THE ROLE OF SILVER CARP AND BIGHEAD IN THE CYCLING OF NITROGEN AND PHOSPHORUS IN THE EAST LAKE ECOSYSTEM

Through the metabolic processes after feeding, some of the nutrients from the food of the silver carp and bighead become part of the fish body, and some return to the water body as faeces and excreta, joining the recycling of substances in the ecosystem. Therefore, the processes of feeding, metabolism and defaecation in the fish are accompanied by a series of feedbacks. This study investigated the excretion rate of nitrogen and phosphorus by the silver carp and bighead during starvation and the rate of release of nitrogen and phosphorus from the fish faeces and Microcystis in waters with and without fish. From these results, the role of the two fish species in the cycling of nitrogen and phosphorus in the East Lake ecosystem can be assessed, using parameters from some other studies for calculation. (1) The feeding processes of the silver carp and bighead accelerated the release of nitrogen and phosphorus from the algae bloom. The rates of release of nitrogen and phosphorus in the water with fish were 1.88 and 1.41 times respectively the rates in the water without fish. However, the amounts of nitrogen and phosphorus released did not significantly influence the dynamics of the production from the algae bloom in the East Lake. The amount of nitrogen released from faeces and excreta was 11.45% of the total nitrogen, and that of phosphorus released was 3.4% of the total phosphorus in the water body. (2) The feeding process of the silver carp and bighead does increase the utilization of the aquatic production, but, on the other hand, it takes large quantities of nitrogen (52.20 tons) and phosphorus (11.36 tons) out of the water, which were 3.01% and 5.28% of the total nitrogen and phosphorus respectively in the seston in the water body. (3) The intensive feeding of zooplankton by the two species decreased the densities of the population, and shortened the turnover period of the prey populations- The production by the prey populations in 1982 was 1.05 times that in 1981, while the number of silver carp and bighead stocked in 1982 was 7 times that in 1981. In these two years, food consumption by these two fish species took 31.73% and 0.63% respectively of the total production by the prey population; the P/B coefficient of the prey populations was 67.92 and 48.01 respectively. Thus, the relative stability of the zooplankton also stablized the phytoplankton populations. From the three points mentioned above, we think that, since the 1970s the fisheries in the East Lake have not only supplied fishery products and increased the economic efficiency, but also slowed down the process of eutrophication of the East Lake. The fisheries have a role of regulation on the ecological balance of the East Lake.