Ecological Studies On The Food Web Structures And Trophic Relationships Of Freshwater Lakes In China Using Stable Carbon And Nitrogen Isotopes

The structure and trophic relationship of food webs are fundamental research topics in modern ecology. Food webs are important for the understanding of patterns and processes of an ecosystem. Stable isotopes of carbon and nitrogen, as a powerful approach to study the structure and dynamics of food webs and trophic relationships, have been increasingly used in the studies on a wide variety of ecosystems. In this thesis, I gave an overall review on sample preparation, analysis, interpretation and applications of stable isotope ratios (δ¹³C and δ¹⁵N) in aquatic ecosystems, and reported the applications of δ¹³C and δ¹⁵N in ecological studies in several Chinese lakes with the main results and conclusions as follows.In the hypereutrophic subtropical Lake Donghu, high external nutrient loading and the presence of abundant detritus from submersed macrophytes were responsible for the high sediment δ¹⁵N and δ¹³C, respectively. (13)C was significantly higher in submersed macrophytes than in other macrophytes. The similar δ¹³C values in phytoplankton, zooplankton, zoobenthos, and planktivorous fish indicate that phytoplankton was the major food source for the consumers. By using a δ¹⁵N mass balance model, it is estimated that the contributions of zooplankton to the diet of silver carp and bighead carp were 54% and 74%, respectively, which is in agreement with previous microscopic observations on intestinal contents of these fishes.Temporal and spatial changes in δ¹³C and δ¹⁵N of seston (mainlyphytoplankton) and isotopic relationship between seston and the lake anchovy (Coilia ectenes taihuensis) were studied in the large eutrophic freshwater Lake Chaohu in China. Much of the spatial and temporal variation in 513C of lake anchovies was explained by variation in seston, indicating a strong link between pelagic primary production and higher order consumers. Because the lake is shallow, there were no significant differences in S13C and S15N of seston between surface and overlying waters. Spatially, the relatively high 513C and 815N of seston in the western part of the lake might be due to high levels of anthropogenically derived N and C introduced from the surrounding cities through sewage drainage systems. The trophic position of the lake anchovy in the food web of Lake Chaohu was estimated to be 2.9-4.1 (3.5±0.4), which agrees well with the previous stomach content analysis and suggests that the lake anchovy fed both on zooplankton and small fishes.There were large variations of stable carbon and nitrogen isotope ratios of lake anchovy {Coilia ectenes taihuensis) from Lake Chaohu. Significantly distinct stable carbon and nitrogen isotopic compositions between small (<130mm) and large (>130mm) lake anchovy imply that they feed on food sources that are overall different in isotopic composition for some considerable time. 813C and S15N signatures of lake anchovy provided evidence for food-web differences between small and large lake anchovy, indicating that small lake anchovy were feeding on a predominantly plankton-based diet while large lake anchovy were living on a more benthic and picivorous feeding mode. Overlapping in isotopic compositions of small and large lake anchovy probably indicated that these fish occasionally shared a common diet as suggested by previous stomach content analysis, and may also imply that some small lake anchovy shifted their diets earlier than others. This study, together with previous studies, demonstrated that intraspecific isotopic variation should be considered when conducting studies on feeding habit and trophic position of aquatic consumers by using stable isotopes.We investigated differences in 813C and 815N of particulate organicmatter (POM) between two connected lakes, Lake Fuxian and Lake Xingyun, which locate in Yunnan Plateau, southwestern China. In the photic layers of Lake Fuxian, 513C and 815N of POM, and Chi. a showed similar trends, which might be driven mainly by phytoplankton physiology. The relatively negative 513C and positive 515N values of POM associated with the development of anaerobic conditions in the hypolimnion of Lake Fuxian were most likely because that sections in the deep layer water were partially synthesized microbiologically by utilizing 13C-depleted and 15N-enriched sources. The difference of stable isotope signatures between these lakes observed in this study demonstrated the dynamic nature in isotopic signatures of different trophic state of lakes.Stable carbon and nitrogen isotopes were used to investigate food web and to determine the importance of external and internal carbon sources in Lake Chaohu. Temporal and spatial variations, and overlap in the carbon and nitrogen signatures of primary producers made it difficult to determine unambiguously the feeding habits of many consumers. However, stable carbon isotopic signatures suggested that autochthonous carbon sources were the main carbon sources in the food web of the lake, while the contribution of external carbon inputs was seemed to be neglectable to the local food chains. Both invertebrates and fish appeared to eat a variety of food (often more than one trophic level), indicating that a continuous rather than discrete measure of trophic position would be useful in describing trophic relationships in this lake.The trophic relationship between flesh-burrowing parasitic isopod Ichthyoxenus japonensis and host fish Carassius auratus auratus was investigated using stable isotopes of carbon and nitrogen. Both 813C and 815N of /. japonensis were significantly depleted with respect to its host, which differs from the fractionation patterns that conventionally observed between consumers and their diets. Our results confirms that endoparasites were significantly and consistently depleted, which may caused by high lipidcontent relative to its host, and the utilization of 15N-depleted amino acids and/or proteins from host and 15N-depleted ammonia from its inhabitance.Food web structure and trophic relationship were investigated in Lake Fuxian using stable carbon and isotope ratios. 813C and 515N of primary production and consumer indicated that internal carbon inputs (phytoplankton and periphyton) were the predominant food sources for both littoral and pelagic consumers, and more than 60% of the studied species relied mainly (more than 50%) on benthic-derived production. Species invasion poses a serious threat to biodiversity and native ecosystems. Stable isotope ratios were used to document the food web impacts of the invasion of the exotic zooplantivorous ice fish on the endemic zooplantivorous Anabarilius grahami. A. grahami had more enriched 513C (-26.4%o versus -27.2%o) and increased trophic position (3.5 versus 2.8) in post-invasion than pre-invasion food webs, reflecting a shift in the diet of A. grahami from zooplanktonic towards benthic and picivorous prey.