Seasonal phytoplankton blooms and associated biological-physical drivers have been studied in various marine systems throughout the world. This report will summarize the mechanism of phytoplankton blooms first, and then introduce our recent research on marine phytoplankton bloom dynamics through two case studies in the Yellow Sea and the Arctic Sea. In the Yellow Sea case, satellite and in situ observations are used to examine spatial heterogeneity in the timing and magnitude of phytoplankton blooms in relation to local and remote physical processes. Satellite ocean color 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 thermal or haline 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. In the Arctic Ocean case, we examined the spatial patterns in bloom timing, bloom magnitude, and primary productivity in relation to ice-retreat timing, using a combination of satellite observations and numerical modeling. We found distinct regional differences in how the phytoplankton bloom relates to ice-retreat timing. In the Arctic shelf regions, earlier and stronger blooms follow earlier annual ice retreats and enhanced light availability. By contrast, in some parts of the central Arctic, especially in the Canada Basin, there have been weakened blooms and reduced primary production in recent years. This reduction is largely due to a chain reaction triggered by earlier ice-melt and enhanced haline stratification, which further suppress vertical nutrient exchange and reduce surface nutrients in an already nutrient-limited system. Recognizing and quantifying strong and regionally distinct links between sea-ice retreat and primary production will improve spatiotemporal projections of biogeochemical cycles and trophic flows in Arctic marine ecosystems. Our results also imply that the spatial heterogeneity of marginal seas must be carefully considered when assessing phytoplankton responses in the context of climate change, due to the uncertainty and complexity of underlying mechanisms.