| As one of China’s five largest freshwater lakes,Chaohu Lake is an important water resource for daily life and industries in the surrounding areas.Pollutions in it can pose a significant threat to human health,biological safety,and economic development.Heavy metal pollution and cyanobacterial bloom are two major environmental problems currently existing in Chaohu and urgently need effective treatment.This study aims to get a further understanding of the temporal and spatial distribution of the two types of pollutions in Chaohu Lake,and figure out the possible relations between them.In this research,formulas have been established to calculate and predict the distribution of heavy metals in Chaohu Lake during the algal bloom period.This study may contribute to the more accurate implementation of pollution prevention and control work in Chaohu Lake.In this study,five times of sampling have been conducted in Chaohu Lake to obtain the spatio-temporal distribution of heavy metal concentrations in the water.The heavy metal data in sediments from early 2015 to 2017 has been collected to analyze the sediment contamination in Chaohu Lake.When it comes to the distribution of cyanobacteria blooms,the ELCOM-CAEDYM model has been established to simulate and analyse its response to meteorological factors.Then,the data of heavy metals,chlorophyll a and turbidity in the water have been combined to analyze the short-term relationship between heavy metals and cyanobacteria blooms.The data of heavy metals in sediment during 2015-2017 and satellite remote sensing images of cyanobacterial blooms during 2014-2017 have been analyzed together to figure out the macroscopic relationship between the deposition of heavy metals in the lake and the spatio-temporal distribution of the outbreaking frequency of cyanobacteria blooms.Based on these analyses,the quantitative relationship between heavy metals and algal bloom-related factors during the algal bloom period has been established.Furthermore,this relationship together with the model results has been used to calculate the distribution of heavy metals in the whole lake.The main findings are as follows:(1)According to Surface Water Environmental Quality Standard(GB2828-2002),from December 2016 to October 2017,the surface water in Chaohu Lake was generally at Level Ⅱ,with little heavy metal pollution,and only Ni,Ti,Cr exceeded the standard.The time series of heavy metals in Chaohu Lake generally showed an upward trend in 2017.Only in September,the concentration of heavy metals in the lake decreased,which might be related to the water transfer from July to August.In terms of spatial distribution,heavy metals were mainly distributed in mouth of Nanfei River,the south coast of central lake,and the southeast coast of the eastern lake.The heavy metals in the lake mainly came from the rivers nearby.The center of the western lake was also an important source of heavy metal pollution for other sites,but it was hard to affect the water sources in the eastern lake.(2)During 2015-2017,Zn,Pb,and Cd were at higher pollution levels in the sediments of Chaohu Lake.There were also mild to moderate Ni and Hg pollutions.Considering interannual viaration,the heavy metal pollution in January 2016 was the worst,followed by April 2015.In terms of spatial distribution,the central area was the cleanest while western part was the worst.Highly polluted regions were mainly located in western district,particularly the mouth of Nanfei River,and Tongyang River,Zhegao River in eastern district.Severe contamination only occasionally occurred in central lake.Bakou,from where drinking water once was taken,experienced excessive Ni,Hg,and As pollution.(3)The simulation results show that the meteorological factors have an important influence on cyanobacterial blooms.Into the fall,the average temperatures of Chaohu Lake maintain at 20-30℃,which is still conducive to the growth of cyanobacteria.When there are algal blooms in the western or central part of the lake,the west wind can blow it to the drinking water source at the fastest speed,and the southwest wind will enable it to cover the entire eastern lake as quickly as possible,bringing about the largest area of algal bloom.When the southwest wind maintains at a small wind speed(1 m/s),it takes only about three days for blooms to migrate to the drinking water source.During the outbreak of algal blooms,the temporary increase in wind speed can only bring about temporary improvement.If the wind speed drops to a low level again in the short term,cyanobacterial blooms may re-emerge.(4)The total and particulate Cd,Co,Ni,Pb,and As concentrations have been found to have significant correlations with the chlorophyll a concentration,turbidity in the water when the wind speeds were small.It could be that the absorption and adsorption of algal blooms were significant during these periods.During 2014-2017,the most serious heavy metal pollution in sediments and the most heavy cyanobacterial blooms have occurred in the same year,2015.Both the outreaking frequency of algae blooms and the level of heavy metal pollution are high in western and central zones.The overlap suggests that the outbreak frequency of cyanobacterial blooms is consistent with sendimental heavy metals pollution in the spatial and temporal distribution to some degree.The outbreak of cyanobacterial blooms may involves more heavy metals in transporting in the lake and bring about considerable contamination in areas with no river.(5)A good linear relationship has been found between Cd and chlorophyll a in surface water of Chaohu Lake,with R of 0.813.Then,quantitative relationships with water temperature,dissolved oxygen,chlorophyll a and nitrate nitrogen have been established for As,Cd,Co and Zn separately using multiple linear regression method,with values of R ranging from 0.639 to 0.738.Finally,combining the multivariate linear regression equation and the cyanobacterial bloom simulation results,a reasonable distribution of the heavy metal in Chaohu Lake has been figured out successfully. |
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