Temporal-spatial variations and developing trends of Chlorophyll-a in the Bohai Sea, China

The patterns of sea surface Chlorophyll-a (Chl-a) have regional and dynamic features. An understanding of the Chl-a dynamics and whether its trends in the past will be persistent in the future is important for restoration of ecosystem. Spatial and temporal variations of sea surface Chl-a concentrations in the Bohai Sea were investigated with data remotely sensed by MODIS from 2003 to 2014. The goals of this research are to identify the phytoplankton dynamics and detect their correlation with environmental changes and anthropogenic activities. Based on an indicator system built with Mann-Kendall Test and Hurst Exponent, our research shows that the Chl-a concentration in the surface layer is heterogeneous in both temporal and spatial scale. It is higher in costal zones, particularly near the Qinhuangdao coast. The occurrence of spring and summer blooms has a one-month time lag from south to north. An increasing trend that was persistent is evident offshore and a decreasing trend that was persistent is seen near the coast, which may indicate an expansion of eutrophication from coast to deep sea. The seasonality of the phytoplankton bloom is basically driven by vertical structure of water column. Climate and mariculture activity are significant correlated with the Chl-a trends. River discharge and suspended sediment also influence Chl-a.