56 - Mercury Speciation in the Southern California Current Ecosystem

The California Current is a productive eastern boundary current with strong seasonal upwelling, which may have significant implications for mercury (Hg) cycling in the region by bringing nutrient- and Hg- enriched waters from depth into the mixed layer. Hg exists in four principal forms in the marine environment: elemental (Hg0), inorganic (HgII), monomethyl (MMHg), and dimethyl mercury (DMHg). Both abiotic and biotic processes can transform one species to another, which impacts distribution, reactivity, and incorporation and bioaccumulation into the marine food web. Understanding the speciation of Hg and the conversions between species can allow us to determine Hg mobility and toxicity. We measured seawater samples for total mercury (THg), Hg0, MMHg, and DMHg following 3 water parcels and at 8 coastal stations within the California Current Ecosystem (CCE). We compare and contrast Hg speciation within water parcels as they age and between water parcels across trophic statuses (oligotrophic vs. eutrophic) to further understand controls on Hg biogeochemistry within the CCE. Presented here are depth profiles of the measured Hg species during this cruise, which reveal that there are higher [THg] and [DMHg] in the upwelled, eutrophic water masses (0.24 to 1.16 pM and 14 to 461 fM, respectively) and lower [THg] and [DMHg] in the oligotrophic water mass (0.08 to 0.53 pM and 1.6 to 100 fM, respectively). This represents roughly a 66% decrease in [THg] and an 80% decrease in [DMHg] on average between eutrophic to oligotrophic conditions. Furthermore, DMHg represents a significant fraction of THg in eutrophic conditions, on average 18%, but it can reach 40-70% of THg at waters deeper than 150 m, whereas it only represents a small fraction of THg in oligotrophic conditions, on average 7%. These results highlight the significance of upwelling and biological activity as a potential source of Hg in the CCE.

Authors: Hannah Adams, Xinyun Cui ,Carl Lamborg, Amina Schartup