Mercury (Hg) is an environmental pollutant that leads to the contamination of forest, soils, lakes and streams. Hg is also a well known neurotoxin and, human and wildlife health concerns are primarily related to exposure to methyl mercury (MeHg) that bioaccumulates in aquatic and terrestrial organisms. Wetlands are important landscapes where ionic Hg (Hg²⁺) is transformed to MeHg, and transported to downstream aquatic ecosystems. This research was conducted in small forest watershed-lake ecosystems in the Adirondacks to better understand Hg cycling in wetlands and lakes.

The study was conducted in three phases. Phase I and II, were designed to investigate Hg transport and fate in two forest wetlands through detailed evaluations of seasonal Hg concentrations and mass budgets. The two wetlands, a riparian peatland and an abandoned beaver meadow, intercept majority of the upland runoff from Archer Creek draining Arbutus Lake Watershed. In Phase III, Hg budgets of two contrasting lakes were evaluated. Arbutus and Sunday lakes differ in landscape, physical and chemical attributes.

The proportion of total mercury (THg) that was methyl mercury (% MeHg) in the upland stream of Archer Creek increased from 2% in the upland system to 6% at the outlet of the wetlands. During the growing season concentrations of THg, MeHg and dissolved organic carbon (DOC) were markedly higher and sulfate (SO₄^2-) was markedly lower compared to the non-growing season. Transport of Hg species was facilitated by DOC production as indicated by significant positive relations with THg and MeHg.

Mass balance calculations revealed that the wetlands were net sources of THg and MeHg to stream water, although the magnitude of the source was a function of wetland connectivity to stream water. The storage of THg and MeHg in wetland soil was a large pool which seems coupled with total carbon and sulfur accumulation. In the current scenario of decreasing Hg emission and atmospheric deposition, the large ‘active’ Hg soil pool in wetlands is a potential short-term and long-term source of Hg and MeHg to downstream aquatic ecosystems.

Concentrations of Hg species increased with increases in DOC concentrations and percent wetland coverage at Arbutus (4%) and Sunday Lake (20%). Negligible removal of THg inputs was observed at Sunday Lake that has a short hydraulic residence time (HRT; 0.02 yrs) compared to approximately 60% removal at Arbutus Lake that has longer HRT (0.6 yrs).