Nutrient budgets for the Bohai Sea: Implication for ratio imbalance of nitrogen to phosphorus input under intense human activities

Eutrophication is a global problem for coastal ecosystems, one that the Bohai Sea (BHS), China, is severely afflicted by due to rapid economic and social development over the last forty years. For sustainable nutrients management in the BHS, comprehensive budgets for Nitrogen (N) and Phosphorus (P) was characterized in 2017, and the relative contributions of river input, submarine fresh groundwater discharge, atmospheric deposition, sediment diffusion, and exchange with the Yellow Sea were quantified. The annual N and P fluxes into the BHS were 362 × 103 t and 10.4 × 103 t, respectively. The terrigenous N inputs occupied the highest proportion, while the largest P input was from sediment diffusion. The ratio of N:P was 77 for total external inputs, while that of the Yellow River was 680; both exceeded the Redfield ratio, indicating an imbalance in the nutrient structure and a P limitation in the BHS.