| The large amounts of organic matter and humus accumulated in the sediments are the characteristics of typical lakes in the Northeast Region,which lead to the high concentrations of COD,TN,TP and other organic matter and nutrients in the lake water.In recent years,the increase of cultivated land area and intensive human activities in the basin have greatly affected the water quality of lakes.Most lakes are threatened by eutrophication,and the ecological security of lakes is facing great challenges.The changes of land use patterns in the basin have directly affected the aquatic ecosystem,and changes in the structure and function of the terrestrial ecosystem also have an important impact on the water quality of lakes.Based on the assessment of water environmental quality and ecological environment,the lake ecological security is synthetically estimated,which can evaluate the current situation of lake ecological security and would provide theoretical basis and technical support for the construction of lake ecological security.Meanwhile,nutrient criteria threshold of typical lakes in Northeast Region is conducive to scientifically characterize the eutrophication and ecological response of lakes in this area,which can effective reduced and controled nutrient input into lakes,offer the legal basis for managers,and are the key to lake basin ecological security.Shankou Lake,a typical mountain lake in the Northeast Region was used to study in this paper.Based on the setting of water quality monitoring sites and the field investigation,the water environment quality and ecological environment were evaluated,and then the ecological security was also comprehensively assessed in Shankou Lake Basin.Stressor-response models(including linear regression model,classification regression tree model and changepoint analysis model),land use-nutrient regression models,and terrestrial ecosystem health states were developed to determine the criterion values of total nitrogen(TN),total phosphorous(TP),and chlorophyll a(Chl a)using field data from lakes and reservoirs in Heilongjiang Province.Analysis of covariance(ANCOVA)and nonlinear regressions were used to quantitatively characterize the impact of climate change on the development of nutrient criteria.The main conclusions are as follows.(1)The water quality of Shankou Lake in 2014 met “the surface water environment quality standard”(GB3838-2002)class ? standard.The radios of IN to TN and IP to TP were more than50%,the TOC contents of sediments in dry season were lower,ranging from 36.20 mg/kg to 65.07mg/kg,while those in wet season were higher,ranging from 39.23 mg/kg to 98.00 mg/kg.Both in the wet season and dry season,nitrogen pollution in sediments from Shankou Lake were in ?level,indicating sediments was polluted by organic matters in some degree.The Cd pollutionlevels in the sediments were relatively high,while Cu,Ni,As,Zn were not polluted or had low pollution level.The heavy metals of sediment in the dry season had high ecological risk,and those in the wet season had higher ecological risk.(2)In 2014,the socio-economic activities in Shankou Lake Basin had less impact on the ecosystem,and the ecological structure was reasonable and stable.The structure of land use in the basin was simple,forest land was the main land use type,followed by dryland,grassland and water.In recent years,a large amount of farmland had been reclaimed by local residents along the upper reaches of Shankou Lake,resulting in a large reduction in water area and a large increase in dryland.From 1988 to 2012,the soil erosion and soil and water conservation in Shankou Lake Basin were relative good.The index of ecosystem service function was 73.1,which was in good condition.The comprehensive indexes of ecosystem health of the whole lake ranged from 43 to 67,with an average value of 55,which indicated that the ecosystem health state had been maintained at a moderate level.From 1988 to 2014,the ecosystem health indexes of Shankou Lake land area were all above 80,which were in excellent condition.However,the indexes tended to decrease because of the influence of human activities after 2008,mainly manifesting in the decrease of vegetation coverage(such as forests)and the increase of the proportion of farmland.(3)In 2014,the ecological security index of Shankou Basin was 72.61,and the ecological security condition of Lake Shankou was normally safe.The assessment results indicated that poor development level of economic,ineffective control of inflowing pollutant,low value of transparency,short aquatic product supply,and low pollutant treatment ability were the main factors influencing ecological security of Shankou Lake.(4)Stressor-response models,land use-nutrient regression models,and terrestrial ecosystem health states were used to determine the criterion values of total nitrogen(TN),total phosphorous(TP),and chlorophyll a(Chl a)for the typical lakes in Heilongjiang Province.The results indicated that there were no significant differences in the nutrient criteria obtained by the various methods.The results of linear regression model analysis suggested that lg Chl a had a significant positive correlation with lg TP and a non-linear relationship with lg TN.The nutrient criteria derived from land use-nutrient regression model were Chl a 3.794 ?g/l,TN 0.770 mg/l and TP 0.059 mg/l.LOWESS curve and changepoint analysis showed that when the thresholds were 0.058(0.060)mg TP/L and 0.865(0.976)mg TN/L,respectively,the concentration of Chl a began to decrease.ANCOVA and nonlinear regressions were used to quantitatively characterize the impact of climate change indicators on the relationships between nutrient and Chl a concentrations and between land use and nutrient concentrations.These results indicated that climate change should be considered for the development of nutrient criteria,and with a warmer climate,achieving a desired water quality without the threat of eutrophication in the future will require stricter nutrient criteria than are required under the current climate conditions. |
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