Does forest harvest increase the mercury concentrations in fish? Evidence from Swedish lakes

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.

Mercury methylating microbial communities of boreal forest soils

“…Indeed, the mobilisation of inorganic Hg (Hg (II) ) and, the more harmful, MeHg from soils by means of OM-mediated transport has been linked to MeHg accumulation in lake sediments within catchments 15,16 and in fish 17 . The role of forest soils is thus evident from the increase in MeHg export from forests to aquatic ecosystems 16,18,19 , and the subsequent bioaccumulation in downstream fish 17,20 . Since forest soils are an important site for MeHg formation 21 , it is crucial to understand the processes and the organisms involved in MeHg formation in boreal soils.…”

Blackfly Larvae (Simulium spp.) Can Intensify Methylmercury Biomagnification in Boreal Food Webs

“…Wetlands can increase the runoff of humic substances and DOM, increasing therefore the transport of DOM-bound Hg into the rivers and lakes, and can act as sinks and facilitate Hg biomethylation (Lane et al 2018). Leaching of MMHg from soils to surface waters may be increased by soil disturbance, which promotes Hg biomethylation (Braaten and de Wit 2016;Hsu-Kim et al 2018;Porvari et al 2003;Wu et al 2018). In the present study, the proportion of open water was negatively related to larval MMHg in the lake outflows.…”

“…Atmospheric depositing Hg is captured effectively by forests, and Hg predominantly enters the soils with leaf litter (Ward et al 2010). Land-use activities such as forest clear cutting, forest and wetland ditching, peat production, and mining are known to increase Hg loading to aquatic ecosystems in boreal catchments by releasing the deposited Hg when the ground and its microbial activity is disrupted (Porvari et al 2003;UNEP 2013;Wu et al 2018). Once having entered aquatic ecosystems, Hg becomes subject of natural biomethylation reactions by anaerobic bacteria, which are capable of transforming inorganic mercury into organic and the most toxic form, methylmercury (monomethylmercury, MMHg).…”

“…MMHg in turn can accumulate in high concentrations in food chains up to predatory fish and humans consuming fish (e.g. Watras et al 1998), and this Hg biomagnification can be enhanced after forest harvesting (Wu et al 2018).…”

Terrestrial diet influences mercury bioaccumulation in zooplankton and macroinvertebrates in lakes with differing dissolved organic carbon concentrations

“…This is also relevant in boreal ecosystems as common forest harvest activities can enhance terrestrial OM input to lake (Meunier et al, 2016), resulting in increased DOC concentrations (O'Driscoll et al, 2006;Glaz et al, 2015), and sometimes higher Hg and/or MeHg input from terrestrial to aquatic ecosystems (Eklöf et al, 2013;Kronberg et al, 2016). This can increase Hg bioaccumulation in food webs (Bishop et al, 2009;Hongve et al, 2012;Wu et al, 2018).…”