Diurnal trends in methylmercury concentration in a wetland adjacent to Great Salt Lake, Utah, USA

Abstract

A 24-h field experiment was conducted during July 2008 at a wetland on the eastern shore of Great Salt Lake (GSL) to assess the diurnal cycling of methylmercury (MeHg). Dissolved (< 0.45 μm) MeHg showed a strong diurnal variation with consistently decreasing concentrations during daylight periods and increasing concentrations during non-daylight periods. The proportion of MeHg relative to total Hg in the water column consistently decreased with increasing sunlight duration, indicative of photodegradation. During the field experiment, measured MeHg photodegradation rates ranged from 0.02 to 0.06 ng L^− 1 h^− 1. Convective overturn of the water column driven by nighttime cooling of the water surface was hypothesized as the likely mechanism to replace the MeHg in the water column lost via photodegradation processes. A hydrodynamic model of the wetland successfully simulated convective overturn of the water column during the field experiment. Study results indicate that daytime monitoring of selected wetlands surrounding GSL may significantly underestimate the MeHg content in the water column. Wetland managers should consider practices that maximize the photodegradation of MeHg during daylight periods.

Introduction

Knowledge of the fate and cycling of Hg in wetland environments is of high importance because of its capability to bioaccumulate in fish and birds. Once Hg has been input to a surface-water system, Hg can be transformed to organic forms such as MeHg, which is the form that can be accumulated in the food chain. As noted by Nimick et al. (in press), published concentration data for toxic elements such as MeHg may not represent accurate organism exposure because a sample collected during a single time period may not reflect the mean daily concentration. Failure to understand the diurnal concentration cycle of MeHg and the underlying physical/chemical processes could lead to ineffective management decisions in ecosystems with potential toxicity issues.

Biogeochemical cycling of Hg in aquatic ecosystems has been extensively studied over the past 20 years; however, only a few studies have examined diurnal cycling of MeHg in surface-water systems. Photodegradation of low levels of MeHg was observed in lakes and the degradation rate was first order with respect to MeHg concentration and intensity of solar radiation (Sellers et al., 1996). Krabbenhoft et al. (2002) quantified MeHg photodegradation rates by conducting an in situ MeHg incubation experiment. Forty liters of surface water was collected into a single container and then spiked to an initial MeHg concentration of 0.40 ng/L. The spiked sample bottles were arranged into five sets of six bottles that were tethered together with a poly rope at varying depths ranging from 0.1 to 8.0 m below the lake surface. Single bottles were then pulled from the lake at time intervals of 0, 2, 14, 29, and 43 days and analyzed for MeHg concentrations. Results from the experiment indicated that MeHg photodegradation processes were active at depths less than 1.0 m where ultraviolet (UV) light wavelengths can penetrate. Photodegradation of MeHg at this lake was found to be of equal importance to sedimentation and stream flow for removing MeHg from the system.

The MeHg photodegradation rates compiled for mid-latitude lakes ranged from 0.01 to 3.1 ng L^− 1 day^− 1 (Chen et al., 2003). Diurnal trends of selected mercury species were observed during field experiments conducted in the Florida Everglades (Krabbenhoft et al., 1998). The dirunal variability in total and reactive Hg was likely controlled by rainfall and photolytic sorption/desorption processes.

In contrast to photodegradation processes, other studies have found that selected surface-water systems may produce MeHg during daylight hours. Siciliano et al. (2005) found that MeHg concentrations increased or remained constant in two lakes during daylight hours. Nimick et al. (2007) provided the first documentation of diurnal Hg cycling in streams and found that during a 24-h period, filtered MeHg exhibited up to a 93% increase from morning minima, with the afternoon peaks possibly due to diurnal variations in pH and water temperature. Nighttime concentration of MeHg in surface-water samples from an agricultural wetland in California increased by twofold the daylight concentration, indicating that monitoring efforts should be designed to account for these potential diurnal variations (Fleck et al., 2009).

The Great Salt Lake (GSL) ecosystem (Fig. 1) was used as a study site to better understand the chemical and physical processes controlling the diurnal concentration of MeHg in a perimeter wetland setting. The 1,920 km² of perimeter wetlands adjacent to GSL is recognized as a vital waterfowl habitat of hemispheric importance (Aldrich and Paul, 2002). The GSL ecosystem receives industrial, urban, mining, and agricultural discharge from a 37,500-km² watershed that includes more than 2.1 million people in northern Utah. The unique combination of saline waters and freshwater wetlands of the GSL ecosystem comprises one of the most important breeding and staging areas for colonial waterbirds, waterfowl, and shorebirds in western North America. Because of the continental and hemispheric importance of GSL to several migratory and breeding waterbirds, it has been designated a site of hemispheric importance in the Western Hemisphere Shorebird Reserve Network (Aldrich and Paul, 2002).

Despite the ecological importance of the GSL ecosystem, little is known about the input and biogeochemical cycling of trace elements, including Hg, in the lake and perimeter wetlands. Recent biogeochemical assessments by federal and state agencies have found elevated levels of the more toxic form of Hg, MeHg, in water and biota associated with the GSL ecosystem (Naftz et al., 2008, Darnall and Miles, 2009, Naftz et al., 2009, Vest et al., 2009). Water samples from GSL have been found to contain elevated concentrations of MeHg exceeding 30 ng/L (Naftz et al., 2008) and total Hg concentration in eared grebe livers were found to increase by almost threefold during the fall molting period on GSL (Darnall and Miles, 2009). Vest et al. (2009) found that Hg concentrations in liver samples collected from three duck species (common goldeneye, northern shoveler, and green-winged teal) overwintering in the GSL ecosystem were among or exceeded the highest values reported in the published literature.

Elevated Hg concentrations in waterfowl resulted in the first health advisory in the United States limiting human consumption of three duck species harvested from the GSL ecosystem (Utah Department of Health, 2006). The wetlands surrounding GSL likely play a key role in the generation of MeHg. Numerous studies (Olson and Cooper, 1976, Branfireun et al., 1996, Driscoll et al., 1998) have indicated that wetlands are important zones of MeHg production and that substantial wetland reclamation could increase MeHg export to downstream water bodies (Marvin-DiPasquale et al., 2003).

In response to increasing public concern, the State of Utah initiated coordinated studies with the USGS to better understand the biogeochemical cycling of Hg in the open water and perimeter wetlands of the GSL ecosystem. Of particular interest was the impact of potential diurnal variations on the concentration and associated loadings of MeHg from perimeter wetlands and how wetland management plans may be modified to take advantage of natural removal processes, such as photodegradation. Specific objectives of this paper are to (1) determine the absence or occurrence of diurnal trends in MeHg and associated chemical constituent concentrations in water discharging from a representative wetland area adjacent to GSL and (2) identify potential physical and chemical processes that may control any observed diurnal chemical variations.