Comparison of the Spatiotemporal Variation of Chl-a in the East China Sea and Bohai Sea based on long time series satellite data

The coastal sea areas constitute an important foundation for the entire marine ecosystem, which is closely related to the survival and development of human beings. However, the rapid developments of economy and society have induced serious damage to the coastal ecological environment, with increasing eutrophication and frequent red tides occurrence. Up to date, few researches ever reported the ecological response to the human activities concerning the different geographical environment and geological structure in different types of sea areas. The comparison between open sea areas, closed sea or semi-closed sea is necessary to observe the ecological responses of different types of marginal seas under different extent of pressure from human interruption, which is very important for the making and implement of marine environment protection and management.Therefore, this paper selects chlorophyll-a (Chl-a) as an indictor to reflect the ecological healthy and aquatic environment quality of the seas. Sea-viewing wide field sensor (SeaWiFS) and Moderate-resolution imaging spectrora-diometer (MODIS) data are used combining with Slope index, correlation to observe the temporal and spatial changes and relevant influencing factors of Bohai Sea (selected as the semi-enclosed sea), and East China Sea (open sea) in China from 1998 to 2020. Preliminary research results show that the Chl-a characteristics of the two types of sea areas are different. The concentration of Chl-a in the Bohai sea first increased and then decreased; while the Chl-a in the East China Sea increased first, and then rose again after a decreasing trend. The Bohai sea has no significant seasonal changes, but the East China Sea has obvious seasonal changes. The main influencing factors of the Chl-a in the open sea are PAR and SST, while the comprehensive effect of the closed sea area factors is dominant, and the changes are more complicated. For semi-closed sea areas, the important role of circulation and convection should also be considered. Combining the land-sea environmental management of the Bohai Bay and the Yangtze River Basin, open sea areas may respond more quickly to human activities, and the effect of long-term governance of closed sea areas is more obvious.The study will be beneficial to the local government in policy making and implement of marine pollution prevention, marine ecosystems and marine resources research and protection.