Abstract:Trophic level, a basic and measurable feature in complex food networks, represents the position of organisms in the ecosystem and reflects the vertical structure of the food web. The study of trophic levels is used to interpret sophisticated ecological relationships through simplified functional groups and is of great significance to the construction of ecosystem models and the study of nutrient cycling. The construction of the Three Gorges Dam and the operation of the reservoir have fragmented the river ecosystem. The influences of reservoir storage and operation vary between river sections with disparate geomorphic characteristics. Environmental heterogeneity may cause temporal and spatial variation in the structure of the fish community of each section of the mainstream from the reservoir. After two decades, research on fish trophic levels has mainly focused on the initial stage of storage and operation of the reservoir, while information on the fish trophic level in the mainstream remains scarce. To evaluate the long-term spatiotemporal variation in fish trophic levels in the Three Gorges Reservoir, nitrogen stable isotopes of 38 fish species and three snail species were collected from the Mudong, Fuling, Yunyang, and Zigui river sections of the main stream of the Three Gorges Reservoir from July 2018 to January 2019, and the continuous spectra of fish species trophic levels in the reservoir were constructed based on stable isotope analysis. The results showed that the average δ15N value of snails was 8.37‰, ranging from 8.29‰ to 8.53‰, and there was no significant spatial difference between the river sections. Except for the Mudong section, the δ15N values of snails in the other river sections were significantly higher in the flood season than in the dry season. Nitrogen stable isotope analysis of fish samples showed that the average value ranged from 5.31‰ to 17.79‰, with an average of 11.02‰. Taking snails as the baseline organism, fish trophic levels were calculated for each river section according to season, and the results showed that the average trophic level of fish species in the reservoir area was 2.67, ranging from 1.47 to 4.12. There was only one fish species with a trophic level higher than 4; 12 species of fish (31.6%) with trophic levels between 3 and 4, mainly piscivorous and benthivorous fish; 19 species of fish (50.0%) with trophic levels between 2 and 3, mainly omnivorous, benthivorous, and planktivorous fish; and six species of fish (15.8%) with trophic levels between 1 and 2, including three herbivorous and three omnivorous fish species. The trophic levels of fish combinations with different feeding habits varied, and were found to be piscivorous > benthivorous > planktivorous fish and omnivorous > herbivorous fish, indicating that fish feeding preference was one of the key factors determining the trophic level of fish. Between different river sections, there were differences in the range of fish trophic levels, food chain length, and trophic levels of fish combinations with the same feeding habits. However, there was no significant spatial difference in the trophic level of the fish community among the river sections. A paired sample t-test showed that there was no significant difference in fish trophic levels between the flood and dry seasons. Within the same river section, the seasonal fluctuation in food chain length decreased with distance from the dam. Compared with the initial stage of reservoir impoundment, the fish trophic level increased significantly, and the community structure and trophic characteristics of high trophic-grade fish changed to a certain extent. In particular, Coilia brachygnathus with higher trophic levels gradually expanded and became the dominant species in the mainstream of the reservoir area, which might affect the trophic structure of fish in the greater Three Gorges Reservoir area. The results of this study provide a reference for the assessment, protection, and rational utilization of fishery resources in the Three Gorges Reservoir area.
