The feeding habits of small-bodied fishes mediate the strength of top-down effects on plankton and water quality in shallow subtropical lakes.

The proliferation of small-bodied fishes in lakes is often accompanied by deterioration of water quality and ecosystem function. However, the potential impacts of different types of small-bodied fish species (e.g., obligate zooplanktivores and omnivores) on subtropical lake ecosystems in particular have been overlooked mainly due to their small size, shorter life spans and lower economic value. Therefore, we conducted a mesocosm experiment to elucidate how plankton communities and water quality respond to different types of small-bodied fishes, including a common zooplanktivorous fish (thin sharpbelly Toxabramis swinhonis) and other small-bodied omnivorous fishes (bitterling Acheilognathus macropterus, crucian carp Carassius auratus and sharpbelly Hemiculter leucisculus). During the experiment, the mean weekly total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), turbidity, chlorophyll-a (Chl.α) and trophic level index (TLI) values were generally higher in treatments where fish were present compared to treatments where fish were absent, but responses varied. At the end of the experiment, phytoplankton abundance and biomass and the relative abundance and biomass of cyanophyta were higher while the abundance and biomass of large-bodied zooplankton were lower in the fish-present treatments. Moreover, the mean weekly TP, COD, Chl.α and TLI values were generally higher in treatments with the obligate zooplanktivore, thin sharpbelly, when compared to treatments with omnivorous fishes. Also, the ratio of zooplankton to phytoplankton biomass was the lowest, and the ratio of Chl.α to TP was the highest in treatments with thin sharpbelly. Collectively, these general findings indicate that an overabundance of small-bodied fishes can have adverse effects on water quality and plankton communities and that small-bodied zooplanktivorous fishes likely induce stronger top-down effects on plankton and water quality than omnivorous fishes. Our results emphasise that small-bodied fishes should be monitored and controlled if overabundant when managing or restoring shallow subtropical lakes. From the perspective of environmental protection, the combined stocking of different piscivorous fish species that feed in different habitat types could be a way forward to control small-bodied fishes with different feeding habits, but more research is needed to assess the feasibility of this approach.