311- Above- and Belowground Plant Mercury Dynamics in a Salt Marsh Estuary in Massachusetts, USA

"Coastal salt marshes are biogeochemical hot spots that show traits typical to wetlands while at the same time experience high primary productivity rates from vegetation . We investigated plant mercury (Hg) cycling in the Plum Island Sound salt marsh estuary in Massachusetts and hypothesized that high Hg levels observed in salt marsh soils are derived from atmospheric gaseous elemental Hg (GEM) uptake by salt marsh vegetation, similar to the dominant vegetation Hg source reported from upland ecosystems.

We conducted monthly quantitative harvests of aboveground biomass and Hg concentration analyses throughout a full growing season, along with belowground biomass separated into live and dead roots and rhizomes, dead organic detritus, and soil sediments. Results showed that seasonally, Hg concentration and mass in current-season aboveground biomass near-linearly increased from June (3.9±0.2 µg kg-1 [mean±SD] and 0.7±0.4 µg m-2, respectively) to November (16.2±2.0 µg kg-1 and 5.7±2.1 µg m-2, respectively). Early growing season atmospheric GEM uptake is supported by micrometeorological atmosphere-surface measurements but not after August. Peak Hg concentrations and pools were observed in previous-year senesced aboveground tissues (21.1±3.6 µg kg-1, 9.8±3.0 µg m-2 ), exceeding levels of current-year biomass Hg and providing evidence of substantial sorption of Hg to senesced vegetation throughout winter. Hg pools in senesced biomass decreased throughout the growing season due to transfer to soils and wrack export to tidal water. Hg concentrations in roots exceeded aboveground biomass concentrations by 35 times and were similar to soil concentrations, suggesting uptake from soils as the likely Hg source. In both above- and belowground biomass, we observed significant species-specific differences in Hg concentrations among dominant salt marsh species resulting in spatial variability of Hg dynamics associated with vegetation. Ongoing stable mercury isotope analyses and continuous surface-atmosphere exchange fluxes will help to distinguish respective sources of Hg in salt marsh vegetation tissues."

Authors: Ting Wang, Inke Forbrich, Jun Zhou, Buyun Du ,Joshua Polen, Keely O’Beirne, Madison Sachs, Silas Bollen, Elsie Sunderland, Prentiss Balcom, Daniel Obrist