Density-dependent effects of zooplanktivorous Thin Sharpbelly (Toxabramis swinhonis) on plankton assemblages and water quality: Implications for lake rehabilitation

Abstract

Because of anthropogenic and natural factors, zooplanktivorous fish often dominate fish assemblages within lake ecosystems along the middle and lower reaches of the Yangtze River Basin (MLYRB), China. Despite their prevalence, the density-dependent effects of zooplanktivorous fish on lake food webs within the MLYRB are poorly understood compared to other omnivorous fish. We conducted a mesocosm experiment to quantify how plankton communities and water quality responded to varying densities of a common zooplanktivorous fish (Thin Sharpbelly Toxabramis swinhonis). During the experiment, the mean weekly total nitrogen (TN), total phosphorus (TP), ammonium nitrogen (NH₄⁺-N), chemical oxygen demand (COD_Mn), turbidity, and chlorophyll-a (Chl.α) values in treatments where Thin Sharpbelly were present (i.e., fish-present) were all significantly higher than those in the control treatment with no Thin Sharpbelly (i.e., fish-absent). In addition, all water quality variables exhibited a significant positive relationship with the density of Thin Sharpbelly. The abundance and biomass of phytoplankton in the fish-present treatments were significantly higher than in the fish-absent treatment, with Pseudanabaena spp., Dolichospermum spp., and Limnothrix spp. dominating the phytoplankton assemblage. The biomass and abundance of small-bodied zooplankton increased with fish density, whereas the relative abundance and biomass of large-bodied zooplankton (cladocerans and copepods) showed the opposite trend. Also, the ratio of zooplankton to phytoplankton biomass declined nonlinearly with fish density, while the ratio of Chl.α to TP was significantly positively correlated with fish density. Collectively, experimental results indicated that zooplanktivorous fish increased the concentrations of nutrients and stimulated phytoplankton growth through feeding on large zooplankton and the strength of these effects increase with fish density. Moreover, results indicate that high densities of zooplanktivorous fish can alter plankton assemblage structure by facilitating growth of undesirable cyanobacteria and shifting the composition of zooplankton to smaller-bodied species and forms. Our results demonstrate how Thin Sharpbelly, and potentially other obligate zooplanktivorous fish, can have adverse effects on water quality and plankton assemblages, but these responses are density dependent. Our findings suggest that managing the density of zooplanktivorous fish could be a useful ecological rehabilitation measure for improving water quality in MLYRB lakes.