Study On The Trophic Level In The Dishui Lake And The Mutlianalysis Between Chlorophyll-a And Other Environmental Factors

Dishui Lake, located in Lingang New City of Shanghai, is the largest artificial lake excavated on the unhardened beach. It began to be watered in October 2003 after 16 months of excavation since June 2002. It is a round lake with the diameter of 2.66km and area of 5.56km2. The average water depth is 3.7m, while the deepest is 6.2m. The water from Huangpu River flows into Dazhi River firstly, then goes through the channel, and finally flows into the lake. As a major construction of artifical landscape lake, Dishui Lake is the heart of Lingang New City river system. It played not only an important role in flood control and water exchanging, but also has invaluably influence on creating urban landscape ecology and optimizing microclimate in Lingang New City.The research was carried out from January 2009 to December 2010 in Dishui Lake and its basin. We investigated the basic situation of water quality and the spatial and temporal distribution characteristics of nitrogen, phosphor and chlorophyll a. At the same time, the trophic level in the Dishui Lake and mutlianalysis between chlorophyll a and other environmental factors were analyed too. Methods and results were as follows:1. According to the"water quality—Guidance on sampling techniques from lakes, natural and man-made"( GB/T 14581-93 ) and actual situation, 16 sampling stations were chosen among Channels, Mixed areas and Lake in 2009. While 11 sampling stations were set among Dazhi River, Channel before the intake sluice, Channel after the intake sluice and Dshui Lake in 2010. The investigations were carried out in the morning every 2 weeks. Thirteen water quality parameters were analyzed which included transparency (SD, m), pH, water temperature (T,℃), dissolved oxygen (DO, mg/L)suspendid substance (SS, mg/L), chemical oxyen demand (COD_Mn, mg/L), nitrite nitrogen(NO₂-N, mg/L), nitrate nitrogen(NO₃-N, mg/L), ammo-nionitro(NH₃-N, mg/L), total nitrogen(TN, mg/L), activated phosphate(PO₄-P, mg/L), total phosphorus (TP, mg/L) and Chlorophyll-a (Chl.a, mg/m³), Comprehensive nutritional statue index (TLI) were calculated for the assessment of eutrophication. SPSS15.0 was used in mutlianalysis between chlorophyll a and other environmental factors.2. The minor fluctuation of pH was in normal range of natural waters(6～9)in 2009. The annual average water temperature in Channels, Mixed areas and Lake were 19.9±8.2℃, 19.7±8.2℃and 19.6±8.1℃respectively. In 2009, Transparency of all the three areas was less than 0.7m. It was higher in winter and early spring than in summer and dropped to less than 0.4m since late April 2009, and rised again until late November. The annual averages of SS were 30.90 mg/L, 29.61 mg/L and 33.18 mg/L. It was lower in winter and early spring than in summer. In all the three areas, the relationship between SD and SS was negative. The content of DO was high in Dishui Lake, which belonged to water quality level I or II. A certain regularity was showed that DO was generally higher in spring and winner than in summer and autumn. In addition, the COD_Mn of the lake was low, going along withⅡ～Ⅲwater quality level.3. In 2009, the maximum of TN appeared in March in all the three areas, which was even worse than water quality level V. TN ranged from 0.80～2.00mg/L and 60% of TN data belonged to level IV～V, and 10% were worse than level V. 65% of the nitrogen forms were TIN which was the main form of nitrogen. NO₃-N had the highest share of TIN, and then NH₃-N. Channels and Mixed areas had medium self-purification, and lower in Lake. There were extremely significant differences of TN among Channels, Mixed areas and Lake.4. TP was under 0.10mg/L in January, February and December in 2009. It became to relatively stable in April to July, and the concentration was about 0.10mg/L. The highest peak occurred in middle March, while the second peak appeared in middle November. TP changed from 0.043mg/L to 0.271mg/L and 65% of the data belonged to water quality levelⅣ～Ⅴ, 2% worse than level V. No significant differences of TP among the three areas were observed.5. The concentration of Chlorophyll a ranged from 0.5 to 139.5 mg/m³ with the annual average value higher than 50mg/m³.The highest concentration occurred in May which was about 110 mg/m³. There were no significant differences among the different areas too. 6. According to the two-year research, Dishui Lake was mainly in Light eutropher. Compared to 2009, it was better in 2010 with the declination of TP and Chlorophyll a The average value of TLI(TP) dropped about 25%, and the maximum and minimum dropped 17% and 32%. The TLI(Chl.a) dropped about 10% too.7. The relationship between chlorophyll a and environmental factors was complex. In different sampling sites, the important environmental factors affecting chlorophyll a were different. Overall, the relationship between chlorophyll a and SD, NO₂-N was positive, and that between chlorophyll a and T, NH₃-N (only in Quay NO.1) was negative. It was different in different sample stations that between chlorophyll a and NO₃-N. There were no significant differences among chlorophyll a and pH, DO (under the control of T), COD_Mn, TN and TP. Meanwhile, different factors were chosen for the multiple stepwise regressions equations in different sample stations. T, DO, NH₃-N and NO₃-N were chosen more than once.8. The relationship between lg(Y_Chl.a) and lg(X_TP) was positive, while that between lg(Y_Chl.a) and lg(X_TN/TP) was negative. The equations obtained were lg(Y_Chl.a)= 2 .146+0 .5301lg(X_TP) and lg(Y_Chl.a)=2.017-0.2751lg(X_TN/TP), which weremore practicable than the multiple stepwise regression equations.