Anthropogenic nutrient inputs drove shifts in phytoplankton productivity and community structure in the northern South China Sea over the past century

Climatic and anthropogenic pressures have caused significant changes in ocean ecosystems, but the long-term impacts on phytoplankton and underlying mechanisms in the continental shelf areas remain inadequately quantified. In this study, we reconstructed century-long records of phytoplankton productivity (PP) and community structure on the northern South China Sea (SCS) shelf using a combination of sedimentary lipid biomarkers (sterols and long-chain alkyl diols) and bulk geochemical proxies. The results revealed that diatoms and dinoflagellates predominated, with PP showing a consistent increasing trend over the past century, particularly accelerating after the 1980s. However, community structure exhibited divergent spatial trajectories. The relative abundance of dinoflagellates increased in the nearshore area, whereas diatoms increased offshore in recent decades. Statistical analyses identified the 1980s as a critical transition period, which was attributed to changes in sea surface temperature (SST) and nutrient levels. The Generalized Additive Model (GAM) results further demonstrated SST and nitrogen exerted cumulative and synergistic effects on promoting PP in the anthropogenically impacted nearshore region. This synergy, characterized by ocean warming coupled with elevated nitrogen loading and N/P ratios, favored the proliferation of dinoflagellates. The findings underscore that under ongoing climate change, proactive measures to control anthropogenic nitrogen inputs are crucial to mitigate the expanding frequency and geographical coverage of dinoflagellate blooms in coastal shelf ecosystems.