Temporal and regional effects of forest harvesting on mercury bioaccumulation and biomagnification in boreal stream food webs.

Forest harvesting can affect mercury (Hg) dynamics in boreal stream food webs by changing the availability and methylation of Hg and dietary exposure to methylmercury (MeHg) through basal resources. The objectives of this study were to quantify the 1) temporal and 2) regional effects of forest harvest on Hg bioaccumulation and biomagnification in streams. From 2019 to 2021, 5 headwater streams were sampled (n = 2 harvested, n = 3 reference) pre-(2019), during (2020), and post-(2021) harvest for basal food sources, macroinvertebrates and fish. In 2021, 7 streams (n = 4 harvested, n = 3 reference) were also sampled to assess the regional effects of forest harvest. All samples were analyzed for total Hg ([THg]) or MeHg ([MeHg]), and stable isotopes of nitrogen (δ15N). Temporally, we observed increases in [MeHg] in water and some predatory macroinvertebrates and [THg] in fish post-harvest at the most impacted stream (i.e., 78 % of watershed harvested, a stream crossing, and narrow buffer zones), but few differences at the stream where more rigorous best management practices were used (i.e, wider buffer zones and machinery avoidance of wet soils). Regionally, we found significantly higher [MeHg] in food sources and macroinvertebrates in harvested compared to non-harvested landscapes, but no difference in [THg] of fish or Hg biomagnification (log Hg vs. δ15N). Overall, results showed that harvesting increased [Hg] in some food web components, and the magnitude of these effects varied with management practices used and proportion of the watershed harvested.