Limnological changes in a pond ecosystem caused by grass carp (Ctenopharyngodon idella Val.) low stocking density

Grass carp (Ctenopharyngodon idella Val.) stocked (29 kg/ha) in a small pond reduced the biomass of aquatic macrophytes from 109 g/m2 to 33 g/m2 during one growing season. The only changes in hydrochemical parameters (pH, alkalinity, acidity, BOD5, CODMn, NH4-N, NO2-N, NO3-N, TN, PO4-P and TP) associated with the grass carp stocking were a decrease in pH (from 8.43 to 7.57) and in NO3-N concentration (from 0.99 mg/l to 0.56 mg/l). The increases in organic matter content and NO3-N concentration in the surface sediment layer were higher in the control pond than in the pond stocked with grass carp. No changes were detected in the other parameters (NH4-N, PO4-P and TP) in the upper sediment layer and between all parameters measured in the lower inorganic layer. The grass carp grazing had no impact on phytoplankton biomass (concentration of chlorophyll-a) or species composition. There were no changes either in the abundance or in the species composition of zooplankton and zoobenthos induced by grass carp. Statistically significant indirect changes (in water and sediment chemistry) following the grass carp stocking were connected especially with a reduction in the biomass of the filamentous alga (Cladophora globulina) or rather with its maintenance in the control pond.