the aquatic biota has exceeded the photosynthetic oxygen production. The indirect relationship exhibited by the quantity of phytoplankton and oxygen is attributed to anthropogenic activities, which resulted in to the eutrophication of the mangrove ecosystem. The general nutrient load was at a higher level. The macrophytic vegetation was dominated by A^icennia marina, Rhizdphora mucronata and Acanthus ilicifolius. The presence of Avicennia and Acanthus in majority of the area that showed decrease in salinity and more freshwater influx. The phytoplankton community was dominated by djatoms represented Naviculaceae followed by Coscinodisceae, which Is evidenced by the presence of high quantity of silicate. The Zooplankton was dominated by copepods. Benthic community is dominated by the infauna such as polychaetes and decapods. Juveniles of common brackish water fishes. Chanos spp., Liza spp., Etroplus spp., Silago spp., Lethrinus spp. and Lutjanus spp., and species of crustaceans like Penaeus spp., Metapenaeus spp., Macrobrachium spp., Acetes spp., Metaplex spp., Sesarma spp., Uca spp., and Scylla spp., have been found to be the residents of the mangrove ecosystem. Avian fauna comprises mostly little cormorants (Phalactocorax niger) and black crowned night heron (Nycticorax nycticorax). Other arboreal fauna is dominated by Indian flying fox (Pteropus giganteus). An evaluation on the biodiversity of the mangrove ecosystem in the light of the present investigations reveals that species diversity Is less, but moderate population density of available species could be observed. To put It In a nutshell, human interventions on the environment has been detrimental and a general degradation of the ecosystem has been evidenced by the emergence of terrestrial vegetation and shrinking of the true mangrove areas.

Thesis (M. Sc.) (Hons) -- University of Western Sydney, 2003.
"Submitted in accordance with guidelines for the Degree of Master of Science (Hons), College of Science, Technology and the Environment, University of Western Sydney, Hawkesbury, Richmond NSW, Australia, March 2003." Includes bibliography : leaves 229 - 254, and appendices.

La mangrove est un écosystème majeur des zones intertidales des côtes tropicales. En Nouvelle-Calédonie, les mangroves sont des zones tampon entre les massifs ultrabasiques, plus ou moins exploités, et un lagon inscrit au patrimoine mondial de l'UNESCO. Le long de l’estran, cet écosystème se décompose principalement en trois zones de végétation qui se développent parallèlement au trait de côte, avec du front de mer vers les terres : Rhizophora spp., Avicennia marina, tannes. Ce gradient botanique dépend de la durée d'immersion des marées, qui impose des gradients de teneur en eau, de salinité de l'eau interstitielle, d’oxygénation, et de teneur en matière organique dans les sédiments. L'objectif de cette thèse était d'améliorer notre compréhension du cycle biogéochimique du fer et du nickel dans les sédiments de mangrove, et de déterminer le rôle des paramètres édaphiques sur la mobilité de ces éléments métalliques. Ce travail est plus spécifiquement focalisé sur l’évolution de la cristallochimie de Fe et Ni en liant avec les (bio)transformations minéralogiques. Les analyses de spectroscopie d’absorption à rayons X montrent clairement que les spéciations de Fe et Ni évoluent en fonction des gradients redox qui marquent d’une part, la zone intertidale, et d’autre part, la profondeur. La goethite et les phyllosilicates, hérités des bassins versants latériques, sont les principaux hôtes du Fe et Ni dans la partie supérieure des sédiments de mangrove. Ces espèces minérales sont intégralement préservés en profondeur des sédiments des tannes, qui sont pauvres en matière organique et bien oxygénés. En revanche, sous les Rhizophora et les Avicennia, la goethite disparait rapidement avec la profondeur . Dans ces horizons inférieures anoxiques riches en matière organique, la sulphato-réduction se développe, et la pyrite et les complexes organiques sont les principales phases porteuses du Ni. A la limite entre les couches oxiques et anoxiques, une intense ré-oxydation du Fe (II) aqueux et des sulfures de fer conduit à la formation de ferrihydrite, lépidocrocite, et probablement de goethite. La proportion relative des oxyhydroxydes de fer néoformés et mal cristallisés, est plus élevé dans la mangrove à Rhizophora. En outre l’incorporation du Ni dans la pyrite est également moins prononcée dans cette zone. Une telle évolution latérale de la spéciation du Ni peut être liée à la réoxydation fréquente des pyrites porteuses de Ni en lien avec le balayage quotidien de cette zone par les marées. Ce cycle tidal, qui touche plus particulièrement la zone à Rhizophora du fait de sa position basse dans l’estran, peut être une cause majeure des cycles de réduction et d’oxydation des phases porteuses de Fe, et pourrait affecter de manière significative les bilans de masse du fer et du nickel dans les mangroves. En effet, le Ni tend à être immobile dans les tannes, à s'accumuler sous Avicennia, et à être partiellement libéré sous Rhizophora. Finalement, le comportement du Fe et du Ni dans des sédiments de mangrove subissant une oxydation intense, en réponse à un isolement de la mer, a été étudié. Au niveau du front d’oxydation, des concentrations très élevées de Ni en solution ont été mesurées , tandis que les concentrations en Ni dans la phase solide étaient quatre fois plus faible que dans l'horizon pyritisé, et 2,5 fois plus faibles que dans le sédiment supérieur. Ces résultats suggèrent que l'oxydation des sédiments de mangrove est une cause de perte en Ni pour l’écosystème. Cette thèse a permis une meilleure connaissance des processus minéralogiques qui conduisent à la fixation ou la libération des élémentes traces métalliques par les mangroves, et est donc utile pour la gestion des mangroves qui sont situés en aval de bassins versants latéritiques
Mangrove forests are the dominant intertidal ecosystem of tropical coastlines. In New Caledonia, mangroves act as a buffer zone between Ni open-cast mines and a lagoon registered as a UNESCO World Heritage site. Across the intertidal zone, mangroves are composed of three main stands; with from the seaward side to the landward side: Rhizophora spp., Avicennia marina, salt-flat. This botanical gradient relies on the duration of tidal immersion, which imposes sedimentary gradients of water content, salinity, oxygenation, and organic content.The objective of this PhD thesis was to improve our understanding of the biogeochemistry of iron and nickel in mangrove sediments and to characterize the role of edaphic parameters on trace metals dynamic. Particular emphasis was focused on the mineralogical (bio)transformation of Fe and Ni bearing phases and on crystal chemistry. Both XANES and EXAFS data showed that Fe and Ni speciation strongly followed the redox boundaries marking the intertidal and depth zonations. Fe(III)-bearing goethite and phyllosilicates, herited from lateritic outcrops, were the major Fe and Ni hosts in the upper mangrove sediments. These mineral species were fully preserved at depth in the dry and oxic salt flat area. By contrast, beneath the vegetated Rhizophora and Avicennia stands, goethite rapidly disappeared with depth. In these anoxic horizons, sulfate reduction occurred, and pyrite and organic complexes became the dominant Ni-species. At the limit between oxic and anoxic layers, intense re-oxidation of aqueous Fe(II) and Fe-sulfides led to the formation of ferrihydrite, lepidocrocite and likely goethite. The relative proportion of the newly formed poorly ordered iron-oxyhydroxides was found to be higher in the Rhizophora mangrove stand. Moreover Ni incorporation in pyrite was less developed beneath Rhizophora stand. Such lateral evolution of Ni speciation may be related to reoxidation of Ni-bearing pyrites in the Rhizophora stand, which is subjected to periodic alternation of reducing and oxidizing events due to daily tidal fluctuations. The latter may be a major cause for continuous Fe reduction-oxidation cycles in the vegetated mangrove stands, and could significantly affect iron and nickel mass balances in mangroves. Indeed, Ni was found to be immobile in the salt flat, to accumulate beneath Avicennia and to be partially leached beneath Rhizophora. Eventually, Fe and Ni behavior in mangrove sediments currently oxidizing in response to isolation from the sea, was studied. In the layer of the oxidation front, really high concentrations of dissolved Ni were measured, while Ni concentrations in the solid phase were 4 times lower than in the pyritized horizon, and 2.5 times lower than in the upper sediment. These results suggest that mangrove sediment oxidation was a cause of Ni loss. This PhD thesis allows a better assesment of the mineralogical processes that lead to the fixation or the release of trace metals by mangroves, and is thus useful for the management of mangroves that are situated downstream lateritic watershed

Mangrove forests are subjected to many environmental factors which influence species distribution, zonation patterns as well as succession. Important driving factors in these forests are salinity, water level fluctuations and available light. This study investigated the response of red mangrove (Rhizophora mucronata Lam.) seedlings to these factors in controlled laboratory experiments. Increase in salinity and prolonged inundation within estuaries are predicted impacts resulting from sea level rise due to climate change. The study investigated the effect of five salinity treatments (0, 8, 18, 35 and 45 ppt) with a semi-diurnal tidal cycle on seedling growth. In a separate experiment the effect of different inundation treatments: no inundation, 3, 6, 9 hour tidal cycles and continuous inundation (24 h) were investigated. Both morphological and physiological responses of R. mucronata seedlings were measured. There was a decrease in growth (plant height, biomass and leaf production) with increasing salinity. Seedlings in the seawater, hypersaline and no inundation treatments showed symptoms of stress, having increased leaf necrosis ("burn marks"). The highest growth occurred in the low salinity (8 ppt) treatment, but the highest photosynthetic performance and stomatal conductance occurred in the freshwater treatment (0 ppt). The typical response of stem elongation with increasing inundation was observed in the 24 hr inundation treatment. In the light and salinity combination study there were ten different treatments of five different light treatments (unshaded, 20 percent, 50 percent, 80 percent and 90 percent shade) combined with two salinity concentrations (18 and 35 ppt). In this study the seedling growth: plant height, biomass, leaf surface area and leaf production were higher in the moderate salinity (18 ppt) treatments compared to the seawater (35 ppt) treatments. Biomass in the 35 ppt experiment decreased with increasing shade as well as in the unshaded treatments. Photosynthetic performance and stomatal conductance were lower for the unshaded treatment in both 18 and 35 ppt salinity compared to all other treatments with the same salinity. This suggests that R. mucronata more shade than sun tolerant, but overall it can be concluded that the species has a broad tolerance range. The results may be relevant in mangrove rehabilitation and predicting responses to climate change. This is important as mangrove ecosystems may adapt to changing sea levels and in order to restore areas it will be necessary to choose the mangrove species which will grow best. The results may also help to increase the protection of existing mangrove habitats.

Acknowledging the increasingly prominent urban character of ecosystems globally, mangroves being no exception, and possible impediments to the viability of these ecosystems (i.e. the inherent capacity or ability to grow, develop or recover after disturbances), we adapt a system‟s approach to establish the viability of the peri-urban mangrove of Tudor creek in Mombasa, Kenya. Three important aspects of the peri-urban mangroves are assessed. These include (i) structural aspects (vegetation structure and regeneration), (ii) functional aspects (productivity) and (iii) human aspects (socioeconomics). Chapter 1 and 2 introduce the study, outlining the objectives and the study area. The chapters lay down an overview of the mangrove trees physiological adaptations and the ecological attributes that make the mangrove ecosystems unique and highly adaptable to a harsh and dynamic intertidal environment. The chapters further outline the extent and status of mangroves in Kenya, their socioeconomic importance, and the legislation that governs their management and conservation.

Chapters 3 and 6 describe the structural attributes and regeneration status of the peri-urban mangroves of Tudor creek. Based on species importance values the dominant mangrove species were Rhizophora mucronata Lam. (Rhizophoraceae) and Avicennia marina (Forssk.) Vierh. (Acanthaceae)&61607; Lumnitzera racemosa Willd. (Combretaceae), reported in an earlier floristic survey, was not encountered. Tree density varied between 1,264 trees ha–1 and 1,301 trees ha–1, which are within the range of values reported for similar forests globally. However, the size-class structure showed the numerical dominance of small trees over larger trees. The spatial distribution pattern of adults and juveniles varied greatly between sites had a close to uniform pattern (Morisita‟s Index Iδ << 1) for adult trees, but a tendency for clustered distribution (Iδ >> 1) for juveniles. This pattern of distribution is not expected for a healthy forest. The distribution of regenerating seedlings was mainly impacted by canopy gap sizes. These chapters shows that unmanaged but exploited peri-urban mangroves are structurally degraded, having enlarged canopy gaps, characterised by spatial and temporal heterogeneity in edaphic conditions that influences regeneration. This enlarged gaps and edaphic heterogeneity imposes longer periods for canopy closure by lowering regeneration and promoting mortality of seedlings. Larger gaps (> 60m²) had lower regeneration levels and intermediate gaps (20-50m²) had adequate regeneration. The occurrence of R. mucronata seedlings and saplings in the understorey under all cover types and inundation confers advantages to this species under the current disturbance regime. Disturbances include sewage pollution, unregulated harvesting and siltation. The current status of the forest is reminiscent of a recovery phase, a multiphase species succession stage, after a major disturbance event, accompanied by recurrent anthropogenic pressure. This study shows that species composition and thus recovery of a mangrove forest after disturbance depends in part on the balance between subsequent large-scale natural and recurrent small-scale human disturbances.

Chapter 4 assesses the human dependence on the peri-urban mangrove. Through questionnaires and field surveys, the study demonstrates the challenges of questionnaire surveys targeting respondents involved in exploitation of sensitive resources. This potentially limits if not inhibit information flow. Firewood is the ubiquitous form of mangrove wood utilisation, exploited at both subsistence and commercial scales. Forest assessments indicate the lack of preferred or specific harvesting sites, with R. mucronata being the most harvested, probably due to its distribution range and ease of access. Sewage pollution was viewed with mixed feelings. Many appreciate the nutrient enrichment of the sewage rather than the filtration capacity of mangroves, resulting in the usage of sewage for irrigating small plots of subsistence farms. The study shows that resource exploitation is intense in an urban setting due to an economic drive and an increasing demand. In the rural setting on the other hand, utilisation included both subsistence and commercial charcoal production. This has promoted more efficient, destructive and unsustainable exploitation levels. Associated benefits of these activities grossly under-value the ecosystem goods and services in addition to degrading the ecosystem. Our observations, consistent with other studies, shows that management of mangroves for wood extraction in urban areas may not be a viable and/or sustainable option, as it conflicts with the essential „filtration‟ and „habitat provisioning‟ functions and services of mangrove ecosystems. These functions and services are increasingly important in a “diminishing” urban environment. This arises out of the lack of adequate alternatives and conflicting interests in growing urban areas. It is recommended that „adaptive‟ and „participatory management’ based on multiple uses and users, with specific legislative, education and institutional interventions. Integrating local ecological knowledge, may further expedite the formulation of sustainable management plans for peri-urban mangroves. Chapter 5 presents insights on the productivity of an under-valued, over-exploited and sewage polluted peri-urban mangrove through litter fall studies on three common mangrove species, R. mucronata, A. marina and S. alba. The study covers a period of two years. The mean annual litter fall was estimated at 12 ± 3 t ha-1yr-1 for the whole stand, which is within the range of values reported for similar forests occupying the same latitudinal range. Litter fall, in both content and quantity was highly seasonal, with high rates occurring in the dry North Easterly Monsoon (NEM) season, January-April (ca. 5 ± 1 g DW m-2 day-1) and lower rates in the cool and wet South Easterly Monsoon (SEM) season, June-October (ca. 3 ± 0.5 g DW m-2 day-1). R. mucronata recorded the highest annual rate of 15 ± 3 t ha-1yr-1. No significant differences in litter fall rates were observed between A. marina and S. alba, (11 ± 3 and 10 ± 5 t ha-1yr-1 respectively). Sewage exposure levels did not appear to influence litter production rates despite higher nutrient levels in completely exposed sites. δ15N varied with species; A. marina (6.48 ± 0.03‰) and S. alba (6.76 ± 0.24‰) having higher composition than R. mucronata (3.88 ± 0.64‰). The leaf C:N ratio strongly correlated with elevated leaf δ15N signature. Higher C:N ratio for R. mucronata corresponding with lower leaf δ15N (3.88 ± 0.64‰) signature, and lower C:N ratio for A. marina and S. alba (6.48 ± 0.03‰ and 6.76 ± 0.24‰ respectively) corresponding with higher δ15N signature. This reflects species specific response to raw sewage exposure. This implies the forest has a more open N cycle, favouring δ15N accumulation within the system. This study shows that sewage exposure does not necessarily translate into elevated productivity in mangroves, but may alter litter nitrate content depending on species, possibly altering the decay of litter and nutrient cycling.

Chapter 7 presents a synthesis linking the findings to possible implications on the general status of the mangrove ecosystem. The major disturbances observed for the peri-urban mangroves of Tudor creek include (i) domestic sewage pollution, (ii) recurrent unregulated harvesting; and (iii) recurrent annual siltation during the rainy season. Our observations indicate that

(i) Raw domestic sewage pollution may not be harmful to the mangrove vegetation, but may affect edaphic conditions through nutrients enrichment. Sewage pollution effects, though not qualitatively proven in our study, enhance growth of mangrove trees. This is due to increase in amounts of nutrients available for biomass formation, observed as leaf nitrates resorption efficiencies. However, the raw domestic sewage is reported to alter the general healthy decomposing aerobic-anaerobic mangrove system to a complete anaerobic system. This tends to lower efficiencies in nutrient cycling, and cause accumulation of nutrients in the sediments. Observations within the same site and other East African mangroves (under the PUMPSEA project) indicate negative effects of sewage on the sediment cyanobacterial diversity, with an increase in microalgal abundance. Furthermore, within Mikindani (a sewage impacted site in Tudor creek), high rates of sediment ΣCO2 production indicate a system under stress due to the presence of easily degradable organic matter. (ii) Un-regulated harvesting creates and enlarges canopy gaps, lowering availability of seed bearing trees, altering species composition and stem size distribution due to its selective nature, and lowers regeneration under the enlarged canopy gaps. This strongly lowers recovery rates after major disturbances. (iii) Siltation stands out as a major cause of degradation. Siltation is extrinsic in nature, a result of poor land use practices. This probably makes it a major issue of concern due to its impact on regeneration. A major siltation event, associated with the 1997-1998 ENSO, is widely identified as a cause of enlarged canopy gaps. Little recovery has occurred 10 years after the event due to recurrent anthropogenic pressure. The combined effects of these factors have important implications on growth, productivity and recovery of the mangrove ecosystem. The effects include shifts or changes in mangrove tree species distribution. This has lowered the system functional diversity and response diversity, and therefore ecosystem resilience - viability of the ecosystem. It is recommended that integrated adaptive management, based on sound knowledge of the system is the recommended approach. The participation of stakeholders (government institutions, the private sector and local communities) is crucial for managing peri-urban mangroves for sustainability. Not intervening may only result in a worst case scenario. Especially with the current global financial crisis, more locals will turn to „cheap‟ mangrove firewood.

En vue de l‟augmentation de l‟urbanisation dans les multiples écosystèmes du monde, les mangroves n'étant pas une exception, on observe des obstacles à la viabilité de ces écosystèmes, c.-à-d. leur capacité inhérente de se développer, de s‟étendre ou de récupérer après des perturbations sous des conditions urbaines. Nous avons adapté une approche systémique pour établir la viabilité des mangroves périurbaines de la baie de Tudor à Mombasa, Kenya. Trois aspects importants des mangroves périurbaines sont évalués. Cela inclus (i) les aspects structurels (structure de la végétation et la régénération), (ii) aspects fonctionnels (productivité) et (iii) aspects humains (socio-économiques). Les Chapitres 1 et 2 introduisent l'étude, en exposant brièvement les objectifs et le domaine d'étude. Les chapitres présentent un panorama des adaptations physiologiques des palétuviers et des attributs écologiques qui rendent l‟écosystème de la mangrove unique et hautement adaptable à un environnement intertidal rigoureux et dynamique. Les chapitres suivants exposent l‟étendue et le statut des mangroves au Kenya, leur importance socio-économique et la législation qui cadre leur gestion et conservation.

Les chapitres 3 et 6 décrivent les caractéristiques structurelles et les étapes de la régénération des palétuviers périurbains de la baie de Tudor. Par ordre d'importance des espèces, les palétuviers les plus souvent rencontrés étaient Rhizophora mucronata (Rhizophoraceae) et Avicennia marina (Forssk.) Vierh. (Acanthaceae)&61607; Lumnitzera racemosa Willd. (Combretaceae) n'a pas été rencontrée lors d‟un voyage floristique précédent. La densité d'arbre varie entre 1.264 arbres ha-1 et 1.301 arbres ha-1, ce qui correspond aux moyennes rapportées pour des forêts comparables dans le monde. Cependant, la structure des classes de taille montre une dominance numérique des palétuviers moins développés par rapport aux plus développés. Le pattern de distribution spatiale des adultes et des juvéniles varie considérablement entre les sites et montre presque un modèle uniforme (l'index de Morisita; Iδ << 1) pour les individus adultes. En revanche, pour les juvéniles on retrouve une tendance de distribution groupée (Iδ >> 1). Ce modèle de distribution n'était pas attendu pour une forêt non altérée. La distribution de la régénération des pousses est principalement dictée par la taille des gaps. Ce chapitre montre que les mangroves périurbaines, exploitées et non aménagées, sont structurellement dégradées, ayant des larges gaps caractérisés par une hétérogénéité des conditions édaphiques qui influencent la régénération. Quant aux gaps les plus larges, on retrouve un taux de régénération plus faible. Les gaps élargis et l‟hétérogénéité édaphique imposent de plus longues périodes de fermeture des canopées, en diminuant la régénération et promouvant la mortalité des pousses. Les gaps plus grands (> 60m2) ont des niveaux de régénération plus bas et les gaps intermédiaires (20-50m2) ont une régénération adéquate. L‟occurrence de pousses et de juvéniles de R. mucronata dominent le niveau inférieur de tous les types de couvertures végétales et d‟inondation. Cela confère un avantage à cette espèce sous le régime de perturbation actuel. Les perturbations incluent les eaux usées non traitées, les récoltes non-réglementées et l‟envasement. L'état actuel de la forêt est une phase de récupération, un stade de succession d‟espèces en multiples phases, suite à un événement majeur de perturbation, accompagné d‟une pression anthropogénique récurrente. Cette étude démontre que la composition d‟espèces et donc de la récupération de la forêt de mangrove après une perturbation dépend en partie de l'équilibre entre les perturbations naturelles à grande échelle et humaines à moindre échelle.

Le chapitre 4 évalue la dépendance humaine à l'égard de la mangrove périurbaine. Par des questionnaires et des travaux de terrains, on a constaté que l‟utilisation des questionnaires est délicate. En effet, ceux-ci ciblent des personnes impliquées dans l'exploitation des ressources sensibles. Ceci limite potentiellement sinon empêche le flux de l'information. Le bois de feu est la forme d‟utilisation la plus répandue du bois de mangrove, utilisé dans le cadre de la subsistance et du commerce. Les résultats indiquent qu‟il n‟y a pas de sites de récolte préférés. R. mucronata est l‟espèce la plus récoltée, probablement en raison de sa distribution et facilité d‟accès. La pollution par les eaux usées a appréciation mixte. Certains l‟apprécient pour l‟irrigation de leurs cultures de subsistance, malgré la capacité de filtration des mangroves. L‟étude montre que l‟exploitation des ressources est intense, dans un cadre urbain, dû à une conduite économique et une demande croissante. Par contre, dans le cadre rural, l‟utilisation inclus la production de charbon de subsistance ainsi que commercial. La limitation des moyens de vie dans le cadre rural, couplés à la pauvreté et le besoin d‟énergie domestique bon marché, créent une dépendance des ressources de

mangroves. Ceci a favorisé des niveaux d‟exploitation plus importants, destructeurs et non viables. Les gains associés à ces activités qui dégradent l‟écosystème, sont beaucoup moins importants que les biens et les services offerts par la mangrove. Nos observations, en accord avec d‟autres études, montrent que la gestion de l‟extraction du bois des mangroves dans des zones urbaines n‟est pas une option viable et/ou durable, puisqu‟il entre en conflit avec les fonctions et services essentiels de „filtration‟ et „d’approvisionnement pour l’habitation‟ de la mangrove. Ce phénomène résulte du manque d‟alternatives adéquates et des conflits d‟intérêts des zones urbaines en expansion. Il est recommandé de mettre en place une gestion „adaptative‟ et „participative‟ fondée sur les utilisations et utilisateurs multiples, ainsi qu‟une législation, une éducation et des interventions institutionnelles spécifiques, intégrant les connaissances écologiques locales, afin de faciliter des plans de gestion durable pour les mangroves périurbaines.

Le chapitre 5 présente un aperçu de la productivité de la mangrove périurbaine sous-évaluée, surexploitée et polluée par des eaux usées à travers des études de la litière de trois espèces communes de palétuvier, R. mucronata, A. marina et S. alba. L‟étude couvre une période de deux ans. La chute moyenne annuelle de litière a été estimée à 12 ± 3 t ha-1an-1 pour la totalité du secteur, ce qui correspond aux valeurs de forêts comparables occupant des latitudes similaires. La composition et la quantité de la litière changent fortement avec les saisons. Les taux élevés (ca. 5 ± 1 g DW m2 j-1) s‟observent au cours de la saison sèche (Janvier-Avril), sous le North Easterly Monsoon (NEM).Tandis que les taux inférieurs (ca. 3 ± 0,50 g DW m2 j-1) s‟observent au cours de la saison humide (Juin-Octobre), sous le South Easterly Monsoon (SEM). Il y a des variations significatives entre les espèces. R. mucronata a enregistré le taux annuel le plus élevé 15 ± 3 t ha-1an-1. Par contre, il n‟y a pas de différences significatives entre A. marina et S. alba, (11 ± 3 et 10 ± 5 t ha-1an-1, respectivement). Le niveau de pollution par les eaux usées ne semble pas influencer les taux de production de litière en dépit de la concentration plus élevées des nutriments dans les zones complètement exposées. Le δ15N a varié entre les espèces, A. marina (6,48 ± 0,03‰) et S. alba (6,76 ± 0,24‰) ayant une composition plus élevée que R. mucronata (3,88 ± 0,64‰). Cela implique que la forêt a un cycle de l‟N plus ouvert, favorisant l'accumulation dans le système. Cette étude démontre que l'exposition aux eaux usées ne se traduit pas nécessairement en une productivité élevée des palétuviers, mais peut changer le contenu de nitrogène total des feuilles, probablement en changeant la décomposition de la litière et le cycle des nutriments. Le chapitre 7 présente une synthèse liant les résultats aux implications possibles sur le statut général de l'écosystème des mangroves. Les perturbations principales observées pour les mangroves périurbaines de la baie de Tudor incluent (i) la pollution par les eaux usées domestiques, (ii) la récolte récurrente irrégulière et (iii) l‟envasement annuel pendant la saison des pluies. Nos observations indiquent que: (i) la pollution par les eaux usées non traitées n‟est pas nocive pour les palétuviers, mais peut affecter les conditions édaphiques par l'enrichissement en éléments nutritifs. Les effets de la pollution des eaux usées, quoique non prouvés qualitativement dans notre étude, augmentent la croissance des palétuviers. Ce par une augmentation des quantités de nutriments disponibles pour la formation de biomasse, observée lors de l‟étude de l‟efficacité de résorption des nitrates par les feuilles. Cependant, on rapporte que les eaux usées domestiques non traitées changent le système de décomposition aérobique-anaérobique naturel des mangroves vers un système uniquement anaérobique. Ce qui diminue l‟efficacité du cycle nutritif et engendre l'accumulation des nutriments dans les sédiments. Les observations sur le même site et sur d‟autres mangroves de l‟Afrique de l‟Est (sous le projet PUMPSEA), indiquent des effets négatifs des eaux usées sur la diversité des cyanobactéries du sédiment et une augmentation de l'abondance des micro-algues. En outre, à Mikindani (site affecté par des eaux usées dans la baie de Tudor), les taux élevés de production de ΣCO2 dans le sédiment indiquent un système sous pression (ou sous stress), suite à la présence de la matière organique facilement dégradable. (ii) L‟exploitation non réglementée va créer et agrandir des gaps. En fonction de la nature sélective de l‟exploitation, il y aura une diminution de la disponibilité des arbres produisant des propagules, un changement de la composition des espèces et de la distribution des tailles des arbres. De plus, elle réduit la régénération sous les larges gaps. Cela diminue fortement les taux de rétablissement après d‟importantes perturbations.

(iii) L'envasement est la cause majeure de la dégradation. L'envasement est extrinsèque par nature, parce qu‟il découle de mauvaises pratiques d‟exploitation de la terre. Ce phénomène est probablement le point de concertation majeur, dû à son impact sur la régénération. Un événement d'envasement majeur, lié à l‟ENSO de 1997-1998, est identifié comme étant une cause d‟agrandissement des gaps. Peu de récupération s'est produite lors des dix années qui suivirent l'événement, suite à la pression anthropogène récurrente. Les effets combinés de ces facteurs ont des implications importantes sur la croissance, la productivité et la récupération de l'écosystème des mangroves. Les effets incluent des variations ou des changements de la distribution des espèces de palétuviers. Ceci a diminué la diversité fonctionnelle et la diversité des réponses du système et a donc diminué la résilience de l‟écosystème - la viabilité de l'écosystème. Il est recommandé qu‟une gestion adaptative intégrée, fondée sur la connaissance orale du système, soit la meilleure approche. La participation des décideurs (institutions gouvernementales, secteur privé et communautés locales) est cruciale pour la gestion des mangroves périurbaines. Dans l‟absence d‟intervention, seul le pire scénario peut être envisagé. En particulier avec la crise financière globale actuelle, la population locale se tournera encore plus vers le bois de feu „bon marché‟ de la mangrove.
Doctorat en Sciences
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Mestrado em Microbiologia
Os mangais são económica e biologicamente importantes. Sendo, contudo, cada vez mais ameaçados. Com o intuito de recuperar estes ecossistemas, têm sido desenvolvidos programas de reabilitação. Todavia, geralmente, estes programas não consideram a importância e os possíveis efeitos das interacções entre microrganismos e plantas no ecossistema dos mangais, devido ao número limitado de estudos em ecologia microbiana neste ecossistema. Sabe-se que as raízes de espécies de plantas terrestres influenciam a composição das comunidades bacterianas do solo. Por sua vez, os microrganismos podem contribuir no crescimento e saúde das plantas. Este estudo teve como objectivos desenvolver um sistema primers para reacção em cadeia da polimerase -electroforese em gel com gradiente desnaturante (PCRDGGE) para o domínio Archaea e o género Nanoarchaeum e determinar se as raízes de plantas de mangal (Rhizophora mangle e Laguncularia racemosa) afectam a composição das comunidades de Archaea e Nanoarchaeum que habitam o solo que está sob a influência das raízes de mangal (efeito “rizosfera”). As comunidades de Archaea e Nanoarchaeum foram analisadas por métodos moleculares, como a PCR e o DGGE. Foi desenvolvido um sistema de primers para PCR-DGGE adequado para o domínio Archaea e para o género Nanoarchaeum com base em novas sequências do gene 16S rRNA recentemente publicadas. Os perfis de DGGE foram analisados com a análise de similaridades (ANOSIM), o método de escalonamento multidimensional não paramétrico (MDS) e o índice de Shannon-Wiener. Os resultados de MDS e ANOSIM sugerem que existem diferenças significativas entre as amostras de sedimento e as amostras de rizosfera de R. mangle e L. racemosa. Por sua vez, a análise de MDS sugere que as raízes de L. racemosa afectam mais a composição da comunidade de Archaea do sedimento do que as raízes de R. mangle. Pelo contrário, os resultados de MDS e ANOSIM sugerem que as plantas de L. racemosa e R. mangle não exercem qualquer efeito na composição de Nanoarchaeum e que não existem diferenças entre as amostras de sedimento e as de rizosfera. A diversidade das populações de Archaea e Nanoarchaeum foi estimada pelo índice de Shannon-Wiener; e mostrou que a diversidade de Archaea era mais elevada do que anteriormente descrito em sedimentos marinhos. Com o propósito de completar este estudo algumas bandas dominantes das amostras de rizosfera vão ser clonados e sequenciados, vão ser criadas bibliotecas de clones para Nanoarchaeum e serão efectuadas análises de pirosequenciação às comunidades de Archaea.
Mangrove forests are economically and biologically important; however, they are also increasingly threatened. In order to recuperate these ecosystems, rehabilitation programs have been developed. However, in general these programs have no knowledge about the importance of plant-microbe interactions in mangrove ecosystem. This happens also because the limited numbers of studies on microbial ecology in this ecosystem. Therefore, they do not consider the possible effects of plant-microbe interactions in mangrove reforestation approaches. It is known that roots of terrestrial plant species influence the composition of soil bacterial communities. In turn, microorganisms can contribute to plant growth and health. In this study we aimed to develop a polymerase chain reaction -denaturing gradient gel electrophoresis (PCRDGGE) primer system suitable for Archaea domain and Nanoarchaeum genus and to determine if roots of mangrove plants (Rhizophora mangle and Laguncularia racemosa) affect the composition of Archaea and Nanoarchaeum communities inhabiting the sediment under influence of mangrove roots (rhizosphere effect). Archaea and Nanoarchaeum communities were analyzed using molecular methods, such as PCR and DGGE. A PCR-DGGE primer system suitable for Archaea domain and Nanoarchaeum genus was developed based on new 16S rRNA gene sequences recently published. DGGE profiles were analyzed with analysis of similarities (ANOSIM), non-metric multidimensional scaling (MDS) and Shannon-Wiener index. Both MDS and ANOSIM results suggest that there are significant differences between bulk and rhizosphere samples of R. mangle and L. racemosa. In turn, MDS analyses suggest that roots of L. racemosa affect more the composition of Archaea community from bulk sediment than roots of R. mangle. On the opposite, ANOSIM statistics and MDS analyses suggest that L. racemosa or R. mangle plants do not influence the nanoarchaeal composition and that there are no differences between bulk and rhizosphere samples. Diversity of Archaea and Nanoarchaeum populations was estimated by using the Shannon-Wiener index; and showed that diversity of Archaea was higher than previously reported in marine sediments. With the purpose to complete this study some dominant bands of rhizosphere samples will be cloned and sequenced, clone libraries for Nanoarchaeum will be generated and pyrosequencing analysis of archaeal communities will be performed.

Mestrado em Engenharia Civil
Mangrove forests are a very important ecosystem in protected coastal zones. These forests provide food and habitat for numerous species and have an important role in the coastal stabilization, trapping sediments and reducing erosion. They also contribute to wave attenuation, which might add to the safety of a coastal environment. Although the importance of mangrove forests in wave attenuation is widely recognized, the amount of comprehensive studies to understand the quantitative effect of the mangroves in the attenuation of wave energy is still insufficient. This thesis aims to increase knowledge on wave attenuation in mangroves by studying this process in the field. Results are presented of a fieldwork campaign executed in the Trang province, Thailand, between November 2010 and May 2011. It is the first known study that includes quantitative information on the vegetation in the field, presenting rates of vegetation density along the studied transects. Two different transects were chosen in Trang province – Kantang and Palian – showing differences in slopes and vegetation. Measurements included surface elevation, vegetation density and pressure fluctuations. After the measurements, the pressure data was carefully selected and analyzed using spectral analysis and wave linear theory concepts. The entire procedure is described in this thesis in order to provide a good basis for future studies in the field. The results show the presence of waves with periods mostly exceeding 5 seconds. At Kantang most of the waves have periods between 10 and 20 seconds. Wave attenuation is found to be about 2.5 J/m2 in 100 meters of mangrove forest at Palian and 0.8 J/m2 per 100 meters of mangroves at Kantang. This difference can be explained by a more accentuated slope, a denser forest and the presence of higher frequency waves that are easily attenuated at Palian. In general, the wave attenuation reaches values around 60% attenuation along the transects. Comparison with other studies shows that the results in this thesis resemble the findings of previous researches. The wave attenuation in mangroves was also compared to wave attenuation in salt marshes and it was concluded that the salt marshes have better wave attenuating capacity if the water depth is not too high, but with increasing water depths they rapidly loose this capacity, in contrary to mangroves.
As florestas de mangue são um ecossistema muito importante em zonas de costa protegida. Estas florestas fornecem alimento e habitat a inúmeras espécies e têm um papel importante na estabilização costeira, retendo sedimentos e reduzindo a erosão. Estas florestas também influenciam a atenuação das ondas, contribuindo para a segurança costeira. Embora seja amplamente reconhecida a importância das florestas de mangue na atenuação da energia das ondas, constata-se que a bibliografia sobre este tema é ainda insuficiente. Esta tese tem como objetivo aumentar o conhecimento sobre este fenómeno. Para tal, são analisados os dados relativos a um trabalho de campo executado na província de Trang, Tailândia, entre Novembro de 2010 e Maio de 2011. Este é o primeiro estudo a incluir informação quantitativa sobre a vegetação na zona em análise, apresentando valores para a densidade da vegetação ao longo dos trajetos estudados. Foram selecionados dois trajetos distintos na província de Trang – Kantang e Palian. Estes locais apresentam diferenças na inclinação do terreno e na densidade da vegetação. As medições realizadas incluem levantamento o topográfico do terreno, a densidade da vegetação e as variações de pressão ao longo do trajeto. Após as medições, os dados relativos à sobre a variação de pressão foram cuidadosamente selecionados e analisados usando análise espectral e conceitos da teoria de onda linear. Todo o procedimento é descrito nesta tese de modo a proporcionar uma boa base para futuros estudos na área. Os resultados mostram, na sua maioria, a presença de ondas com períodos superiores a 5 segundos. Em Kantang, a maior parte das ondas têm períodos entre os 10 e 20 segundos. A atenuação da energia hidrodinâmica num percurso de 100 metros de floresta é estimada em cerca de 2,5 J/m2 em Palian e 0,8 J/m2 em Kantang. Esta diferença pode ser explicada pela mais acentuada inclinação do terreno, uma maior densidade de vegetação e pela presença de ondas com períodos menores (que são mais facilmente atenuadas) em Palian. Em geral, a atenuação das ondas atinge valores de cerca de 60% ao longo dos trajetos. A comparação com outros estudos mostra que as conclusões retiradas nesta tese são semelhantes às conclusões de pesquisas anteriores. A atenuação da energia hidrodinâmica em florestas de mangue também foi comparada com a atenuação em pântanos salgados. Conclui-se que estes ecossistemas têm maior capacidade de atenuação se o nível do mar for baixo, mas que com maiores profundidades esta capacidade reduz-se rapidamente, ao contrário das florestas de mangue.

Vietnam, with 3,260 km of coastline, is one of the most vulnerable countries to global warming. The mangrove area in Vietnam has declined markedly in the past 60 years. Many afforestation and reforestation projects have been undertaken in Vietnam but these efforts have not always been successful. This PhD study was undertaken in Thai Binh, Thanh Hoa and Nghe An Provinces, in north-central Vietnam, where mangrove forests play a vital role in reducing the impacts of natural disasters from typhoons and storms. From an understanding of historic change in mangrove cover, the thesis explored mangrove growth, health and survival in field experiments. Chapter 2, through a review of the literature, provides background to the research undertaken in the thesis with particular reference to (i) mangrove health and tree decline, (ii) mangrove restoration projects; and (iii) the sustainable management of mangroves. Chapter 3 used satellite imagery from 1973 to 2020 for two provinces to determine the spatial extent of mangroves, and identified factors responsible for the temporal change in mangrove cover. The drivers of change over this period were in the order of aquaculture > other land use > natural factors > afforestation. By comparing the data in project documents and government audits, it was estimated that the average rate of success of mangrove restoration programs is about 30%. Afforestation trials (Chapter 4) were established in Thanh Hoa and Thai Binh provinces to explore two species (Sonneratia apetala and S. caseolaris), two seedling ages at planting (12-month and 18-month), shore position (upper shore and lower shore), and planting configuration (monoculture, mixed culture) on mangrove survival and growth. After 18 months, the survival rate of both species was >90%. In the upper shore site, 12-month-old seedlings grew faster than 18-month-old seedlings, but the opposite occurred in the lower shore site. For the same age of seedlings, trees in the lower shore site grew faster than in the upper shore site. Also, tree growth was influenced by planting configurations in both the upper and lower shore site. The potential contribution of fungal pathogens to canopy decline in the afforestation trials was assessed in Chapter 5. Fungi were isolated from the main symptoms observed in the field, namely shoot dieback, and pink and black leaf spots. Using morphological and molecular methods, it was found that Neopestalotiopsis sp.1 and Pestalotiopsis sp.3 were associated with shoot dieback; Pestalotiopsis aff. humus, Pestalotiopsis aff. neolitseae and Pestalotiopsis sp.1 were associated with pink leaf spots; whilst Curvularia aff. tsudae, Pestalotiopsis sp.2 and Pestalotiopsis sp.4 were associated with black leaf spots. Pathogenicity of the main isolates was confirmed by inoculating Sonneratia apetala and S. caseolaris seedlings in a nursery. Data on the spatial and temporal extent of mangrove cover in Thanh Hoa and Nghe An provinces should assist local authorities to better manage coastal mangrove forests in the future. The early results from the afforestation trials showed that where care is taken in site selection and planting that it is possible to achieve good outcomes in afforestation projects in Vietnam. The data will increase our knowledge of best practices for design and species selection in mangrove afforestation. Although tree survival was high, the presence of fungal pathogens may be a threat for the long-term health and survival of planted mangroves. The field trials provide a base for future long-term studies on mangrove performance in northern Vietnam.

There are large gaps in the coverage of critical ecological processes related to the movement of individuals or genes (i.e. dispersal), which is critical for determining the spread and persistence of populations across space. In this dissertation we investigate understudied but important aspects of the dispersal process in mangroves, with as the main objective the reduction of parameter and model uncertainty. Models rarely incorporate realism and complexity at the level of emigration, transfer and immigration phases, hampering reliable predictions of dispersal patterns and long-term population dynamics under different climate change scenarios.

Mangrove ecosystems function at the edge of land and sea, often covering large intertidal areas along (sub)tropical coastal regions worldwide. Mangroves can live in these highly dynamic and demanding environmental conditions via a series of remarkable adaptations. They produce buoyant seeds and fruits (propagules) that disperse at the ocean surface (i.e. hydrochory - see cover image).

Despite their ecological and economical value, about 40 % of original mangroves have been lost worldwide during the last 50 years due to excessive exploitation and development. Deforestation, degradation and conversion to other land uses like intensive shrimp farming and agriculture have reduced and fragmented these ecosystems at an alarming rate. Climate change, probably most pronouncedly via changes in sea level, poses another important threat.
Doctorat en Sciences
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I compared wading bird foraging ecology in commercial shrimp ponds and natural mangrove mudflats in the Muisne River Estuary in northwest Ecuador. I estimated foraging habitat suitability by observing the foraging efficiency, diet, and behavior of great (Ardea alba) and snowy (Egretta thula) egrets, censusing birds, and measuring prey availability (i.e. prey density, standing crop, water depth, and diversity). Great egrets had greater foraging efficiency in shrimp ponds, while snowy egret foraging efficiency was greater on mudflats. Over 85% of prey items in snowy egret boluses were from shrimp ponds. Mean density, standing crop, length, and mass of prey items was significantly greater in shrimp ponds, but availability was limited by water depth and diversity. Great and snowy egrets utilized shrimp ponds as their primary foraging grounds, while all other diurnal wading bird species foraged primarily on mudflats, where the diversity of wading birds and their prey was greatest.

Mangrove habitats are considered as the ideal fish nursery as they are known to increase the growth and survival of juvenile fishes by providing enhanced food availability and protection. However, most studies have focused on tropical mangroves with a few recent warm temperate studies finding conflicting results. Furthermore, the nursery value of South African mangroves to fishes remain understudied in subtropical areas, while warm temperate mangroves are yet to be evaluated. This study aimed to assess whether mangrove presence leads to any advantage to the larvae of an important estuarine resident fish species, Gilchristella aestuaria, by comparing the food patch quality of South African warm temperate mangrove and non-mangrove estuaries. Results indicate that larvae fed primarily on the dominant prey species, Pseudodiaptomus hessei, Paracrtia longipatella, and Acartiella natalensis. However, postflexion larvae consumed more of the larger species, P. hessei, within the two mangrove estuaries (16.09 %V in Nahoon and 13.79 %V in Xhora) than the two nonmangrove estuaries (12.20 %V in Gonubie and 7.05 %V in Qora), despite other prey species occurring at similar densities. Results indicate that mangrove habitats acted as sediment sinks, slightly reducing the turbidity of these estuaries which resulted in postflexion larvae actively selecting larger, more nutritious prey, which in turn, significantly increased their individual instantaneous growth rates (0.11 ± 0.21 Gi) when compared to postflexion larvae in non-mangrove estuaries (0.09 ± 0.12 Gi). This study found that mangrove presence was significantly related to postflexion larval densities when coupled with abiotic (such as temperature and turbidity) and biotic factors (such as predator-prey interactions). Understanding the spatial and temporal dynamics, predator-prey interactions as well as the growth and survival of G. aestuaria is particularly important as they are key zooplanktivores that are prey to other species in estuarine food webs.

Mangrove forests provide wide ranges of roles supporting aquatic and terrestrial biota and providing ecosystem services to humans. Over the past century, mangroves have been impacted by human activities, leading to widespread habitat loss and degradation, while efforts to restorator and rehabilitate were not very successful in most projects. However, it will always be more effective to ensure the conservation of existing mangrove habitats. Accordingly, an adequate assessment method with appropriate evaluation components is required to monitor the habitat status. The current assessment approaches more often address the structural conditions of trees and mangrove ‘forests’ which are too insensitive to pressures. There are other indicator variables that describe processes and attributes that underpin mangroves’ structures and functions. These indicator variables are likely to be more sensitive to the impacts resulted by human activities. They can therefore be used to trigger a warning and management process when conditions inside the habitat start to go wrong. The approach in this thesis was to develop an assessment method which used quantitative indicators, as indicator-based approaches and measurable information interest the stakeholders who are managing and reporting habitat status more than purely descriptive assessments. This approach initially started from a literature review and was supplemented with the use of systematic expert judgments. During this process, the competence of a suite of potential indicator variables was obtained. This competency included the variables’ ability to reflect mangrove health and their capacity for delivering ecosystem services. I also considered other criteria such as ecosystem integrity, easy, fast and cost-effectiveness of measurement as well as the time scale for the indicators to respond the pressures impacts (Chapter 2). The indicator variables must be sensitive to anthropogenic pressures, which is another critical criterion to be considered. To address this, there was a need for a scale representing the degree of human pressure / influence. This was dealt by quantifying human activities as proxies of human pressures. They were measured in Pressure Groups (PG) including land-use, hydro-morphology alteration and water quality. This method was implemented in a case study of the Moreton Bay, South East Queensland, Australia. The results reported in Chapter 3 are dimensionless quantified pressure data providing an early detection of vulnerability in mangrove communities. The results can be also used for the calibration of assessment models in future studies concerning how biotic and abiotic indicators are reflecting the pressures. In Chapter 4, the sensitivities of the competent potential indicator variables (using the results of Chapter 2) were tested against the quantified human activities (using the results of Chapter 3). I provided a set of sites ranging from the least to the highest level of anthropogenic pressure in the Moreton Bay study area. The values of the potential variables to different levels of human activities assist the selection of sensitive mangrove health metrics. The habitat characteristics such as vegetation indices in canopy level, and sediment features i.e. metals, nutrients and Chl-a showed a degree of sensitivity to human activities and accordingly were identified as capable variables for an early warning process. Identifying the appropriate health metrics may lead to the development of an Index of Mangrove Ecosystem Integrity (IMEI); this is discussed in Chapter 5. The identified indicator/field variables may be amongst the most sensitive variables to human activities. This is because the field data collection was done in those mangrove forests which are still quite stable (see Chapter 4). A higher number of variables could be included in models by extending this field work to more degraded habitats and in areas with larger magnitude of human activities. Consequently, other field variables may be discovered with lower levels of sensitivity to human activities. These sets of variables can be used as higher levels of warning process. These health metrics will assist in developing IMEI.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

Mangrove forests are a diverse assemblage of trees and shrubs that are adapted to a saline and tidally inundated environment. The global spread of mangrove species is affected by climate, with most confined to areas that are warmer and moist. At a global scale, temperature limits the distribution of mangrove forests but on a regional scale and local scale, rainfall, river flow and tidal exchange have a strong effect on the distribution and biomass of mangrove forests. Other factors that play a role in limiting growth and distribution of mangroves include accessibility of suitable habitats for growth and also conditions that are suitable for propagule dispersal. The objectives of this study were to review the distribution of mangroves in South Africa and determine the number of extreme temperature events that may limit further distribution and secondly to measure growth rates of mangrove species at Mngazana and Nahoon Estuary and the nutrient pools in the sediment of these forests. In 1982, Ward and Steinke published a list of estuaries where mangroves were present. The current study sampled the population structure, microclimate and level of expansion in two estuaries within and outside of the published range. The minimum, maximum temperature and number of extreme temperature events were measured using iButtons. Mangrove expansion will be limited by minimum temperatures (1.1 ° C) and an increase in extreme temperature events (830) (<5 ° C and 5- 10 ° C) at the latitudinal limits. Expansion of A. marina at Kwelera and Tyolomnqa Estuary was evident but sapling survival was low. Mangroves have expanded within and outside the range proposed by Ward and Steinke (1982), but are limited by physical factors, restricted by the presence of saltmarsh and other estuarine macrophytes and natural disturbance regimes. An increase in population growth was recorded over the years at both Mngazana and Nahoon Estuary. Nutrients, (ammonium, nitrates + nitrites and soluble reactive phosphorus) varied amongst sites and were related to seasonality. Nitrogen in both estuaries was available in the form of ammonium (NH4) and its concentration was generally higher (1.3-76.2 pm) than other forms of nitrogen (0.07-6.3 µm). Soluble reactive phosphorus (SRP) was higher during the wet seasons in both estuaries. An increase in porewater salinity since 2007 (41.3 practical salinity unit (PSU)) was measured at Mngazana and this is a result of freshwater abstraction and low rainfall. A. marina saplings and adults grew significantly faster at Nahoon Estuary (the distributional limit) (11.1 ± 1.1 cm year-¹) compared to Mngazana Estuary (5.3 ± 1.8 cm year-¹). Different mangrove species and forests respond differently to environmental factors and changes in mangrove distribution is expected in South Africa but changes are expected to happen slowly and opportunistically.

La mangrove est un écosystème majeur des zones intertidales des côtes tropicales. En Nouvelle-Calédonie, les mangroves sont des zones tampon entre les massifs ultrabasiques, plus ou moins exploités, et un lagon inscrit au patrimoine mondial de l'UNESCO. Le long de l’estran, cet écosystème se décompose principalement en trois zones de végétation qui se développent parallèlement au trait de côte, avec du front de mer vers les terres : Rhizophora spp., Avicennia marina, tannes. Ce gradient botanique dépend de la durée d'immersion des marées, qui impose des gradients de teneur en eau, de salinité de l'eau interstitielle, d’oxygénation, et de teneur en matière organique dans les sédiments. L'objectif de cette thèse était d'améliorer notre compréhension du cycle biogéochimique du fer et du nickel dans les sédiments de mangrove, et de déterminer le rôle des paramètres édaphiques sur la mobilité de ces éléments métalliques. Ce travail est plus spécifiquement focalisé sur l’évolution de la cristallochimie de Fe et Ni en liant avec les (bio)transformations minéralogiques. Les analyses de spectroscopie d’absorption à rayons X montrent clairement que les spéciations de Fe et Ni évoluent en fonction des gradients redox qui marquent d’une part, la zone intertidale, et d’autre part, la profondeur. La goethite et les phyllosilicates, hérités des bassins versants latériques, sont les principaux hôtes du Fe et Ni dans la partie supérieure des sédiments de mangrove. Ces espèces minérales sont intégralement préservés en profondeur des sédiments des tannes, qui sont pauvres en matière organique et bien oxygénés. En revanche, sous les Rhizophora et les Avicennia, la goethite disparait rapidement avec la profondeur . Dans ces horizons inférieures anoxiques riches en matière organique, la sulphato-réduction se développe, et la pyrite et les complexes organiques sont les principales phases porteuses du Ni. A la limite entre les couches oxiques et anoxiques, une intense ré-oxydation du Fe (II) aqueux et des sulfures de fer conduit à la formation de ferrihydrite, lépidocrocite, et probablement de goethite. La proportion relative des oxyhydroxydes de fer néoformés et mal cristallisés, est plus élevé dans la mangrove à Rhizophora. En outre l’incorporation du Ni dans la pyrite est également moins prononcée dans cette zone. Une telle évolution latérale de la spéciation du Ni peut être liée à la réoxydation fréquente des pyrites porteuses de Ni en lien avec le balayage quotidien de cette zone par les marées. Ce cycle tidal, qui touche plus particulièrement la zone à Rhizophora du fait de sa position basse dans l’estran, peut être une cause majeure des cycles de réduction et d’oxydation des phases porteuses de Fe, et pourrait affecter de manière significative les bilans de masse du fer et du nickel dans les mangroves. En effet, le Ni tend à être immobile dans les tannes, à s'accumuler sous Avicennia, et à être partiellement libéré sous Rhizophora. Finalement, le comportement du Fe et du Ni dans des sédiments de mangrove subissant une oxydation intense, en réponse à un isolement de la mer, a été étudié. Au niveau du front d’oxydation, des concentrations très élevées de Ni en solution ont été mesurées , tandis que les concentrations en Ni dans la phase solide étaient quatre fois plus faible que dans l'horizon pyritisé, et 2,5 fois plus faibles que dans le sédiment supérieur. Ces résultats suggèrent que l'oxydation des sédiments de mangrove est une cause de perte en Ni pour l’écosystème. Cette thèse a permis une meilleure connaissance des processus minéralogiques qui conduisent à la fixation ou la libération des élémentes traces métalliques par les mangroves, et est donc utile pour la gestion des mangroves qui sont situés en aval de bassins versants latéritiques
Mangrove forests are the dominant intertidal ecosystem of tropical coastlines. In New Caledonia, mangroves act as a buffer zone between Ni open-cast mines and a lagoon registered as a UNESCO World Heritage site. Across the intertidal zone, mangroves are composed of three main stands; with from the seaward side to the landward side: Rhizophora spp., Avicennia marina, salt-flat. This botanical gradient relies on the duration of tidal immersion, which imposes sedimentary gradients of water content, salinity, oxygenation, and organic content.The objective of this PhD thesis was to improve our understanding of the biogeochemistry of iron and nickel in mangrove sediments and to characterize the role of edaphic parameters on trace metals dynamic. Particular emphasis was focused on the mineralogical (bio)transformation of Fe and Ni bearing phases and on crystal chemistry. Both XANES and EXAFS data showed that Fe and Ni speciation strongly followed the redox boundaries marking the intertidal and depth zonations. Fe(III)-bearing goethite and phyllosilicates, herited from lateritic outcrops, were the major Fe and Ni hosts in the upper mangrove sediments. These mineral species were fully preserved at depth in the dry and oxic salt flat area. By contrast, beneath the vegetated Rhizophora and Avicennia stands, goethite rapidly disappeared with depth. In these anoxic horizons, sulfate reduction occurred, and pyrite and organic complexes became the dominant Ni-species. At the limit between oxic and anoxic layers, intense re-oxidation of aqueous Fe(II) and Fe-sulfides led to the formation of ferrihydrite, lepidocrocite and likely goethite. The relative proportion of the newly formed poorly ordered iron-oxyhydroxides was found to be higher in the Rhizophora mangrove stand. Moreover Ni incorporation in pyrite was less developed beneath Rhizophora stand. Such lateral evolution of Ni speciation may be related to reoxidation of Ni-bearing pyrites in the Rhizophora stand, which is subjected to periodic alternation of reducing and oxidizing events due to daily tidal fluctuations. The latter may be a major cause for continuous Fe reduction-oxidation cycles in the vegetated mangrove stands, and could significantly affect iron and nickel mass balances in mangroves. Indeed, Ni was found to be immobile in the salt flat, to accumulate beneath Avicennia and to be partially leached beneath Rhizophora. Eventually, Fe and Ni behavior in mangrove sediments currently oxidizing in response to isolation from the sea, was studied. In the layer of the oxidation front, really high concentrations of dissolved Ni were measured, while Ni concentrations in the solid phase were 4 times lower than in the pyritized horizon, and 2.5 times lower than in the upper sediment. These results suggest that mangrove sediment oxidation was a cause of Ni loss. This PhD thesis allows a better assesment of the mineralogical processes that lead to the fixation or the release of trace metals by mangroves, and is thus useful for the management of mangroves that are situated downstream lateritic watershed

Coastal environments can be highly susceptible to environmental changes caused by anthropogenic pressures and natural events. Both anthropogenic and natural perturbations may directly affect the amount and the quality of water flowing through the ecosystem, both in the surface and subsurface and can subsequently, alter ecological communities and functions. The Florida Everglades and the Sian Ka’an Biosphere Reserve (Mexico) are two large ecosystems with an extensive coastal mangrove ecotone that represent a historically altered and pristine environment, respectively. Rising sea levels, climate change, increased water demand, and salt water intrusion are growing concerns in these regions and underlies the need for a better understanding of the present conditions. The goal of my research was to better understand various ecohydrological, environmental, and hydrogeochemical interactions and relationships in carbonate mangrove wetlands. A combination of aqueous geochemical analyses and visible and near-infrared reflectance data were employed to explore relationships between surface and subsurface water chemistry and spectral biophysical stress in mangroves. Optical satellite imagery and field collected meteorological data were used to estimate surface energy and evapotranspiration and measure variability associated with hurricanes and restoration efforts. Furthermore, major ionic and nutrient concentrations, and stable isotopes of hydrogen and oxygen were used to distinguish water sources and infer coastal groundwater discharge by applying the data to a combined principal component analysis-end member mixing model. Spectral reflectance measured at the field and satellite scales were successfully used to estimate surface and subsurface water chemistry and model chloride concentrations along the southern Everglades. Satellite imagery indicated that mangrove sites that have less tidal flushing and hydrogeomorphic heterogeneity tend to have more variable evapotranspiration and soil heat flux in response to storms and restoration. Lastly, water chemistry and multivariate analyses indicated two distinct fresh groundwater sources that discharge to the phosphorus-limited estuaries and bays of the Sian Ka’an Biopshere Reserve; and that coastal groundwater discharge was an important source for phosphorus. The results of the study give us a better understanding of the ecohydrological and hydrogeological processes in carbonate mangrove environments that can be then be extrapolated to similar coastal ecosystems in the Caribbean.

Patterns of distribution of crabs inhabiting burrows in three temperate mangrove swamps near Sydney, New South Wales, were described. Heloeoius cordiformis (an ocypodid) did not exhibit a consistent pattern of distribution among three tidal zones, but was always associated with well— drained mounds of substratum within zones. Paragrapsus laevis (a grapsid) tended to be most abundant in the lower two zones on the shore and usually inhabited burrows in the moist or submerged flats between mounds. Sesarma erythrodactyla (a grapsid) was distributed fairly evenly among the three tidal zones and also between mounds and flats. H. cordiformis hibernated in burrows from June through August or September. Overall abundances also varied seasonally, but trends were inconsistent among the three swamps sampled.

The key findings of this thesis link mangrove diversity with the diversity of abiotic environments and, for the first time, provides a consistent method for the development of a comprehensive scientific framework for the management of mangrove ecosystems at national level. In Australia (and globally) a lack of information on the different mangrove habitats that support the diverse range of mangrove ecosystems has inhibited both management and conservation of mangrove ecosystems. Currently there is no agreed and commonly used national classification scheme for mangrove information. This gap weakens future conservation and sustainable development of the Australian coastline. This thesis addresses that gap in knowledge. The physical framework of the Australian coast forms the habitat template in which mangrove diversity develops. The key hypothesis examined is that an understanding of the abiotic setting within which mangroves are found markedly improves an understanding of the distribution and diversity of mangrove ecosystems. To test that hypothesis, detailed examination of the four overarching physical drivers that shape the Australian continental edge: geology, physiography, climate and oceanography, was undertaken to create a comprehensive picture of the abiotic coastal environment. This showed that coastal, fluvial, and hydrological processes that lead to coastal landform development form the basis for understanding the various coastal landforms and mangrove habitats developed along the Australian coast. The results demonstrate that physical drivers and coastal process do shape the distribution of mangrove species, and how mangrove vegetation is developed, maintained, changed or lost. Inter‐relationships between these overarching physical drivers provide a classification of Australia’s coast into major coastal sectors, including those which do not support mangroves. To elaborate the results, a description of each sector containing mangroves and the types of mangrove habitats present is provided. Mangrove diversity is not simply variation in floristics (species richness) but also must include structural and physiognomy diversity. An individual species of mangrove may perform various ecosystem roles in different physical settings. This concept is elaborated through constructing eight hypothetical expressions of mangrove diversity examining the changes in mangrove habitats under increasing complexity of tidal and climate influence, showing how the interaction between tidal, climate and environmental gradients results in different mangrove expression around the Australian coast. Building on the interactions between abiotic and biotic diversity, seeking ways and means to improve mangrove conservation in Australia, lead to ideas for the development of a mangrove focused national conservation strategy. A set of three criteria is put forward to identify the significance of mangrove sites globally, nationally and/or regionally (sub‐nationally). Besides these practical approaches, an assessment of each State and Territory’s mangrove management and conservation mechanisms shows the scope of existing policies and conservation strategies, and reveals little coordination of mangrove conservation or management issues at the national level, or between the jurisdictions. Building on these findings, and in the framework of the proposed national conservation strategy, a series of recommendations are made to improve the collection and access to mangrove information, including through the implementation of a national level data and information plan providing standards for biophysical and management information on mangroves.

Sesarmid crabs have important ecological roles in the mangroves of the Indo-west-Pacific biogeographic region. By processing large amount of leaf litter, these crabs retain organic matter in the mangroves, increase the rate of mineralisation, and fuel additional trophic links with the processed material. However, the dietary composition of the mangrove sesarmids is still a controversy. Gut content analyses suggest a dominant contribution of mangrove leaf litter in the crab diet but stable isotope data seem to support the assimilation of organic matter derived from the microphytobenthos but not mangrove litter. In addition, ability of the sesarmid crabs to obtain sufficient nutrition from mangrove litter, which is rich in structural carbon (cellulose) but poor in other nutrients such as nitrogen, has been questioned. The over-arching goal of this thesis was to fill these gaps of knowledge in detritivory of the mangrove sesarmids, using Parasesarma erythodactyla (Hess, 1865), the dominant shredder in sub-tropical Australian mangroves, as an animal model.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

With only around 80 species globally, mangroves generally support low local tree species diversity while sustaining significant ecosystem services. The relationship between biodiversity and ecosystem function in mangroves has rarely been studied, especially when related to local forest diversity. The interdependence between biodiversity and function is important to healthy and functional mangrove ecosystems. Among the many functions of mangroves, juvenile nekton are known to utilize mangroves as their nursery habitat but the justification for this association is not clearly understood. Different plant species are expected to produce different quality of organic content derived from their respective decomposed leaf litter. These leaf litters, which mix into surrounding sediments, are expected to contain different carbon to nitrogen ratios and specific secondary metabolites, influencing its quality. Information on how organic content quality affects the juvenile nekton assemblages in mangroves is limited. This thesis assessed these questions quantitatively, using both field experiments and a laboratory experiment. The aim of this study was to enhance our understanding on how mangrove forests of different local species diversity (mixed forest of Avicennia marina and Rhizophora stylosa and monospecific forests of the two species) can affect the leaf litter decomposition rate, organic content and organic content quality in the sediment, including their adjacent mudflats. This information was then used to assess if the organic content quality from decomposing leaves and sediment influenced the juvenile nekton and invertebrate assemblages associated with the forest stands. The influence of the structural complexity in terms of root density of different forests on the juvenile nekton and invertebrate assemblages was also assessed. Finally, the influence of local tree diversity on the micro-habitat selection by the juvenile caridean shrimp (Palaemon debilis) was investigated in a laboratory choice experiment. Sampling of juvenile nekton and invertebrates was done in three mangrove forests at three locations in the wet and dry seasons in eastern Australia using the Simulated Aquatic Microhabitat (SAMs) method (Kneib, 1997). The animals’ assemblages in the three mangrove forests were characterized and quantified. For each forest, (i) the decomposition rate of A. marina and R. stylosa leaves; (ii) the relationship between animal assemblages and the organic content quality in the sediment; (iii) the relationship between animal assemblages and the structural complexity (root density); and (iv) the relationship between the isotopic signatures of the animal assemblages and the isotopic signatures of the forests were investigated. Juvenile nekton and invertebrate assemblages were highly variable among replicate samples, among different locations, forest types and seasons. The organic content of decomposing leaves and sediment, as well as the structural complexity in terms of root density, also varied spatially. While the factor location was a significant driver of the differences, the effect of forest types (local forest diversity) did not show a similar pattern across locations. The pattern also varied between the mangrove and mudflat habitats. For the decomposition study, it was anticipated that both A. marina and R. stylosa leaves would decompose faster in the mixed forest, however, there was no significant difference in the decomposition rates of the two species in different habitats. In mangrove habitats, sediment organic content was higher in the monospecific R. stylosa forest compared to the mixed forest and monospecific A. marina forest. However, in mudflat habitats significant difference in sediment organic content was only found when adjacent to monospecific R. stylosa forests or mixed forests. In mangrove habitats, the sediment was of lower quality in the mixed forest than in the R. stylosa forest. However, in mudflat habitats, there was no difference between the mixed and monospecific forests. The abundance of juvenile fish and invertebrates was higher only in the mangrove habitat of the mixed forest but not on the adjacent mudflat. In mangrove habitats, the number of species per family and species richness showed significant differences between the mixed forest and both monospecific R. stylosa and A. marina forests. In mangrove and mudflat habitats, Spearman’s rho showed that sediment quality was not related to animal abundance. The quality of decomposed leaf litter was higher in the mixed forest except for A. marina leaves in the adjacent mudflat. As expected, the quality of decomposed leaf litter improved over the decomposition time for all forest types in both habitats. Irrespective of forest type, animal abundance was not related to root density. However, based on location, Spearman rho showed that root density in Tallebudgera Creek was related to animal abundance in all forest types. Animal abundance related to root density during the wet season was related to mixed forest and monospecific Avicennia marina forest. Therefore, local forest diversity showed no clear influence in the relationship between animal abundance to both sediment quality in both habitats and root density in mangrove habitats. The δ13C of the leaves of A. marina and R. stylosa was generally different from those of the animals. Even though the mudflat adjacent to the mixed forest had higher quality of decomposed leaf litter (R. stylosa only), it was unrelated to the animal abundance and assemblages. The δ13C of R. stylosa leaves was close to the δ13C of one of the fish species from the same forest and this indicates R. stylosa potentially contributed to the diet of this fish. Results of the laboratory choice experiment indicated that the identity of mangrove forests influenced micro-habitat selection in the juveniles of Palaemon debilis. This response is probably mediated by olfactory cues from the mangrove leaf litter. This study is the first to show that P. debilis is able to detect and distinguish water-borne chemical cues from three different types of mangrove leaves. The chemicals released by the leaf litter of R. stylosa apparently acted as a positive cue that attracted the juvenile shrimps.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

South Australian mangroves consist of only one single species Avicennia marina (Fosk.) Vierh.var australasica (Walp) Moldenke, 1960. They are distributed discontinuously within St. Vincent Gulf and Spencer Gulf and provide significant valuable habitat both in economical and ecological terms. The Fisheries Act 1971-1982 protected the existence of mangroves and the Harbour Act 1936-1981 controlled removal of mangrove areas in coastal development. To date very few ecological studies have been conducted in the South Australian mangroves, particularly on the infaunal organisms that have an important role in maintaining the ecological dynamic within the estuaries systems. As this is the first study on infaunal mangrove communities in the inverse estuaries of South Australia, there was no prior data for ecological comparison. The study was conducted at three mangroves location (Garden Island, Middle Beach and Saint Kilda) close to Adelaide in May 2000 and 2001. Overall the study has reported that the infaunal mangrove assemblages of South Australian mangroves were comparable to other temperate mangroves. The infaunal communities were characterised by lower diversity and abundance compared to the tropical or subtropical mangroves. The infaunal zonation related to the tidal gradient and habitat variation was detected. Most infauna organisms occupied the surface layers and substantially decreased towards the deeper layers. The study also suggested that sediment structure of mangrove systems were complex and infaunal communities responded differently to the change of environmental conditions both in small scale and larger scale. Thus, assessing the infaunal communities structure in mangrove systems should be based on ecological characteristics rather than geographical positions. The examination of dominant polychaetes families showed that different species have different responses to the environmental cues within mangrove systems. The study did not find that any polychaete species was restricted to mangroves only as they all were also found in the habitat adjacent mangrove forest.
Thesis (Ph.D.)--School of Earth and Environmental Sciences, 2003.

Despite their important ecosystem benefits for terrestrial and marine flora and fauna and the human livelihood mangrove forests suffer a high loss rate mainly due to human activity. Aside from these impacts, natural forest disturbances exist more commonly in mangroves compared to other forests as a direct consequence of their exposed coastal location. Within this thesis I investigate the influence of natural disturbance regimes on the mangrove forest dynamics focusing in particular on the ecological role of disturbances, disturbance patterns, forest structure, succession behavior and long-term vulnerability evaluation. The study areas were set in the Indian River Lagoon in Florida (USA) and in Can Gio an UNESCO Biosphere Reserve (Vietnam). In addition, theoretical simulation studies were carried out to complement the field studies. Thereby, in our study at the Indian River Lagoon site I investigated the ecosystem response to hurricane events of an artificially impounded mangrove forest. In Can Gio, the suitability of lightning strike – caused gaps for setting a homogenous plantation into more natural-like state according to species composition and forest structure was analyzed. Finally, a theoretical simulation study was carried out to compare lightning strike and hurricane events regarding their homogenization and heterogenization effects on the spatio-temporal forest structure. The findings of the field study in the Indian River Lagoon indicate that hurricane events had a severe impact on forest areas in higher successional stages by creating open patches, whereas areas in lower successional stages remained largely undisturbed. Furthermore, the impoundment determines the species selection of the post-hurricane succession by favoring flooding-tolerant species. However, regeneration was found to be impaired by the artificially high inundation regime at some disturbed patches. The lightning-strike disturbances enhance the species composition in the monospecific plantation in Can Gio by providing a sufficient light regime for entering seeds to establish. In addition, lightning-strike gaps increased the plantation structure complexity. Regenerating lightning-strike gaps remained as “green islands” within windthrow sites in the plantation due to their low stature and provided seeds for surrounding disturbed areas thereby accelerating their recolonization. The results of the simulation analysis of a theoretical landscape showed that in the simulated highly complex natural mature forests all disturbance regimes entail homogenization on the spatial structure compared to an undisturbed scenario. The hurricane scenario showed an increased temporal variation of the forest dynamics whereas lightning-strike gaps were not able to contribute to additional heterogeneity in the simulated area, despite of having the same tree mortality probability during disturbances. The interaction of the large-scale impoundment in the Indian River Lagoon and medium-sized hurricane events is characterized by partially impeded post-hurricane regeneration. In contrast, small-scaled lightning strikes influenced the regeneration of medium-sized windthrow sites positively within the homogenous plantation. We therefore suggest management activities aimed at creating small clearances within the plantation in Can Gio to simulate additional small-scale disturbances in order to facilitate heterogenization of the plantation structure. Natural disturbances are found to be able to enhance the species diversity and the interactions of ecological processes. In particular, where sustainable management strategies focused on maintaining ecosystem services especially in restored sites or plantations act as a supportive part. Natural disturbances are an integral component of mangrove forests and fulfill specific ecological functions. However, our findings indicate that these disturbances, on top of altered environmental conditions associated with climate change and direct human impacts, might jeopardize the natural development in unnatural forest structures as on plantations or restored sites. This thesis gives an extensive overview about the effect of various disturbances in different mangrove forest systems, including semi-natural forests and strongly modified plantations, on species composition and forest structure. Field studies and simulation analyses contribute in equal parts to the results of the thesis.

Mangroves in South Africa are prone to anthropogenic pressures such as browsing and trampling by livestock and by natural disturbances such as drought and floods. These mangroves exist at one of the most southerly limits in the world providing a unique opportunity to study recovery and resilience to change. This study used long term data at three South African study sites located in the subtropical (St Lucia Estuary) and warm temperate regions (Nxaxo and Kobonqaba estuaries) to compare mangrove growth rate, population structure and responses to disturbance. The first objective was investigated at St Lucia Estuary where site specific or microhabitat differences were measured to assess the influence of these on mangrove growth and population structure. It was suggested that site - specific variability would ensure mangrove survival and analysis showed that seedlings were present at different sites in different years. There was always some recruitment but often little survival to the next size class. Over time seedling numbers were quite variable and self – thinning of adults was evident. The second objective was to identify the environmental factors influencing population density and growth. Results showed that sediment moisture and salinity influenced seedling and adult density due to fluctuations in estuary water level. Mangrove growth rates for Avicennia marina in terms of height was faster (40 – 75 cm yr-1 ) at Site 1 where conditions were waterlogged and moderately saline and slower (5 – 25 cm yr1 ) in dry and hypersaline conditions at Site 4. Overall mangroves at St Lucia have shown persistence through drought and fluctuations in environmental conditions however resilience may be hindered at sites that are subjected to partially flooding and prolonged waterlogging. The second site was Nxaxo Estuary in the Eastern Cape where cattle exclusion plots (nonbrowsed) and control plots (browsed) were used to assess the recovery of A. marina trees from cattle browsing. Trees in the browsed plots showed no vertical growth while trees in the non-browsed plots grew significantly faster (5.4 ± 0.5 cm yr-1). When cattle were prevented from entering the mangrove area, the trees recovered fairly quickly and within 3 years growth was similar in both the non – browsed (17.9 ± 3.2 cm yr -1) and browsed plots (18 ± 1.6 cm yr -1). Lastly, mangrove recovery was investigated at Kobonqaba Estuary where long-term closure of the mouth to the sea resulted in high water levels, inundation of pneumatophores and die-back of the majority of the mangroves. This study measured the recovery of the mangroves by assessing changes in vegetation cover and sediment characteristics along transects. High initial porewater nutrient concentrations promoted salt marsh growth and plants increased average cover from 0% in 2011 to 18.9% (2013) and 50% in 2015. The total number of A. marina individuals increased from only seven to 27 individuals over four years. Salt marsh competition and facilitation will likely influence mangrove recovery in the future. Overall it was concluded that when a short term pressure (cattle browsing) is alleviated, mangrove forests even at warm temperate sites are able to recovery rapidly by increasing growth and seedling establishment. However, mangroves in the subtropical sites show more resilience and recovery potential to long term pressures such as fluctuating environmental conditions because of faster growth rates.

Mangroves are intertidal ecosystems found along the tropical and subtropical coastlines.

Though globally recognised as ecosystems of ecological, biological and economical

remarkable importance, these ecotone formations are characterised by a continuously

increasing anthropization. However, very little studies have been focused on the impact of

various anthropogenic activities on the mangrove vegetation structure.

We have firstly (a) assessed the commercial and subsistence utilization of mangrove

wood products in the Littoral region (Cameroon estuary). Then, we have confronted the

subsistence usages of mangrove wood products in the Southern region (close to the mouth of

the Nyong River and Mpalla village (Kribi)) in comparison with the Littoral region. By doing,

we have compared the local residents‟ perceptions on environmental changes that occurred

within the two regional mangrove forests. Also, we have (c) studied the structural dynamic of

mangrove vegetation neighbouring the Douala city (Cameroon). Always in the vicinity of this

town, we have (d) reconstructed the original structure of largely disturbed mangrove forests.

Moreover, we have (e) map the mangrove structure in a non peri-urban setting located within

the Cameroon estuary. Here, we have finally (f) analysed the spatial distribution of a black

mangrove namely Avicennia germinans (L.) Stearn.

Our results underlined an excessive utilization of mangrove wood products in the

Cameroon estuary. We have showed that the frequency of mangrove harvesting was relatively

fewer in Kribi (Mpalla) and the mouth of the Nyong River. The local people inhabiting these

two localities perceived mangroves as less degraded areas. In contrast, those established

within the Cameroon estuary stated that mangroves were largely disturbed. When combining

the local people statements with our field observations, we recorded that it a complex mix of

causes (e.i. clear-felled corridors, agriculture, sand and gravel extraction, over-harvesting and

anarchic urbanization) that have led to the largely degradation (vegetation and sediment) of

the peri-urban mangroves in Cameroon. A diachronic analysis (1974, 2003, 2009) of their

coverage revealed that over the 35-year period, mangrove had decreases in cover of 53.16%

around Douala. We have also showed that in the peri-urban settings, wood harvesting was

commonly applied on the structurally more complex (highly dense stands neighbouring the

habitations) mangrove forests (Mboussa Essengue) and, in a lesser extent, on the structurally

more developed mangrove stands (fewer dense stands faraway from Douala). On the other

hand, the mapping analysis of the non peri-urban mangroves (distant from Douala) has

revealed that the structure of these intertidal forests was relatively less impacted. In the

Cameroon estuary, we also showed that A. germinans trees were randomly distributed on

almost one-half of the sampling plots and clumped at some scales on the remaining plots.

Accordingly, this species might play a significant role in the recovery process of artificial

gaps found in the non peri-urban areas.

The multi-disciplinary approach employed in this study has allowed a better

understanding of the direct and indirect impacts of anthropogenic activities on the mangrove

vegetation structure in Cameroon. These results constitute a fundamental data base quite

useful for the multi-temporal monitoring of these littoral ecosystems perpetually disturbed.

The application of similar approach in other mangroves facing high anthropogenic pressures

appears important.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished