October 2023:  In bodies of water, higher sulfate levels are correlated with increased mercury methylation, leading to higher mercury concentrations in the food chain, says US Forest Service researcher Randy Kolka. In his recent talk “Factors influencing mercury cycling and bioaccumulation in fish,” Kolka described how that process is accelerated by the sulfate left behind by mining and other industrial processes.

The presence of elevated sulfate speeds mercury methylation: elemental mercury (Hg) is released from the biomass in a water body— think of the mud and decaying plant matter in a lake or slow-moving stream bottom— becoming methylmercury (MeHg). Both toxic and soluble, methylmercury can move through the ecosystem, entering the bodies of fish and insects, for example. 

To test those sulfate effects, Kolka and colleagues created research zones in the lakes and bogs at the Forest Service’s Marcell experiment station near Grand Rapids, Minnesota. Using sprinklers, they deposited sulfate in controlled amounts and compared those areas over time to nontreated areas, using the naturally occurring sulfate levels as a baseline. They found that when that the natural sulfate level was quadrupled, the rate of mercury methylation doubled. When further magnified by addition of carbon (against the background of carbon-rich peat), mercury methylation rose by a factor of ten.

Another finding: mercury levels in mosquito larvae rose in proportion to added sulfate. Since waterborne insects can be ingested by birds and other wildlife, methylmercury can easily spread beyond the aquatic food chain and into the wider food web.

Kolka’s talk was hosted by the Tuesday Group lecture series in Ely, Minnesota September 5, 2023. View it here (56 min): Factors influencing mercury cycling and bioaccumulation in fish. The six-minute section on sulfate starts at the 23:16 mark.

Image: This slide from Kolka’s talk (at 23:54) shows the result of adding sulfate to one of the experiment zones at the Marcell station.