Contrasting roles of Changjiang diluted water on phytoplankton dynamics near a temperate volcanic island: Implications for the island mass effect

Changjiang diluted water (CDW) substantially affects physicochemical and phytoplankton dynamics in the East China Sea, yet its ecological role near oceanic islands remains poorly understood. We investigated how CDW modulates primary productivity and phytoplankton community structure in the coastal waters of Jeju Island in August 2024. When CDW strongly influenced the area, phytoplankton primary productivity increased markedly in the weakly stratified, nitrogen-replete western coast, but was constrained in the strongly stratified, nitrogen-depleted southern coast. Nitrate uptake measurements revealed that the productivity enhancement was closely tied to nitrate availability and assimilation rates. Under strong CDW influence, phytoplankton community structure became simplified, with dinoflagellates dominating the highly productive western coast and cyanobacterial prevailing in the nutrient-limited southern coast. Small-sized phytoplankton groups were prevalent when the CDW influence decreased. Dinoflagellates displayed a bifurcated response to nutrient limitation, increasing under phosphorus limitation and decreasing under nitrogen limitation. Multivariate analysis identified nitrate concentration and stratification intensity as integrative indicators of the relationship between physical forcing and low-trophic-level responses. The CDW-driven process illustrates an amplification of the island mass effect through physical mixing and nutrient enrichment around the island located along the CDW transport pathway, promoting episodic blooms and modulating phytoplankton communities. • DIN and stratification regulate phytoplankton responses to CDW in Jeju coastal waters. • Contrasting CDW roles derive bloom enhancement and nutrient limitation across Jeju coasts. • Phytoplankton productivity and community shifts function as indicators of CDW variability.