Unraveling seasonal and interannual nutrient variability shows exceptionally high human impact in eutrophic coastal waters

Abstract

AbstractNutrient import into eutrophic estuarine‐coastal zones is modulated by river discharge and anthropogenic activities (agriculture, industry, sewage, etc.). Comparing anthropogenic vs. hydrological influences on nutrient loading is important for ecosystem managers but has been a long‐standing challenge. As an attempt to fill this gap, a retrospective study was conducted using multiyear (2012–2022) data in one of the most eutrophic coastal seas in China, the Jiangsu coastal waters. Of the two main pollutants on the Jiangsu coast, dissolved inorganic nitrogen (DIN) is extremely overloaded relative to phosphate (PO4). Their spatial distribution shows clear terrestrial sources as concentrations decrease offshore. Although in each survey DIN and PO4 are enriched in low‐salinity waters, their spatially averaged concentrations are out of phase with river discharge on the seasonal and interannual scales. A box model was applied to decompose contributions of river discharge and nutrient import by anthropogenic activities. Results show that the human impacts on the interannual nutrient variability exceed those of hydrological factors on the Jiangsu coast. Anthropogenic nutrient loading decreases during 2012–2020, paralleled with the reduced fertilizer use and other nonpoint sources. Sporadic nutrient fluxes from point sources are likely responsible for the aseasonal nutrient‐rich patches. Our study demonstrates that water quality in the Jiangsu coastal waters is highly affected by anthropogenic activities. Findings in this study have broad implications for local monitoring strategies and restoration efforts and provide insight into assessing human stress from field data in global eutrophic ecosystems.