Responses of Phytoplankton Communities to the Effect of Both River Plume and Coastal Upwelling

River plumes and coastal upwelling systems are both nutrient-abundant habitats compared to the open ocean, and the mechanisms that structure the phytoplankton community when these occur together are unclear. In this study we investigated the dynamics and drivers of phytoplankton biomass and community composition in the Pearl River plume-coastal upwelling system on the northern South China Sea shelf during summer. We found that phytoplankton biomass was lower in the plume than in upwelled water. Diatoms were the only dominant group in the upwelled water. In contrast, diatoms and Synechococcus were co-dominant in the plume, and the negative correlation between the proportions of these two groups indicated that they occupied distinct niches. The lower phytoplankton biomass in plume waters was due to nutrient limitation, and there was a transition from limitation by phosphorus to limitation by nitrogen in the plume along its path. This bottom-up control limited only diatoms, whereas Synechococcus was limited mainly by top-down control via microzooplankton grazing. This difference led to niche differentiation of the dominant phytoplankton groups in the plume-upwelling system. This discovery of niche differentiation enhances understanding of food web structure and will facilitate modeling of marine biogeochemical cycles.