8892_book.fm Page 47 Friday, January 5, 2007 3:59 PM Monitoring and Evaluating Trends in Sediment and Water Indicators David Krabbenhoft, Daniel Engstrom, Cynthia Gilmour, Reed Harris, James Hurley, and Robert Mason ABSTRACT As recently as a decade ago, a paucity of geographically dispersed and reliable data on mercury (Hg) and methylmercury (MeHg) in water and sediments would have made discussions of large-scale monitoring programs difficult to conceive or implement Methodological advancements made over this time period, as well as substantial improvements in our overall scientific understanding of mercury sources, cycling and fate in the environment, have enabled scientists, land managers, and regulators to consider how environmental responses to changing mercury emissions and deposition could be monitored A program whose ultimate goal is to assess environmental responses to changes in atmospheric Hg deposition will undoubtedly rely on sediment and water indicators as critical program components For both water and sediment, a well established set of sampling protocols and analytical procedures will enable reliable data collection across a diverse set of aquatic ecosystems Waterbased indicators of Hg and MeHg have already been useful for documenting decadalscale changes in Hg and MeHg concentrations in the Everglades of Florida and a seepage lake in northern Wisconsin At both sites, changes in Hg deposition were also measured and linked to the environmental response Unfortunately, there are very few other long-term records of Hg and MeHg in water and/or sediment, thus establishing widespread baselines or current trends is presently difficult With increasing numbers of studies and monitoring efforts that utilized the collection of water and sediment samples, however, a growing database on Hg and MeHg is evolving that would be useful for site selection and establishing general contamination levels for a more coherent monitoring effort Within an aquatic ecosystem, water-based indicators are expected to be the first environmental compartment to respond to altered mercury loading and where change can be detected The response would likely first manifest itself as a change in aqueous 47 © 2007 by Taylor & Francis Group, LLC 8892_book.fm Page 48 Friday, January 5, 2007 3:59 PM 48 Ecosystem Responses to Mercury Contamination: Indicators of Change total Hg (HgT) concentration, and then later as a change in MeHg concentration The MeHg/Hg ratio (also expressed as percent MeHg) is a measure of the efficiency of ecosystems to convert the load of inorganic Hg(II) into MeHg Shifts in the value of this ratio could reflect changes in ecosystem conditions affecting methylmercury production or elimination other than Hg loading, thus helping to distinguish the effects of Hg loading from other confounding factors that can affect MeHg concentrations Temporary changes in MeHg/Hg ratios could also reflect the time required for MeHg concentrations in ecosystems to respond to changes in Hg concentrations and methylation rates These types of insights make the MeHg/Hg ratio a very useful indicator In addition, a significant advantage to this indicator is that it requires no additional funding support, assuming Hg and MeHg measurements on sediment and water will be part of a routine monitoring plan Sediment-based indicators are also critically important for monitoring changes in Hg inputs to aquatic ecosystems, and are often better indicators (compared to water-based indicators) of changes to Hg loading that occur over several years to decades Mercury researchers commonly sample sediments because they are good indicators of overall contamination levels, but also because near-surface sediments (95%), in water it is almost always a relatively small fraction of total Hg in aqueous solution (