Trophic structure in response to land use in subtropical streams

Abstract

Land use in the uplands can substantially impact the biological community in stream ecosystems, but how the trophic structure of aquatic communities within riverine food webs respond to catchment land use changes within their respective catchment remains unclear. Here, we investigated the impact of land use on aquatic communities in two streams in subtropical China. We measured isotopic compositions of basal resources, four macroinvertebrate functional feeding groups (scrapers, collectors, shredders, and predators with 132 taxa), and three fish functional feeding groups (herbivores, omnivores, and piscivores with 26 species) in three land uses (woodland, agricultural, and urban). Isotopic values of δ13C and δ15N were used to estimate the dietary proportions of consumers (macroinvertebrates and fish) and community-level metrics (niche width and trophic diversity). We found that isotopic values of basal resources, macroinvertebrates, and fish varied significantly among sampling sites adjacent to the three land uses. Dietary analysis showed that terrestrial resources contributed relatively less (~20%) than aquatic resources to consumers (macroinvertebrates and fish) in higher trophic levels, especially in the urban sites. Compared to woodland sites, food webs in the urban sites had lower niche width and higher trophic redundancy, and agricultural sites had higher trophic length and lower niche width. Trophic structure alterations in response to land use primarily resulted from availability and variability of basal resources in the upstream river channel and the riparian zone, and also the feeding plasticity of macroinvertebrates and/or fish. These empirical findings indicate the importance of diversity across all trophic levels in the management of stream ecosystems in urban and agricultural landscapes.