Anthropogenic perturbations in the Yellow River drive decadal phytoplankton shifts in the Bohai Sea and North Yellow Sea

Understanding long-term phytoplankton dynamics is essential for explaining contemporary ecological change and informing coastal management. In this study, we reconstructed decadal-scale variations in phytoplankton biomass and community structure in the Central Bohai Sea (CBS) and North Yellow Sea (NYS) using sedimentary sterol biomarkers from multiple sediment cores, with brassicasterol and dinosterol serving as indicators of diatoms and dinoflagellates, respectively. The results revealed that both regions experienced substantial 4–8-fold increases in proxy-inferred diatom and dinoflagellate biomass after 1980. The proxy-inferred diatom to dinoflagellate ratios (Diatom/Dino ratios) exhibited interdecadal oscillations, characterized by a decreasing shift around 1960 and an increasing shift around 2002. The proxy-inferred long-term biomass increase was mainly associated with anthropogenic nitrogen enrichment, whereas the shifts in Diatom/Dino ratios were likely linked to anthropogenic perturbations in the Yellow River. Furthermore, inter-regional comparisons based on rate-of-change (RoC) analysis indicated that both proxy-inferred diatom and dinoflagellate biomasses and their relative ratios in the NYS were more sensitive to allochthonous nutrient inputs than those in the CBS. This study provides valuable long-term baselines and new insight into the mechanisms underlying phytoplankton dynamics in the CBS and NYS, and highlights the important role of Yellow River perturbations in regulating diatom and dinoflagellate community shifts.