Sources of water column methylmercury across multiple estuaries in the Northeast U.S.
Introduction
Methylmercury (MeHg) is a potent neurotoxin and much of the human exposure to MeHg comes from consumption of coastal and estuarine fish. The sources of MeHg to the base of estuarine food webs are still poorly constrained (Kwon et al., 2014, Hammerschmidt and Fitzgerald, 2006), but water column MeHg has been identified as an important driver of Hg bioaccumulation in estuarine pelagic organisms (Chen et al., 2014). MeHg production in sediment has been measured in many systems (e.g., Schartup et al., 2013, Hollweg et al., 2009, Hollweg et al., 2010, Hammerschmidt et al., 2004, Hammerschmidt et al., 2008), and a number of studies have found a strong relationship between Hg in sediments and biota (e.g., Taylor et al., 2012, Gehrke et al., 2011), suggesting the importance of sediment MeHg production in driving bioaccumulation into biota. However, active MeHg production in sediment is not always predictive of water column MeHg concentrations. Chen et al. (2014) found that MeHg concentrations in water column particulate material, but not in sediments, were predictive of MeHg concentrations in fish across estuaries. Even in systems with contaminated sediment, Chen et al. found that transfer of MeHg into estuarine food webs may be driven by the efficiency of processes that determine MeHg input and bioavailability in the water column rather than by the rate of MeHg production in the sediment.
Most studies have focused on MeHg relationships in a single estuarine system such as Chesapeake Bay, New York/New Jersey Harbor (NY/NJ Harbor) or Long Island Sound (LIS) (e.g., Hollweg et al., 2009, Hollweg et al., 2010, Balcom et al., 2004, Balcom et al., 2008, Hammerschmidt et al., 2004, Hammerschmidt et al., 2008, Mason et al., 1999). These studies examined relationships among Hg species in either the water column or sediment, but studies that report relationships between water column and sediment Hg (e.g., Gosnell et al., 2015, Sunderland et al., 2010, Heyes et al., 2004) are rare. Only a few studies have taken a multi-estuary approach (e.g., Chen et al., 2014, Schartup et al., 2013, Schartup et al., 2014) to look at MeHg sources in estuarine areas. The multi-estuary approach allows for identification of predictive relationships for MeHg that apply across a broad geographic range in both the water column and sediment.
To assess the role of sediment MeHg concentration and production on water column MeHg levels, we sampled intensively at 10 sites in 4 estuaries extending from Maine to New Jersey in July and August of 2009. Potential sources and pathways of MeHg transfer to the water column were examined using the relationships among water column and sediment MeHg concentrations across estuaries.
Section snippets
Sampling sites
Ten sites in four estuaries with contrasting sediment and water characteristics were sampled on the East Coast of the U.S. (Fig. 1). The sites are described by Schartup et al. (2014), and detail maps showing sampling locations are presented in the supporting information (Fig. S1). Drakes Rd. (WD), Harbor Rd. (WH) and Mile Rd. (WM) were sampled in July 2009 and are located near the relatively pristine Wells National Estuarine Research Reserve (ME, Webhannet Estuary, Gulf of Maine). Jackson
Methylmercury concentrations and trends in water across estuaries
Dissolved MeHg concentrations were relatively high (0.01 to 0.68 pM) among estuaries in the Northeast U.S., somewhat elevated at Wells, and highest in LIS and Hackensack (MC; Fig. 2, Table S1). Dissolved MeHg was higher than seasonal levels reported for NY/NJ Harbor (0.04 to 0.22 pM; Balcom et al., 2008), but similar to the elevated MeHg concentrations measured in the Hudson River estuarine turbidity maximum (ETM; 0.05 to 0.8 pM; Heyes et al., 2004), and the Delaware River ETM (0.02 to 1.0 pM;
Conclusions
The lack of a relationship between sediment compartment MeHg and water column MeHg indicates that sediment is not the only source of MeHg to the water column in the four estuaries studied. Moreover, increased concentrations of MeHg during low as compared to high tides further suggests that upstream sources may be important, particularly in wetland dominated systems. However, the highly variable water column to sediment MeHg ratios suggest that the contribution of external sources may vary
Acknowledgments
This study was funded by the National Institute of Environmental Health Sciences (NIH Grant Number P42 ES007373). For assistance in the lab and the field, our thanks to Genevieve Bernier, Brian DiMento, Susan Gichuki, Kati Gosnell, Terill Hollweg, Allan Hutchins, Veronica Ortiz and Udonna Ndu (Dept. of Marine Sciences, University of Connecticut); Deenie Bugge and Darren Ward (Dept. of Biological Sciences, Dartmouth College); and Luis Carrasco (Institute of Environmental Assessment and Water
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Present address: Department of Environmental Health, Harvard T.H. Chan School of Public Health, Boston, MA 02215, USA.