Multiscale nutrient load assessment and total maximum allocated load optimization in the land area of Liaodong Bay, China

• Establishing a systematic management framework is necessary for pollutant control. • Multi-scale analysis is beneficial for pollutant source apportionment. • A relatively small spatial area contributes more to the river pollution load. • The hydroperiod-optimization approach has a significant advantage. Liaodong Bay has been receiving substantial land-based pollutants, leading to severe coastal water pollution. Excess nitrogen and phosphorus have therefore caused serious water eutrophication. This study aims to establish a systematic framework in which the multi-scale loads and different sources of land-based total phosphorus (TP) and total nitrogen (TN) in Liaodong Bay are first quantified. Then, the total maximum allocation load (TMAL) for TP is calculated using a combination of bottom–up linear and nonlinear optimization methods based on the dynamic water environmental capacity during different water periods. Overloaded control units are identified by comparing them with actual pollution loads. Results show that the TN and TP loads from the land areas of Liaodong Bay are 53.7 × 10 3 and 1.71 × 10 3 tons/year, respectively, with 81 % of TN and 78 % of TP originating from the Liaohe and Daliaohe River Basins. A quarter of the control units account for more than half of the pollution. In the rationality analysis, the flux results show good agreement with the actual pollution loads. The TP overload units cover 48 % of Liaodong Bay’s land area, with an average load reduction of 47.23 %. Among the 13 municipalities across 10 river basins, Shenyang, Huludao, and Yingkou exhibit the highest TP load reduction requirements. Most TP pollution problems in Liaodong Bay can be solved by mitigating pollution from wastewater treatment plants (WWTPs), decentralized livestock and poultry farming (DLPF), and untreated urban living (UUL) emissions. This study provides a scientific basis for integrated watershed management.