Assessment of the summer-autumn bloom in the Bohai Sea using satellite images to identify the roles of wind mixing and light conditions

Abstract

In shallow coastal ecosystems, summer–autumn phytoplankton blooms require thorough investigation because of their large variability in space and time. This variability results primarily from the complex interplay of different governing factors, e.g., wind mixing and light availability. To identify the effects of these governing factors on the summer–autumn blooms in the Bohai Sea, we first established a statistical approach to define the biological variables that characterize summer–autumn blooms, i.e., the bloom onset time (denoted as bloom onset), the bloom magnitude, and the chlorophyll-a (Chl-a) increase rate during the bloom period (denoted as Chl-a increase rate). These variables were quantified using daily Medium Resolution Imaging Spectrometer (MERIS) satellite scenes from 2003 to 2009. We observed a remarkable onshore–offshore gradient in all three biological variables. For example, bloom onsets were delayed in coastal areas compared to deep offshore waters. Spatially, the bloom onset exhibited a high correlation with wind mixing prior to the bloom period (R > 0.6), whereas the Chl-a increase rate and bloom magnitude were highly correlated to the distribution of the light attenuation coefficient (Kd(min), R = − 0.65 and R = 0.69, respectively). Furthermore, the empirical orthogonal function (EOF) analysis revealed similar interannual variability in the Kd(min), bloom onset and Chl-a increase rate. We also observed similar interannual trends in the bloom magnitude and wind mixing during the bloom period. From the analysis above, we believe that light is a crucial limiting factor for summer–autumn phytoplankton dynamics in the Bohai Sea. Wind influenced Chl-a dynamics in many ways and at various scales. Spatially, weak wind speeds prior to bloom events tended to provide favorable light conditions for the development of blooms. Over the long-term, however, enhanced wind mixing tended to supply the surface layer with nutrients more effectively and thus promoted a higher bloom intensity.