Mysteries in the sediment: UMD’s Nathan Johnson is focused on the St. Louis River Estuary’s complex mercury problem

Just south of Duluth Minnesota, the St. Louis River Estuary forms a 12,000 acre network of critical habitat for North American wildlife, water and migratory birds, and native plants and fish. Located at the confluence of the St. Louis River and Lake Superior, the estuary also plays a key role in Minnesota’s wild rice trade, commercial fishing industry, and cultural and recreational heritage.  

Over the first half of the past century, however, the estuary’s water quality suffered dramatically as a result of industrial activity and the consequent chemical contamination. The surface water quality has largely been cleaned up, but concern remains about contaminants in sediment and the area has been federally designated an Area of Concern in the Great Lakes.


Johnson's students hope to hit pay dirt in the St. Louis River Estuary's mercury mystery. WRS PhD student  Amanda Brennan displays a sediment sample.

University of Minnesota Water Resources Science faculty Nathan Johnson and his colleagues are working alongside state and federal resource management agencies from around the region to shed that designation by restoring the estuary’s health and biological productivity. A Civil Engineering professor at the University of Minnesota - Duluth, Johnson’s research focuses on mercury contamination in the estuary’s sediment and understanding why some bodies of water are more sensitive to the bioaccumulation of methylmercury than others.

Specifically, Johnson is looking at why mercury in the estuary is acting differently than in other parts of the St. Louis River and Lake Superior. His research team’s results show that there is significantly more mercury in the food web of the St. Louis River Estuary than the immediately adjacent Lake Superior and St. Louis River.  For some reason, the mercury in the estuary in particular is persisting in its more bioaccumulative form known as methylmercury.

Methylmercury’s effects on ecosystem health are well known – it’s a powerful neurotoxin that even in low levels can impair the reproductive ability of fishes, birds, reptiles, and mammals, as well as the developmental ability of humans and other species. Most mercury in the environment exists in an inorganic form and is emitted through the combustion of fossil fuels, incineration and other industrial activities into the atmosphere. And since mercury spends a lot of time in the atmosphere, says Nathan, “much of the mercury that falls in Minnesota has come from other areas of the globe. It’s a global pollutant that most often enters watersheds via rainfall. Certain biological and geochemical processes make the conversion to bioaccumulative methylmercury more efficient in some waterbodies.”

While Johnson believes one answer may be found in the first layer of the estuary sediments, the hydrologic setting, its strong wetland influence, and elevated sulfate levels from upstream sources have created a complex situation. “It’s unclear what’s controlling the mercury production and accumulation in the base of the estuary’s food chain and I suspect that whether we can fix the problem will require a complex understanding,” he says.

Understanding the biogeochemical processes is key to allowing state and federal resource management to make informed decisions concerning the use of dredging materials and habitat restoration in the St. Louise River Estuary as well as the control of upstream mercury and sulfur sources.

Says Johnson, “In addition to understanding why there’s less mercury in the food web of Lake Superior and the St. Louis River than in the estuary, we’re working to see if there’s a role humans can play in helping to improve the water quality and whether cleaning contaminated sediment in the estuary will lower mercury levels.”

In a separate study, with support from the Minnesota Pollution Control Agency, Johnson and colleagues from Gustavus Adolphus College, University of Toronto Scarborough, and University of Wisconsin-La Crosse, are also investigating the transport and bioaccumulation of mercury in the watersheds of several Minnesota rivers that have shown exceptionally high mercury levels in fish tissue.  The project seeks to identify characteristics among watersheds that affect mercury cycling including landscape-to-stream delivery of mercury, processes that increase the rate at which mercury is transformed into the methylmercury form, and rates of bioaccumulation in the food web.