Epsilon Archive for Student Projects

Abstract

Methylmercury (MeHg) is a widespread toxic pollutant of major concern in boreal aquatic ecosystems. The inundation caused by hydropower impoundments is well-known to elevate the concentrations of MeHg in water and biota. This study will investigate how a specific hydropower impoundment in northern Sweden has affected mercury (Hg) levels in surface waters and perch during a period of 30 to 32 years. The study focuses on the evolution of Hg following the initial, well-known elevation of MeHg. Temporal and spatial analyses are used to specify how the reservoir is affecting Hg levels through time, for levels both inside the reservoir as well as upstream and downstream. An analysis of potential drivers of the Hg concentrations was used to relate the concentration trends to environmental processes. From the results, the main potential driver for all three Hg variables in the reservoir is water colour, however, several potential drivers, such as oxygen content, have not been tested. The results from temporal and spatial analyses suggest that the elevated MeHg concentrations caused by inundation subsequently decreased for about two decades before the concentrations stabilised at a concentration two times higher than those in an adjacent reference site. Total Hg (THg) concentrations in surface water and THg concentrations in perch are also stabilised at a concentration two times higher than the reference lake three decades after inundation. These continuously elevated levels are suggested to be caused by either the hydropower management, the impact the hydropower impoundment has on the water regime, or potential variations in catchment Hg and organic matter discharge between the hydropower sites and the reference site.