Open access publications in the SLU publication database

Abstract

The bioaccumulating neurotoxin methyl mercury (MeHg) is largely recognized as a problem in the boreal landscape. Both forest harvest sites and wetlands have been identified as MeHg sources. However, studies on logging effects have reported mixed results, and furthermore a black alder swamp has been recognized as a significant and consistent MeHg sink. This thesis aimed at clarifying the influence of these environments on the fate of MeHg in boreal landscapes.
The impact of forest harvest on MeHg production and export was studied in two types of landscapes, undulating and flatter terrain. Based on MeHg soil pools and fluxes of MeHg in streams, I conclude that MeHg was net produced in the soils after clear-cutting, with up to a 10-fold increase. Further, clear-cutting in undulating terrain approximately doubled the MeHg export to surface waters. The increase in load corresponded to 14% of the MeHg export from forested land in Sweden. When comparing to both forested and wetland areas, the increase in load was almost 8%. Moreover, laboratory incubation experiments using stable Hg isotopes were used to determine factors influencing methylation and demethylation rates.

Experiments with
soils from clear-cuts and reference sites suggested that the increase in methylation after clear-cutting was controlled by methanogens and sulfate reducing bacteria. In contrast, clear-cutting did not affect demethylation rates.
Furthermore, we wanted to know whether degradation or retention in the soil caused a black alder swamp to be a MeHg sink. A spatial analysis showed that MeHg was net degraded in the soil. In addition, based on snapshot budgets of nine additional swamps, net degradation of MeHg appeared as a common feature of alder swamps. Finally, incubation studies including sterile controls were performed to understand the processes affecting Hg methylation and MeHg demethylation in the black alder swamp. By targeting specific microbial guilds, we identified active microbial communities, showing that in particular methanogens were responsible for the biotic demethylation. Also, high rates of abiotic MeHg degradation were observed throughout the swamp.

Altogether, these findings are important for forestry practice recommendations and landscape planning. Forestry operations clearly increase the MeHg load while black alder swamps mitigate MeHg produced in upstream environments. Therefore, I propose that riparian buffer zones and restoration of black alder swamps situated downstream MeHg sources would decrease the MeHg load to downstream surface waters.