More than two decades after University of Regina students found fish in Katepwa Lake had higher than expected methylmercury levels, researchers believe they finally have an answer as to why.
The mystery started in 1997, when Tyler Cobb and Mark Graham — U of R students at the time — did a research project where they tested fish and zooplankton in Katepwa Lake for methylmercury over a 24-hour period. What they found surprised them.
Katepwa Lake is a productive lake, meaning it contains a lot of nutrients that allow algae to grow. Normally, this dilutes the methylmercury and the fish have a low level of the neurotoxin. But Cobb and Graham found that Katepwa’s fish still had a relatively high methylmercury level, and that it was higher overnight than during the day.
This data became part of Peter Leavitt’s decades-long research into the ecology of Katepwa Lake. Leavitt is a professor of limnology at the U of R and Canada research chair in Environmental Change and Society.
Researchers also noted a type of transparent water flea in the lake called Leptodora — which Leavitt dubbed “ghost fleas” — that had higher methylmercury levels.
Methylmercury is a toxic form of mercury, and can cause permanent brain damage ranging from mild to severe, if too much of it is consumed. Pregnant women, breastfed infants and young children are the most at risk of methylmercury poisoning.
“We had an idea that the Leptodora was actually contributing to the higher levels of mercury in fish, but the piece that was missing was that we didn’t really understand the ecology of these Leptodora and how the fish were eating them,” said Britt Hall, a biology professor at the U of R.
Richard Vogt joined the team trying to solve the mystery around 2012. The former U of R post-doctoral student figured out that, although the fish in the lakes couldn’t see the ghost fleas because of their transparency, the fish could sense the pressure waves they make when they swim.
When the Leptodora come up from the mercury-rich sediments at the bottom of the lake to feed near the surface at night, the fish snap them up, accumulating the methylmercury they carried.
Once that piece was in place, Leavitt turned to Hall in 2015, with her background in how mercury moves through the environment, to see if she could make heads or tails of the data and turn it into a scientific paper . Using Leavitt’s wealth of data on the lake and the contributions of other researchers, Hall recently published the paper Mercury Elevator in Lakes: A Novel Vector of Methylmercury Transfer to Fish via Migratory Invertebrates.
“This just gives us a mechanism as to why the concentrations in fish might be unexpectedly high in these productive lakes,” Hall said.
Around the world, Hall hopes this serves as a reminder to fishery managers to always test even productive lakes for their methylmercury levels rather than assume the levels will be low.
In Saskatchewan, the provincial government provides consumption guidelines for fish in all lakes based on their mercury levels.
Copyright Postmedia Network Inc., 2020
