Study On The Evolution Trajectory Of Eutrophication And Its Driving Factors In Lake Gehu For The Last Three Decades

Lake eutrophication has become a global environmental pollution problem.Climate change affects the effectiveness of nutrient control in eutrophication lakes.By analyzing the process of lake degradation and restoration,we can clarify the degree of impacts of climate change and human disturbance on lake ecosystems,and formulate nutrient control thresholds to cope with climate change,which will help to accurately implement regulations and reduce management costs.Based on 34 years of long-term water quality monitoring data of Lake Gehu,this study identified the process of degradation and restoration used the filtering trajectory method(FTM).And the reasons of responses trajectory of the ecosystem at different stages were analyzed.Principal component analysis(PCA)and Spearman correlation analysis were combined to identify the main controlling factors of Chla.The generalized additive model(GAMs)was used to quantify the influence and contribution of nutrient and climate factors on algal production efficiency(ETP).Finally,the double probability method(DPM)was used to formulate water quality objectives of nutrients for the algal control in Lake Gehu and strategic recommendations were provided for lake management.The main conclusions are as follows:(1)The recovery trajectory of Chla-TP and Chla-TN in Lake Gehu is nonlinear.And TP have a better interpretation on Chla than TN.When TP came across the threshold of 120 μg/L,Lake Gehu shifted from a clear state to a turbidity state.The decline of Chla showed a short lag time,while the recovery of aquatic vegetation was delayed.(2)ETP was the direct factor affecting Chla,and adverse climate change does not fully offset the positive effects of nutrient load reduction.On the interannual scale,the interpretation of TP for ETP changes is28.9%,and ETP was positively correlated with TP when TP<100 μg/L.At the seasonal scale,the effect of climate factors on ETP was much greater than that of nutrient concentration.TN,temperature and wind speed were the best GAMs models for seasonal variation of ETP,which deviation explainability was 48.2%.The short and long term effects of climate change requried further attention and had significance for eutrophication control.(3)During the evolution of Lake Gehu,the carrying concentration of TP in the water decreased significantly by 40% ～ 60% in 2005,and the sensitivity of eutrophication increased.Finally,the water quality target for algae control was set as TP = 40 μg/L(Chla = 10 μg/L),which is 20%stricter than the national standard(TP = 50 μg/L).In addition to reducing nutrient load,aquatic vegetation restoration and biological regulation are effective alternatives to control eutrophication.