Mercury Export from a Salt Marsh Ecosystem to Tidal Water in the Great Marsh, the Parker River Wildlife Estuary in Massachusetts

Among aquatic ecosystems, saltmarshes are among the most productive and play an important role at the interface between land and sea ecosystems. They provide critical ecosystems services and are important sources and sinks for nutrients and contaminants. In this study, we aim to address the dynamics of mercury (Hg), an important global contaminant, in the Great Marsh, Parker River Wildlife Estuary in Massachusetts. This area, together with larger parts of New England, is known to be a hot spot of Hg contamination, with high Hg levels observed in the blood of birds and other wildlife. The goal of this study is to assess if saltmarsh ecosystems (vegetation and soils) can serve as potential sources of Hg to tidal water via twice-daily tidal flooding and lateral export of Hg from saltmarsh vegetation/soils to the tidal water.

We collected water samples across a large, 20-km, tidal gradient by boat during multiple seasons along 10 sampling locations from the ocean through the intertidal area to a freshwater stream at the head of the estuary. The approach is based on an end-member mixing model whereby concentrations in tidal water are expected to follow the salinity gradient of mixing of freshwater and ocean water. Any deviation from a mixing line is indicative of additional sources (with concentrations above) or sinks (concentrations below) relative to the mixing line. Analysis of dissolved and particulate mercury show concentrations of Hg strongly enriched compared to the freshwater-saltwater mixing line, in support of a source of Hg to the tidal water. Average filtered Hg (FHg) concentrations in tidal water with low salinity, upper part of the estuary, averaged 1.19 ± 0.65ng L^-1, and its total Hg (THg) concentrations averaged 6.13 ± 4.43ng L^-1; compared to freshwater FHg concentrations of 0.90 ng ± 0.52L^-1, and its THg concentrations averaged 1.00 ± 0.41ng L^-1; and ocean water FHg concentrations of 0.41 ± 0.10 ng L^-1 with its THg concentrations of 0.62 ± 0.21 ng L^-1.

Relative to salinity, Hg concentrations show highest enhancement in the upper parts of the estuary compared to areas closer to the mouth of the estuary. In spite of strong overall correlations between filtered Hg and DOC (P < 0.05, r² = 0.87), the patterns observed for Hg strongly contrast patterns of dissolved organic matter which showed little deviation from the freshwater-saltwater mixing line indicative of little sources or sinks of DOC in the tidal area. Our data is consistent with a source of Hg to the tidal water in the salt marsh region of this estuary, in support of possible lateral transport of Hg from the saltmarsh soils to the water. Our study is continuing to analyzer these patterns in order to assess if the source of this Hg originates from sediments or from salt marsh soils.