Marcell Experimental Forest – Mercury and Lead in Peat

Peat cores were collected from the S2 ombrotrophic bog at the Marcell Experimental Forest (MEF) to determine if the lead inventory in the bog has decreased over time. Lead concentrations in the outflow of the bog measured from 2009-2015 indicated continued mobilization and export of lead out of the bog despite dramatic decreases in atmospheric deposition. A seminal study conducted by Urban et al. (1990) from 1981-1983 calculated a mass balance of lead in the S2 watershed which included a lead inventory in peat based on the approximate time frame of 1930-1983. We collected peat cores in 2016 to compare peat inventories of lead. We found the lead inventories in the peat have decreased over time, consistent with lead being mobilize by dissolved organic carbon (DOC) and gradually flushed out of the bog, but this trend is unclear. In contrast to lead concentrations in the outflow water, upland runoff and the surface sphagnum moss layer have dramatically lower lead concentrations compared to 1980s levels indicating fast ecosystem responses to a decease in lead inputs in these compartments. However, the deeper peat layers near the water table are responding slower to the decrease in lead inputs and historical lead inputs continue to be mobilized and transported from the bog. Our results would be applicable to other trace metals, such as mercury, that bind strongly to DOC. For example, a dramatic increase in mercury deposition would not result in near-term decreases in mercury fluxes out of the bog.

img_8928
Erin Baumann sectioning a peat core