2.1 Mangrove Litter Fall
Litter fall study was conducted in order to quantify the amount of litter falling on sediments and assess its impact on adjacent intertidal flat surface sediments in Pulau Sekeping and Bakau Tinggi Mangroves.
Litter fall is a basic need in an entire ecosystem process as their dynamics supports the whole ecological continuous and being the most important source of mangrove organic matter in tropical estuaries. However, most of the total litter fall was exported from the mangrove by the tidal fluctuations and the rest which remains on the floor were directly grazed by the organisms or carry out the decomposition stages. Apart of total litter fall, leaf material is reported to be the main component of total litter fall and supported as main contributor to decomposing litter in most of the year (Lee, 1989, Smail, 1995, and Mfilinge, 2002).
Mangrove litter often plays an important role in the estuarine food webs (Dick and Osunkoya, 2000) as litter fall can be a valuable indicator of mangrove productivity and of the energy and materials that may be exported to adjacent coastal waters. The input of litter to mangrove systems forms the basis of a detritus food chain and also provides an energy resource for variety organisms; bacteria, fungi, protista and invertebrates (Osborne, P.L., 2000). The soluble compounds in the litter constitute an easily available source of energy for microorganisms (Goma-Tchimbakala and Bernhard-Reversat, 2006).
Mangrove litter production varies seasonally related to rainfall (Tam et al., 1998) and wind (Osborne, P.L., 2000). Amarasinghe and Balasubramaniam (1992) reported that the physiology of the trees and location of the mangal from the river also may have an effect on the seasonality in litter production. Mangal situated far from the rivers may receive lower input of freshwater and nutrients ensue infrequent and insufficient tidal flushing. This explains the lower productivity in the back-mangrove zones than the water-front zones which subject to daily flushing.
Leaf fall was seasonal with a maximum in the wet, summer season and wood fall correlated with periods of high wind (Osborne, P.L., 2000). During the dry season, low soil moisture, low tides and high evapotranspiration rates impose water stress on mangroves that enhances both leaf senescence and litter fall while flower and fruit fall were greatest during wet season. Leaves form the largest portion of the total litterfall and hence constitute a significant input of organic matter (Amarasinghe and Balasubramaniam, 1992). Although leaf fall is the dominant single component of litterfall, the potential contribution of other components, specifically mass of reproductive structures, is also evident (Meier et al., 2006).
Nutrient concentrations in leaf litter significantly decreased compared to green leaf. These microorganisms utilize the easily decomposable constituents of the litter and were responsible for the greater decay rate in plantation (Pandey et al., 2007). The quantity and decomposition of forest litterfall depends on the tree species, the age of the stands and their development and is influenced by the environmental conditions, particularly water and nutrient availability (Goma-Tchimbakala and Bernhard-Reversat, 2006).
2.2 Leaves Decomposition
This study aimed to examine in detail changes in total lipid and total FA concentrations (including those of major FA classes) in mangrove leaves during microbial decay of two fresh common mangrove species in the Pulau Sekeping and Bakau Tinggi Mangroves; Bruguiera gymnorrhiza and Rhizhophora apiculata and to study their comparative mangrove lipid and FA input to marine ecosystem. By studying decomposing mangrove leaves, how rapidly total lipids and FAs are removed from decaying leaves and therefore how much mangrove lipids and FAs might be contributed to the estuarine ecosystem by each species could be determined (Mfilinge et al., 2005b).
In some studies, drying has been used to allow storage of leaves until experiments could be performed as this may represent the natural situation for most terrestrial litter. However, most mangrove litter falls on wet sediments or in water and remains wet. On this basis then, the use of dried leaves would likely give erroneous results (Mfilinge et al., 2002). This experiment was using naturally abscised leaves without dried as the primary aim is to examine the actual degradation rates naturally on site.
Litter fall provides a basic essential to the whole mangrove ecosystems. Litter fall does not only support as direct grazing materials for the mangrove dweller, but also available in the form of decomposed materials and are also being transported to the adjacent estuary. It is through the decomposition process that the fresh materials of litter fall were broken down and the changes of chemical constituents take places. This process provides the essential organic input to the mangrove ecosystem and one of the most important is fatty acid. Even though the scientific reports on the lipid profile of mangrove litter components are rare, but through the literature of Xu et al., (1997) and Meziane et al., (2007), it seems that mangrove leaf litter is the largest contributor of mangrove-derived unsaturated fatty acids to estuarine food chains. Hence, due to the prevalence of mangrove as a source of organic matter, fatty acid composition of the mangrove leaves can always be used as tracers of mangrove-derived organic matter.
What is the definition of decomposition?. What is the differences between the decomposition and degradation?. In the case of organisms consuming the litter fall what is the status of the leaves?
Decomposition is a term given to the process of breaking down organic matter into its constituent inorganic components. The major input to this process is litter (dead leaves, branches, twigs, flowers and fruits). Decomposition is a staged process in which a resource (detritus, organic matter) is broken down through a combination of physical, chemical and biological process. It is through the degradation processes that nutrients such as carbon, nitrogen, phosphorus and others are released to the estuarine ecosystem and the open ocean (Mfilinge et al., 2002).
Most mangrove organic matter is probably decomposed by microorganisms (bacteria and fungi) that live in prodigious numbers in the sediment (Osborne, 2000). As decomposition proceeds, the quality of the litter will change. The concentration of labile (easy to breakdown) organic compounds decline and more refractory materials such as lignin, cellulose and cutin remain. Decomposition is a fundamental process of ecosystem functioning because it is a major determinant of nutrient cycling (Moretto, et al., 2001). The process of litter decomposition is undeniably critical for maintaining site fertility and productivity (Cindy, 2005).
Typically, decomposition is measured as mass and nutrient loss from dead organic material placed in that part of the ecosystem in which it normally occurs-usually the soil surface for senescent plant leaves, branches, stems, and other aboveground litter, and various soil horizons for roots, fungi, and other belowground organisms or parts thereof (Robertson and Paul, 2000). It is through the decomposition process that nutrients and other organic compounds such as lipids are released to estuarine waters and sediments via tidal transport. The lipids and nutrients are vital for the functioning of the estuarine communities e.g. crabs and fish (Mfilinge et al., 2005b). On the other hand, Silva et al., 2007 reported that decomposing leaf litter does not contribute significantly with nutrient output to estuarine waters since the leaf litter only becomes enriched in P and N at the end of the leaf decomposition process. The amount of leaves decomposing in and on the forest floor is a function of input (litter fall and import from adjacent areas) and output/removal (export by tides, decomposition and removal by leaf-eating crabs) (Nielsen and Andersen, 2003).
2.2.1 Factors Influence Decomposition Process
Litter decomposition rates vary significantly between plant species, affected by leaf anatomy and chemical composition (in particular, the internal nutrient and lignin concentrations) and also vary geographically (Tam et al., 1998). In tropical environment, the climatic seasonality characterized by alternating wet and dry periods plays a vital role in regulating the rates of litter decomposition (Tripathi and Singh, 1992a) by changing the population of microbial community on decomposing organic matter (Arunachalam et al., 1997). The rates of decomposition are also particularly influenced by temperature, oxygen supply and the nutritional status of the resource.
The supply of essential nutrient elements such as nitrogen and phosphorus may limit the growth of decomposer organisms. Changes in original nutrient concentrations are due to the effects of leaching, decomposition, accumulative adsorption process, and microorganism growth. Leaching is a natural process in which leaves on the mangrove sediment lose mass due to chemical and physical processes. When leaves fall to the sediment and are exposed to water, there appears to be a rapid loss of dissolved organic matter and phosphate (Silva, 2007). Anaerobic carbon mineralization in mangrove sediments is also limited by the high content of structural carbohydrates (lignocelluloses) (Holmboe et al., 2001).
Mass loss from dried leaves was slower than that from fresh yellow leaves which is the dried leaves has a higher rates of leaching of C and P than fresh leaves, but still decayed more slowly. It is obvious that any kind of drying kills microflora, removes water from plant cell cytoplasm and changes the nutritional and chemical status- important media for microbial growth. In such cases, recolonization by microflora may take significantly longer (Mfilinge et al., 2002). Since pre-drying affects the degradation process, previous studies may have underestimated the rates of degradation of mangrove leaves and nutrient transfer within the mangrove ecosystem. In addition, drying leaves does not accelerate the breakdown of the leaf tissues, but increases the rate of leaching during the early stage of decay and then slows the overall decay process due to low microbial abundance (Mfilinge et al., 2002).
2.2.2 Site differences in decomposition
Substrate quality and susceptibility to microbial activity, varies among the different compounds contained in plant litter and these compounds are therefore often decomposed at different rates. Alongi et al., (2000) suggesting that there was sufficient labile organic matter fueling greater bacterial activity at increasing temperatures in forests of both Rhizophora stylosa and Avicennia marina. Small amino-acids and low molecular-weight are decomposed faster than high molecular-weight sugars and cellulose, which are decomposed faster than lignin. Some of these compounds may further contain fractions that are resistant to microbial degradation and plant litter therefore often contains a refractory fraction that is only decomposed over very long time scales. Decomposition and mineralization rates thus vary widely among detritus originating from different plant groups because the composition (i.e., content of nutrients, structural tissues) of such litter may differ substantially (Banta et al., 2004).
The nutritional quality of the substrate may determine whether microbial activity becomes limited by lack of energy (i.e., carbon) or by nutrients and hence, determine how the elemental composition of detritus will change during decomposition (Banta et al., 2004). Fungi and bacteria are the active decomposer organisms responsible for the early phase of litter decay and nutrient release by rapidly increasing in numbers (Pandey et al., 2007). The decrease in TOC content and C/N ratio indicate a rapid decomposition of the organic matter derived from roots in the young mangrove and from litter in the mature mangrove (Marchand et al., 2003).
The lack of difference in the concentrations of interstitial DOC, DON, and DOP among forests is not surprising, considering that bulk concentrations of these poorly understood pools tell us little about their availability and lability. It is likely that these pools are largely refractory (Alongi et al., 1998). Phosphorus cycling is also likely to be influence by leaf fall and decomposition. The burial of this organic matter is likely to be of importance to the phosphorus compounds in the sediment (Nielsen and Andersen, 2003).
There are several essential nutrients required by plants and algae. The two nutrients that are most generally in short supply and that limit growth of primary producers are nitrogen and phosphorus. Nitrogen, a key component in amino acids, DNA, and RNA, is used in proteins, genes, and chlorophyll. Phosphorus is also a key component in DNA, as well as ATP, and is particularly important in energy transfer and storage in primary producers (Hauxwell, and Valiela, 2004). The role of nutrients as factor determining decomposition process is largely unclear. The initial substrate quality of litter such as concentrations of cellulose, hemicelluloses and lignin, and nitrogen (N), phosphorus (P) and potassium (K) have been found to play a major role in litter decomposition in different ecosystems (Pandey et al., 2007).
Nutrient recycling is enhanced on the tidal side, especially the transfer of fixed carbon to substrate organic matter pools. It is also possible that the quality of the detritus on the tidal side, as measured by lower C:N and/or C:P ratios, may have also improved leading to a higher decomposition rate (Dick and Osunkoya, 2000). Differential flooding (Mfilinge et al., 2002) has been found to play a significant role in determining leaf detritus decay rates among lower and upper intertidal treatments. Litter in the lower intertidal remained submerged in water for a longer time than those in the high intertidal. Litter in high intertidal areas compounded with less input of nutrients (especially N) from seawater, slowed down leaching and saprophytic decay more effective when leaves are wet (Mfilinge et al., 2002). This has be supported by Alongi et al., (1998) whose saying that microbial activity also varies with frequency and duration of tidal inundation.
The better substrate conditions experienced on the tidal side may also have contributed to more favourable conditions for estuarine invertebrate habitats, thus allowing a greater abundance of fauna to access, utilise and fragment the litter (Dick and Osunkoya, 2000). Tidal side receiving daily tidal inundation is contributing more favorably to the basis of the estuarine food web than the landward side. It can be argued that lower decomposition rates at the landward side are due to tides not reaching the landward side (Dick and Osunkoya, 2000).
The comparatively slow rates of organic matter mineralization in the Malaysian forests can be attributed to the some factors for slow mineralization rates in some other mangroves: (1) the refractory, peat-like nature of sediment organic matter; (2) low concentrations and quality of dissolved nutrients; (3) competition between the trees and microbes for some nutrients; (4) desiccation in the dry season; (5) bioturbation activities of sesarmid and ocypodid crabs; and (6) the oxidizing and assimilative capacities of mangrove roots and rhizomes (Alongi et al., 1998). Constant levels or immobilization of nutrients are generally observed during the first stage of decomposition of Rhizophora leaves, followed by mineralization later (Nielsen and Andersen, 2003).
The initial decrease in C indicated dry weight loss per sample, which was caused by rapid release of total nonstructural carbohydrates such as sugars, and starches which can be easily utilized by microbes, but were only present in low concentrations. The increase is due to rising concentrations of structural carbohydrates (such as lignin and hemicelluloses) per dry matter as a result of the loss of other constituents (sugars and starches) in the detritus. The structural carbohydrates often constitute the major portion of the detrital biomass and are usually very resistant to decomposition (Mfilinge et al., 2002).
2.2.3 Lipid and Fatty Acids in Decomposing Mangrove Leaves
Microbial activity in decaying leaves appears to cause major changes in the composition and concentration of plant-produced fatty acids and lipids. Major changes included an increase in BrFAs and MUFAs, and a decrease in SAFAs. The changes were rapid early in the decay process and varied between the mangrove species. The percent of SAFAs in the total FA, together with the palmitic fatty acid 16:0, may indicate the state of decay of the mangrove leaves and may have potential for the characterization of mangrove detritus. The BFAs and FFAs were suitable indicators of microbial activity in the detritus, supporting the hypothesis that changes in fatty acid composition during leaf decomposition may provide indications of the state of decay and microbial activity (Mfilinge et al., 2003).
Mangrove sediments, which represent the end of the degradation process, contained significantly lower lipid concentration than the leaf detritus at the end of the experiment, probably due to low microbial abundance. The Fatty Acid composition in Bruguiera gymnorhiza leaves changed from predominantly saturated straight chains to the monounsaturated FAs and more branched saturated FAs, as the leaf detritus aged. Mfilinge et al., 2003 indicated that a higher amount of PUFA in decomposing Bruguiera gymnorhiza was detected in the old detritus (8-18 weeks) than in the fresh detritus (Mfilinge et al., 2003).
The rapid increase in total lipid content early in decomposition process can be partly attributed to rapid growth of bacteria probably fungi, and the products of their metabolic activity. Apart from chemical changes in the lipids, the dramatic increase in lipid concentrations can probably due to weight loss resulting from the removal of more labile compounds e.g. sugars and starches, by leaching, microbial colonization and input of organics from external sources (Mfilinge et al., 2005b). The increase in total lipids also could be caused by the increasing other lipid compounds, such as sterols, during microbial decay. Although the invertebrates were entirely removed from the detritus, some microorganisms could have remained and might have accounted for the increase in sterol, and consequently total lipid concentration (Mfilinge et al., 2005b). Bacterial activity also plays a significant role in lipid and fatty acid transformation.
The highest lipid concentration, which was also significantly higher than in the senescent leaves of both species, was found in the second week of incubation in the field (Mfilinge et al., 2003). The increase in branched components in the detritus is indicative of a significant bacterial population. This suggests that microbial colonization and growth on the litter made a significant contribution to the increase in total lipid content (Mfilinge et al., 2003). Therefore, increases in the composition and percent concentration of BrFAs, MUFAs, and the total FA concentration during leaf decay, are most probably related to increases in bacterial colonization and growth on the leaves (Mfilinge et al., 2003).
Microbial degradation did not affect LCFAs in the detritus and that the LCFAs may remain in detritus for a long time. Since waxes decompose very slowly in nature, LCFAs in the detritus may remain in sediments for several years (Mfilinge et al., 2003). Mangrove leaves contain higher ratios of SAFAs to PUFAs in their green leaves and detritus. The declining trend in 16:0 during decomposition is related to the loss of C (Mfilinge et al., 2003).
2.2.4 Weight loss
Decay rate is thought to play a pivotal role in determining how certain factors influence nutrient availability and there is little evidence that faster decomposition plays a large role in postharvest increases in soil nitrate concentrations (Cindy, 2005). The detritus weight loss is possibly due to the rapid microbial growth, breakdown of bound lipids originally present in the yellow leaves, diatom colonization and input of lipids from the surrounding sediment. The loss in weight is faster at higher ambient temperature as warm temperatures enhanced microbial activity and decay (Mfilinge et al., 2005b).
Lipid contributions to estuarine estimated by:
TLF × LWT/xg,
TLF = Total annual leaf fall in the 10 ha mangrove forest
LWT = Weight of a lipid or FA
The initial rapid weight loss rates were most likely due to the fast release of non-structural carbohydrates such as sugars and starches (dissolved organic materials DOM) easily utilized by microbes (Mfilinge et al., 2002), which subsequently colonized and initiated the breakdown of leaf material. Sesarmid crabs (where they occur) initiate litter breakdown and thus enhance microbial processes in the detrital food chain. Desiccation coupled with impoverished faunal abundances could also be playing a role in retarding litter degradation/nutrient recycling (Bosere et al., 2005). Drying of any kind kills or limits growth of microflora, removes water from plant cell cytoplasm and changes the chemical status of leaf material, which are important media for microbial growth (Mfilinge et al., 2002).
2.3 Fatty Acid
Fatty acids are the major constituents of lipids, which are a compact and concentrated form of energy for plant and animals as well as essential constituents of cell membrane lipids, precursors of bioactive metabolite and chemical messengers (Napolitano et al., 1997). Fatty acids are carbon-rich and easy metabolized compounds as part of the animal's diet that is ubiquitous in all organisms (Andrea et al., 2006) as the phospholipids are important chemical constituents of the membrane lipids and play a major structural role in the cytoplasmic membranes of animals and plants, including bacteria and fungi (Mfilinge et al., 2005b). Fatty acid constituents of marine lipids are present in a great structural variety associated with the vast biological diversity of marine life (Napolitano et al., 1997).
Fatty acids may contribute energy resources, essential nutrients for survival and growth, and may be integral components of cell membrane structures and function (Hazel et al., 1991; Parrish, 1998; Sargent et al., 1987; Parrish et al., 2000). Their biological specificity, and the fact that they are transferred from primary producers to higher trophic levels without change, makes fatty acids suitable for use as biomarkers (Parrish et al., 2000). These biomarkers can then be used to trace the origin and trajectory of organic matter in the ecosystem (Alfaro et al., 2006). Lipids found in biological membranes have received by far most attention as they show major differences between microbial groups and are relatively easily extracted from natural samples (Boschker and Middelburg, 2002).
What is saturated,Pufa,mufa, branced?
Table 2.1: Fatty acid classes based on the carbon chain
Fatty acid forms
2.3.1 Fatty Acid as Biomarkers
Many studies have successfully used FAs to trace the transfer of organic matter in coastal and estuarine food webs (e.g. Canuel et al., 1995; Kharlamenko et al., 1995; Napolitano et al., 1997) while previous studies have also demonstrated that the specific fatty acid markers available for different groups of primary producers and consumers (Perry et al., 1979; Volkman et al., 1989) could represent a useful tool, especially since information on dietary composition is integrated over a long time scale than in conventional approaches, i.e. gut-content analysis (Bachok et al., 2006). There is an increasing focus on the application of the FA trophic tracer technique to the relatively higher trophic levels of these food webs such as macroinvertebrates (Meziane and Tsuchiya, 2000, 2002; Bachok et al., 2003; Copeman and Parrish, 2003), which have both important ecological and economical roles (David et. al, 2006). Thus the concentration of particular fatty acid markers could provide insight into the food sources utilized by animals (Kharlamenko et al., 2001).
Fatty acids are major constituents of every living cell, and eventually become the predominant lipids of marine particulate organic matter. To be useful as a trophic marker, a fatty acid must be synthesized at low trophic levels and then transferred unchanged (or in a recognizable form) to upper levels of the food web. Some polyunsaturated fatty acids (PUFA), in particular 20:5w3 and 22:6w3, are essential for marine animals, and therefore their synthesis at higher trophic levels is negligible (Napolitano et al., 1997).
Chamberlain et al., (2005) have reported the conditions that must be fulfilled if particular FAs are to be used as biomarkers in trophic studies: (i) the FA compositions of the possible diets must each contain unique compounds in relatively high abundances, (ii) these compounds must be either absent from, or only minor components of, those FAs biosynthesized by the consumer, and (iii) these compounds must be assimilated into the consumer FA composition as intact molecules at significant abundance when the diet is consumed.
For identification of microorganisms, specificity should be high in the sense that the biomarker is only produced by the organism of interest; otherwise interference from other microorganisms may occur (Boschker and Middelburg, 2002). For example, previous studies have used fatty acids as biomarkers for bacteria (Rajendran et al., 1993), diatoms (Parrish et al., 2000), dinoflagellates (Parrish et al., 2000), zooplankton (Falk-Petersen et al., 2002), macroalgae (Johns et al., 1979; Khotimchenko and Vaskovsky, 1990), and vascular plants (Wannigama et al., 1981). These biomarkers can then be used to trace the origin and trajectory of organic matter in the ecosystem (Andrea et al., 2006).
Lipids also have been used in the past as biological markers for algae and bacteria in marine sediments (Perry et al., 1979): to determine the biomass of bacterial symbionts in ascidians (Gillan et al., 1988): and as possible organic tracers for establishing benthic microbial community structure (Bobbie and White, 1980). For biomass determination, the biomarker should be present in relatively constant amounts in the organisms of interest. Biomarkers occurring in storage products should not be used if biomass has to be quantified because their content varies with the physiological condition of the organism (Boschker and Middelburg, 2002).
Recent studies have used fatty acid markers to trace the origin and flow of organic matter from mangroves through marine food webs (Meziane and Tsuchiya, 2000, 2002). For example, fatty acids with more than 24 carbon atoms are synthesized only by vascular plants (Volkman et al., 1980), and consequently can be used as markers of material of vascular plant origin in sediments and animal tissues (LeBlanc et al., 1989, Meziane and Tsuchiya, 2000, 2002). The branched-chain fatty acids (BrFAs) 15:0 and 17:0, iso and anteiso, and the monounsaturated fatty acid (MUFA) 18:1w7, are generally considered to be predominantly synthesized by bacterial communities (Jeffries, 1972, Volkman et al., 1980) and consequently, are useful as bacterial biomarkers and indicators of bacterial biomass (Parkes, 1987). The abundant PUFAs in the mangrove leaves, namely, 18:2ω6 and 18:3ω3, were useful tracers for the first trophic transfer and that they are likely to be nutritionally important FAs as opposed to the LCFAs (Hall et al., 2006).
PUFAs are generally labile in nature which subjects them to rapid losses through zooplankton grazing and/or bacterial degradation (Smith et al., 1983). PUFAs are therefore not expected to be preserved in their original abundances, but their presence could be related to a fresh input of algae, diatoms, and dinoflagellates to sediments (Meziane and Tsuchiya, 2000, 2002). While LCFAs (>C24) are generally used as biomarkers for materials of vascular plant origin. They are associated with the waxy leaf coatings of higher plants and are therefore considered indicative of vascular plant input (Meziane and Tsuchiya, 2002). Relatively high amounts of C26, C28, and C30 (which are associated with degraded OM input) were detected in sediment of the burrow chamber, indicating the presence of mangrove detritus inside the burrow.
2.3.2 Fatty Acids Markers
18:2w6, 18:3w3 and 18:3w6
Diatoms / Microalgaea 20:5w3, 16:1w7/16:0
Dinoflagellatesa 18:4w3 and 22:6w3
Bacteriaa Odd branched FA 15:0 iso, 15:0 anteiso, 17:0 iso, 17:0 anteiso,
Monounsaturated FA (MUFAs) 18:1w7
Palmitic acida 16:0
Terrestrial planta Long Chain Fatty Acids (LCFAs; >C24)
Seagrassa 18:2w6, 18:3w3
Zooplanktona 20:1, 22:1
Brown algaea 18:1w9
Red algaea 20:5w3, 20:4w6
(aAlfaro et al., 2006; Mfilinge et al., 2005; bMchenga et al., 2007)
Poor nutritional quality of mangrove detritus does not stimulate bacterial colonization as much as algal matter does. The significant increase in bacterial abundance is likely due to the decomposition of benthic algae and their detritus (Mfilinge et al., 2005). Green macroalgae contain higher concentrations of w3 and w6 PUFAs than mangrove leaves and mangrove sediments (Mfilinge et al., 2003). Invertebrates may influence PUFA concentrations in surface sediments. Invertebrates such as fiddler crabs are known to appear abundantly and active on the surface sediments.
LCFAs may be preserved in mangrove sediments and nearby ecosystems for a long period without undergoing further transformation, and therefore may be useful as indicators of the contribution of mangrove lipid to the estuarine ecosystem and the ocean (Mfilinge et al., 2005b). Long chain fatty acids (LCFAs) were found in high concentrations in mangrove leaves either fresh or decomposing as previously reported by other researchers (Alfaro et al., 2006 and Meziane and Tsuchiya, 2000, 2002). The major difference between the fatty acid profiles of fresh versus decomposing mangrove leaves was a decrease in 18:2w6 and 18:3w3 in decomposing leaves, while the amount of LCFAs was unaffected. These saturated fatty acids were not found in any significant quantity in the other major sources of primary production such as seagrass and brown algae, thus providing a good biomarker for mangrove organic matter (Alfaro et al., 2006).
2.4 Outwelling of Particulate Organic Matter
The applications of using fatty acids as taxonomic tool and tracers of organic matter have been worldwide discussed in research publications. However, an inherent assumption of this application is that populations of the same species demonstrate smaller degrees of variations compared to inter-specific differences in chemical signature, plus the additional factors like geographic location that may influence the universality of chemical tracer signatures; compromising their usefulness as indicators of trophic relationship (Meziane et al., 2007). Therefore, in studies concerned with the export of mangrove organic matter basically through litter production, another aspect to consider is the behavior of these fatty acids while there are in the decomposing litter, during sediment diagenesis process, and when ingested by benthic consumers.
In this study, the variations in fatty acid profiles of two most common mangrove species, Bruguiera gymnorrhiza and Rhizophora apiculata in their fresh and decomposed stages, and of several species of mangrove organisms were analysis and compared to the fatty acid profiles in the sediments and surface water of mangrove and six stations of Kuala Kemaman estuary, with the aim to determining the outwelling of organic input and also examining the prospect of using individuall or groups of fatty acids as taxonomic and trophic markers.
It is believed that a significant part of the organic detritus was flushed out until the ocean and thus plays an important role in the dynamics of near shore coastal ecosystems (Franscisco et al., 1987)
As direct grazing by crabs and other organisms substantially reduces exports of leaf litter by tidal transport and accelerates its breakdown (Robertson, 1986, 1991), most litter decomposes by microbial action (bacteria and fungi) that live in prodigious numbers in the sediment, or is transported by tides to adjacent habitats (Osborne, 2000). On this basis, it was assumed that the small amount of litter produced in the mangrove might be available for export (Mfilinge et al., 2005). Leaves and pneumatophore (aerial root) tissues are the major sources of mangrove organic matter available to the estuarine ecosystem, either through direct grazing or decomposition (Alfaro et al., 2006). Previous researchers have identified the importance of mangrove leaf litter as one of the main sources of organic matter in estuaries, with great potential for exportation to adjacent habitats (Wafar et al., 1997 and Woodroffe, 1982). Organic matter processed in the mangroves is exported to the intertidal flats in both the rainy and the dry season (Meziane and Tsuchiya 2000).
Organic matter can leave as large pieces (branches, trunks), smaller plant parts (leaves, twigs, flowers), as tiny particles or in solution. Certainly, significant quantities of the buoyant litter and organic matter mainly leaves, is exported from mangrove ecosystems by tidal waters either fresh, or in various states of decomposition, in the form of large and fine pieces of leaf detritus which enrich the intertidal sediments with nutrients (Bano et al., 1997 and Wafar et al., 1997) and particulate organic matter. Mangrove-derived nutrients may contribute to adjacent areas as detritus rather than as fresh material, which would make it available to filter feeders but not grazers (Alfaro et al., 2006). Organic matter can also leave when organisms depart (Osborne, 2000). Marchand et al., 2003 reported that algal-derived organic matter is much reactive to decomposition than higher plant-derived organic matter. The amount of energy and organic matter export to the estuary from mangrove ecosystems depends on the degradation rate of mangrove litter (Mfilinfe et al., 2002).
Mangrove-derived POM is the dominant source of organic matter to the estuary (Meziane and Tsuchiya, 2000). Many studies have stressed the importance of organic matter export from these systems to the productivity of offshore waters. Dissolved organic carbon may provide a significant and readily utilizable carbon source to offshore heterotrophs (Osborne, 2000). The organic matter derived from marsh plants may be incorporated into the macrozoobenthos of the intertidal flats (Meziane, et al., 1997). Tropical mangrove forests are probably efficient biogeochemical barriers to the transport of P and N in tropical coastal areas. Hence mangroves could be an important and efficient ally against eutrophication in tropical coastal areas (Silva et al., 2007). The restricted organic accumulation may also be partly linked to the tidal flushing; however the older forest, the greater organic content. The organic content quickly decreases with depth in the sediment, due to decay processes thought to be suboxic and induced by biological processes rather than physical processes (Marchand et al., 2003).
The temporal variability in the magnitude of organic matter outwelling from mangroves and its contribution into the intertidal surface sediments is mainly influenced by mangrove litter fall dynamics, and to climatic conditions, particularly wind speed (accompanied with rainfall during typhoon storms), rainfall and temperature (Dittmar et al., 2001 and Mfilinge et al., 2005). The amount of litter produced in the mangrove forest directly affects the outwelling of particulate organic matter (POM) between mangrove forests and adjacent marine habitats (Mfilinge et al., 2005). Outwelling is reduced when rainfall is low and that during the rainfall season the forest may receive some terrestrial inputs. The cause for the departure from the expected importance of mangroves to the productivity of the estuarine waters resides in the differences in the extent of mangrove canopy cover and the area of estuarine waters they are expected to supply with C, N and P (Wafar et al., 1997).
The export of N and P from the mangrove forest to coastal marine systems through leaf litter is a function of the amount of leaf litter exported and its nutrient concentrations. Nutrient concentrations depend on the residence time of leaf litter on the forest floor and the enrichment of nutrients during leaf litter decomposition (Silva et al., 2007). The combined optical and geochemical approaches give evidence that the sedimentary organic content and the decay processes are influenced by the growth of the mangrove forest (Marchand et al., 2003). Alongi et al., 1998 reported that the pathways, but not total rates, of organic carbon oxidation in mangrove sediments are affected by forest age.
In addition to organic matter derived from halophytes, autochthonous carbon sources that can be used by these macro-invertebrates are microalgae (mainly diatoms) and benthic bacteria. The presence of specific constituents (various lipids, fatty acids and sterols) may provide information on the origin of organic matter within the ecosystem (Meziane et al., 1997).