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Response of phytoplankton to silver carp, Hypophthalmichthys molitrix (Val.), stocking under phosphorus fertilization: a case study in a large reservoir in central China

Phosphorus fertilization is employed in some reservoirs to increase fish production. To clarify whether silver carp regulate phosphorus (P) concentration or if fertilizing leads to phytoplankton blooms, a 6-week experiment was conducted with 18 enclosures in Nanwan Reservoir, central China. Treatments were three levels of P concentration (low: 45 μg/L; medium: 90 μg/L; high: 180 μg/L) with or without silver carp stocking (10 g/m3). The effect of silver carp stocking on phytoplankton community changed with P fertilization in the water column. No significant fish effect was observed on total phytoplankton biomass at the low P level, while the chlorophyll a concentration, total phytoplankton biomass, and biomass of phytoplankton > 30 μm were enhanced significantly by silver carp at the medium P level. In addition, at the medium P level, the biomass of Bacillariophyta was significantly greater, whereas the biomass of Chlorophyta was significantly lower in the enclosures with silver carp stocking. Potentially, silver carp enhanced the total phytoplankton biomass by nutrient recycling. Zooplankton had a weak grazing efficiency and had limited effects on control of phytoplankton biomass at the three P levels. The 6-week experiment confirmed that P fertilization could consistently cause greater fish productivity with phytoplankton biomass increasing at the same time. We therefore propose a mechanism for the observed relationship between silver carp stocking and phytoplankton. Under the condition of ambient fish density (10 g/m3), silver carp could not control phytoplankton biomass with phosphorus fertilization; instead, the fish may have promoted phytoplankton growth via nutrient recycling. Silver carp seem to have only a weak effect on the phytoplankton community without P fertilization. Therefore, from the viewpoint of phytoplankton regulation, P fertilization should be considered as a cautious operation for water protection and reasonable fish enhancement.