| Three Gorges Reservoir (TGR) of the Yangtze River is currently the largest water conservancy and hydropower project all over the world. After its completion, the Three Gorges Reservoir provided huge integrated benefit, such as flood control, power generation, water supply, irrigation and improving the ship transportation etc. On the other hand, the building of the Three Gorges reservoir changes the natural features of rivers artificially, and puts forward a new and serious test to protection of ecological environment of the reservoir area. The city of Chongqing, which locates at the upstream of TGR is populous, and due to some historical reasons, infrastructure in environment is behindhand. Huge quantities of unprocessed industrial wastewater, sewage from cities, pollutants from ships and surface runoff from large areas of farmlands are poured into the Yangtze River and its secondary rivers. Water flow in the main river and tributaries has been slowing down gradually especially after the impoundment at the designed water level since 1997. In addition to the long length of backwater areas, eutrophication has arisen obviously in some secondary rivers since 2003 and the situation tends to get worse. There are many studies aiming at eutrophication in one specific tributary or a few ones at home and abroad, but few are about some important rivers combing with their tributaries on a large scale. This paper elicits a comprehensive evaluation about nutritional status of water in the secondary rivers based on the studies of the nutritional condition about the 32 warehousing rivers of the TGR basin from 2002 to 2009. Analysis of the disposal capacity of waste and sewage of the exiting facilities as well as the actual emission are combined and the overall trend of eutrophication in the reservoir area is predicted. It can be found that the algal bloom in the tributaries of the reservoir areas is seasonal, and eutrophication in the various sub-rivers is getting worse and worse. Thus, after the 175m above sea level water strorage level in TGR, the risk of eutrophication of water in some areas particularly in bay area of will be further increase. Study of prevention and control of such hazardous phenomenon as algal bloom is an imminent issue.Based on the previous studies, given that algal bloom breaks out frequently in Pengxi River, a first-class sub-river of TGR area and the outbreaks in large areas has seriously affected the ecological environment, this author of this paper monitored the water quality of backwater area of Pengxi River continuously form 2007 and 2009. The "Environmental Quality of Surface Water Standards (GB3838-2002)" and the Carson Index are employed to asses eutrophication of water in Pengxi River. The results are as the following:Chlorophyll a (Chla) ranges from 13.870 to 98.550μg.L-1; the density of Total Phosphorus (TP) ranges among 0.021～0.118mg.L-1;the density of total nitrogen (TN) ranges from 0.913 to 2.023mg.L-1; SD was in range of 0.4 to 2.5m; the water temperature was in the range of 8.5～31℃; Chla and TN, TP shows significantly positive correlation (P<0.01), but Chla and SD had significantly negative correlation (P<0.01); TN and SD shows significantly negative correlation (P<0.01); TP and SD also shows negative correlation (P <0.01). Chla has more correlation with TP (p= 0.831), but transparency has more negative correlation with TP (p=-0.584); it is obvious that the TP index had close relation with other indexes, and it can be inferred that phosphorus is the major limiting factor to restrict water eutrophication in backwater area of Pengxi River.The three sampling sites from upstream to downstream are Quma Ferry, Lake-like Gaoyang and Shuangjiang Bridge in turn. Eutrophication has been increasing from Quma Ferry downstream to Shuangjiang Bridge. It can be predicted that in suitable environmental condition algal bloom in backwater area of Pengxi River is likely to break out again seriously. While determining the main limiting factors to restrict outbreak of eutrophication in Pengxi River, it can be concluded that the limiting factors of eutrophication are different in different years or even in the same year but in different sampling sites. What is sure is that nitrogen is not the limiting factor of eutrophication and phosphorus in specific years and specific areas can be a limiting factor for phytoplankton growth in Pengxi River. It is shown through research that although the water flow gets slow after sluice of the TGR, it still possesses some features as a river. It is not accurate to apply the determining methods of limiting nutrients of lakes to rivers. Determining methods of limiting factors of eutrophication in rivers remain for further study.Finally, to test the influence of temperature on the growth of phytoplankton in Pengxi River, the author of this paper selected Gaoyang section which has the most serious eutrophication in recent years as a sampling site on April 9,2009 and simulaed in laboratory to explore the correlation between temperature and the growth of algae under different temperatures, TN and in Pengxi River. The result is as the following:algae can grow in water when the temperature ranges from 5 to 30℃, and especially at 20℃algae grows most vigorously. The growth of algae can absorb and remove some of nitrogen and phosphorus. After the experiment, the density of TN and TP decreased by 61.4% and 50.6% respectively comparing with those in initial water samples. At the same time, algal biomass (which can be indicated by Chlorphyll a) showed some correlation between temperature, but the correlation was not significant(P=0.651), which indicates that temperature has influence on the growth of algae and outbreak of algal bloom in water of Pengxi River but it is not the decisive factor. Thus, the main factor for the outbreak of algal bloom in Penxi River may be nutrient, light or velocity or interaction of these factors, which remains to be further studied. |
PDF Full Text Request