Abstract:

The physicochemical characteristics of water samples collected from 20 sampling sites in the Hongze Lake were periodically monitored in 2010.Modified Carsons trophic state index (TSIm) was used to evaluate the nutritional status of water quality.Spatialtemporal distribution variation characteristics of water quality were analyzed by using principal component analysis (PCA) and selforganizing feature map (SOM),and the spatialtemporal differences were discussed.The results indicated that the water physicochemical parameters had significant seasonal variations except water depth (WD) and secchi depth (SD).Total phosphorus (TP) and permanganate index (CODMn) were the highest in winter,while nitrogen content was the highest in spring.The water quality was mainly influenced by the ionic and nitride concentration,and the total phosphor was not the dominating control factor.The results from TSI indicated that water quality was in the state of eutrophication at the four seasons and was the most severe in summer.The comprehensive evaluated scores based upon PCA were 0178,0404,-0651 and 0071 in spring,summer,autumn and winter,respectively,which also indicated that water quality in the Hongze Lake was the best in autumn and the worst in summer.In 2010,TP declined significantly and TN scarcely differed from that in 1989.However,the contribution of inorganic nitrogen such as ammonia nitrogen (NH+4N) and nitratenitrogen (NO-3N) to TN increased,which might be relevant to mass use of fertilizer and feed in the process of continuous development of planting,aquaculture and livestock industry in the watershed of the Hongze Lake.The results of spatial characteristics of water quality from SOM and hierarchical cluster analysis (HCA) had some differences,but most of the results were consistent with each other.Compared with the HCA,the SOM results were further accord with the actual situation and were able to more clearly and accurately reflect the spatial characteristics of water quality.In addition,SOM model diagram had strong visibility,which could intuitively reflect the differences of each water parameter at different sampling sites.Thus,the method can be applied to water quality evaluation research.The SOM results showed that 20 sampling sites were divided into three categories,G1,G2 and G3.G1 represented the relatively closed north part of the Hongze Lake with some submersed macrophytes distribution,in which the water quality was mainly affected by the pollution of blockedweb aquaculture.G2 represented the east and south part where there were busy shipping and the water of the Huaihe River directly flows into,in which the water quality was mainly influenced by the river water and shipping.G3 represented the west part of the Hongze Lake where the ponds of aquaculture closely distributed and lots of people lived along the coast,in which the water quality was affected mainly by the waste water of aquaculture and domestic sewage.It is suggested that the control of external pollution and submersed macrophytes recover are the effective measures to ease and solve the water eutrophication of the Hongze Lake