Characteristics Of Chlorophyll-a And Its Relationship With Environmental Factors In Different Types Of Sea Areas Based On Long Time Series Remote Sensing Data

Under the global climate change,the sea surface temperature(SST)is rising,resulting in seawater stratification and hypoxia,which makes the marine phytoplankton biomass show a downward trend.However,this downward trend is mainly concentrated in the ocean,and phytoplankton in offshore sea areas still shows an obvious upward trend.Eutrophication is considered to be the main factor stimulating the rise of phytoplankton biomass in offshore sea areas.However,the response of offshore phytoplankton to global climate change and the future change trend under the pressure of global warming and human activities have not been determined.Offshore sea areas can be divided into two types of sea areas with significantly different geographical environment and structure.One is an open sea area with only one side adjacent to the land,the other is a closed sea area connected to the sea or ocean by only one outlet,and the rest is surrounded by land or islands.Compared with the open sea area,the closed sea area may be more vulnerable to damage because of its relative closeness.Therefore,under the complex offshore environmental conditions and multiple pressure sources,the changes of offshore phytoplankton and its response to the environment are more complex and diverse.In order to explore the changes of phytoplankton in offshore sea areas under the pressure of global warming and human activities,and analyze its internal main driving mechanism,this study takes China’s Bohai Sea and the United States’Gulf of Mexico as typical closed sea areas,and East China Sea and the east coast of United States as typical representatives of open sea areas.Taking the monthly chlorophyll-a(chl-a)concentration products provided by Sea Wi FS and MODIS Aqua as the data source,the long-time series data of chl-a concentration from 1998 to 2020 are formed through the correction and combination of the two data.Slope index analysis,M-K analysis and C_V variation analysis were used to explore the variation trend of chl-a concentration in different types of sea areas.At the same time,the environmental data(sea surface temperature(SST),wind speed(WS),photosynthetic active radiation(PAR))collected by satellite remote sensing technology are used for partial correlation and principal component analysis with chl-a so as to deepen the understanding of phytoplankton impact factors and response mechanism to global climate change in two different types of sea areas,and put forward corresponding environmental protection suggestions for the two types of sea areas.The results show that due to the different geographical structures of open and closed sea areas,the spatial and temporal distribution of phytoplankton,changing trends,and response mechanisms to environmental factors are different:(1)The chl-a concentration in open sea area and closed sea area shows an upward trend as a whole,but the difference is that the annual growth rate of chl-a concentration in closed sea area is large,and the growth trend passes the significance test.Especially in the small enclosed sea area of Bohai Sea,in addition to the coastal area,the central area of the sea area also shows an upward trend,indicating that the human impact on the sea area has spread from the coastal area to the central sea area,deteriorating the environment of the central sea area;The annual growth rate of chl-a concentration in the open sea area is small,and the growth trend fails to pass the significance test.The significant growth area of chl-a is mainly concentrated in the offshore sea area,while the continental shelf sea shows an obvious downward trend of chl-a concentration.Therefore,the growth of chl-a in the open sea area is a comprehensive manifestation of the significant increase in the nearshore sea area and the small downward trend in the continental shelf sea area.(2)The overall relationship between chl-a and SST in the four sea areas is negative,and the decreasing trend of chl-a concentration is dominant in the East China Sea,the eastern coastal waters of the United States and the Gulf of Mexico,indicating that the offshore sea areas are also inhibited by the increase of SST under climate change.Due to the large disturbance and rich nutrients in the nearshore and circulation areas,the universal negative correlation between chl-a concentration and SST is weakened,showing a positive correlation.That is,in the coastal waters,the impact of human activities is dominant,which is manifested in the emission of a large number of nutrients and promotes the growth of phytoplankton;In the open sea area,the impact of climate change is dominant,which is manifested in the stratification caused by the rise of sea surface temperature,which inhibits the rise and intersection of bottom water bodies,thus destroying the growth environment of phytoplankton and inhibiting the growth of phytoplankton.Compared with the closed sea area,the inhibition of phytoplankton by the rise of sea surface temperature in the open sea area is more obvious under the global climate change.Therefore,it can be judged that although the offshore sea area is more affected by human activities,the impact of climate change is also gradually increasing.At the same time,it should also be noted that the Bohai Sea and the East China Sea with high degree of eutrophication show the change characteristics of"bell shaped"uplift in the interannual change of chl-a,that is,they have experienced a period of rapid growth of concentration,and then have a gradual downward trend,which is obviously different from the continuous rise of chl-a in the coastal waters of the eastern United States and the Gulf of Mexico.Therefore,under the continuous protection and management of human offshore marine environment,if the degree of eutrophication decreases,with the decreasing trend of chl-a concentration caused by global warming,it may lead to the continuous decreasing trend of phytoplankton production in offshore waters.This also puts forward a greater test for the governance of offshore ecological environment,maintaining the health of offshore ecosystem and maintaining offshore primary productivity.