Mercury dynamics in salt marsh of the Plum Island Sound estuary in Massachusetts, US

Coastal ecosystems are biogeochemical hot spots, where material exchanges rapidly between land, ocean, and atmosphere. These transformation processes have been studied intensively for carbon and nitrogen, but less for mercury (Hg). We study Hg cycling in the Plum Island Sound estuary in Massachusetts with a focus on the role of vegetation assimilation for Hg transformation and storage. Estuarine transects indicate coastal waters receive substantial inputs of inorganic Hg in areas dominated by salt marshes. We hypothesize these Hg inputs to derive from atmospheric Hg taken up by the highly productive salt marsh vegetation and stored in the organic rich sediment. To test our hypothesis, we are measuring atmosphere-surface fluxes of gaseous atmospheric Hg (Hg(0)) with a micrometeorological tower system. First results from summer 2021 show some emission and an average deposition of -1.8 ± 32.4 ng m-2 hr-1, indicating Hg stocks in salt marsh plants and soils indeed may derive from plant and soil uptake of atmospheric Hg during the growing season. Our study site shows distinct vegetation zonation with either Spartina alterniflora or Spartina patens dominating high marsh habitats. To analyze whether these species differ in their Hg uptake dynamics, we are sampling above- and belowground biomass throughout the growing season and taking sediment samples for bulk analysis. We find Hg concentrations in S. patens are twice as high as in S. alterniflora (e.g. in June: 3.7 ± 2.2 µg kg-1 versus 1.7 ± 0.9 µg kg-1). Since aboveground biomass stocks also differ by a factor of two (S. patens: 817 g m-2, S. alterniflora 501 g m-2), areas dominated by S. patens (3.0 µg m-2) show up to four times higher mass accumulation of Hg in above-ground biomass compared to areas dominated by S. alterniflora (0.8 µg m-2). Temporally, both Hg concentrations and Hg mass contained in above-ground biomass strongly increased throughout the growing season. Bulk Hg concentrations in soil (1116.5 to 440.9 µg kg-1 ) indicate Hg storage belowground. We are currently separating the different belowground components (roots, rhizomes and soil) to better understand and constrain possible pathways of Hg cycling (atmospheric vs. root uptake) using stable isotope analysis.