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Decades-long Prairie lakes mystery solved by U of R researchers has global implications

By University Advancement and Communications Posted: July 20, 2020 12:00 p.m.

The translucent ‘Ghost flea’ acts as an elevator, bringing toxic methylmercury up from sedimentary beds to the surface of Prairie lakes.
The translucent ‘Ghost flea’ acts as an elevator, bringing toxic methylmercury up from sedimentary beds to the surface of Prairie lakes. Photo: blickwinkel/Hecker

Midnight feedings, ghostly one-eyed creatures lurking on lake beds, scientists working late into the night to crack unsolved mysteries – these are not the spine-tingling hallmarks of a late-night horror movie but are rather the elements of a recent paper published by researchers at the University of Regina in Environmental Science & Technology Letters of the American Chemical Society. 

The paper “Mercury Elevator in Lakes: A Novel Vector of Methylmercury Transfer to Fish via Migratory

07-201.jpg
Dr. Britt Hall led the data analysis to
help solve the decades-long
mystery of why fish in southern
Saskatchewan lakes showed high
levels of methylmercury.
Photo: U of R Photography

Invertebrates” began with Dr. Britt Hall dusting off the data set started 23 years ago. Hall, a professor of biology in the Faculty of Science at the University of Regina, whose research focuses on how mercury moves in the environment, led the data analysis to help solve the decades-long mystery of why fish in Katepwa Lake in southern Saskatchewan showed high levels of methylmercury. 

Methylmercury is a neurotoxin, which can permanently damage brain activity in humans. Those most at risk of methylmercury poisoning are pregnant women, infants who are breastfed, and young children. The range of impacts can be incredibly severe (creating life-long physical and mental disabilities) or mild (including poor hand-eye coordination or intellectual delays), so it is important that fisheries managers around the world understand the pollutants that exist in recreational lakes and provide consumption warnings. 

“Prairie lakes, such as Katepwa, are productive – meaning they contain lots of nutrients that allow algae to grow, typically diluting the methylmercury and resulting in a lower level in fish,” explains Hall. “But in Katepwa Lake, the data was showing that methylmercury concentrations are actually quite high, and we didn’t know why that was.” 

According to the researchers, the high mercury levels could be caused by a particular kind of water flea called Leptodora which migrates up from the mercury-rich sediments at the bottom of the lake at night to feed near the surface of the water, transferring the methylmercury to fish when they are eaten. They carry mercury kind of like an elevator carries people from ground level to their offices or apartments on upper floors. 

“This migration to the surface increases the amount of toxin in the fish, which are in turn eaten by larger and larger fish, resulting in a bioaccumulation of mercury in the top predator – us,” says Dr. Peter Leavitt, Canada Research Chair in Environmental Change and Society, and professor of limnology at the U of R. 

He and his undergraduate and graduate researchers have been collecting the rich data on methylmercury levels in the Qu’Appelle lake system since 1994, as part of the Qu’Appelle Valley Long-Term Ecological Research program. The program has provided the basis for research findings of hundreds of academic papers since it began, including this one. 

Leptodora are very interesting creatures,” explains Hall. “They’re big for water fleas, about 1.7 centimetres long, and almost completely clear. They developed a lack of coloration as a prey-avoidance mechanism, because if they’re clear, it’s difficult for their hunters to see them.” 

As there are several different varieties of water fleas, Dr. Leavitt has affectionately coined the term “Ghost fleas” to differentiate the Leptodora

So, the researchers had discovered that there were abnormally high levels of methylmercury in Lake Katepwa and the research was clear that fish were ingesting the Ghost fleas that snacked on the mercury-laden sediment at the bottom of the lake, but the remaining mystery was how could fish, which use their vision to hunt, see these translucent creatures at night?

“Approximately six years ago, Dr. Richard Vogt, a post-doctoral student at the University of Regina at the time, conducted an experiment that determined the fish in this lake have a way of hunting without being able to see their prey,” said Hall. “Instead, the fish can sense the pressure waves Leptodora make as they swim through the water, which allows the fish to target them at night.” 

The missing piece of the puzzle that researchers didn’t have back in 1997 to explain the higher than expected methylmercury levels now slid into place, finally bringing the whole story together in Hall’s one publication. 

“The important message to fisheries managers is that productive lakes in the Prairies may have high levels of methylmercury, which does not fit our expectations. Knowing this can help people reduce the risk of mercury poisoning,” says Hall. “Managers can use this information to produce consumption advisories to warn sports fishers about how much fish they should be eating from the lakes.” 

07-202.jpg
When consumed by larger and larger
fish, mercury found in Ghost fleas
accumulates up the food chain,
resulting in a danger to humans.
Photo: Yale Peabody Museum of
Natural History / CC0

With this paper complete, Hall is returning her focus to her own research program where she and her students are studying why Prairie wetland ponds or sloughs have such diverse mercury biogeochemistry.  

“Mercury is a global pollutant and is emitted when we burn coal,” explains Hall. “It stays in the atmosphere for a year and a half before it is deposited into wetlands and lakes around the globe through precipitation. When I moved to Regina in 2005, no one was doing mercury research in Saskatchewan. It presents a great opportunity to do some novel research here that will increase our understanding of how climate change may impact the production of neurotoxic methylmercury.” 

In the meantime, University of Regina scientists are continuing their long-term studies of the Qu’Appelle lakes. Now in its 27th year, the Qu’Appelle Valley Long-Term Ecological Research program is one of the longest-running lake programs in Canada. 

Fish consumption guidelines can be found on the Saskatchewan Ministry of Parks, Culture, Heritage, and Sports webpage

Funding for the Qu’Appelle Valley Long-Term Ecological Research program is provided by the Natural Sciences and Engineering Research Council of Canada, Canada Research Chair program, and Canada Foundation for Innovation. 

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