Spatial heterogeneity of seasonal phytoplankton blooms in a marginal sea: Physical drivers and biological responses

Abstract

Satellite and in situ observations are used in this study to examine spatial heterogeneity in the timing and magnitude of phytoplankton blooms in relation to local and remote physical processes in the Yellow Sea (YS), a marginal sea in the northwestern Pacific Ocean. Satellite ocean colour data reveal that annual chlorophyll maximums vary significantly in both timing and magnitude over different subregions of the YS. Strong summer blooms were found off estuary regions, and widespread spring blooms were found in the central trough. Localized autumn and winter peaks were found in small patches around Jeju Island and in nearshore regions. A statistical analysis of in situ measurements of the western YS suggests that variability in hydrographic properties could explain most of the spatial heterogeneity observed in both bloom timing and magnitude. The spatial heterogeneity of hydrographic properties, such as stratification and nutrient availability, are controlled by a suite of physical forcings, including the extent of the YS Cold Water Mass, river discharge, warm slope water intrusion, and seasonal warming/cooling. Our results imply that the spatial heterogeneity of marginal seas must be carefully considered when assessing phytoplankton responses in the context of climate change, because of the complexity of underlying mechanisms.