Abstract
A number of studies have evaluated the effects of forest harvest on mercury (Hg) concentrations and exports in surface waters, but few studies have tested the effect from forest harvest on the change in fish Hg concentrations over the course of several years after harvest. To address this question, mercury (Hg) concentrations in perch (Perca fluviatilis) muscle tissue from five lakes were analyzed for two years before (2010−2011) and three years after (2013–2015) forest harvest conducted in 2012. Fish Hg concentrations in the clear-cut catchments (n=1373 fish specimens) were related to temporal changes of fish Hg in reference lakes (n=1099 fish specimen) from 19 lakes in the Swedish National Environmental Monitoring Programme. Small (length<100mm) and large perch (length≥100mm) were analyzed separately, due to changing feeding habitats of fish over growing size. There was considerable year-to-year and lake-to-lake variation in fish Hg concentrations (−14%–121%) after forest harvest in the clearcut lakes, according to our first statistical model that count for fish Hg changes. While the effect ascribed to forest harvest varied between years, after three years (in 2015), a significant increase of 26% (p<0.0001) in Hg concentrations of large fish was identified in our second statistical model that pooled all 5 clearcut lakes. The large fish Hg concentrations in the 19 reference lakes also varied, and in 2015 had decreased by 7% (p=0.03) relative to the concentrations in 2010–2011. The majority of the annual changes in fish Hg concentrations in the clearcut lakes after harvest were in the lower range of earlier predictions for high-latitude lakes extrapolated primarily from the effects of forest harvest operations on Hg concentrations in water. Since the risk of forest harvest impacts on Hg extends to fish and not just surface water concentrations, there is even more reason to consider Hg effects in forestry planning, alongside other ecosystem effects.
Highlights
The raw materials provided by forest harvest are of great socioeconomic value
A number of studies have evaluated the effects of forest harvest on Hg concentrations and exports in surface waters, but few studies have tested the direct effect from forest harvest on fish Hg concentrations
The six lakes sampled in this study were oligotrophic with relatively high dissolved organic carbon (DOC) concentrations (N 5 mg L−1) (Vollenweider and Kerekes, 1980) (SI Table S3)
Summary
There are, concerns about how forestry operations influence water quality (Futter et al, 2016; Roberge et al, 2016). One of these concerns is the possible effects on mercury (Hg) concentrations in freshwater biota in boreal waters that already pose a risk to human and wildlife health (Åkerblom et al, 2014). Since there is increasing evidence that the export of total mercury (THg) and the bioaccumulative methylmercury (MeHg) is affected by forest operations (Eklöf et al, 2016), the limited information on the biotic effects of forest harvest on fish Hg concentration is a concern in itself. Further investigations on the connection between forestry and Hg concentrations in biota, especially fish, are needed to provide a better basis for managing the many trade-offs inherent in forest management
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