Effects Of Nutrient Enrichment And Fish Stocking On Zooplankton Communities

In southern China, fish is usually stocked in early winter when here exists increasing external nutrients, we performed two enclosure experiments in the large deep impoundment, Liuxihe reservoir to know the effects of the fish stocking on zooplankton communities.In the first experiment which was carried out from December 2009 to January, phosphorus was added into the enclosures to modify the external input.The experiment comprised two parts, i.e. one phosphorus adding without fish stocking treatments and two phosphorus adding with fish stocking treatment, and one control with neither fish stocking nor nutrient adding.The abundance and biomass of the zooplankton increased significantly in the phosphorus adding treatments without fish. The abundance and biomass of the Daphnia increased in the treatment.Fish predation had little influence on the total abundance of the zooplankton but obvious effect on the biomass in the treatment of phosphorus adding. In the absence of fish the phosphorus adding triggered a increase of the abundance of Daphnia galeata. In the presence of fish, the abundance and biomass of cladocera especially Daphnia galeata and Leptodora kinditt decreased. The abundance of rotifers did not show a obviously varying trent changes in the phosphorus adding treatments without fish, but increased in the treatments with fish stocking.In the second experiment which was carried out from November 2010 to January 2011, silver carps, bighead carps and tilapia were added solely or in mixture to detect their effects on the crustacean community.In all the fish treatments, there was lacking of large size species in cladocera.,Bosmina fatalis was the dominant species of the Cladocera, the abundance and biomass of the Cladocera decreased, but there was no significant difference between three fish treatments.In all the fish treatments, the abundance and biomass of the copepods decreased, but the biomass of copepods in the silver carp treatment was higher than the silver carp-bighead carp treatment and silver carp-bighead carp-tilapia treatments. the high abundance of the Phyllodiaptomus tunguidus was observed in the silver carp treatment. However, there was no significant difference between these three fish treatments.