Remote Sensing Monitoring And Improvement Evaluation Of Algae In Qinghai Lake

Lakes,one of the most important freshwater resources on the earth,are an important foundation for sustainable economic development and human existence in lake basin areas.At present,many inland lakes in China are facing serious problems of eutrophication of water bodies,which are characterized by algae proliferation.As the largest inland saltwater lake in China,Qinghai Lake is an important water resource for maintaining ecological security in the northeast of the Qinghai-Tibet Plateau and an important place for the concentrated breeding of waterfowl in the plateau lake area of China.In recent years,although the overall aquatic ecological environment of Qinghai Lake has remained stable,large-scale outbreaks of Cladophora blooms have occurred near the main river inlets and along the coast,which have seriously damaged the Qinghai Lake plateau wetland ecosystems,naked carps,water quality,and the senses of Qinghai Lake.This article takes Qinghai Lake algae as research object,analyzes Qinghai Lake algae spectral characteristics and Sentinel-2 image band setting based on measured hyperspectral data,and selects nine methods suitable for Qinghai Lake algae remote sensing monitoring.Drone aerial images were used to evaluate the accuracy of various methods,then the algorithm with the best inversion effect was selected to monitor the algae of Qinghai Lake in different periods,and the algae treatment effect of Qinghai Lake in 2019 was evaluated.Results show that:Using the area around Bird Island in Qinghai Lake as a sample to perform algae inversion based on different methods,the single-band threshold method has good algae inversion results in the four vegetation red-edge bands and near-infrared band,bands with 10 m spatial resolution has a better inversion effect than those with 20 m spatial resolution,but the difference is small.The overall inversion effect of the band difference method is better,and the classification accuracy of the combination of B8 and B2 is the highest.Band ratio method has generally poor inversion effect,and the classification accuracy of combination of B8 and B2 is the highest.The normalized chlorophyll index(NDCI)method has a modest algal inversion effect,and the classification accuracy of combination of B8 and B4 is the highest.The overall inversion effect of the virtual-baseline floating macroalgae height(VB-FAH)is better,and the combined inversion with band 8 has the highest accuracy.The overall inversion effect of floating algal index(FAI)is modest,and the combined inversion with B8 has the highest accuracy.Among the three-band method,surface algal bloom index(SABI)and the maximum chlorophyll index of Sentinel-2 images(S2MCI)classification method based on SNAP software,the S2MCI has the highest inversion accuracy.Algorithms with the best inversion effect of each method combined with the condition of Kappa coefficient higher than 0.9 were selected to perform algae inversion on each algae distribution area of Qinghai Lake.The results show that:the normalized chlorophyll index method(NDCI)has the highest accuracy of algae retrieval in Quanji Estuary,Shaliu Estuary,Jiaowuqu Estuary and coastal areas,and QiadangQu Estuary and coastal areas,however the virtual-baseline floating macroalgae height(VB-FAH)has the best inversion effect in the Buha Estuary,Quanwan and Heima Estuary areas.The algae of Qinghai Lake are mainly distributed in Quanji Estuary,Shaliu Estuary,Jiaowuqu Estuary and coastal area,QiadangQu Estuary and coastal area,Quan Bay and Heima Estuary.The maximum algae area of Qinghai Lake in 2019 is 4.10 km~2,and the minimum area is 1.79 km~2.Among them,the algae area in the Jiaowuqu Estuary and its coastal areas is the largest,followed by the Qiadangqu Estuary and its coastal area,the algae in Quanji Estuary,Buha Estuary,Heima Estuary and Quan Bay are less,and the Shaliu Estuary is the least.The change of the total area of algae in Qinghai Lake in 2019is firstly reduced,then increased,and then decreased,but there are differences of the algae area change in each region.Among them,the Shaliu Estuary,Jiaowuqu Estuary and coastal areas and Quan Bay remain in line with the total area change trend,showing a change characteristic of first decrease,then increase,and then decrease.Algae area of Buha Estuary and Quan Bay increased first and then decreased.Quanji Estuary and Qiadangqu Estuary have the characteristics of increasing first,then decreasing,then increasing and finally decreasing.From 2016 to 2019,the total area of algae in Qinghai Lake showed a trend of decreasing first and then increasing.On July 30,2016,the total area of algae in Qinghai Lake was the largest in recent years,and the minimum was on July 30,2018.The Buha Estuary-Bird Island of Qinghai Lake was selected as the implementation area for the algae treatment.The ecological interception zone,grass planting ditch,and endogenous clean-up measures have a very good effect on algae control in the treated area.In 2019,the algae area in bay area on the north side of the Buha Estuary decreased by 93.55%compared with 2018.Manual salvage of algae can play a certain inhibitory effect on algae and can effectively reduce most of the algae in the treatment area.However,because salvage is only for dead algae floating on the lake,which is affected by late algae outbreaks and wind blowing,therefore the treatment effect is characterized by instability and repetition.