Effects of nitrogen and phosphorus from fish cage-culture on the communities of a shallow lake in middle Yangtze River basin of China

In recent decades, net-cage aquaculture has become one of the main patterns of the intensive fish-culture in the lakes, reservoirs and even rivers in China. This aquaculture pattern results in enriching exogenous nutrients in water and, consequently, accelerates the process of lake eutrophication. To ensure that normal environmental conditions and fisheries in a lake remain sustainable, qualitative estimations of nutrients in relation to ecosystem changes are essential. A study, mainly on nitrogen (N) and phosphorus (P) influences due to cage fish-culture was carried out in a shallow 35.5 ha bay in Niushanhu Lake, a shallow lake located in middle Yangtze Basin, during the period from March to December 2000. Net-cages in total covered an area of 1000 m2 and the annual fish yield was 16.0 metric tons (MT). Fish feeding residue entering the water during the period was equivalent to 1532.9 kg of total N and 339.2 kg of total P. Sampling and analyses of the total N and total P concentrations, diversity and biomass of plankton and Chl a were made monthly, while data on zoobenthos were collected twice, respectively, at the beginning and the end of the study. Results showed that the Chl a content in water was correlated negatively to distance from the cage. The Chl a content that is converted into wet biomass of phytoplankton may be expressed by the regression: B=2.673-0.0016D (B, biomass in mg/l; D, distance in km; r=0.9362; n=7). The biomass of rotifers inside or near the cages was higher than that in areas more distant, while that of the cladocerans was the opposite. No significant difference of copepod density or biomass was detected between cages and open water. Changes of zoobenthic community were remarkable. At the beginning of fish farming, there were nine zoobenthic taxa inside and 13 outside the cages. Only two saprophilous taxa, chiefly oligochaetes, were present in the cages at the end of the culture. Density and biomass of benthic animals decreased as well. Several bio-indices, such as Shannon-Wiener index, Simpson index, and Margelef index, also exhibited a declining tendency. Through this study, the authors are of the opinion that mass-input of exogenous nutrients may cause negative effects on water quality in areas from the cage to a distance of 50 m outwards.