The spatial and temporal changes of nutrients of net-pen aquaculture area in Yangcheng Lake and its water quality evaluation
阳澄湖网围养殖区水体营养盐的时空变化与水质评价

Beginning in May 2006 and ending in May 2009, surface and bottom water samples were collected in net-pen culture area in the middle of Yangcheng Lake, and were analyzed for water quality. The trophic state index (TSI) values were calculated and adopted to comprehensively evaluate the trophic levels in the net-pen culture area. The results showed that 1) the nutrient concentrations in net-pen culture area exhibited obvious spatial and temporal variations, all the measured parameters, except for the concentration of NO3--N, were higher in culture period than in other periods, meanwhile, they were also higher in high-density culture area than in low-density area, and in bottom water than in surface water, which might reflect that massive nitrogen and phosphorus sediment formed due to high-density culture; 2) the trophic status increased to a high level in high-density culture area, and also increased to a moderate level in low-density area; and 3) water-quality monitoring data from the same period in May 2008 and in May 2009 indicated that no obvious change of water-quality in the lake was found in the single culture period. It suggested that short-term aquaculture had less effect on the water quality, while long-term aquaculture could bring up obvious effects. The eutrophication in net-pen culture area had become more and more serious, and the culture area was currently considered to attend nearly-severe eutrophic level, and phosphorus pollution posed more threat to the lake than nitrogen pollution did.