Freshwater mussels can enhance zooplankton size and their grazing potential on phytoplankton with implications for water clarity in tropical eutrophic shallow lakes

Filter-feeding freshwater mussels are ecosystem engineers that significantly modify the structure of biological communities and trophic status, affecting water quality. An 8-week mesocosm-experiment, with and without the freshwater mussel Sinanodonta woodiana, tested the hypothesis that this filtering-feeding mussel alters the plankton community by reducing chlorophyll a (Chl a) of larger phytoplankton (> 2.0 μm), reducing small zooplankton such as rotifers and increasing the biomass of large zooplankton such as cladocerans and copepods. These changes would enhance the ability of zooplankton to control phytoplankton in eutrophic shallow waters. Compared with the mussel-free control, treatment with mussels led to a decrease in the Chl a of phytoplankton, particularly microphytoplankton (> 20 μm) and nanophytoplankton (> 2.0 μm), and an increase in the biomass of large zooplankton such as cladocerans and copepods, while the biomass of rotifers did not change. As a result, mussels enhanced the biomass ratio of zooplankton to phytoplankton, indicating stronger grazer control of phytoplankton. Total suspended solids were reduced, and light intensity and benthic algal Chl a increased in the mussel treatment. Our study shows that this native freshwater mussel may directly and indirectly facilitate a reduction in Chl a of phytoplankton thereby improving the water quality in tropical eutrophic shallow lakes.