Satellite-Observed Interannual Variations in Sea Surface Chlorophyll-a Concentration in the Yellow Sea Over the Past Two Decades

Ocean chlorophyll-a concentration is a reliable indicator of phytoplankton biomass that plays an important role in controlling the marine ecosystem. Here, we investigated the interannual variations in sea surface chlorophyll-a concentration (SSC) in the Yellow Sea and underlying mechanisms with 22 yr (1998-2019) of satellite ocean color observations. Results indicated that SSC showed a positive trend (0.039 mg m(-3) yr(-1) on average) during 1998-2010 and a negative trend (-0.058 mg m(-3) yr(-1) on average) during 2011-2019. Similar SSC trends occurred in the four seasons. These trends had larger magnitudes in inshore waters than in offshore waters. SSC variations were primarily driven by rainfall and anthropogenic nutrient emissions with a possibly secondary contribution from sea surface temperature (SST). During 1998-2010, increased rainfall and anthropogenic nutrient emissions largely enhanced the terrestrial nutrient inputs into the Bohai Sea and Yellow Sea. Coincidentally, decreased SST might have led to a weakened stratification in summer, favoring upward turbulent diffusion of deeper waters rich in nutrients and chlorophyll-a. These processes resulted in significant increases of surface nutrient concentrations and SSC. During 2011-2019, the opposite processes occurred to generate decreased SSC. The current study highlights the change of SSC trends in the Yellow Sea in the first two decades of the 21st century and physical processes at work, enriching our understanding of ocean chlorophyll-a dynamics in the global shelf seas.Plain Language Summary The Yellow Sea is a shallow marginal sea of the western Pacific Ocean. Combined effects of the complicated hydrodynamic conditions and human activities make the Yellow Sea one of the important marine ecosystems in the world, which contributes greatly to the socioeconomic development of bordering coastal countries, namely China, North Korea and South Korea. Previous studies indicated that increased anthropogenic nutrient emissions have predominantly caused a positive trend in sea surface chlorophyll-a concentration (which is usually used as a reliable indicator of phytoplankton biomass in the marine ecosystem) in the Yellow Sea. In the current study, based on satellite ocean color observations, we found that sea surface chlorophyll-a concentration in the Yellow Sea showed a positive trend during 1998-2010 and a negative trend during 2011-2019. These variations were primarily driven by rainfall and anthropogenic nutrient emissions with a possibly secondary contribution from sea surface temperature. The current study highlights the important role of the natural factors (primarily rainfall) in causing the change of sea surface chlorophyll-a concentration trends in the Yellow Sea in the first two decades of the 21st century.