Spatio-temporal Dynamic Analysis Of Phytoplankton Biomass In National Lakes Based On Google Earth Engine

Lakes are not only natural freshwater resource reservoirs,but also important natural resources for human survival and development.With the impact of global warming and human economic development,China’s lakes are facing huge challenges such as eutrophication,water quality decline and lake pollution.Therefore,there is an urgent need for effective monitoring of phytoplankton biomass in lakes nationwide.Chlorophyll a(chl-a)concentration is used as an important indicator for evaluating phytoplankton biomass in lakes.Monitoring the concentration of chl-a,and then analyzing its temporal and spatial distribution characteristics,changes and influencing factors are of great significance in the work of lake ecological assessment and resource development and utilization.In this paper,756 lakes with an area of more than 10km2 are selected as the research object.With the help of the Google Earth Engine(GEE)platform,based on the inversion and processing of the chl-a concentration data in the study area from 2003 to 2018,the seasonal and annual average nutrition status and the spatial and temporal changes of the lake were analyzed.Correlation analysis was carried out on various factors affecting the concentration distribution of chl-a such as precipitation,air temperature and lake surface temperature(LST).In view of the obvious regional characteristics of the concentration distribution of lake chl-a in the study area,this paper selects two scales and typical lakes to analyze the spatial and temporal distribution characteristics of the area chl-a concentration.The main research conclusions are as follows:(1)The distribution of lake trophic state in China has obvious seasonality and regionality.From 2003 to 2018,about 90%of lakes were in a mesotrophic state,and a small number of lakes were eutrophic and poorly trophic.The overall performance was a trend of low concentration of chl-a in winter lakes and a high value of summer.Most of the oligotrophic state occurs in winter,especially in the Mengxin Lake area and the Northeast Plain and mountain lake areas.Due to the low temperature,the concentration of chl-a in the lake is relatively low.There are relatively many eutrophic lakes in summer,especially in the eastern plain lake area and Shandong plain and mountain lake area.The mesotrophic lakes have obvious performance in spring and autumn.The interannual variability of lake chl-a concentration shows that 82%of lake chl-a concentration shows a slight change trend(the absolute value of annual change rate is less than 0.5).62%of the lakes with drastic changes appeared in the Qinghai-Tibet Plateau lake area,and 19%in the Mengxin lake area,while other lake areas with less dramatic lake distribution.(2)The spatial distribution of the concentration of chl-a is obviously affected by factors such as LST,precipitation,and the proportion of grassland in buffer zone.Studies have shown that:the concentration of chl-a in more than 70%of lakes in China is positively correlated with LST and precipitation,most of which are distributed in the north and east,and are significantly affected by geographical location.On the contrary,it is negatively correlated with the proportion of grassland in the buffer zone and related to the inhibition of the grassland surrounding the lake on the nitrogen and phosphorus content of the lake;(3)Air temperature,precipitation and wind speed have an effect on the spatial distribution of chl-a concentration in regional lakes.The spatial distribution of chl-a concentration in the climate zone and provinces has a strong correlation with air temperature and precipitation,and the correlation is affected by the geographical location of the lake and the climate environment.(4)The concentration of chl-a in the waters of Taihu Lake and Poyang Lake has obvious seasonal changes,and is seriously affected by human activities.The waters of Taihu Lake are in a state of eutrophication throughout the year.From 2003 to 2018,the concentration of chl-a in Taihu Lake has been increasing year by year.Due to the influence of human activities,the value of the concentration of chl-a in the near-shore waters of Taihu Lake in summer reached its peak;The waters of Poyang Lake are in a mesotrophic state all year round.The concentration of chl-a is higher in summer and autumn,and the interannual change of the concentration of chl-a in Poyang Lake is stable between 2003 and 2018.