Forest harvest effects on mercury in streams and biota in Norwegian boreal catchments

Abstract

Forest harvesting practices can potentially increase mercury runoff from catchments. A paired catchment experiment was conducted in a boreal forest in southern Norway, to test effects of forest harvest operations on i) concentrations and fluxes of methylmercury (MeHg), total mercury (HgT), nutrients and dissolved organic matter (TOC), and on ii) MeHg bioaccumulation in stream foodwebs. Thirty percent of a catchment was harvested in winter time with snow cover but no soil frost, resulting in wheel tracks and soil compaction. Pre-harvest differences included higher streamwater MeHg, HgT and TOC, and lower pH in the treated catchment compared to the reference. No significant treatment effects on concentrations of MeHg, HgT and TOC were detected, whereas concentrations of nutrients (ammonium, nitrate, phosphorus (P)) increased significantly. Estimated catchment export of nitrate and ammonium increased fourfold, as a combined effect of changed discharge and concentrations. Export of MeHg and HgT increased weakly, primarily because of an increase in discharge. Levels of MeHg in stream invertebrates mirrored differences in aquatic MeHg between the two streams, resulting in higher MeHg in biota in the harvest catchment in pre-harvest conditions. After harvest, MeHg levels in primary consumers (herbivorous stoneflies) were no longer different between the two streams, despite continued exposure to higher aqueous MeHg in the harvested catchment. Simultaneously, dietary biomarkers (δ 15 N signature, lipid-and algal fatty acid content) in the stoneflies had changed significantly, consistent with higher nutrient loadings and associated higher diet availability in the harvested stream. The results of our experiment do not support that forest management has a strong impact on catchment MeHg production. Catchment disturbance through forest harvesting may decrease MeHg in aquatic biota, because of higher stream productivity and associated higher quality of dietary sources, at least in the short-term. Other studies on catchment MeHg production to disturbance have shown a range in responses, from strong to none. So far, no factor has emerged to explain such range in responses. Predictions of forest management effects on MeHg in streamwater and aquatic food webs are hampered by limited understanding of catchment controls on MeHg production.

Forest harvest contribution to Boreal freshwater methyl mercury load

“…In Table all studies reporting MeHg/DOC ratios after clear‐cutting are listed. Our reported average MeHg/DOC ratios for the 10 REFs (0.014 g g −1 above and 0.016 g g −1 below the ML) are similar to the average for Boreal coniferous forest reference stands (0.015 g g −1 , Table ) reported by Eklöf et al [] and de Wit et al []. In these two studies clear‐cutting only had minor effects on the MeHg/DOC ratio in the former and no effect in the latter study.…”

“…In most of these studies, single or a few sites have been followed for several years (preharvest and postharvest) and reference stands have been used to correct for interannual variations. In de Wit et al [] the THg/DOC ratio decreased after harvest, but the period of pretreatment was only extended for 0.5 years and after that time the THg/DOC ratio reached a level similar to the REF. Only in one study [ Skyllberg et al , ] was the THg/DOC ratio reported to be significantly ( p < 0.05) higher in streams draining 1–4 year old CCs, as compared to streams draining REFs, but only at sites above the ML.…”

“…All sites were dominated by Norway Spruce ( Picea abies ) forests and located in the boreal region of Sweden, Norway [ De Wit et al , ], and Finland [ Porvari et al , ]. Percentage of clear‐cut area, number of sites ( N ).…”

“…Also, single sampling event comparisons of multiple sites have come to the conclusion that clear‐cutting boreal coniferous forests significantly increases concentration of MeHg in stream runoff [ Skyllberg et al , ]. Other studies did not find any significant effects on either concentrations or export of MeHg or Hg by clear‐cutting single sites [ de Wit et al , ]. Thus, because of few studies with partly deviating results, the magnitude of possible forest harvest effects on MeHg and their variability among different types of soils and forest landscapes remain unclear and limit upscaling to larger regions [ Bishop et al , ].…”

Linking stream ecosystem integrity to catchment and reach conditions in an intensively managed forest landscape

“…, De Wit et al. ). Because DOM quality controls its reactivity, influences ecosystem functions, and affects microbial food webs (Weishaar et al.…”