Study On The Water Quality Change Rule And Eutrophication Of A Stratified Reservoir In North China

Zhoucun Reservoir,located in the northeast of Zaozhuang,is an important drinking water source for Zaozhuang City.For now,the Zhoucun Reservoir still experienced water quality pollution and eutrophication.In this study,field surveys were conducted from 2012 to 2015.Temporal and vertical variations of the physical,chemical and phytoplankton indexes were investigated during monitoring.Combined with the hydrology and climate conditions,seasonal evolution of the water quality and eutrophication of the Zhoucun Reservoir was studied,and the combined effects of inflow rivers,thermal stratification and the El Ni?o induced drought on the water quality was analyzed.The main results are as follows:(1)The inflow rivers had a significant influence on the spatial distribution of water quality and algae in the Zhoucun Reservoir.There were serious nutrient pollution in the three inflow rivers.The average total nitrogen concentration of the three inflow rivers was 3.14~12.48 mg/L and the primary nitrogen species was nitrate.The average total phosphorus concentration was 0.024~1.630 mg/L.The conductivity,total phosphorus,nitrate,ammonia and total phosphorus near the entrance of the inflow rivers was higher than that in the lacustrine zone of the reservoir.Due to the high nutrient concentration,the algae abundance near the entrance of Xiashi River and Xuwa River was higher than the other area of the reservoir.The undercurrent drived the concentration of total nitrogen and nitrate at the depth of 5 m increased dramatically during the summer of the standard years.(2)The thermal stratification had a significant influence on the water quality of the Zhoucun Reservoir.The Zhoucun Reservoir,which was stratified from April to October and mixed from November to the next March,belonged to the holomictic reservoirs.The oxycline appeared with the straitification in the Zhoucun Reservoir.The hypolimnion turned into anaerobic condition in the mid-May each year due to the hypolimnion oxygen consumption and sediment oxygen consumption.The hypolimnion water was polluted seriously during stratification.The ammonia,total phosphorus,manganese and sulfide exceeded surface water Class III water standard 1.9 times,2.2 times,8.9 times and 5.0 times,expectively.The stratification also promoted the algae growth.From May to August,the large amounts of algae in the Zhoucun Reservoir made the depth of the light compensation point was only about 1-3m.When the wind power was small,there was relatively high gradient of temperature and oxygen,which made part of the epilimnion(<3m)easily turn into anaerobic condition.(3)The El Ni?o event facilitated the water quality deterioration of the reservoir.During the El Ni?o year,the precipitation and inflow volumes both decreased.The water level decreased continuously and so was the volume of hypolimnion.The concentrations of ammonia,total phosphorus,iron,manages and sulfide in the hypolimnion in the El Ni?o year were significantly higher than that in the standard years.The continuously decrased in the water level made both the upper and lower metalimnion limit dropped down,shoten the duration of fall overturn and increased the pollutants concentration(such as managanese and total phosphorus)in the metalimnion.All the above promoted the water quality deterioration during the overturn in the El Ni?o year.The manages and total phosphorus in the epilimnion bothe exceeded the water quality standard,reaching 0.222 mg/L and 0.122 mg/L,respectively.(4)The algae in the Zhoucun Reservoir experienced different seasonal succession rules under different eutrophic states.The Zhoucun Reservoir reached eutrophic level from June to November and belonged to mesotrophic waters from last December to May during the standard years.The microcystis bloom occurred in the summer of the standard years and the maximum algal abundance was 1.9×108 cells/L.During the El Ni?o years,the Zhoucun belonged to mesotrophic waters in the whole year.The green algae was dominated during the summer and the algal bloom did not occurr.The results of RDA and the general linear model indicate that water temperature,Schmidt index and total phosphorus were the main environmental factors that influenced the phytoplankton community structure,while the total phosphorus was the primary factor for the algale abundance in the standard years.In contrast,water temperature,Schmidt index and nitrate was the main factors for algal community structure and water temperature was the critical factor for the algal abundance.