Unveiling hidden mercury and methylmercury sources: the role of submarine groundwater discharge in the largest western Mediterranean Sea coastal lagoon
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Mercury (Hg) loads from Submarine Groundwater Discharge (SGD) may represent an understudied source of methylmercury (MeHg) to the ocean. SGD comprises three components: terrestrial fresh groundwater discharge, long-scale seawater recirculation through the sediments, and short-scale porewater exchange. Existing studies disagree on the SGD relevance as a source of Hg to the coastal ocean and rarely report the Hg fluxes supplied by the different SGD components. Hence, we aimed to quantify the SGD-driven Hg fluxes to the Mar Menor, the largest coastal lagoon in western Mediterranean subject to polluted SGD. We measured dissolved total Hg (THg) and MeHg in lagoon waters, streams discharging to the lagoon and coastal porewaters during summer and autumn. Porewaters were enriched in both THg and MeHg compared to the lagoon. Lagoon shore waters and porewaters with high MeHg concentration and labile DOM were prone to MeHg formation. The input of THg through SGD to the Mar Menor lagoon (650 g THg yr-1) was an order of magnitude greater than the stream input and dominated by long-scale lagoon water recirculation. These findings have significant implications for the Hg budget in the region and raise awareness of the importance of legacy Hg remobilized by SGD in coastal areas.