S.D. Study Links High Mercury to Lake Expansion
Prairie lakes that expand greatly in area during wet cycles are more likely to have elevated mercury levels soon after, South Dakota State University (SDSU) research suggests.
That's one finding in a study by recent Ph.D. graduate Trevor Selch and his adviser, adjunct associate professor Steve Chipps of SDSU's Wildlife and Fisheries Sciences Department. Chipps is also assistant unit leader for fisheries with the South Dakota Cooperative Fish and Wildlife Research Unit, a part of the U.S. Geological Survey.
The finding makes sense because there is mercury in the atmosphere from sources such as coal-fired power plants, and some of that mercury is continually deposited on the Earth's surface. When lakes expand their surface area, more of that mercury ends up in the water and in the food web.
"It's tied pretty strongly to the atmospheric deposition, which is to a large extent human-induced. But what's incorporating it into our lakes is the natural cycle of wet and dry periods," Chipps said.
(Selch is now a fisheries pollution control biologist for Montana Fish, Wildlife & Parks.)
Chipps said the study helped answer why South Dakota first began to see high mercury levels in fish from some lakes in the years after about 2000—first in Bitter Lake, then in other lakes that include Highway 81 lakes, Lake Isabel, Lake Hurley, North Island Lake, Pudwell Dam, Roosevelt Lake, and Twin Lakes.
"By the time we got started with the research in 2004, we had about six lakes that had mercury advisories on them," Chipps said. "The question was, why do these particular lakes have high mercury concentration? Prior to that in South Dakota we really hadn't had any mercury advisories to speak of."
Chipps said that research from other Midwestern lakes doesn't explain high fish mercury concentrations in South Dakota's lakes. The scientific literature suggested that lakes with low pH and low in productivity tend to have higher mercury levels. But in South Dakota, many lakes tend to have elevated pH levels, and those with high mercury levels are generally productive water bodies.
Chipps said Selch studied Landsat satellite imagery for the years since the 1980s to pin down the connection to surface area.
"He was able to look at the satellite images and then digitize the lakes before they flooded and then after they flooded," Chipps said. "And what we found was that lakes that expanded dramatically tended to have higher mercury concentrations than those that didn't expand as much."
Some South Dakota lakes with elevated mercury concentrations don't show very good natural reproduction in walleyes and perch, Chipps noted—a fact that could be related to a range of factors, including lack of suitable spawning habitat. But the SDSU study included lab experiments to assess mercury as a factor in reproductive success.
"We created five different mercury concentrations in the water. Then we had some eggs and some milt, or sperm from the males, that we used to fertilize the eggs at different mercury concentrations," Chipps said. "What Trevor found there was that the higher the mercury concentration in the water, the fewer eggs that were actually fertilized."
The control—with no mercury in the water—showed fertilization success of about 70 percent. But there was a steep decline in fertilization success as mercury concentrations increased.
"Our highest concentration was about 1 milligram of mercury per liter, which is pretty high for methylmercury concentration in the water. It's higher than what we would see in the natural environment. We just wanted to be sure we were covering the extremes. And there our fertilization success was only about 10 percent," Chipps said.
Though the experiment doesn't address other factors in reproductive success in fisheries, Chipps said, it clearly shows that high concentrations of mercury in water have a negative effect.
A map and a table of study results from different lakes are available at the following links: http://agbiocom.sdstate.edu/photos/Hg-Manuscript-map.jpg & http://agbiocom.sdstate.edu/photos/Hg-Manuscript-table.jpg .