Carl J. Watras

Bioaccumulation of mercury in pelagic freshwater food webs

Current paradigms regarding the bioaccumulation of mercury are rooted in observations that monomethyl mercury (meHg) biomagnifies along pelagic food chains. However, mechanisms regulating the formation of meHg, its initial incorporation at the base of pelagic food chains, and its subsequent trophic transfer remain controversial. Here we use field data from 15 northern Wisconsin lakes, equilibrium aqueous speciation modeling, and statistical modeling to revisit several hypotheses about the uptake, distribution, and fate of inorganic Hg (HgII) and meHg in aquatic biota. Our field data comprise determinations of total Hg (HgT) and meHg in surface waters, sediments, microseston, zooplankton, and small fish in each of the study lakes. For these lake waters, strong positive correlations between DOC and aqueous concentrations of mercury along with negative correlations between DOC and the seston-water partitioning of mercury indicate that organic ligands bind HgII and meHg strongly enough to dominate their apparent aqueous speciation. In the microseston, zooplankton and fish, meHg concentrations and bioaccumulation factors (BAFs) increased with increasing trophic level while biotic concentrations of HgII decreased--indicating that meHg was indeed the biomagnified species of mercury. For all trophic levels, meHg concentrations varied positively with the calculated aqueous concentration of meHg+ (free ion), especially when coupled with pH, or meHgOH (hydroxide) species but not with meHgCl0, the neutral chloride complex. These findings suggest that: (1) the passive uptake of meHg does not control bioaccumulation at the base of aquatic food webs in nature (i.e. phyto- and bacterioplankton); (2) correlation with pH and DOC largely reflect the supply and bioavailability of meHg to lower trophic levels; and (3) meHg concentrations at higher trophic levels reflect uptake at low trophic levels and other factors, such as diet and growth. Low concentrations of meHg in surficial sediments indicate that the fates of biotic HgII and meHg are different. Most biotic meHg is demethylated rather than buried in lake sediments.

Observations of methylmercury in precipitation

Abstract Using a newly developed technique, methylmercury species have been quantified in several precipitation and lake water samples. Mercury species are determined by aqueous-phase ethylation to the volatile dialkyl analogs, followed by cryogenic gas chromatographic (GC) separation. Mercury-specific detection by cold vapour atomic fluorescence spectrometry permits detection limits of about 0.1 pg as Hg. Snow samples collected from north-central Wisconsin contained monomethylmercury levels of about 0.25 p M (0.05 ng l −1 Hg) and total mercury concentrations of 20 p M (4 ng l −1 Hg). A time series of rain samples collected during a storm passing over the North Olympic Peninsula in western Washington State showed average monomethylmercury levels of 0.75 p M (0.15 ng l −1 Hg), with total mercury concentrations from 10 to 25 p M (2–5 ng l −1 Hg). Total mercury showed a strong washout effect over the course of the storm, while methylmercury appeared to show a diurnal pattern, with elevated levels during the daylight hours. No dimethylmercury was observed in any precipitation sample. Methylmercury was observed in most lakes studied, with a high of 3.1 p M (0.64 ng l −1 Hg) in Onadaga Lake, New York, and a low of M in Lake Crescent, located in the Olympic Mountains of Washington State.

Methylmercury production in the Anoxic Hypolimnion of a Dimictic Seepage Lake

Experimental results and field data indicated that methyl-Hg was produced within a layer of bacterioplankton near the top of the anoxic hypolimnion of Pallette Lake. In situ incubations at ambient Hg concentrations indicated that the net flux of methyl-Hg from the layer was between 50 and 100 pmol/m2*d. This input was sufficient to account for the summer accumulation of methyl-Hg in the entire hypolimnion and it exceeded atmospheric inputs by 2 orders of magnitude. Maximum rates of net methylation occurred in the same region of the water column where we observed maximum rates of sulfate reduction. The measured rates were: 100 fmol rriethyl-Hg/L*d and 90 nmol SO4/L*d. Sulfate reducing enrichment cultures isolated from the hypolimnion were also able to methylate Hg in the laboratory. Sulfate reduction did not occur in anoxic profundal sediments during summer and we infer from ancillary data that methylation in profundal sediments was also low. Whole-lake rates of sulfate reduction in the hypolimnetic layer and shallow sediments were roughly equivalent, but we cannot yet compare methylation rates at these sites due to large uncertainties in the littoral flux of methyl-Hg. We propose that zones of Hg methylation and SO4 reduction follow the oxic/anoxic boundary in both the watercolumn and sediments. The relative importance of watercolumn and sediment processes will depend on the physical and chemical structure of a given lake.

Impact of acidification on the methylmercury cycle of remote seepage lakes

Concentrations of monomethylmercury [CH3Hg] were measured in the water and seston of five nearly pristine Wisconsin lakes, which span a range of pH from about 4.6 to 7.2. Previous studies had established a clear inverse relationship between [CH3Hg] in fish and the pH of lakes in this region. Here, we examined the pH dependency of [CH3Hg] in lake water and explored the partitioning of CH3Hg between water, seston, and fish as a function of pH. Results indicate that [CH3Hg] in lake water tends to increase as pH decreases, but that seasonal and spatial variability of [CH3Hg] in individual lakes confounds a simple analysis of the relationship. The partitioning of CH3Hg was related only weakly, if at all, to pH. Average partitioning coefficients (log kd=log (Cp/Cw)) were higher for yearling yellow perch (6.0 to 6.5) than for seston (5.5 to 6.0) but did not vary significantly between lakes. This suggests that acidification has a stronger effect on the supply of CH3Hg to the ecosystem than on specific rates of uptake by the biota.

Mercury in surficial waters of rural Wisconsin lakes

Abstract As part of an ongoing program to investigate mechanisms regulating the aquatic biogeochemistry of Hg, unfiltered surface waters of eight rural lakes in northcentral Wisconsin were collected and analyzed for reactive and total Hg. Samples were collected during autumn mixis using ultra-clean, trace-metal-free protocols which have been applied successfully in the marine environment. Results indicate that Hg concentrations are considerably lower than previously reported, and approach levels observed in remote ocean waters. Concentrations of reactive Hg ranged from 0.7 to 2.9 pM in the eight lakes studied. Total Hg, determined following strong oxidation using BrCl, ranged from 4.7 to 9.7 p M in four of the lakes. Although the data set is limited, these concentrations are 20–100 times lower than previous estimates for lakes in the region. The results reemphasize the importance of uncompromised clean laboratory protocols in the collection and analysis of trace constituents in natural waters. They also imply a reevaluation of ideas regarding the sources and distribution of Hg in lakes. A simple mass balance for Little Rock Lake clearly shows the potential importance of both atmospheric Hg deposition and sedimentary remobilization to the geochemical cycling and bioaccumulation of Hg.

Recent declines in mercury concentration in a freshwater fishery: isolating the effects of de-acidification and decreased atmospheric mercury deposition in Little Rock Lake

The atmospheric deposition of H+, SO4, and Hg to Little Rock Lake in northern Wisconsin has declined substantially during the past decade. Parallel decreases have been observed in the surface waters of the lake. Here we extend the observations to the fish community and we present evidence of a contemporaneous decline in levels of Hg in fish tissue. By comparing data from two separated basins of the lake, we then make an initial effort to isolate and quantify the relative importance of de-acidification and reduced Hg deposition on mercury contamination in fish. Statistical modeling indicates that fish Hg in both basins decreased by roughly 30% between 1994 and 2000 (-5%/y) due to decreased atmospheric Hg loading. De-acidification could account for an additional 5% decrease in one basin (-0.8%/y) and a further 30% decrease in the other basin (-5%/y), since the basins de-acidified at very different rates. These results are consistent with the hypothesis that depositional inputs of SO4 and Hg(II) co-mediate the biosynthesis of methyl mercury and thereby co-limit bioaccumulation. And they suggest that modest changes in acid rain or mercury deposition can significantly affect mercury bioaccumulation over short-time scales.

Experimental acidification of Little Rock Lake, Wisconsin

The controlled acidification of a two-basin lake is described. The lake was divided by a vinyl curtain in 1984; acidification of one basin began in 1985. Target pH values of 5.5, 5.0 and 4.5 are planned for 2-yr increments. Biotic and chemical responses and internal alkalinity generation are being studied. Baseline studies, initial results at pH 5.5, and predictions of lake responses to acidification are described.

Mercury methylation in the hypolimnetic waters of lakes with and without connection to wetlands in northern Wisconsin

Rates of Hg methylation and demethylation were measured in anoxic hypolimnetic waters of two pristine Wisconsin lakes using stable isotopes of Hg as tracers. One of the lakes is a clear-water seepage lake situated in sandy terrain with minimal wetland influence. The other is a dark-water lake receiving channelized inputs from a relatively large terrestrial wetland. Methyl mercury (MeHg) accumulated in the anoxic hypolimnia of both lakes during summer stratification, reaching concentrations of 0.8 ng center dot L-1 in the clear-water lake and 5 ng center dot L-1 in the dark-water lake. The stable isotopic assays indicated that rate constants of Hg-(II) methylation (K-m) ranged from 0.01 to 0.04 center dot day(-1) in the clear-water lake and from 0.01 to 0.09 center dot day(-1) in the dark-water lake, depending on the depth stratum. On average, K-m was threefold greater in the dark-water lake. Hypolimnetic demethylation rate constants (K-dm) averaged 0.03 center dot day(-1) in the clear-water lake and 0.05 center dot day(-1) in the dark-water lake. These methylation rates were sufficient to account for the observed accumulation of MeHg in hypolimnetic water during summer in both lakes. Despite substantial export of MeHg from the wetland to the dark-water lake, our study indicates that in-lake production and decomposition of MeHg dominated the MeHg cycle in both lakes.

Mercury cycling in a northern wisconsin seepage lake: The role of particulate matter in vertical transport

During summer stratification, total mercury (Hgτ) reached maximum concentrations in the O2 :depleted, hypolimnion of Little Rock Lake, Wl. Initially, the hypolimnetic increase was attributed solely to redox-controlled release of Hg from bottom sediments. However, subsequent depth profiles of Hg indicated that hypolimnetic Hg enrichment could also result from the downward transport and recycling of particulate Hg prior to incorporation in the sediments. Contrasts between Fe and Hg cycles in this lake reinforce this notion. Increases in hypolimnetic Fe were observed during both summer and winter O2 decreases. In contrast, hypolimnetic Hg concentrations declined during winter. In the ice-free season, the distribution of particulate mercury (Hgp) correlated with the distribution of chlorophyllous particulates in this lake, re-emphasizing the importance of biotic processes in controlling Hg cycling in the hypolimnion.

Little Rock Lake (Wisconsin): Perspectives on an experimental ecosystem approach to seepage lake acidification

Ecosystem-level experiments are essential in assessing the effects of environmental perturbations like acidification. To date in North America, such experiments have been rare and geographically limited to drainage lakes in south-western Ontario and to streams in the northeastern US. Seepage lakes, which are the dominant hydrologic type in large regions of the US, have received limited attention from many perspectives, including whole-system manipulation. The Little Rock Lake Acidification Project was initiated to expand insights from previous acidification experiments with whole drainage lakes to a seepage lake system. It involves the gradual acidification of a small (18 ha), seepage lake in northcentral Wisconsin. The lake has been divided into a treatment and reference basin using a flexible, inert barrier; and the treatment basin is being acidified in steps of 0.5 pH units/2 yr period from a starting pH of 6.1 to a final pH of 4.6 (roughly the average pH of rain in this region). The goals are to document the biological and chemical changes which occur, to identify the direct and indirect mechanisms which regulate responses, and to expand insights to a class of lakes previously understudied. In this paper, we describe the history and rationale of the project and we discuss in general terms the utility and constraints of whole-ecosystem manipulations.