Methyl mercury dynamics in a tidal wetland quantified using in situ optical measurements

Abstract

We assessed monomethylmercury (MeHg) dynamics in a tidal wetland over three seasons using a novel method that employs a combination of in situ optical measurements as concentration proxies. MeHg concentrations measured over a single spring tide were extended to a concentration time series using in situ optical measurements. Tidal fluxes were calculated using modeled concentrations and bi-directional velocities obtained acoustically. The magnitude of the flux was the result of complex interactions of tides, geomorphic features, particle sorption, and random episodic events such as wind storms and precipitation. Correlation of dissolved organic matter quality measurements with timing of MeHg release suggests that MeHg is produced in areas of fluctuating redox and not limited by buildup of sulfide. The wetland was a net source of MeHg to the estuary in all seasons, with particulate flux being much higher than dissolved flux, even though dissolved concentrations were commonly higher. Estimated total MeHg yields out of the wetland were approximately 2.5 mg m 22 yr 21 -4-40 times previously published yields-representing a potential loading to the estuary of 80 g yr 21 , equivalent to 3% of the river loading. Thus, export from tidal wetlands should be included in mass balance estimates for MeHg loading to estuaries. Also, adequate estimation of loads and the interactions between physical and biogeochemical processes in tidal wetlands might not be possible without long-term, high-frequency in situ measurements.

Mercury Dynamics in a San Francisco Estuary Tidal Wetland: Assessing Dynamics Using In Situ Measurements

“…Many studies of Hg cycling in the sediments of estuarine open waters have been reported (reviewed in Merritt and Amirbahman 2009), but little is known about cycling in and exports from estuarine tidal wetlands (Mitchell and Gilmour 2008;Bergamaschi et al 2011;Bergamaschi et al 2012). Such wetlands are potentially important in regional mercury dynamics because they typically trap river-borne sediments (Ganju et al 2005); they host elevated levels of dissolved organic carbon (DOC), which can solubilize Hg (Amirbahman et al 2002;Bergamaschi et al 2011); and they are important sites of methylmercury production (Hall et al 2008).…”

“…Such wetlands are potentially important in regional mercury dynamics because they typically trap river-borne sediments (Ganju et al 2005); they host elevated levels of dissolved organic carbon (DOC), which can solubilize Hg (Amirbahman et al 2002;Bergamaschi et al 2011); and they are important sites of methylmercury production (Hall et al 2008). Further, tidal exchange between estuarine wetlands and the surrounding waters can drive material exchanges of fine sedimentary material or dissolved organic matter (Eckard et al 2007;Hall et al 2008;Kraus et al 2008), which are relevant to mercury transport.…”

“…Studies seeking to quantify tidal fluxes commonly sample discretely over a small number of tidal cycles and then extrapolate over longer time periods (Murray and Spencer 1997). This approach, however, can lead to large errors in the final flux estimates (Ganju et al 2005), and it may miss or obscure important interactions between physical and biogeochemical processes (Bergamaschi et al 2011). Continuous in situ measurements have the potential to address some of these problems.…”

“…High-resolution in situ measurements have been successfully used to measure constituent concentrations and fluxes in both tidal (e.g., Ganju et al 2005;Downing et al 2009;Bergamaschi et al 2011) and non-tidal (Downing et al 2008;Saraceno et al 2009) systems. Turbidity measurements, a consequence of light scattering by particles, is commonly used as a proxy for fine sediment concentrations and often for Hg because sediment-associated Hg is the largest fraction of Hg in aquatic systems (Domagalski 2001).…”

Recent Advances in Understanding Flow Dynamics and Transport of Water-Quality Constituents in the Sacramento–San Joaquin River Delta

“…Appreciation and consideration of tidal transport and effects were once limited to physical studies, but it has become more common in water quality and ecological studies, and has helped to improve our understanding of these fields (e.g., Bergamaschi et al 2011;Bennett and Burau 2015).…”