| Summary: | Mercury (Hg) contamination in aquatic ecosystems is a serious concern because it can bioaccumulate and biomagnify to harmful concentrations within a food web and consequently end up in humans that eat polluted fish. The Pasvik watercourse, located in the border area between Norway, Finland and Russia, is strongly affected by the emissions of heavy metals from nearby Russian metallurgic smelters. In this study, the feeding ecology of the fish species present in Pasvik watercourse is examined. Further, Hg contamination in relation to their position in the trophic network and their distance from the local pollution sources is explored. Seven fish species (including three different morphotypes of whitefish, vendace, perch, pike, burbot, brown trout and grayling) were collected from two lakes in the Pasvik watercourse. Lake Vaggatem is located 40 km upstream from the main smelters and Lake Skrukkebukta 16 km downstream. Analyzes of stomach contents, habitat use and stable isotope signatures (δ15N and δ13C) revealed similar food web structures in the two study lakes, especially in respect to the piscivorous species. However, some trophic differences were evident between the lakes for vendace and two of the whitefish morphs seemingly caused by a more pronounced dominance and ecological effect of the invasive vendace in Vaggatem. In Skrukkebukta, there were significant differences in Hg concentrations between the littoral feeding LSR whitefish and the pelagic foraging DR whitefish. This was not evident in Vaggatem, probably due to the vendace dominance in the pelagic zone of this lake, resulting in a habitat relegation of DR whitefish from the pelagic to the littoral, and a more various diet also including benthic prey for this morphotype. The piscivorous species had higher Hg concentrations than the invertebrate feeders and the concentrations increased with size for the piscivores and for vendace. Pike, perch, vendace and the profundal feeding SSR whitefish had significantly higher Hg concentrations in Skrukkebukta. In addition, the downstream Lake Skrukkebukta showed a significantly higher biomagnification rate than in Vaggatem. It is plausible that the nearby smelters contributed to the significantly higher Hg concentrations in fish from Lake Skrukkebukta. However, the differences in feeding ecology also seems to play an important role. The invasive vendace has restructured the food web to a larger extent in the upstream Lake Vaggatem, which consequently may affect the transfer of Hg in the food web.
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