Trends of satellite derived chlorophyll-a (1997–2011) in the Bohai and Yellow Seas, China: Effects of bathymetry on seasonal and inter-annual patterns

• It provides a decadal variation of sea surface Chl-a in the Bohai and Yellow Seas.
• Seasonal Chl-a pattern in coast shows difference with the Yangtze River mouth and offshore region.
• The decadal trend of Chl-a deceased in coastal waters, whereas increased in offshore region.
• Hydrography and circulation are important in determining the spatial variability of Chl-a.

The spatial and temporal variability of sea surface chlorophyll-a (Chl-a) concentrations in the Bohai and Yellow Seas were analyzed, using satellite-derived Chl-a products from SeaWiFS and MODIS sensors over the period of September 1997–September 2011. A set of monthly and cloud-free Chl-a data was produced by the Data Interpolating Empirical Orthogonal Function (DINEOF) method. The results indicate that there are different Chl-a seasonal patterns existing in the Yangtze River mouth, coastal and offshore waters, respectively. In the Yangtze River mouth, a long-lasting Chl-a peak (May–September) is seen in summer. In coastal waters, two significant Chl-a maxima occur in winter–spring and late summer, respectively. In offshore waters, only one significant spring (March–April) Chl-a maximum is evident with a time lag of 1–3 months to coastal waters and the signal of autumn maximum is very weak. In coastal waters, wind-tide-thermohaline circulations and East Asia summer rainy monsoon may important physical factors to impact the seasonal pattern of Chl-a, but increased human activity (e.g., eutrophication, dam) could significantly enhance this process. In offshore waters, the impact on the circulation of the YSWC in winter and YSCW in summer in the central Yellow Sea could be important physical factor in explaining the variability of Chl-a in seasonal patterns. The decadal trends of Chl-a and sea surface temperature are decreasing in coastal waters, with a significantly positive correlation. In offshore waters, the decadal trends of Chl-a is increasing but a slight decreasing sea surface temperature trend is seen, and they indicate a negative correlation.