Greenhouse gases emissions and dissolved carbon export affected by submarine groundwater discharge in a maricultural bay, Hainan Island, China

Abstract

Greenhouse gases (GHG) emissions in coastal areas are influenced by both mariculture and submarine groundwater discharge (SGD). In this study, we first conducted a comprehensive investigation on carbon dioxide (CO2) and methane (CH4) emissions affected by SGD in a typical maricultural bay in north-eastern Hainan Island, China. A radon (222Rn) mass balance model revealed considerable high SGD rates (179 ± 92 cm d−1) in the bay, and the fluxes of SGD-derived dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) were 150.36 and 3.65 g C m−2 d−1, respectively. Time-series measurement results, including those for 222Rn, CH4, CO2, and physicochemical parameters, indicated that GHG dynamics in the maricultural bay mainly varied with tidal fluctuations, and isotopic evidence further revealed that acetate fermentation was the main mechanism of methanogenesis in the maricultural waters. The water-air fluxes in the maricultural area were 1.05 ± 0.32 and 9.49 ± 3.96 mmol m−2 day−1 for CH4 and CO2, respectively, implying that Qinglan Bay was a potential source of GHG released into the atmosphere. At the bay-scale, the CO2 emissions followed a spatial pattern, and the CH4 emissions were mainly affected by mariculture. The high CH4 emissions in the maricultural waters caused by maricultural activities, SGD, high temperature, and special hydrology resulted in the formation of the CH4-dominated total CO2-equivalent emissions model. Our study highlights the importance of considering the link between SGD and GHG emissions in maricultural bays when constraining global GHG fluxes.