Search

Estuaries are a conduit of mercury (Hg) from watersheds to the coastal ocean, and salt marshes play an important role in coastal Hg cycling. Hg cycling in upland terrestrial ecosystems has been well studied, but processes in densely vegetated 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 Hg associated with plants, with a 4-fold increase in Hg concentration and an 8-fold increase in standing Hg mass from June (3.9 ± 0.2 µ g kg -1 and 0.7 ± 0.4 µ g m -2 , respectively) to November (16.2 ± 2.0 µ g kg -1 and 5.7 ± 2.1 µ g m -2 , respectively). Hg did not increase further 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 and 2 times higher than concentrations in live aboveground biomass, respectively. Furthermore, live belowground biomass Hg pools (Hg in roots and rhizomes, 108.1 ± 83.4 µ g m -2) were more than 10 times larger than peak standing aboveground Hg pools (9.0 ± 3.3 µ g m -2). A ternary mixing model of measured stable Hg isotopes suggests that Hg sources in marsh aboveground tissues originate from about equal contributions of root uptake (∼ 35 %), precipitation uptake (∼ 33 %), and atmospheric gaseous elemental mercury (GEM) uptake (∼ 32 %). These results suggest a more important role of Hg transport from belowground (i.e., roots) to aboveground tissues in salt marsh vegetation than upland vegetation, where GEM uptake is generally the dominant Hg source. Roots and soils showed similar isotopic signatures, suggesting that belowground tissue Hg mostly derived from soil uptake. Annual root turnover results in large internal Hg recycling between soils and plants, estimated at 58.6 µ g m -2 yr -1. An initial mass balance of Hg indicates that the salt marsh presently serves as a small net Hg sink for environmental Hg of 5.2 µ g m -2 yr -1. [ABSTRACT FROM AUTHOR]

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 ¹g m-2) is more than ten times larger than peak standing aboveground Hg pools (9.0±3.3 ¹g 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Our previous work has documented a correlation between Hg concentrations and 210Pb activity measured in wet deposition that might be used to help apportion sources of Hg in precipitation. Here we present the results of a 27-month precipitation collection effort using co-located samplers for Hg and 210Pb designed to assess this hypothesis. Study sites were located on the east and west coasts of North America, in the continental interior, and on the Florida Peninsula. Relatively high variability in Hg/210Pb ratios was found at all sites regionally and seasonally (e.g., overall: 0.99–9.13ngdpm−1). The ratio of average volume-weighted Hg concentrations and 210Pb activities showed consistent trends (higher in impacted area), with Glacier Bay in southeast Alaska, exhibiting the lowest value. Assuming that Glacier Bay represents a benchmark for a site with no regional contribution, we estimate less than 50% of the Hg input was “global” at the Seattle and Florida sites. Differences in Hg/210Pb in wet deposition could be due to either a regional/local source contribution of Hg, or a regional/local enhancement in the removal of Hg from the atmosphere (i.e., oxidants), however, this approach is not capable of discerning between these two possibilities. Thus, this method of source apportionment represents an estimate of the maximal amount of Hg contributed by regional sources and may be limited in regions of deep convective mixing. [Copyright &y& Elsevier]