WATERSHED RESPONSE TO MERCURY DEPOSITION IN THE ADIRONDACK REGION OF NEW YORK: CRITICAL ISSUES FOR RECOVERY

We synthesize multi-disciplinary research on atmosphere-watershed mercury dynamics and effects on ecosystems in the Adirondack region of New York, including factors influencing recovery following emission controls. The Adirondacks have exhibited marked changes in net atmospheric mercury deposition since about 1900 including seven-fold increases followed by decreases since the 1970-1980s. Litterfall is the dominant influx pathway of mercury (~17 µg/m²-yr) to the Adirondacks, throughfall is nearly balanced by soil evasion losses, and the major loss is soil accumulation (~15 µg/m²-yr). Stream loss of dissolved mercury is closely coupled with the mobilization of DOC fractions, while particulate mercury is largely associated with particulate organic carbon. Long-term measurements of Adirondack lakes have generally shown increases in DOC. Limited drainage losses of ionic mercury are converted to methyl mercury largely in wetlands, particularly during the summer growing season, increasing transport to downstream surface waters. A recent resurvey of 25 Adirondack lakes shows the water columns have largely exhibited decreases in total and methyl mercury concentrations, while changes in the mercury content of yellow perch have been mixed. These changes in fish mercury appear linked to changes in lake pH and fish condition, as well as lake elevation and watershed area. Critical issues for the recovery of Adirondack ecosystems are the effects of changes in atmospheric mercury, sulfate and nitrate deposition, changes in DOC and its influence of mercury bioavailability, and the legacy of mercury that has accumulated in soils.