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

Abstract. Estuaries are dominant conduits of mercury (Hg) to the coastal ocean and the salt marshes within play an important role in coastal Hg cycling. While Hg cycling in upland terrestrial systems has been well studied, processes in salt marsh ecosystems are poorly characterized. We investigated Hg dynamics in vegetation and soils in the Plum Island Sound estuary in Massachusetts, USA and specifically assessed the role of marsh vegetation for Hg deposition and turnover. Monthly quantitative harvesting of aboveground biomass showed strong linear seasonal increases in plant Hg, with a four-fold increase in Hg concentration and an eight-fold increase in standing Hg mass between June (3.9±0.2 µg kg^-1 and 0.7±0.4 µg m^-2, respectively) and November (16.2±2.0 µg kg^-1 and 5.7±2.1 µg m^-2, respectively). Hg ceased to increase in aboveground biomass after plant senescence, indicating physiological controls of vegetation Hg uptake in salt marsh plants. Hg concentrations in live roots and live rhizomes were 11 times and two times higher than concentrations in aboveground live biomass, respectively. Furthermore, live belowground biomass Hg pools (roots and rhizomes, 108.1±83.4 μg m^-2) is more than ten times larger than peak standing aboveground Hg pools (9.0±3.3 μg m^-2).

A ternary mixing model suggests Hg sources in marsh aboveground tissues originates from a mix of root uptake (~35 %), precipitation uptake (~33 %), and atmospheric gaseous elemental mercury (GEM) uptake (~32 %). The results suggest a more important role of Hg transport from belowground (i.e., roots) to aboveground tissues in salt marsh vegetation compared to upland vegetation, where GEM uptake is generally the dominant Hg source. GEM deposition via uptake and subsequent senescence (5.9 µg m^-2 yr^-1) and throughfall (1.0 µg m^-2 yr^-1) hence is lower in this salt marsh ecosystem compared to upland vegetation and is similar to open field wet and dry deposition (6.2 µg m^-2 yr^-1). Hg contained in salt marsh aboveground tissues leads to direct Hg export to tidal water and oceans via wrack (tidal flushing of vegetation), which accounts for ~1.6 µg m^-2 yr^-1. Hg consumption by herbivory ranges between 0.5 and 2.4 µg Hg m^-2 yr^-1. The similarity in isotopic signatures between roots and soils suggest that belowground plant tissues mostly take up Hg directly from soils. Annual root turnover results in large internal Hg recycling between soils and plants accounting for 58.6 µg m^-2 yr^-1. An initial mass balance of Hg in this whole estuarine salt marsh ecosystem considering atmospheric inputs (atmospheric GEM and precipitation Hg(II), throughfall, including plants) and losses (wrack export and lateral exchange of dissolved and particulate Hg) shows that the salt marsh presently serves as a small net Hg sink for environmental Hg of 5.2 µg m^-2 yr^-1.