Multiple stresses from a single agent: Diverse responses to the experimental acidification of Little Rock Lake, Wisconsin

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.

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“…3A. During acidification, meHg concentrations in phytoplankton, zooplankton, and fish increased significantly (Frost et al 1999).…”

Section: Methodsmentioning

confidence: 99%

“…1; Brezonik et al 1993;Frost et al 1999). This clear-water lake receives little wetland runoff, and the concentration of waterborne meHg follows a well-defined annual cycle, increasing during summer, when the lake is warm and open to atmospheric deposition and declining during winter, when the lake is ice covered (Fig.…”

Section: Meili 1994mentioning

confidence: 99%

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

et al. 2006

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“…Another important source of variation in this study was pH, which has been implicated by other workers in studies of aquatic bacterial communities (44,47,53) and communities of organisms likely to influence BCC (21,24,25,38). pH may reflect the influence of geology on water chemistry and is itself an important control of the biogeochemical transformations which can take place in a given environment.…”

Section: Sources Of Variation In Bccmentioning

confidence: 99%

Geographic and Environmental Sources of Variation in Lake Bacterial Community Composition

Yannarell

Triplett

2005

Appl Environ Microbiol

332

259

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This study used a genetic fingerprinting technique (automated ribosomal intergenic spacer analysis[ARISA]) to characterize microbial communities from a culture-independent perspective and to identify those environmental factors that influence the diversity of bacterial assemblages in Wisconsin lakes. The relationships between bacterial community composition and 11 environmental variables for a suite of 30 lakes from northern and southern Wisconsin were explored by canonical correspondence analysis (CCA). In addition, the study assessed the influences of ARISA fragment detection threshold (sensitivity) and the quantitative, semiquantitative, and binary (presence-absence) use of ARISA data. It was determined that the sensitivity of ARISA was influential only when presence-absence-transformed data were used. The outcomes of analyses depended somewhat on the data transformation applied to ARISA data, but there were some features common to all of the CCA models. These commonalities indicated that differences in bacterial communities were best explained by regional (i.e., northern versus southern Wisconsin lakes) and landscape level (i.e., seepage lakes versus drainage lakes) factors. ARISA profiles from May samples were consistently different from those collected in other months. In addition, communities varied along gradients of pH and water clarity (Secchi depth) both within and among regions. The results demonstrate that environmental, temporal, regional, and landscape level features interact to determine the makeup of bacterial assemblages in northern temperate lakes.

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“…Both fish and benthic invertebrates can tolerate a lower pH in naturally acidic high DOC systems than they can in clear anthropogenically acidified systems, although DOC concentrations Ͼ20 mg liter Ϫ1 may be toxic (Collier et al 1990). In acidified lakes organisms are subjected to multiple stresses in addition to pH (Frost et al 1999), and DOC and pH have interactive effects on the toxicity of contaminants (Knulst 1992;Miskimmin et al 1992;Welsh et al 1993).…”

Section: Increases Attenuation Of Solar Radiationmentioning

confidence: 99%

Dissolved organic carbon and nutrients as regulators of lake ecosystems: Resurrection of a more integrated paradigm

Williamson

Morris

Pace

et al. 1999

Limnology & Oceanography

Self Cite

359

298

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The primary interpretive paradigm used to study lakes is their trophic status. Oligotrophic lakes have low nutrient loading and low productivity, while eutrophic lakes have high nutrients and high productivity. The strong empirical relationship between nutrient loading and productivity is a valuable tool for teaching, for research, and for management of lakes. In order to incorporate the variety of other known anthropogenic impacts on lakes, however, lake characterization needs to extend beyond the nutrient-productivity paradigm. For example, acid precipitation, heavy metal and toxic organic contaminants, increases in UV radiation, and global warming are all recognized threats to lake ecosystems. One of the key characteristics of lakes that determines how they respond to disturbances such as these is their concentration of colored dissolved organic carbon (CDOC). Here we argue that a paradigm that includes CDOC (using the absorption coefficient at 320 nm as a proxy) as well as nutrients will be useful in predicting and understanding the response of lake ecosystems to multiple stressors. We propose to resurrect the CDOC axis that was proposed by investigators earlier this century and to extend it by adding some operational definitions to permit placing some of the major lake types on the axes in a way that will help us to better understand the structure, function, and response to disturbance of lake ecosystems that are subject to natural and anthropogenic environmental changes at the local, regional, and global scales. Data from a few diverse lakes and a successional sequence in Glacier Bay, Alaska, are used to illustrate the potential utility of the 2-axis model in separating lake types.The most commonly used paradigm for studying lake ecosystems defines lakes in terms of their trophic status. Lakes with low nutrients and low organic production are considered oligotrophic, while those with high nutrient inputs and organic productivity are eutrophic (Wetzel 1983). This paradigm has been adopted by the general ecological community as well as limnologists and is in general use in introductory ecology and limnology textbooks. The trophic status paradigm is supported by a particularly strong empirical relationship between chlorophyll and phosphorus (Vollenweider 1968;Dillon and Rigler 1974;McCauley et al. 1989), by comparative studies and whole-lake experiments that have clearly demonstrated the role of nutrients and grazers in eutrophication Pace 1984; Carpenter et al. 1991), and by the widespread success of remediation of AcknowledgmentsWe thank Dan Engstrom for stimulating discussions on the role of DOC in the succession of lakes in the Glacier Bay chronosequence, Sheri Fritz, Tom Frost, and Bruce Hargreaves for providing feedback on some of these ideas in their early stages of development, Dave Krabbenhoft for discussions on Hg, and Philip Singer and James Symons for introducing us to the literature on water treatment disinfection by-products and DOC. Sheri Fritz provided the unpublished data on Brush La...

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Multiple stresses from a single agent: Diverse responses to the experimental acidification of Little Rock Lake, Wisconsin

Cited by 46 publications

References 54 publications

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

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

Geographic and Environmental Sources of Variation in Lake Bacterial Community Composition

Dissolved organic carbon and nutrients as regulators of lake ecosystems: Resurrection of a more integrated paradigm

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