Environmental drivers and key taxa shaping diatom–dinoflagellate ratios in eutrophic coastal waters

Diatoms and dinoflagellates are two pivotal phytoplankton groups present in coastal ecosystems that play key roles in marine food webs and biogeochemical cycles. The diatom-to-dinoflagellate ratio (diat/dino ratio) serves as an indicator of ecosystem status and phytoplankton community dynamics; however, the specific taxa that contribute to its variability remain poorly understood. This study investigated the phytoplankton community composition and diat/dino ratios in the coastal regions of the East China Sea and northern South China Sea during summer using quantitative PCR (qPCR) and 18S rRNA gene pyrosequencing. The qPCR results revealed that diatoms dominated in the estuarine and nearshore waters, whereas dinoflagellates prevailed in the offshore regions. Random Forest analysis identified dissolved oxygen (DO) and the nitrogen-to-phosphorus (N:P) and silicon-to-nitrogen (Si:N) ratios as the primary drivers of variation in the diat/dino ratio. The influence of N:P ratios was further modulated by the absolute nitrogen and phosphorus concentrations. Taxonomic profiling revealed that Thalassiosiraceae and Chaetocerotaceae were enriched in nutrient-rich estuarine waters, while Leptocylindraceae, Bacillariaceae, and Skeletonemaceae dominated in regions with low N:P ratios. In contrast, dinoflagellate families, such as Thoracosphaeraceae, Pyrocystaceae, Peridiniaceae, and Heterocapsaceae, were more abundant in environments with high DO and elevated N:P ratios. Notably, the northward expansion of Scrippsiella (Thoracosphaeraceae) drove changes in the bloom dynamics that threaten the coastal ecosystem balance. These findings demonstrate that nutrient stoichiometry and oxygen availability influence the diat/dino ratio by favoring distinct phytoplankton taxa, thus offering insights into how anthropogenic nutrient inputs shape community structure and guide coastal ecosystem management.