Kuroshio Intrusion Combined with Coastal Currents Affects Phytoplankton in the Northern South China Sea Revealed by Lipid Biomarkers

The northern South China Sea (NSCS) is significantly influenced by the Kuroshio intrusion and the coastal currents. Our knowledge on the roles of both currents on phytoplankton spatial variations is still inadequate. Here, we investigated the concentrations of phytoplankton biomarkers and their proportions in surface suspended particles from 47 sites of the NSCS during summer of 2017 and 2019. Brassicasterol/epi-brassicasterol, dinosterol, and C 37 alkenones were used as proxies of biomass for diatoms, dinoflagellates, and haptophytes, respectively, and their sum indicating total phytoplankton biomass. A three end-member mixing model was applied to quantitatively assess the influence extent of the Kuroshio intrusion and the coastal currents. Our results showed that the Kuroshio intrusion and the coastal currents contributed equally to the overall surface water masses in the study area; however, the two currents had distinct effects on the spatial distribution of phytoplankton. For phytoplankton biomass, the eutrophic coastal currents were likely to be the main controlling factors, while the impact of the Kuroshio intrusion was weak and stimulated significant increases in phytoplankton biomass only at certain boundary sites. For phytoplankton community structures, the Kuroshio and its intrusion were the main factors, resulting in an increase in the proportions of dinoflagellates and haptophytes. The proportion of diatoms slightly increased due to the influence of the coastal currents. Our study quantifies the effects of the Kuroshio and the coastal currents on phytoplankton in the NSCS in terms of hydrological parameters, providing an important basis for the understanding of ecological functions and biogeochemical cycles in marginal sea-open ocean boundary regions.