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It’s always interesting for me, being an aquatic biologist and angler, to look down into the water and see the diversity of life on the bottom of a beautiful stream.  It wasn’t until recently during my time researching invertebrates, that I now go out to a stream while fishing and wonder how much mercury, a neurotoxin, is in the body of the small critter my fly is trying to resemble.

Billowing smoke stacks of coal burning industries fill the Midwest.  These rolling fumes contain mercury (Hg) that gets combusted into the atmosphere and deposited over the landscape and into our sensitive wetlands.  Widespread deposition and the extensive nature of mercury sensitive landscape across the Midwest have produced methylmercury problems in some of our most remote aquatic ecosystems including Voyageurs National Park.

photo courtesy of - Megan Hess

Once in aquatic ecosystems, Hg sinks down to the sediments of the wetland where it is methylated by sulfate reducing bacteria.  In a new form, Methylmercury (MeHg), is now an organic molecule that organisms can take up into their tissues that get transferred up the food chain to the fish we love to catch.  MeHg is a neurotoxin that has led the Environmental Protection Agency (EPA) to establish fish consumption advisories for most of the states in the Great Lakes region.

photo courtesy of - Megan Hess
photo courtesy of - Megan Hess

Scientists from the University of Wisconsin – La Crosse’s River Studies Center have investigated the effects of atmospheric Hg deposition in lakes of our National Parks in the Great Lakes Region using larval dragonflies as biosentinels to measure MeHg. A biosentinel is an organism that uptakes a contaminant from the environment and can then be used to monitor the contaminant concentration and health of the ecosystem from analyzing the organism’s tissues (Haro 2014).

The highest level of MeHg contamination was in the most remote national park of the study, Voyageurs National Park.  The MeHg levels in dragonflies from lentic (still water) systems positively correlate to the amount of MeHg in predatory fish as the concentrations bioaccumulate up the food chain (Figure from Haro 2013).

photo courtesy of - Megan Hess

Riverine food webs in watersheds dominated by forested wetlands are also extremely vulnerable to the effects of methylation contamination.  My questions are, what about the methylation process in riverine ecosystems?  How do landscape changes and different land use practices in these watersheds influence the methylation process?  I decided to look at this in the Black River Watershed of west-central Wisconsin.

There is a gradient of land use in the Black River Watershed ranging from the dystrophic (acidic, low oxygen) peat-lands and abundant cranberry operations in the north-central part of the state to gently rolling agriculture influenced streams in La Crosse county where it empties to the Mississippi River.  Because of the differences in topography and land use, it is important to look at how these differences impact the methylation potential for sulfur-reducing bacteria as this could contribute to the bioaccumulation of MeHg in larger game fish that local fisherman catch and consume from the Black River.

photo courtesy of - Megan Hess

Using larval dragonflies as biosentinels, I have collected a wide range of species in the Black River Watershed of Wisconsin.  In particular, the larvae in the clubtail family (Gomphidae) and spiketail family (Cordulegastridae) burrow into sandy substrates, which is also just below the water-sediment interface where anaerobic sulfur-reducing bacteria convert inorganic Hg into organic MeHg.  Because of their habitat, large body size and relatively sedentary behavior, they are the biosentinel to use for detection of methylmercury.  These mercury concentrations can then be extrapolated for game fish that eat these critters to inform us of the levels we may be consuming in local fish.

In the Black River watershed there is a clear gradient in Hg levels from higher concentrations in the northern peat-lands to lower concentrations in the southeastern trout streams. For the Green-Striped Darner species, Aeshna verticalis, for example, the average Hg concentration in the north was 276 ng/g dry weight.  In lakes, when dragonflies contain these levels of Hg it correlates with predatory fish having Hg levels that exceed EPA limits.  The dominance of peatland in the northern region of the Black River watershed may explain why the larval Green Striped Darners possess higher Hg concentrations than similar larvae found in the southern part of the watershed. These northern peat-lands are also providing water downstream of the Black River and thus potentially transporting this MeHg (Branfireun 1999).

It is important to understand how mercury gets into our beloved water ecosystems and to have knowledge of areas that could be hot spots for methylation of mercury for human, organism, and environmental health.  Being able to identify these hot spots, resource managers can then better advise the fishing public through fish consumption advisories.

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