15 December 2010

Pumping oxygen into lakes may reduce mercury contamination

Posted by mohi

Twin Lakes

Colville National Forest's Twin Lakes were the site of an experiment to see if adding oxygen to the deep water changes the lake's mercury content. Photo courtesy of USDA Forest Service

Doctors sing the praises of the fatty acids found in fish, but lament the toxic affects of eating fish contaminated with mercury. Fisherman are warned against taking fish from nearly one-third of lakes in the U.S. because of high concentrations of mercury. In some freshwater lakes, the most harmful form of mercury, methylmercury, tends to build up in oxygen-free layers that occur at the bottom of the body of water. In B23F-0451 “Effects of Hypolimnetic Oxygenation on Mercury Cycling in Twin Lake, Washington,” Marc Beutel of Washington State University described how injecting oxygen just above the lake’s bottom affects the levels of methylmercury in the water.

Groups of bacteria that live in the oxygen-free zones are thought to be responsible for why lower levels of the lake contain high concentrations of methylmercury. Beutel hypothesized that sending oxygen into the oxygen-free zone of a lake would reduce its methylmercury levels.

Along with other researchers, Beutel was able to test this hypothesis in the Twin Lakes of Washington state, a site where oxygen is already being pumped into one of the lakes. Populations of redband trout in the Twin Lakes drop down in warm summer months when the bottom layers of water become oxygen-free. To counteract this, a group of Native American tribes called the Colville Confederated Tribes are adding pure oxygen gas into the bottom of the North Twin Lake.

The Washington State research team tested the amount of dissolved methylmercury in the oxygenated North Twin Lake and compared the levels to those of the untreated South Twin Lake. They found that the mercury levels were 10 times lower in newly oxygenated lake.

The team predicted that the amount of methylmercury in zooplankton in the North Twin Lake would also be lowered by the oxygen treatment. Surprisingly, they found the opposite: there was more mercury per kilogram of zooplankton in the treated North Twin Lake than in the untreated South Twin Lake. Beutel suspects the results may be due to different numbers of zooplankton or the location that the zooplankton occupy in each lake. The research team plans to see if the amount of methylmercury compared to total zooplankton is lower in the oxygenated lake.

Beutel’s study sheds light on the complex ecology of mercury accumulation suggests that lake management strategies such as oxygenation can affect mercury cycling in our freshwaters. And the more we know about this toxin behaves in freshwater, the sooner regulators can get moving on cleaning up our local fishing holes.

–Susan Young is a science communication graduate student at UC Santa Cruz