Impact Of Timber Harvest On Mercury Mobilization In A Mixed Pine And Hardwood Forest And Adjacent Lake In Mississippi

With 65% of land area covered in forest, Mississippi is a leader in the timber industry. Whereas forestry provides numerous benefits for humans and wildlife, forest harvest has been hypothesized to increase mercury (Hg) mobilization to aquatic systems. In a forest, Hg mostly accumulates in the upper layer of soil, where associates with organic matter. This top soil is exposed after clear-cutting and thus more susceptible to runoff. Harvest may also change the net balance between Hg methylation and demethylation by microorganisms through changes in redox conditions and addition of carbon sources from decaying logging residues. To better understand the impact of timber harvest on Hg in mixed pine and hardwood managed forest in the southeast United States, Hg was determined in soil, sediment, water and biota from the forest floor, a forest stream, and adjacent lake (Dorroh Lake located in Webster County, Mississippi) prior to and after harvest. Mean surface soil-Hg concentrations (ppb ± SE) were 50.0 ± 3.9 before harvest and 36.0 ± 3.8 after harvest. Before harvest, the majority (82%) of the Hg in the forest surface soil was bound to the oxidizable phase, which includes humic acids and other organic matter. After harvest, the distribution of Hg in the surface soil changed, and more Hg was bound to the reducible phase (78%), which primarily consists of iron/manganese oxides. This points to a decrease in the amount of organic matter present in the surface soil, possibly as a result of a mixing of surface and deeper soils containing lower Hg levels, erosion, and increased Hg volatilization. In the lake, concentrations of THg and MeHg increased in the month following harvest. Mean-THg increased from 0.82 to 5.0 ng/L and MeHg increased from 0.04 to 0.17 ng/L. In the stream, dissolved THg decreased after harvest from ~3 to ~1.5 ng/L. Dissolved MeHg was similar before and after harvest (0.48 and 0.40 ng/L, respectively), but the proportion of MeHg increased in the stream from ~10 to 30%. Timber harvest also seemed to decrease oxygen levels in the stream and lake, at least temporarily. Nevertheless, the unharvested riparian zone adjacent to the stream seemed to serve as a buffer retaining organic matter and minimizing effects on the stream and lake. Suggestions for future work include a long-term study of fish-Hg concentrations in a lake whose watershed was disturbed by deforestation, and a study site closer to Oxford that will allow for a greater frequency of sampling. Methylation rate measurements are also needed to fully address impacts of forest harvest on in-situ production of MeHg