| Carbon flow within ecosystem, as a key process controlling the sustainable production of the ecosystem, is an important portion in biogeochemical cycle and ecosystem. The research on the carbon flow of various ecosystems under multi-pressure is meaningful to achieve the sustainable production of ecosystems. The major objective of this thesis is to analyze the carbon flow in various ecosystems based on the stable isotope and fatty acid composition of organisms. We collected the specific organisms in north, middle and south of China Sea. For instance, three species macro jellyfishes in the Yellow Sea; Coilia mystus in different migration stages around Changjiang Estuary and adjacent region; benthos and predators in Wenchang/Wenjiao Estuary in eastern Hainan province. Stable carbon and nitrogen isotopes and fatty acid composition of them were analyzed. The diet sources of the organisms were analyzed based on their stable isotopes and fatty acid composition by multivariate analysis. Then the transfer character of various food webs indicated by the diet compositions of organisms was discussed. Finally, the usage of biomarkers in carbon transfer within food web was summarized.Firstly, carbon sources of organisms can be traced by the stable isotopes and fatty acid composition. For example, the producers (i.e. mangrove leaves, seagrass, macroalgae, phytoplankton) in estuarine ecosystem in Wenchang/Wenjiao Estuary can be separated based on the stable isotopes and fatty acid composition, which can provide information for tracing the carbon sources of the food web. The stable isotopes of benthos in Bamen Bay mangrove swamp (δ13C,-24.6%o--12.6%o;δ15N,0.9%o-11.3%o) indicated that the various carbon and nitrogen sources of them. The fatty acid signature of them can reflect the variance in their diet composition. For example, the fiddler crab preyed dominantly on benthic diatom, but for Sesarmidae crab and Cassidula nucleus, mangrove leaves were the dominant diet source. Littorina sp. may forage on epiphyte on the mangrove root and mangrove detritus and suspended particle comprised the diet of bivalve. For Sipunculus nudus, bacteria were important carbon source. For the macro jellyfishes in the Yellow Sea, their stable carbon isotope values indicated that they mainly depended on marine-derived carbon source. Further, the low18:In-7/n-9ratio and high20:1,22:1concentration indicated that they were carnivorous and zooplankton was important diet item of them.Secondly, ontogenetic diet shift experienced by the organisms can also be identified based on the stable isotope and fatty acid composition of them. Taking Coilia mystus as an example, diet changes during their migration were traced using stable isotope and fatty acid composition. Both δ13C and δ15N values were enriched within their migration, with changes in the concentrations of some fatty acid biomarkers. The variation in benthic biomarkers (18:In-7,20:4n-6) and pelagic biomarkers (18:In-9,20:1+22:1) of Coilia mystus indicated that benthic food and pelagic food predominated their diet items alternatively in their various migration stages. Those young individuals in their feeding stages and winter-over stages preyed dominantly on benthic food (e.g. mysidaceas). As their growth, the contribution of pelagic food to larger individuals in their previous spawning stages increased significantly, with copepods as their major diet items. However, the contribution of copepods to those in spawning stages decreased, with that of benthic food increased at the same time. Comparison with the temporal and spatial changes in the zooplankton in Changjiang Estuary and adjacent region revealed that the diet changes of Coilia mystus were closely related to the changes in zooplankton. Even in the same temporal and spatial scales, the diet variation among individuals in various body sizes can also be identified by stable isotope and fatty acid composition. Fro instance, variation in specific fatty acids in jellyfishes of different body sizes suggested that diet shift may happen during their growth. The larger individuals contained high20:1+22:1, preying more zooplankton, but for those smaller ones, the high concentration of15:0+17:0and20:4n-6in their body indicated that the contribution of detritus food chain was bigger. For Sillago sihama, the20:4n-6concentration and δ13C,δ15N vales of the larger individuals (>20cm) all were higher than those of the smaller individuals(<20cm), which suggested that ontogenetic diet shift (from pelagic to more benthic) happened during their growth.Research on the matter transfer routes of various types food webs based on the stable isotopes and fatty acid composition. As filter-feeding species,Coilia mystus was one part of pelagic food web. So the changes in diet composition of them can reflect the character of matter transference within the food web in Changjiang Estuary and adjacent region. The result indicated that the matter transference in the pelagic food web exhibited temporal and spatial variation, and was closely related to the plankton community. As for benthic food web, the diet composition of zoobenthos in mangrove swamp suggested that the contribution of mangrove detritus to the food web was species-dependent, and exogenous carbon (e.g. benthic algae, bacteria, plankton) was also important carbon sources of mangrove benthic food web. The result suggested that the carbon source of benthic food web was multiplex, the transference route was complicated. Diet composition of fishes, shrimps and crabs in BamenBay were analyzed to investigate if the mangrove detritus support the pelagic food web. Wide ranges of δ13C and δ15N values and difference in the fatty acid compositions of the organisms indicated that their diet sources were complicated and they preyed on various items. The lower δ13C and δ15N values of Lutjanus argentimaculatus may due to their feeding on organisms related to the mangrove detritus. The higher18:2n-6concentration in them confirmed the contribution of mangrove detritus. So, it can be concluded that the transference of mangrove detritus to higher trophic level was only limited to specific species. Instead, phytoplankton, benthic diatom and epiphytes were the important nutrient provider in the estuarine ecosystem. Coilia mystus preyed on both pelagic and benthic food, and the plankton also contributed to the diet mangrove zoobenthos. The diet of Sillago sihama even changed from pelagic to benthic food. The interaction between pelagic and benthic food web can be identified from the above research.The δ13C values of fatty acids in predators were influenced by dietary sources and metabolism process. During the transference within food web, the813C values of fatty acids became depleted or enriched, which may be related to diet species and fractionation during metabolism.δ13C values of polyunsaturated fatty acids were consistent, but variation of δ13C values of saturated fatty acids was larger. Fatty acids with persistent613C values in consumers can indicate their dietary sources. The result suggested that stable carbon isotopic ratio of fatty acid had advantage when used as biomarkers to trace the carbon source of organism under various factors.
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