A simulation-optimization approach based on the compound eutrophication index to identify multi-nutrient allocated load.

Eutrophication with nutrient enrichment is a global marine ecosystem concern that threatens human health, economic activities, and ecosystem functions. Therefore, a nutrient load optimization method is required to help control marine eutrophication. However, eutrophication-based nutrient allocated load optimization is a multi-objective project due to a series of eutrophication pressures, such as cross-regional land-based nutrient loads and multi-nutrient regimes and ratios. In this study, a synergistic multi-nutrient control method was developed for the Bohai Sea (BS), China, which links multi-nutrient pressures with eutrophication states. Based on the eutrophication control standard, which is the second level of compound eutrophication index (CEI), the total maximum allocated loads (CEI-based TMALs) of total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and the chemical oxygen demand (COD) were calculated by a simulation-optimization approach. Using the end year of China's 13th Five-Year Plan (2020) as the reference year, 154 high load pressure jurisdictions (HLPJs) that contribute to eutrophication response segments in the BS were identified. Accordingly, practiced the optimized annual reduction rates of TDN, TDP, and COD in the HLPJs at 15 %, 11 %, and 2 % according to CEIII, respectively, the proportion of eutrophicated areas gradually decreased from 32 % in 2020 to 15 % in 2025 and might be 0 % in 2035 with ecosystem resilience in 2035. In particular, under the annual reduction rates of TDN and TDP optimized based on CEIII, the DIN/DIP molar ratio in the BS decreased to 16:1 by 2035. The simulation-optimization approach associated with the CEI-based TMALs for multi-nutrient control in this study might make implementing land-sea coordination more efficiency and marine nutrient regime stably. This can provide scientific and technological support for improving the health of coastal ecosystems.