The contribution of fish and seaweed mariculture to the coastal fluxes of biogenic elements in two important aquaculture areas, China.

Carbon, nitrogen, phosphorus and oxygen (CNPO) are essential biogenic elements, driving life activities in marine environments. However, the integrated research of fish and seaweed culture on the fluxes of CNPO is scarce. To bridge the research gap, the contribution of mariculture of fish and seaweeds to the fluxes of CNPO in two important mariculture provinces, Fujian and Guangdong, in China, was investigated for the first time. Data from published literature and this study were integrated to calculate the CNPO fluxes using relative formulas. CNP release and O loss caused by fish mariculture increased with year (2003-2020) and reached 185.55 ± 3.18 Gg C, 35.92 ± 0.51 Gg N, 7.27 ± 0.24 Gg P and 644.18 ± 11.05 Gg O for Fujian and 215.81 ± 2.51 Gg C, 41.77 ± 0.40 Gg N, 8.47 ± 0.19 Gg P and 749.23 ± 8.71 Gg O for Guangdong in 2020. The averaged P fluxes due to fish mariculture in Fujian and Guangdong during 2016-2020 are 2.2 folds of the Min River and 69 % of the Pearl River, respectively. CNP removal and O generation by seaweed culture in Fujian also increased with year (2003-2020) and reached 555.74 ± 16.45 Gg C, 58.44 ± 4.83 Gg N, 7.80 ± 1.41 Gg P and 1481.97 ± 43.86 Gg O in 2020. In contrast, seaweed culture in Guangdong resulted in maximal C (39.81 ± 1.43 Gg), N (4.33 ± 0.26 Gg) removal and O (106.15 ± 3.82 Gg) release in 2013 and maximal P (0.41 ± 0.03 Gg) removal in 2019. The averaged N and P fluxes due to seaweed culture in Fujian during 2016-2020 are 69 % and 2.4 folds of the Min River, respectively. The different mariculture structure leads to a net CNP sink in Fujian but a net CNP source in Guangdong. The net CNP source may lead to seawater acidification, eutrophication and deoxygenation in coastal areas. These findings supply solid data for adjusting mariculture structure to achieve CNPO neutrality within mariculture.