Biodiversity is the variety in living organism that can be found at a given scale. Magurran (2004) stated that biodiversity comprised of two fundamental levels, species diversity and genetic diversity. Field et al., 1998 defined biodiversity as encompassing a wide ranging scale, which may cover from genotypic diversity within a population, up to the biome diversity within continents. Biodiversity is often measured as the number of species or genotypes, although a few studies have manipulated evenness (Wilsey and Polley, 2002).
Ecosystem function comprise of three major functions, biogeochemical, ecological and anthropocentric (Daily, 1977; Field at al., 1998). Biogeochemical functions includes primary and secondary production, decomposition, nutrient cycling, hydrology, soil development and soil fertility, regulation of climate cycles, stabilization of substrates, and purification of the air and water; ecological functions includes resilience and resistance to biotic and abiotic perturbations, maintenance of food-web integrity, and provision of habitat for a range of trophic levels; and anthropogenic function includes medicinal compounds, aesthetic and recreational values, maintenance of fisheries, and management of sediments on a range of time scales (Field et al., 1998).
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A more focused concept of ecosystem function is provided by Pacala and Kinzig (2002), who suggested that ecosystem function comprise of stocks of energy and materials (biomass), fluxes of energy or material processing (productivity, decomposition), and the stability of rates or stocks over time.
2.1.2 The Relationship between Biodiversity and Ecosystem Function
Both biodiversity and ecosystem function are broad concepts and may link to each other at several scales. The relationship between biodiversity and ecosystem function is complicated yet the interdependence between these two are important in order to maintain a healthy ecosystem. Conserving biodiversity is necessary to maintain the ecosystem function. There is an uncertainty about the way in which the diversity of population and species in an ecosystem is related to the functional properties of the ecosystem. It is important that biodiversity is preserved wherever possible to maintain the regulation of ecosystem function.
According to Schulze and Mooney (1983); it is unclear how and why a change in biodiversity might change the ecosystem function. It might be due to limited knowledge about the population biology, the functional properties of most species, the mechanisms which underlie the self-assembly and organization of species in communities and the effects of variations in the arrangement of components in a complex systems.
Direct and manipulative experiments can produce important findings into the role of biodiversity in ecosystem function, but they are limited when it comes to aspects of this relationship that occur over long temporal and large spatial scales (Field et al., 1998). Research on the relationship between biodiversity and ecosystem function is now focusing on the biodiversity loss at large spatial scales, which involves reduction and change in species at different trophic levels (Raffaelli, 2006). The majority of manipulative experiments designed to address the relation between biodiversity and ecosystem function have focused largely on a single trophic level within a large food web (Raffaelli et al., 2002, Bulling et al., 2006).
The challenges in assessing the ecosystem function implications in biodiversity are related to scale, both temporal and spatial. Quantifying the consequences of biodiversity for ecosystem function is a challenge under any circumstances (Huston, 1997). Direct experimental studies of biodiversity effects on ecosystem function (Naeem et al. 1995; Hooper and Vitousek, 1997) use model systems that provide reasonable access to some, but not all, of the scales over which biodiversity effects could be important. Therefore, more research is needed in order to improve the understanding on this complex relationship.
2.1.3 Biodiversity and Ecosystem Function in Mangroves
The main challenge faced by biodiversity and ecosystem function research in marine ecology is dealing with the large scales of marine systems and the difficulties of trying to conduct a complex experiment like what have done in terrestrial ecology (Naeem, 2006). Mangrove ecosystems are interesting and important for studies on the role of biological diversity for ecosystem function.
Mangroves support a low diversity of the dominant higher plants (Duke, Ball and Ellison, 1998) (Duke et al., 1998). The diversity of other life form (arthropods, molluscs, fish and birds) are much greater. The diversity of mangrove plant species changes based on distance from centres of diversification, dispersal ability, the viability of propagules prior to rooting, and the directions of the ocean currents. On a large area, mangrove plant diversity increase with precipitation and the area of watershed (Duke, 1992) and decrease with increasing latitude (Smith and Duke, 1987; Duke et al., 1998).
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Mangroves ecosystems compress a broad range of habitat variation into a compact spatial scale. The gradient from fully submerged to fully exposed sites expands the diversity of mangrove habitats. Therefore, mangrove ecosystems provide an efficient model to evaluate interactions between environmental variation and functional consequences of biodiversity (Field et al., 1998).
One of the potentially important limitations of direct experiments on the biodiversity or function relationships is the limited nature of biotic interactions in small plots operated over short periods. Biotic interactions in mangrove ecosystems are limited by other factors, including direct human impacts (Sasekumar and Chong, 1998), alterations of surrounding terrestrial habitat, and biogeographical constraints.
There are a number of studies done on quantifying ecosystem function in mangrove ecosystems. Observations on plant physiology (Ball, 1996), primary production (Ong et. al., 1982), nutrient cycling (Gong and Ong, 1990), and biotic interactions (Farnsworth and Ellison, 1993) provide complementary studies in a number of mangrove regions, yet far from being comprehensive. Currently, remote sensing tools become more appropriate on spatially extensive analysis in large and separated sites.
Other than mangrove plants, biodiversity and ecosystem function studies on other taxa has also been conducted. For example, Sasekumar and Chong (1998) focus on the major impacts of human activities on invertebrate diversity and Farnsworth (1998) reviewing the evidence for the potential role of tree species richness and net primary production in regulating the diversity of consumers. Allen (1998) stated that little is known about the role of biodiversity in maintaining services like flood protection, nutrient and organic matter processing, sediment control, and fisheries support underlie the economic foundations of many tropical regions.
Mangrove ecosystems is ideal for comparative biodiversity or function studies that take a broad view of ecosystem function, integrating biogeochemical, ecological, and anthropogenic functions across a range of time scale. Comparative studies should be in sites that are similar except for biogeographic effects on biodiversity. It is also important to think of biodiversity gradients in climate zones as replicates on a core study (Field et al., 1998).
2.2 Mangrove Plant Diversity Distribution
2.2.1 Factors that Influence the Mangrove Plant Diversity Distribution
The distribution of mangroves comprises the separate ranges of many individual species, which cannot be explained by physical or climatic limitations. The actual number of species differs, not only geographically, but between sites within a geographical region.
The geographical distribution of mangrove plant species and mangrove habitats are limited by temperature. Mangroves normally occur outside the range, delimited by the winter position of 200C isotherm and the number of species tends to decrease as it approaches the limit. In the southern hemisphere, ranges extend further south on the eastern margins of land masses than on the western, reflecting the pattern of warm and cold ocean currents (Hogarth, 2007).
The ecological role of precipitation is different in mangrove than in most terrestrial ecosystems, because mangroves are located in standing water or saturated soil. Therefore, precipitation influences salinity more than water availability. Strong influences of salinity, waterlogging and anoxia lead to physical and biological similarities in substrate conditions (Boto and Wellington, 1984). Direct human influences play a major role in the biodiversity of all taxa in mangrove ecosystems (Ewel, Ong and Twilley, 1998).
22.214.171.124 Regional Diversity Distribution
The latitudinal range of mangroves is affected by temperature and the longitudinal distribution of species is affected by physical barriers. Mangroves are separated between the Indo-West Pacific (IWP) and the Atlantic-Caribbean-East Pacific (ACEP) regions. The number of species of mangrove declines eastwards across the Pacific, where the easternmost limit being Samoa (1700W). Beyond this, no mangroves occur naturally until the west coast of the Americas. The ACEP region falls into three subregions: the eastern Pacific, the Carribean and western Atlantic and the Atlantic coast of West Africa. The IWP region is biogeographically separated into East Africa, Indo-Malesian subregion and Australasia (Hogarth, 2007).
The IWP and ACEP regions have comparable total areas of mangrove habitat, but widely different numbers of genera and species (Table 1). It is obvious that mangrove species are not evenly distributed around the tropics. More species and genera are present in the IWP (Australasian and Indo-Malesian subregions), than in the ACEP region (Ricklefs and Latham, 1993). The IWP contains more than three times as many genera (23 compared with seven), and nearly five times as many species (58 compared with 12) (Hogarth, 2007).
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Table 1: Area of mangrove habitat, and genera and species of mangrove, found in the different zones of the Indo-West Pacific (IWP) and Atlantic-Carribean-East Pacific (ACEP) regions (Ricklefs and Lathan, 1993).
Region and subregion
Mangrove area (km2)
Among the most important groups of mangrove animal, there are more species of sesarmid crab in the IWP; 37 species have been recorded in Australia and 44 species from Malaysia, compared with eight or so from West Africa. Approximately 211 species of molluscs have been recorded from the Indo-Malesian and 145 from Australasia, compared with 124 from the West Atlantic or Carribbean (Hogarth, 2007).
2.2.2 The Importance of Mangrove Root to Juvenile Nekton
Juveniles of many species are found more in mangrove habitats compared with adjacent habitats such as mudflats and seagrass beds (Robertson and Duke, 1990; Laegdsgaard and Johnson, 1995). The nursery role in mangroves is well established, but it is still uncertain why mangroves attract juvenile fish to its habitat. There are three main hypotheses, which leads to this phenomenon; the 'structural heterogeneity' where juvenile fish are attracted to the structural heterogeneity of mangroves, the 'predation risk' where risk or predation is lower in mangroves due to increased structural complexity, and the 'food availability' where the availability of food is greater in mangroves (Laegdsgaard and Johnson, 2001).
Mangrove plant species vary in terms of their root structural heterogeneity (type, height density, and structure of their aerial roots) (Tomlinson 1986). Therefore, the quality of shelter provided to juvenile nekton may vary with mangrove forest type. Juvenile nekton normally utilizes the root area as shelter, protection from the predators and food (detritus). Therefore, mangrove root serves important role to accommodate the needs of juvenile nekton.
2.3 Mangrove as Nurseries
2.3.1 The Importance of Mangrove as Nurseries
Mangrove forest has been recognized to be of high ecological importance as feeding and nursery area for commercially important fish and invertebrate species (Chong et al., 1990; Chong & Sasekumar, 2002). Mangrove forest can be found in estuary areas. Mangrove forest has been proven to be highly productive in terms of providing good quality organic matter, where in turn provides food source for juvenile nekton. Therefore, one of the important roles of mangrove is providing feeding and nursery area to juvenile nekton.
Estuaries provide migration route, where anadromous and catadromous species migrate between rivers and sea (McDowall, 1988). In general, few species complete their life cycles within estuaries and the distribution of freshwater teleosts occasionally extends into estuarine waters (Haedrich, 1983; Dando, 1984). The fish found in estuaries are mainly marine species which enter these systems either infrequently and in small numbers, or regularly and in large numbers (Haedrich, 1983; Dando, 1984; Claridge et al. 1986; Potter et al. 1990). Flooded mangrove forests and tidal streams often contain high abundance of resident and transient nekton (fish and decapod crustaceans), some are of recreational or commercial importance (Odum et al. 1982; Robertson and Duke, 1990).
Although most species of nekton found in estuaries are marine visitors, the few that complete their life cycle entirely within the estuary tend to be very abundant (Haedrich, 1983). These permanent residents, which have adapted to the fluctuating conditions of this environment, are important but often ignored components of the estuarine nekton assemblage. Permanent resident species often reside in shallow vegetated habitats, thus they and their young may not be well represented in trawl or plankton samples taken in deeper estuarine waters. These species are more abundant than marine transients in samples from mangrove, salt marsh, and seagrass habitats (Robertson and Duke, 1987; Hettler, 1989; Peterson and Turner, 1994).
2.3.2 The Movement of Juvenile Nekton in Estuaries
Nursery is a special place for juvenile nekton (fishes and decapods crustaceans) where density, survival and growth of juveniles and movement to adult habitat are improved over those in adjoining juvenile habitat types (Sheridan and Hays, 2003). Early life stages of estuarine resident species, particularly those with demersal young, are not affected by the same physical processes influencing larval supply and recruitment variability in marine-spawned species
Variability in early life stages of species that are permanent residents of the estuarine nekton is poorly understood, especially in systems with extensive areas of emergent vegetation. Sampling small mobile nekton in these shallow intertidal habitats presents a difficult methodological challenge. Kneib, 1997 used a Simulated Aquatic Microhabitats (SAMs) method to collect the early life stages of resident nekton that remained on the emergent marsh surface after it was exposed by the tide and could not be sampled by traditional methods.
The estuarine-dependent species typically enter estuaries or shallow inshore waters from the sea for a period each year, but do not stay there permanently. They are usually the most abundant fish. In addition there are stray marine organisms which enter estuaries only irregular, and are often restricted to the seaward end. These increase species diversity and are usually low in numbers of individuals. Many residents leave the estuary for deeper water during winter, and some may breed outside the estuary. There is migrant species which use the estuary as a route from rivers to the sea or vice versa (Little, 2000).
The majority of estuarine opportunists drift into estuaries as larvae from eggs spawned in coastal waters and when as young fish they become demersal, they take advantage of the rich benthic food sources in the sublittoral sediments, on intertidal mudflats and in salt marshes and mangroves. Estuaries and coastal waters worldwide thus contain fish less than one year old that use them as nursery grounds before emigrating to the open ocean as recruits to their adult populations (Little, 2000).
Robertson and Duke, 1987 indicated that the nursery ground value of mangroves remain unclear, due to few studies used balanced sampling strategies, which provide concurrent data on the densities of fish and crustaceans in mangroves and control (non- mangrove) site.
2.3.3 The Distribution of Juvenile Nekton in Estuaries
There are three factors that determine the distribution of fish in estuaries; fish respond to the physical and chemical characteristics of their habitat, food source and to find refuges from predators.
A study using multivariate analyses in the Humber estuary, United Kingdom, concluded that salinity is the most important variable, closely followed by temperature. Oxygen levels are also relevant, but perhaps turbidity has little effect on bottom populations (Marshall and Elliot, 1998). Food supply is perhaps the most obvious of the biological factors. Among the benthic feeders are the mullet, which graze algal films from soft substrata. The species avoid competition by selecting these different sand types (Blaber, 1997). Some of the estuarine opportunists feed exclusively on benthic infauna and so their distributions depend upon the distribution of the benthos.
The migrations of some crustaceans such as shrimps and mysids mimic those of estuarine-opportunist fish. In general terms, numbers are high in estuaries during summer. This may be either because the larvae drift in from spawning grounds outside, or because the species breed within the estuary. For example, the shrimp Penaeus merguiensis spawns at sea off the coast of northern Australia and its postlarvae settle inshore and in estuaries with dense fringes of mangrove. Here, their distribution is related to that of mangroves rather than to characteristics of the water column such as salinity (Vance et al., 1990).
High turbidity may reduce the effectiveness of predators, so the crustaceans may use turbid zones to escape from predator. Where turbidity is low, crustaceans seek refuge from their fish predators in very shallow water, as large predators tend to remain in deep water. In the tropics, it is probable that shrimps and fish as well use the structural complexity of mangrove roots and pneumatophores as protection from predators, although experimental proof of this is still lacking (Vance et al., 1990).
Many estuaries experience seasonal migrations of fish that do not stay for long periods like the estuarine opportunists, but use the estuary as a route between rivers and the sea. Most of these species are 'anadromous', meaning that they spawn in fresh water and the young hatch and spend their juvenile life there before migrating back to the sea. Fish that perform the reverse migration, and breed in the sea, are called catadromous.
2.4 Organic Matter in Mangroves
2.4.1 The role of Organic Matter in Mangroves
Organic matter is composed of organic compounds that are derived from decaying organism. The process of decaying is due to the activity which is caused by decomposers such as bacteria, fungi and other detritus feeding organisms. In mangrove habitats, the organic matter is abundant due to high productivity of mangroves. Odum and Heald (1975) stated in their 'outwelling hypotheses' where mangrove litter becomes the dominant food source for higher trophic levels. However, Robertson et al., (1992) and Lee (1995) argued that the fate of the mangrove litter is uncertain in the 'outwelling' of carbon and nutrients, whether it forms the basis of coastal food webs or accumulate in the sediments. Lee (1995) stated that mangrove organic matter is incorporated into coastal foodwebs in a limited quantity. Therefore, organic matter plays an important role in mangrove habitats as it provides food source to the mangrove's organism.
2.4.2 The Importance of Organic Matter to Juvenile Nekton
Mangrove-derived detritus is an important food source for decomposer food web, such as sesarmid crabs, fiddler crabs and gastropods. Organic matter that is not exported by tidal actions will enter the sediment. The decomposer such as sesarmid and fiddler crabs will consume soil rich in organic matter. Bacteria and fungi accumulate on the organic matter providing additional food items for juvenile nekton.
2.5 Decomposition of Mangrove Leaf
2.5.1 Decomposition Rate of Mangrove Leaf
Mangrove leaf litter may be rapidly broken down by herbivours (Odum and Heald, 1975; Leh and Sasekumar, 1985; Roberstson, 1986); decomposed by microbial action; flushed out of the mangrove forest by tides, depending on the degree and frequency of tidal inundation (Twilley, 1985; Twilley et al., 1986). It may also accumulate within the mangrove mud. The decomposition rate of mangrove leaf litter exhibit wide variation among sites, habitats and forest types due to the interaction of these multiple factors.
Mangrove leaf litter will be exported to adjacent waterways (Boto and Bunt, 1982; Robertson, 1986), some may accumulate in the forest where it will be consumed by macroinvertebrates (e.g., Poovachiranon, et al., 1986) after the decomposition process. In high-intertidal forests, flushed only by spring tides (e.g., dominated by Ceriops spp., Bruguiera exarista and Avicennia marina), there is some variation in the quantities of leaf litter removed or consumed by leaf eating crabs and/or decomposed by microbial action (Robertson, 1988).
Lee (1998) stated that grapsid crab that feeds on fallen mangrove leaves and may play significant parts in recycling organic matter. Crabs are important in determining the structure and function of mangrove ecosystems. Their function is they feed on fallen mangrove leaves, which helps in recycling organic matter and pull the fallen mangrove leaves into their burrows, thus retaining organic matter.
2.5.2 The Nutritional Value of Mangrove Leaf
Mangrove leaf litter is an important source of carbon, nitrogen, and other nutrients for estuarine food webs (Odum and Heald, 1975; Thongtham and Kristensen, 2005). The mangrove crab consumption preferences are likely due to differences in leaf structure and chemical components (Ashton, 2002; Skov and Hartnoll, 2002), both of which affect palatability and digestibility. Mangrove leaves differ in structure and chemical components (Tomlinson, 1986; Erickson et al. 2004; Li and Lin, 2006).
Leaves of different mangrove species vary significantly in their nitrogen and tannin contents (Robertson, 1988; Erickson et al. 2004). Therefore, the nutritional quality that may determine consumption preferences also varies (Ashton, 2002; Skov and Hartnoll, 2002; Alongi, 2009). Avicennia marina has been found to be the preferred species in laboratory experiments (Kwok and Lee, 1995; Ravichandran et al. 2007) because of its low tannin and low C/N ratio. Avicennia marina leaves have lower tannin and higher initial nitrogen content that Rhizophora stylosa and Bruguiera sexangula leaves (Alongi, 2009).
126.96.36.199 Carbon and Nitrogen Content of Mangrove Leaf
Change and decay will deter the nutritional value of mangrove leaves. A useful measure is the ratio of carbon to nitrogen (C/N ratio). The higher this ratio, the less valuable the food. A C/N ratio below 17:1 is regarded as being necessary for a nutritious food. The C/N ratio tends to decrease with time, therefore the older the leaf, the better (Giddins et al., 1986). Sediment detritus appear to be better value than leaves (Skov and Hartnoll, 2002).
Mangrove leaf litter in estuaries provides an important nutrient base for food webs leading to commercially important food fishes and invertebrates. A convenient indicator of the nutritional value of the food is the ratio of the carbon to nitrogen (C : N) content. The dietary requirement of protein for most animals is 16.5 percent of the dry weight of the diet, which corresponds to a C : N intake of about 17 ; 1. In cases, where the C : N intake is above 17 : 1 there is a protein deficiency (Russell-Hunter, 1970).
Nutritive enrichment of Rhizophora mangle leaf litter results from loss in carbon content and an increase in final nitrogen; the C : N changes from 120 in senescent leaves to 43 in partially decomposed leaves (Fell and Master, 1980). Carbon represents 45 percent of the dry weight of senescent Rhizophora mangle leaves; about half of this carbon is leached and half becomes particulate detritus (Fell and Master, 1980). The soluble carbon enters the food chain either as uptake by heterotrophs (Cooksey and Cooksey, 1978) which are then consumed by filter feeders or by direct ingestion after flocculation.
Fell and Master (1984) in their study of decomposition of leaves from Rhizophora mangle, pointed out that C : N ratio is an indicator of the nutritional value of food. A decrease in C : N ratio from senescent leaves to partially decomposed leaves of Rhizophora mangle has been observed (Fell and Master, 1980; Newell, 1984). Similar studies in various locations in Africa, Australia and USA confirm the change in C : N ratio (Steinke et al., 1983; Van der Valk and Attiwil, 1984; Twilley et al., 1986; Steinke and Ward, 1987; Robertson, 1988).
Lee (1999) stated that more research attention should focus on the interplay between physical and biotic influences in the ecology of mangrove ecosystems. An example of this interaction is provided by sediment organic carbon dynamics, as the amount and origin of organic carbon in mangrove sediments should be influenced by both physical and biological factors, thus may influence the quality and availability of food sources for benthic faunal communities.
2.6 Composition of Stable Isotope Signature in Mangrove Leaf
2.6.1 Carbon and Nitrogen Stable Isotope Signature in Mangrove Leaf
The source and the fate of organic matter in the ecosystem can be traced using the stable isotope analysis. It is also used as an indicator to detect and identify sources of nutrients and tracing the flow of organic material in coastal food webs (Fry and Sherr, 1984; Macko and Ostrom, 1994; Peterson, 1999).
Stable isotope analysis measures the ratio of heavy and light stable isotopes of certain elements present in a sample of material. The isotopic ratio of each sample is related to a standard. The difference between the sample isotopic ratio (Rsa) and the standard isotopic ratio (Rstd) gives the isotopic signature (Î´) of the sample material and is measured in units of parts per thousand (â€°) (Lajitha and Marshall, 1994).
The source of organic material can be traced by this ratio as terrestrial sources usually have low values of the isotope 13C as compared to marine sources. In theory, detritus brought down by rivers can be can be distinguished from that of marine origin simply by measuring its 13C value, hence stable isotopes such as carbon, nitrogen and other isotopes can be very useful in investigating estuarine food chains.
Animals eating plants and detritus reflect the Î´13C ratio of the material they consume. The Î´13C ratios of mangrove plants (example: Rhizophora sp., Bruguiera sp., Sonneratia sp., and Avicennia sp.) fall in the range of -26 to -29 â€° and phytoplankton, -15 to 25 â€°, in Malaysia (Gearing et al., 1980). The ratios of plant (consumer) will not change if it dies or being eaten.
Analysis of Î´13C can be used to determine the dominant food source and to estimate its relative contribution. By comparing Î´13C values of different organisms, it is also possible to track mangrove carbon through successive stages of food chains. When organic matter is assimilated by an animal, there is some preferential absorption of 13C rather than 12C, but this effect is small and raises the Î´13C value (make it less negative) by perhaps only 1-2% at each trophic level. This means that the Î´13C value of an animal's tissues is a good indicator of the value in its food (Hogarth, 2007).
Several studies using stable isotope ratios of carbon, nitrogen and sulphur have evaluated the relative importance of plant matter and algae in marsh (Peterson and Howarth, 1987; Peterson et al., 1985; Sullivan and Montcreiff, 1990) and mangrove (Rodelli et al., 1984; Newell et al., 1995; Ambler et al., 1994) food webs. This approach involves the measurement of the relative abundance of different light and heavy isotopes of each element that naturally occur when the organic matter is created. These differences, or ratios, are signatures incorporated into the consumers that ingest the organic material, recording long-term nutrition. Isotope ratios differ among plants, animals, and microbes (Peterson and Howarth, 1987; Peterson et al., 1985). This method has limitations (Peterson and Howarth, 1987; Peterson et al., 1985) as wide variations in isotope ratios for a given organism are common; also the ratio may not be conserved because of further fractionation along a food chain.
2.7 Fatty Acid in Mangrove Leaf
Fatty acid is an important component in the tissue of living organisms. Fatty acid acts as biomarkers for prokaryotes, fungi, diatoms, dinoflagellates or vascular plants (Kristensen et al., 2008). Fatty acids have been used to distinguish sources of organic matter in sediments and trophic markers to determine the transfer of matter within the food web, and therefore highlighting the diet of marine invertebrates (Meziane and Tsuchiya, 2000).
Saturated fatty acids (SAFA) dominate the fatty acid composition of mangrove leaves with Palmitic acid (16:0) as the most abundant (Hall et al., 2006). The high content of polyunsaturated fatty acids (PUFA), in particular 18:2v6 and 18:3v3, has been identified as useful biomarkers of mangrove leaves in estuarine food chains (Hall et al., 2006; Meziane et al., 2007).