Mercury cycling in hydroelectric reservoirs of northern Manitoba decades after impoundment

As the global climate changes and demand for renewable electricity increases, construction of hydroelectric dams is increasing globally though the impacts of regulating the worlds rivers are still understudied. Northern Manitoba, Canada, has extensive hydroelectric development since the 1950s; fish mercury (Hg) concentrations in on-system lakes were observed to have increased above human consumption guidelines upon impoundment and have taken decades to decrease towards natural concentrations. To better understand the long-term impacts of hydroelectric regulation on Hg in fish and other biota in Northern Manitoba, we determined methylmercury (MeHg) production potential in soil from the water fluctuation zone in on- and off-system lakes through a soil flooding incubation experiment in the laboratory. We further studied the historic flux of MeHg and Hg to the sediments in on- and off-system lakes and links to organic matter in these waterbodies. We found that MeHg production was highest in the water fluctuation zone of the on-system lakes, which may represent an increased source of MeHg to the food web in these environments even decades after impoundment. In addition, sedimentation rates were found to greatly affect Hg fluxes to the sediment in those waterbodies where increased water flows result in higher erosion and sedimentation. These findings provide new insight in our understanding of the long-term recovery of Hg cycling within on-system lakes decades after impoundment.