Body size mediates the trophic cascade effect of filter-feeding fish [ Hypophthalmichthys molitrix (Val.)] on phytoplankton in large enclosures

Stocking of silver carp [ Hypophthalmichthys molitrix (Val.)] is a common tool used in eutrophic aquatic systems to control phytoplankton. As the mesh size of gill rakers undergoes change as growth proceeds, the particle selectivity and dietary importance of phytoplankton in younger fish may be different than those in older individuals. We conducted an enclosure experiment to assess how a gradient of fish size affected grazing pressure. The presence of fish strongly reduced zooplankton biomass and phosphorus (P) excretion, and P sequestered by fish could largely account for the lower total phosphorus (TP) concentrations observed in the presence of fish than in their absence. Moreover, the nutritional importance of phytoplankton in fish decreased with increasing fish size. Both the overall P excretion and grazing pressure decreased, whereas phytoplankton biomass and the yield of chlorophyll a per TP increased with increasing fish size. This suggested that phytoplankton might become more limited by P availability but less controlled by grazing with increasing fish size. Stocking with large silver carp would strongly reduce grazing pressure and increase the yield of phytoplankton per TP.