Title

Mercury dynamics in streams, lakes, and fish in the northeastern United States

Date of Award

2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Charles T. Driscoll

Keywords

Mercury, Streams, Lakes, Fish, Methylmercury, Dissolved organic carbon

Subject Categories

Civil Engineering

Abstract

Mercury (Hg) is a potent neurotoxin that poses a risk to the health of both humans and wildlife. The majority of Hg that accumulates in freshwater fish as well as other biota originates from anthropogenic activities that release Hg to the atmosphere. Upland forested watersheds in the northeastern USA contain large pools of legacy Hg from historical deposition and are potentially a source of Hg to downstream aquatic ecosystems. The linkages between changes in Hg emissions and response of ecosystems are not well understood. This research was conducted to improve understanding of Hg export from upland watersheds in the northeastern U.S. (NY, VT, NH) and factors contributing to changes in fish Hg concentrations.

The research for this study was conducted in three phases. Phase I was designed to investigate upland stream export of Hg and methylmercury (MeHg) in relation to specific carbon (C) fractions to help identify shifting sources and controls on Hg transport. The second objective of Phase I was to evaluate the effect of changes in hydrology and seasonal differences on Hg and DOC dynamics. Phase II and III were designed to investigate factors influencing changes in water column and fish Hg concentrations.

For Phase I of this research, I observed an increase in Hg and organic matter concentrations with discharge at all three upland stream sites; however the partitioning of Hg fractions (dissolved vs. particulate) differed among sites and seasons. Increased discharge caused a shift in flow paths from mineral soil under base flow to upper soil horizons during high flow. As flow paths shifted, greater concentrations of DOC richer in aromatic substances and lignin were flushed from upper soil horizons to stream water. Due to the strong correlation between Hg and DOC, I hypothesize that there was a concurrent shift in the source of Hg with DOC from lower mineral soil to upper soil horizons.

Research for Phase II included a synoptic survey for water column and yellow perch ( Perca flavescens ) Hg concentrations in 25 Adirondack lakes that had previously been sampled in 1992-93. From the changes in results from the two surveys, I observed that four variables appear to influence yellow perch Hg concentrations in the Adirondacks: watershed area, elevation, change in pH, and change in fish body condition. As the acidity in lakes is attenuated, the lakes may become more productive and/or water quality conditions less stressful to fish leading to improving fish body condition. Improvements in fish body condition may cause fish to exhibit "growth dilution" of tissue contaminants leading to lower fish Hg concentrations.

For Phase III of my research, I used a bioenergetics model in conjunction with empirical data for the eight lakes examined in Phase II to examine the influence of growth rates and MeHg dietary exposure on yellow perch Hg concentrations. Results from this research suggest that fish diet may include zooplankton but also likely includes prey items with MeHg different from measured zooplankton MeHg. Differences in water chemistry (pH, DOC, Al) influenced both the supply and bioavailability of Hg at the base of the aquatic foodchain. Fish with slower growth rates tended to have higher Hg concentrations than in lakes where fish growth was faster and Hg concentrations were lower.

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