Seasonal responses of nutrient to hydrology and biology in the southern Yellow Sea

Seasonal nutrient dynamics regulated by hydrological and biological variables in the southern Yellow Sea (SYS) were studied based on the datasets collected during four cruises: spring (April 28-May 10, 2014), summer (August 17-September 22, 2015), autumn (November 7–17, 2014) and winter (January 13-February 1, 2016). In the present study, by dividing the study area into three distinct current-dominated subregions based on T-S distribution, cluster analysis, and principal components analysis (PCA), we revealed that the response mechanisms of nutrients to physical and biological processes were important. Among all factors, the physical processes, mainly including the Yellow Sea Cold Water Mass (YSCWM) onset, the Changjiang (Yangtze) Diluted water (CDW) and Taiwan Warm Current (TWC) intrusion and the Yellow Sea Warm Current (YSWC) invasion, were the principal driving force, which significantly changed the inherent balance of nutrient structure. Meanwhile, biological depletion was also a significant factor, especially in the stratified seasons, even though its impact on the nutrient structure could be balanced by vigorous water exchange or vertical mixing. The regional variation of nutrient regime and the regulatory mechanism behind it were revealed at the same time. The coastal area showed strong signs of potential P limitation as a result of excessive dissolved inorganic nitrogen (DIN) entering via terrestrial runoff and anthropogenic activities . The 32 °N transect, which is adjacent to the boundary of the SYS and the East China Sea (ECS), showed varying hydrological features due to the strong intrusion of CDW and TWC. Therefore, P or Si were the main limiting factor for the development of algal biomass. The central study area possessed a comparatively stable water column with the specific existence of the YSCWM present at the bottom, thus acting as a potential nutrient pool. Hence, we hypothesized that the offshore side of the study area tended to be more P and N co-limited, whereas the nutrient cycles in the nearshore side showed significant seasonal and regional variations, which corresponded to the change in physical oceanographic processes and algal biomass.