Closing the loop on hydrologic and mercury mass balances for a temperate forested park
Sarah J. Nelson
(click here to go to Sarah's web site)
Scientists often use watershed mass balances to understand ecosystem processes at the watershed scale. Watershed mass balances are detailed measurements of inputs (such as dissolved substances in rain) to a land area drained by a stream or lake, compared to measurements of export of those same substances in the stream or lake. The difference between inputs and export indicates whether a substance is being stored in watershed soils, vegetation, or biota; or flushed out of the watershed. For a substance such as mercury, a toxic element that accumulates in fish and other biota, scientists at Acadia National Park in Maine are using watershed mass balances for two watersheds to unravel a mystery: why do some watersheds export larger amounts of mercury than other seemingly similar watersheds nearby? Early information shows that the type of vegetation and the aspect, or direction a watershed faces, are affecting the relative inputs of mercury and acid rain-related components to monitored watersheds. The disturbance history in these watersheds also affects how much of the substance leaves the watersheds in streamwater. However, research on watershed inputs has only been conducted during the growing season; in Maine, only about half of the year. In the paired watersheds that are being studied at Acadia National Park, more than half of annual precipitation and stream water discharge occur in winter. This research will improve mass balance estimates of inputs from the atmosphere, including mercury and ‘indicator’ ions sulfate, chloride and sodium. The project will be part of a multi-institute effort to relate the enhancement of inputs to landscape features such a forest type. Using these relationships, the research will predict inputs to other areas of the Park and will survey streamwater across Acadia to determine whether inputs actually do relate to export levels in streamwater. Measurements of winter inputs will fill a gap in our understanding of these processes and in the data record for these long-term monitoring sites. Park resource managers will be able to use the deposition model to assess the effects of changes outside Park boundaries - such as emissions of mercury and acidifying substances, and changes inside Park boundaries - such as fire or forest type. These changes may affect patterns of bioaccumulation of toxic substances, or altering the water regime in watersheds. This research will provide information on how such changes could affect the Park’s water resources.