Seasonal and spatial variations in nutrients under the influence of natural and anthropogenic factors in coastal waters of the northern Yellow Sea, China

Analysis of the common and most influential natural and anthropogenic activities on the spatiotemporal variation in nutrients at a multiannual scale is important. Eleven cruises from 2015 to 2017 were carried out to better elucidate the seasonal and spatial variations in nutrients, as well as the impact factors on dissolved inorganic nitrogen (DIN), phosphorus (DIP) and silicate (DSi). Both nutrient concentrations and forms showed similar and significant seasonal variations over the 3 years, and were closely related to the biomass and species of phytoplankton. Terrestrial inputs had significant effects on the spatial distribution of nutrients throughout the year, especially in the surface water, which showed DIN > DIP>DSi. In summer, shellfish aquaculture and hypoxia jointly affected the spatial distribution of nutrients. The bottom water nutrient concentrations in the aquaculture area were 1.1–2.3 times higher than those outside of the aquaculture area. Seasonal hypoxia can increase the release of DSi and NH₄⁺ from the sediment to the water. In summary, anthropogenic activities and physical conditions jointly influenced the nutrient distributions.