Stable isotope (13C and15N) analysis of fish food web of the Xiaojiang Bay in Three Gorges Reservoir

Abstract

The Xiaojiang River is a tributary of Three Gorges Reservoir (TGR). In the past few years, the fish biodiversity and resources in Xiaojiang Bay decreased due to anthropic activities such as dam construction, overfishing, and environmental pollution. Therefore, it is vital to study the food web structure to understand the consequences of biodiversity and resource changes in Xiaojiang Bay. The traditional gut contents analyses required many samples and do not always reflect exactly the trophic relationships among organisms in the food webs. In the present study, the food webs structure in Xiaojiang Bay were investigated during the dry and wet seasons in 2010 by using stable carbon and nitrogen isotopes. The values of particulate organic matter (POM) and periphytic algae samples varied from -23.33‰to - 21.05‰(mean (-24.85±0.38)‰), from -25.13‰to -24.54‰(mean(-22.19±1.33)‰) for δ13C and 3.99‰-6.25‰(mean (5.46± 1.29)‰), 4.64‰-4.79‰(mean (4.71±0.18)‰) for δ15 N, respectively. The fish food webs in the Xiaojiang Bay was fueld by the primary producers (POM and periphytic algae) in both dry and wet seasons. Moreover, the terrestrial carbon sources was an important supplementary carbon flow pathway for the fish food webs' energy with the impoundment' s processed of the TGR in wet season. There were three trophic groups for 17 fish species in Xiaojiang Bay including plant- based omnivorous, animal-based omnivorous and carnivores. An average trophic enrichment factor 3.4‰was used to calculate the trophic levels based on average values for δ15N among primary consumer (sails and bivalve) and its diet. The results indicated a food web having three TLs in dry or wet seasons in Xiaojiang Bay, which suggested that the aquatic ecosystem has formed a relatively stable food webs structure in the early running period of the Three Gorges Project. Stable isotope signatures suggested that the fish food web of Xiaojiang Bay passed on emerges from the primary producers to the higher predators through four main trophic pathways. These findings have important implications for the fish resources' management and the ecological restoration in Xiaojiang Bay and other tributaries in TGR.