Long-term changes in episodic acidification of streams in Shenandoah National Park,Virginia (U.S.A.)

Episodic acidification-the short-term loss of acid neutralizing capacity (ANC) in surface waters during periods associated with rainfall and snowmelt runoff-has been shown in previous field studies to be a ubiquitous process that can have long-term adverse effects on fish populations. A systematic field study of episodic acidification in five gaged Shenandoah National Park (SNP) streams was initiated in 1991, while a long-term study of changes in acid-base chemistry in four gaged SNP streams has been in operation since 1979. Statistical analysis of the long-term record of mean daily discharge and weekly streamwater chemical composition was performed to identify the dominant processes that control episodic acidification of SNP streams; an objective hydrological separation technique was used to determine whether each weekly sample was taken during antecedent baseflow or stormflow conditions. Using this technique, more than 100 stormflow/baseflow pairs were identified in the long-term (1980–1992) record for White Oak Run from which episodic chemical changes were estimated. Statistical analysis of the resultant data suggested that mean episodic depressions in ANC have increased by nearly a factor of two in White Oak Run since the first outbreak of forest defoliation by the gypsy moth caterpillar during the summer of 1990.     

The Effect of Fire on Mercury Cycling in the Soils of Forested Watersheds: Acadia National Park,Maine, U.S.A.

This study compares mercury (Hg) and methylmercury (MeHg) distribution in the soils of two forested stream watersheds at Acadia National Park, Maine, U.S.A. Cadillac Brook watershed, which burned in 1947, has thin soils and predominantly deciduous vegetation. It was compared to the unburned Hadlock Brook watershed, with thicker soil and predominantly coniferous vegetation. Soils in both watersheds were primarily well drained. The fire had a significant impact on the Cadillac watershed, by raising the soil pH, altering the vegetation, and reducing carbon and Hg pools. Total Hg content was significantly higher (P > 0.05) in Hadlock soils (0.18 kg Hg ha^-1) compared to Cadillac soils (0.13 kg Hg ha^-1). Hadlock O horizon had an average Hg concentration of 134±48 ng Hg g^-1 dry weight, compared to 103±23 ng Hg g^-1 dry weight in Cadillac O horizon. Soil pH was significantly higher in all soil horizons at Cadillac compared to Hadlock soils. This difference was especially significant in the O horizon, where Cadillac soils had an average pH of 3.41±0.22 compared to Hadlock soils with an average pH of 2.99±0.13.To study the mobilization potential of Hg in the O horizons of the two watersheds, batch adsorption experiments were conducted, and the results were modeled using surface complexation modeling. The results of Hg adsorption experiments indicated that the dissolved Hg concentration was controlled by the dissolved organic carbon (DOC) concentration. The adsorption isotherms suggest that Hg is more mobile in the O horizon of the unburned Hadlock watershed because of higher solubility of organic carbon resulting in higher DOC concentrations in that watershed.Methylmercury concentrations, however, were consistently higher in the burned Cadillac O horizon (0.20±0.13 ng Hg g^-1 dry weight) than in the unburned Hadlock O horizon (0.07±0.07 ng Hg g^-1 dry weight). Similarly, Cadillac soils possessed a higher MeHg content (0.30 g MeHg ha^-1) than Hadlock soils (0.16 g MeHg ha^-1). The higher MeHg concentrations in Cadillac soils may reflect generally faster rates of microbial metabolism due to more rapid nutrient cycling and higher soil pH in the deciduous forest. In this research, we have shown that the amount of MeHg is not a function of the total pool of Hg in the watershed. Indeed, MeHg was inversely proportional to total Hg, suggesting that landscape factors such as soil pH, vegetation type, or land use history (e.g., fire) may be the determining factors for susceptibility to high Hg in biota.