Controls of Primary Production on Southern Ocean Mercury Cycling (COPPSOM)

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Mercury (Hg) is a toxic element that is globally dispersed from natural and anthropogenic sources. In the global biogeochemical Hg cycle, a substantial amount of Hg emitted to the atmosphere reaches the ocean through atmospheric deposition and ocean sediments are regarded as the ultimate Hg sink. Hg in the oceans biomagnifies in the marine food chain as toxic methyl-Hg which is formed in the water phase or sediment by microbial processes and consumption of marine fish is the crucial pathway of Hg to humans. The biogeochemical Hg cycle in the terrestrial environment is comparatively well understood, much less is known about the fate of Hg in the oceans, specifically about the role of the marine biological pump. Previous studies, most of them from oligotrophic areas, suggested that Hg shows a nutrient like behaviour in the water column and that Hg scavenging by algae is an important vector for the export of Hg to the sediments. However, the effect of Hg scavenging, Hg species transformation and export in highly productive areas has only been sparsely investigated and not yet been quantified. In this project we aim to investigate total Hg and methyl Hg in seawater, suspended matter, phyto- and zooplankton as well as in sediment cores. Samples were collected in a highly productive area in the South Atlantic around South Georgia during the “ISLAND IMPACT” Polarstern research cruise (Oct./Nov. 2022) at several Deep- and Process Stations. Seawater was sampled down to a max. depth of 7000 m while suspended matter and plankton was sampled to max. 350 and 600 m respectively using nets and large volume in-situ pumps. Sediment cores were taken directly below the sampled water column at a maximum depth of 8000 m (Sandwich Trench). The data of Hg and methyl Hg concentrations will be interpreted in context with basic and extended hydrochemical- and physical parameters such as chlorophyll a, nutrients, redox indicators, particle transformation and fluxes as well as analyses of the chemical composition and the biological origin (phytoplankton, macro or mesozooplankton, fecal pellets etc.) of suspended matter and sediments which will be provided by other participants of the ISLAND IMPACT project. With this comprehensive data set we aim resolve the biogeochemical cycling of Hg and methyl Hg and to quantify the Hg export in highly productive areas. We hypothesize that phytoplankton blooms cause depletion of Hg in the water column and are hot spots for methyl-Hg formation and Hg export to the sediment. Moreover, we want to prove if decomposition of organic seston on its way through the water column causes depletion of methyl Hg and enrichment of total Hg in sinking particles. We will further decipher if MeHg and THg in zooplankton of areas of intense phytoplankton blooms becomes higher enriched compared to zooplankton of oligotrophic zones.