太湖梅梁湾生物控藻围栏中颗粒有机物与鲢、鳙鱼的稳定同位素组成表明了显著的季节变化，并且POM的碳同位素值(δ13C)与鲢鱼的δ13C值正相关，表明颗粒有机物δ13C值的季节变化通过食物链储存到滤食性鱼类中。颗粒有机物、浮游动物与鲢、鳙鱼的δ13C值之间差异很小(0.2~1.7‰), 表明浮游生物生产力是鲢、鳙鱼的主要食物源。建立在浮游动物与鲢、鳙鱼的氮同位素值(δ15N)基础上，我们通过同位素质量平衡模型评价浮游动物对鲢、鳙鱼的食物贡献分别是45.7% 和 54.3%。
结合稳定同位素与肠含物分析两种技术手段，我们对鲢、鳙鱼的生长和可利用的食物进行了为期两年的研究。与2004年相比，2005年鲢、鳙鱼均具有相对富集的δ13C值，这主要归因于两个因素：(i) 两年之间浮游食物网基础食物源(浮游植物)稳定同位素值的差异；(ii) 两年之间食物种类组成与同位素值的差异。 鲢、鳙鱼的生长参数(体长、体重)与相应的稳定同位素值正相关，表明鲢、鳙鱼的稳定同位素值的变化主要是由于它们的快速生长导致了生物量的积累。由于2005年鲢、鳙鱼食物中浮游动物比例的显著降低, 鲢、鳙鱼2004年的生长速度要快于2005年。 与以前的结论不同, 鳙鱼2005年的营养级低于鲢鱼, 这可能是鲢、鳙鱼之间的食物种类组成与食物质量水平的差异所引起的.
通过调查太湖梅梁湾食物网中初级食物源与消费者的稳定同位素值与营养级的季节变化, 表明: 绝大多数优势种类的稳定同位素组成(δ13C 和δ15N)在季度之间表明了统计学上的重要差别。水体消费者稳定同位素值的季节变化主要是基础食物源的稳定同位素值的季节变化和鱼类在生活史过程中发生的食性转移两者共同作用的结果。与杂食性鱼类相比较，肉食性鱼类和底栖动物的稳定同位素值更容易产生季节变化，然而它们的营养级相对于杂食性鱼类表明了较小的时间变化。颗粒有机物(POM)与附着藻类均表明了支持食物网的重要性，然而，水体消费者主要利用了来自于颗粒有机物的碳源。食物链长度的评价揭示了梅梁湾食物网存在由于可利用的食物资源引起的不稳定性。
|Other Abstract||Naturally-occurring stable isotopes of carbon (δ13C) and nitrogen (δ15N), as a powerful tool, are applied extensively to all aspects in the studies of aquatic ecosystems. In particular, the stable carbon and nitrogen isotopes are used to solve various problems in aquatic food webs, e.g., food sources and trophic relationships among aquatic organisms, diet shift, intra- and interspecific competition, species invasion, migration, biological accumulation and amplification of heavy metal and organic pollutants in tissues of aquatic consumers and so on. We have conducted a two-year investigation on the stable isotope ratios and growths of two biomanipulation fishes in a large fish pen in Meiliang Bay (Lake Taihu). The main objectives were to elucidate the carbon sources and trophic relationships among particulate organic matter (POM), zooplankton, silver and bighead carps, and to compare the interspecific difference in trophic levels of silver and bighead carps. Furthermore, stable carbon and nitrogen isotopes were utilized to depict the food web characteristics in Meiliang Bay (Lake Taihu) and Nujiang River ecosystems.
Both POM and carps in the fish pen showed outstanding temporal variations in isotopic compositions. Moreover, positive correlation was found between δ13C of silver carp and POM, suggesting that temporal variation of δ13C in POM was preserved in planktivorous fish via food chain. The differences of δ13C among seston, zooplankton, and muscle tissue of silver carp and bighead carp ranged only 0.2~1.7‰, indicating that plankton production was the primary food source of filter-feeding fishes. According to a mass balance model, we estimated that the contributions of zooplankton to the diets of silver carp and bighead carp were 45.7% and 54.3%, respectively, based on the δ15N values of zooplankton and planktivorous fishes.
We examine the growth and food availability of silver carp and bighead in two consecutive years of 2004 and 2005 using stable isotope and gut content analysis in the fish pen in Meiliang Bay. Both silver carp and bighead exhibited significant higher δ13C in 2005 than in 2004, which was probably attributed to different isotopic compositions at the base of pelagic food web and different compositions of prey items and stable isotopes. The significantly positive correlations between body length, body weight and stable isotope ratios elucidated that isotopic changes of silver carp and bighead resulted from the accumulation of biomass concomitant with the rapid growth. Due to the outstanding decrease of zooplankton in the diet in 2005, silver carp and bighead grew faster in 2004 than in 2005. Bighead carp showed a lower trophic level than silver carp in 2005 as indicated by stable nitrogen isotope ratios, which was possibly explained by the interspecific difference between the prey species and food quality of silver carp and bighead.
Stable carbon and nitrogen isotopes were used to examine the changes in food web structure of consumers and primary food resources quarterly from September 2004 to August 2005 in Meiliang Bay, Lake Taihu. Most predominant species showed statistically significant differences in δ13C and δ15N signatures between quarters. Temporal variations in isotopic compositions of consumers were caused by the combined impacts of temporal variations in basal resources isotope signatures and diet shift during the life cycles. Carnivorous fishes and zoobenthos were more subject to temporal variations in isotopic compositions than omnivorous fishes, while their trophic levels (TL) showed less temporal fluctuations than omnivorous fishes. Particulate organic matter (POM) and periphyton showed the importance in supporting the food web. However, consumers derived their food sources primarily from POM. Assessment in food chain length (FCL) implied the instability of food web structures in this eutrophic system with the dynamics of resource availability.
In the food web structure in the upstream Nujiang River, there was only one fish, the schizothoracin Schizopygopsis thermalis. The aquatic plant (Potamogeton pectinatus) showed the main contribution to sediment detritus. Food sources of Schizopygopsis thermalis were primarily from periphyton and sediment detritus. In the midstream of Nujiang River, aquatic consumers consisted of Nujiang schizothoracin (Schizothorax nukiangensis Tsao), Gongshan schizothoracin (Schizothorax gongshanensis Tsao) and Bilobed lip schizothoracin (Ptychobarbus kaznakori Nikolsky) in the food web. Nujiang schizothoracin showed a trophic level similar to Gongshan schizothoracin. Bilobed lip schizothoracin occupied the highest trophic level among the three schizothoracins. Terrestrial frutex litter and lichen played an important role in supporting the carbon source of the three schizothoracins. C4 plants elucidated very little contribution to the growth of schizothoracins. Species composition was abundant in the food web in downstream Nujiang River. Most consumers possessed an identical trophic level, indicating high diet overlap among consumers in the downstream Nujiang River. Great variation was found between δ13C of POM and Nujiang schizothoracin, suggesting that POM was not the main food source of Nujiang schizothoracin. δ13C of Nujiang schizothoracin was progressively enriched from upstream to downstream Nujiang River, which was possible attributed to the input of terrestrial carbon sources from C4 economic crops.|