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

摘要

In shallow coastal ecosystems, summer-autumn phytoplankton blooms require thorough investigation because of their large variability in space and time. This variability results primarily fromthe complex interplay of different governing factors, e.g., windmixing 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 variableswere quantified using dailyMediumResolution 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 bloomperiod (R > 0.6),whereas the Chl-a increase rate and bloommagnitude were highly correlated to the distribution of the light attenuation coefficient (K_(d(min)), R=?0.65 and R = 0.69, respectively). Furthermore, the empirical orthogonal function (EOF) analysis revealed similar interannual variability in the K_(d(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, enhancedwindmixing tended to supply the surface layerwith nutrientsmore effectively and thus promoted a higher bloom intensity.