The Impacts Of Characteristic Pollution Sources On The Water Quality And Plankton Communities Of Inflowing Rivers In Dianchi Lake Basin

In this paper,characteristic pollution sources of inflowing rivers in Dianchi Lake Basin are extracted and verified by using the statistical methods of cluster analysis and factor analysis,as well as field investigation and laboratory experiment.Based on the evaluation of pollution degree through factor analysis,the impacts of different characteristic pollution sources on the water quality and plankton communities of different inflowing rivers are further studied.The main conclusions are as follows:?1?In terms of all the seven inflowing rivers in the north or east of Dianchi Lake Basin,domestic sewage is the most important pollution source,and it is indicated by biochemical oxygen demand?BOD₅?and chemical oxygen demand(COD_Cr);sewage treatment plant?STP?effluent that fails to meet the discharge standard is the second important pollution source,and it is indicated by ammonia nitrogen?NH₃-N?;STP effluent that meets the discharge standard is indicated by nitrate nitrogen?NO₃-N?;surface washing&runoff is indicated by suspended solid?SS?;water self-purification is indicated by pH and dissolved oxygen?DO?;incomplete denitrification is indicated by nitrite nitrogen?NO₂-N?.?2?In terms of the inflowing rivers in the south of Dianchi Lake Basin such as Chai River,farmland runoff is the most important pollution source,and it is indicated by NO₃-N;domestic sewage is the second important pollution source,and it is indicated by BOD₅ and COD_Cr;surface washing&runoff is indicated by SS and total phosphorus?TP?;water self-purification is indicated by DO and pH.?3?From the perspective of water quality,domestic sewage pollution is the main factor that causes the significant increase of total density of phytoplankton communities?p<0.01?,the domiance of cyanophyta species and the signicant decrease of species diversity?p<0.01?,and it can also make protozoa thrive in the river water but simplify their population structure.?4?By increasing the concentration of NO₃-N,STP efluent that meets the discharge standard can increase the proportion of bacillariophyta species?diatoms?and decrease the dominance of cyanophyta species,and therefore significantly increase the species diversity of phytoplankton communities in the inflowing rivers?p<0.01?.However,the effects of STP efluent that meets the discharge standard mainly occurr in the dry seasons?i.e.winter and spring?;in the rainy seasons?i.e.summer and autumn?,phytoplankton communities are still decided by the pollution of domestic sewage and STP efluent that fails to meet the discharge standard.When the concentration of NO₃-N is very low but the organic matter and NH₃-N are relatively high,cyanophyta species will become dominant in the river.As to the zooplankton communities,when the concentration of NO₃-N is above 10 mg/L,the density of cladocerans will decrease substantially,and it may be related to the lack of food sources with the shift of phytoplankton communties induced by high concentration of NO₃-N in addition to the possible direct inhibiting effect of high concentration of NO₃-N.?5?The density of rotifers has a good linear relationship with the numbers of zooplankton species in the inflowing rivers?R=0.828,p<0.01?,and when the rotifers increase by about 20 ind./L in the range of 0 to 800ind./L,the total number of zooplankton species can increase by one.When the number of zooplankton species is over 30,the phytoplankton density will be lower than 10⁷ cells/L.This indicates that rotifers may play a key role on the ecological balance of the river ecosystem in Dianchi Lake Basin.The analysis of zooplankton monitoring samples in summer further shows that flood water from the upstream regions can significantly increase the density of rotifers?R=0.554,p<0.01?.In light of this,the interception of the upstream river water can be an important reason that the number of zooplankton species in the urban rivers is obviously lower than that in the suburban rivers.?6?In the greenhouse farmland area of Dianch Lake basin,NO₃-N is the main pollutant in the farmland runoff,and its influences on the phytoplankton and zooplankton communities are very similar to the STP effluent that meets the discharge standard.However,when the stormwater makes the concentration of NO₃-N very high,it will have an impact on the survival and reproduction of protozoa in the river.This paper proves that constructing a complete separate sewage system,ensuring that STP effluent meets the discharge standard and the rivers have an acceptable concentration of NO₃-N,and increasing the connectivity of river courses with its upper and lower reaches as well as the tributaries are the three most important measures to solve the water pollution problems of the inflowing rivers of Dianch Lake and further restore the river ecosystems.