Seaweed and shellfish mariculture as nature-based solutions: Mitigating nutrient pollution from coastal fish...

Fish mariculture and sewage discharge have triggered severe eutrophication in China's coastal waters. Seaweed and shellfish culture are nature-based solutions while their effectiveness in mitigating nitrogen (N) and phosphorus (P) pollution from fish mariculture and sewage remains unclear. To fill this knowledge gap, this study comprehensively analyzed 20-year (2003-2023) data on N/P release by maricultured fish and removal by seaweed and shellfish in China. The findings showed that seaweed and shellfish mariculture could remove 89.57 gigagrams (Gg) yr^-1 N with 11.41 Gg yr^-1 P and 104.41 Gg yr^-1 N with 8.09 Gg yr^-1 P in 2023, respectively. The combined removal of seaweed and shellfish could fully offset the N emissions from fish mariculture, but not P emissions; it could completely offset sewage discharge for both N and P due to continuously decreasing discharge. Seaweeds demonstrated considerably higher N and P removal capacity than shellfish. Gracilariopsis lemaneiformis and Saccharina japonica showed the highest N (99.61 ± 52.73 g m^-2 yr^-1) and P (12.54 ± 3.88 g m^-2 yr^-1) removal capacity among seaweeds, respectively. The highest N (21.51 ± 0.08 g m^-2 yr^-1) and P (1.57 ± 0.22 g m^-2 yr^-1) removal capacities among shellfish were found in mussel and oyster, respectively. To remediate N and P released by fish per unit area, it requires 2.25- and 2.55-fold areas of seaweed, or 15.65- and 48.44-fold areas of shellfish, respectively. The findings in this study provide solid contributions to understanding the nutrient removal capacity of seaweed and shellfish and their application in mitigating eutrophication.