Water Type And Phosphorus Forms Of Sediments In Dishui Lake Basin

Dishui Lake, which is located in the southeast corner of Lingang New City of Shanghai, is the largest artificial lake on the tide beach. The lake is round, with a total area of 5.56 km2 and average depth of 3.7m. As the core of water scheduling and the heart of the landscape in Lingang New City, it was dug in June 2002, and water was reserved since October 2003. According to the survey, Dishui Lake is in eutrophication state now and the water quality of lake may directly affect the development of local environment protection and economy.Detailed basic research about the lake water quality was required. Our study investigated the major ions (HCO₃^-,SO₄^2-,Cl^-,Ca²⁺,Mg²⁺,K⁺,Na⁺) of Dishui Lake every two weeks from January to December in 2009, in order to prove the water type of Dishui Lake and temporal-spatial distribution of major ions. And then from January to December in 2010, 15 water quality indicators were measured every two weeks from 11 sampling points in Dazhi River, Rivers outside waterlock, Rivers inside waterlock and Dishui Lake. 15 water quality indicators included water temperature, pH, conductivity, dissolved oxygen (DO), transparency (SD), total nitrogen (TN), total phosphorus (TP), ammonia (NH3-N), nitrate (NO3-N), nitrite (NO2-N), total dissolved phosphorus (TDP), reactive phosphorus (AP), alkaline potassium permanganate index (CODMn), BOD5 and chlorophyll a (Chl. a). Through this monitoring we would get the quality status of entire water system of Dishui Lake and find the limiting factor for the algae growth in Dishui Lake basin. Sediments from 9 sampling points in River outside waterlock, River inside waterlock and Dishui Lake were collected every four weeks in 2010. Using the improved Ruttenberg method by Li Yue, we measured the contents of various phosphorus forms in the sediments, and got the status of internal phosphorus pollution in Dishui Lake basin accurately. The results were as follows:(1) The average total ion content of the lake was 1.82±0.22 g/L and the water of Dishui Lake was brackish.The major cation and anion were Cl- and Na+ respectively. The water type was according to Alekin classification. The total ion content decreased gradually during 2009, and the ion composition of the lake changed significantly with the proportion of Cl^- and Mg²⁺ decreasing and HCO₃^-, SO₄^2-, Ca²⁺ increasing. Significant difference laid in the total ion contents among different sampling sites. The area around water sports club had the highest total ion contents while the lowest site was South Island district. As to the ion composition, no significant difference was observed among five sampling sites.(2) In 2010, There were significantly difference among the water quality of Dazhi River, Rivers outside waterlock, Rivers inside waterlock and Dishui Lake. In Dazhi River and Rivers outside waterlock, conductivity,dissolved oxygen of water were relatively high,and various indicators of nitrogen and phosphorus were significantly higher than Rivers inside waterlock and Dishui Lake. In Rivers inside waterlock and Dishui Lake ,the pH of the water were slightly alkaline, and conductivity, dissolved oxygen was relatively low,and various indicators of nitrogen and phosphorus were significantly lower than Dazhi River and Rivers outside waterlock.(3) In 2010, the atom number ratio of nitrogen and phosphorus in Dazhi River, Rivers outside waterlock, Rivers inside waterlock and Dishui Lake were 63.85±29.92, 90.01±65.05, 71.70±38.50 and 65.53±45.76 respectively. The N:P ratio in all sampling sites were higher than 20:1, so the whole lake basin belonged to phosphorus limitation.(4) In 2010,all kinds of phosphorus in the 9 sampling sites was fluctuating during the year, and did not show significantly increasing or decreasing. Total phosphorus content of Rivers outside waterlock were 2395.64±829.61 mg/kg, Total phosphorus content of Dishui Lake and Rivers inside waterlock were 539.39±5.69 mg/kg and 767.08±177.65 mg/kg. Total phosphorus contents of Rivers outside waterlock were significantly higher than Dishui Lake and Rivers inside waterlock.(5) There were significantly difference among the phosphorus forms of Rivers outside waterlock, Rivers inside waterlock and Dishui Lake.The easy exchanged phosphorus (Ex-P), aluminum bound phosphorus (Al-P), iron bound phosphorus (Fe-P) in the proportion of total phosphorus from Rivers outside waterlock were 7.99%±3.48%, 8.17%±1.54% and 47.37%±10.61% respectively, which were significantly higher than that from Rivers inside waterlock (6.50%±2.51%, 2.87%±1.56%, 17.76%±6.52%, respectively) and Dishui Lake (1.75%±0.14%, 0.93%±0.10%, 2.63%±0.14%, respectively). The De-P and Ca-P in the proportion of total phosphorus from Rivers outside waterlock were 11.70%±3.93% and 5.50%±2.67% respectively, which were significantly lower than that from Rivers inside waterlock (25.88 %±1.75%, 31.46%±11.67%, respectively) and Dishui Lake (24.26%±1.60%, 62.65%±1.67%, respectively).(6) There were significant positive correlation between the total phosphorus, total dissolved phosphorus, reactive phosphorus in water and the proportions of easily exchangeable P (Ex-P), aluminum bound phosphorus (Al-P), iron bound phosphorus (Fe-P), occluded phosphorus (Oc-P) to the total phosphorus in sediments.There were significant negative correlation between the total phosphorus, total dissolved phosphorus, reactive phosphorus in water and the proportions of De-P,Ca-P to the total phosphorus in sediments. The proportions of Organic Phosphorus (Or-P) to the total phosphorus in sediments were not significantly related with the total phosphorus, total dissolved phosphorus, or reactive phosphorus in water.(7) Through the cluster analysis about phosphorus content in Dishui Lake basin and in lakes in the lower reaches of the Yangtze River, we concluded that the phosphorus composition in Dishui Lake and River inside waterlock was very special, and different from the other lakes in the lower reaches of the Yangtze River. However, the composition of phosphorus forms in sediments of Rivers outside waterlock were similar with the other eutrophic lakes such as Taihu Lake (Wuli Lake) and Chaohu Lake (Northwest).