The methylmercury cycle in Little Rock Lake during experimental acidification and recovery

Abstract

The cycle of waterborne methylmercury (meHg) in Little Rock Lake is characterized by a period of accumulation during summertime (when the lake is warm and open to the atmosphere) and a period of decline during winter (when the lake is sealed by ice). We followed this cycle for 16 yr, during which time the lake was acidified with H 2 SO 4 and then allowed to recover naturally as part of a long-term field experiment on acidic rain. Mass balance was used to quantify meHg sources and sinks during acidification and recovery. Although year-to-year variability in the summertime accumulation of meHg was high during both acidified and de-acidified years (C.V. ϭ 0.7 and 0.5, respectively), on average 65% more meHg accumulated in the water column during acidification. Most of the meHg mass accumulated in the anoxic hypolimnion (Ͼ70%), even though the hypolimnion constituted Ͻ5% of the lake volume. In hypolimnetic waters, we observed a direct correlation between the maximum meHg concentration and the sulfate deficit for each year (r 2 ϭ 0.5-0.9) and a direct correlation between meHg and sulfide concentrations (r 2 ϭ 0.7). Sulfide was directly related to dissolved organic carbon at concentrations between 300 and 600 mol L Ϫ1 carbon (C). Seasonal changes in waterborne Hg (II) , meHg, and sulfate reduction covaried with the atmospheric deposition of Hg (II) and SO . Across all years, the interaction term [SO ϫ Hg (II) ] explained 70% of the variationin the meHg accumulation rate during summer. These results indicate that meHg production was co-mediated by several simultaneously occurring processes that affect the supply of Hg (II) substrate to the anoxic hypolimnion and the activity of methylating bacteria that are present there. They imply that meHg levels in lakes may respond to future changes in atmospheric Hg deposition in a rapid but complex way, modulated by environmental variables that can interact synergistically with Hg (II) supply. Such variables include sulfate in acid rain, organic carbon in terrestrial runoff, and temperature.Acid rain and methylmercury (meHg) contamination emerged as limnological issues during the 1970s, but it was not until the late 1980s that both were linked to atmospheric deposition and widespread human perturbation of the sulfur and mercury cycles. A direct connection between acid deposition and the aquatic meHg cycle was first indicated by early synoptic surveys that documented strong negative correlations between lake water pH and the concentration of meHg in fish (Spry and Wiener 1991; Wiener and Spry 1996). Today, pH remains the strongest environmental cor-1 Corresponding author (cjwatras@wisc.edu). AcknowledgmentsWe thank Nick Bloom, Emily Greenberg, Steve Claas, Beth Kuhn, Rick Back, Vanessa Visman, Jeff Rubsam, Lana Fanberg, Bruce Rodger, and many others for assistance throughout this longterm project. Mercury analyses prior to 1993 were conducted at Frontier Geosciences, Seattle, Washington.

Temporal trends of mercury concentrations in Wisconsin walleye (Sander vitreus), 1982–2005

“…Furthermore, other factors, including lake chemistry, atmospheric deposition, lake productivity, and fish growth rates, are also associated with latitude and alkalinity, so that several explanations for patterns in time trends are possible. Regional and local differences in walleye Hg trends are likely because of the complex interplay between atmospheric deposition of Hg, lake alkalinity and pH, and factors that affect microbial community composition and activity, such as the nutrient loading from runoff, or climate change (Watras et al 2006).…”

“…In Wisconsin, pH and alkalinity, lake productivity, and fish growth rates all tend to decrease with increasing latitude (Lillie and Mason 1983;Nate 2004). Alkalinity and pH themselves are probably correlated with causative factors such as the activity of sulfate-reducing bacteria leading to increased rates of methyl Hg production (Watras et al 2006). Increased lake productivity and warmer water temperatures in southern Wisconsin lead to faster fish growth rates, which Simoneau et al (2005) have found to be associated with lower Hg concentrations in fish of the same length.…”

Does lake thermocline depth affect methyl mercury concentrations in fish?

“…Warm, shallow, organic rich lake sediments are often important zones of net methylation, and the extent of these sediments in a lake may affect the conversion of Hg to MeHg (Bodaly et al 1993;Munthe et al 2007). Further, the presence of an anoxic hypolimnion significantly enhances net MeHg bioaccumulation in stratified lakes (Watras et al 2006). Additional mechanisms may be important for the bioaccumulation of MeHg, such as the overall response of the phytoplankton community and the food chain structure from producers to fish (Gorski et al 2008).…”

Bi-phasic trends in mercury concentrations in blood of Wisconsin common loons during 1992–2010

“…In our loon data, the second best fitting model for adult blood Hg trends included an interaction between pH and year, suggesting that the rate of change in Hg concentration over time depends on lake pH, with more rapid decreases over time in more acidic lakes (Online Resource 1-parameter estimates in Table 3). The interplay between atmospheric deposition, lake chemistry and other factors that affect microbial community composition, activity and trophic transfer (e.g., Watras et al 2006;Simoneau et al 2005;Chen and Folt 2005) likely leads to spatial differences in loon blood trends. Nevertheless, our biphasic time trend data for Wisconsin loons are largely consistent on a regional scale with contemporaneous trends in waterborne SO 4 and meHg observed in Little Rock Lake.…”