As a result of controls that have been recently implemented and that are proposed for atmospheric emissions of mercury (Hg), there is a critical need to design and implement a program to monitor ecosystem response to these changes. The objective of this chapter is to review the state of Hg monitoring activities and programs that are currently being conducted for airsheds and watersheds, and to make recommendations to strengthen and add to these programs in order to quantify future changes that may occur as a result of changes in atmospheric emissions of Hg and subsequent deposition. In this regard we identified a series of airshed and watershed indicators that, when measured over a long period of time, should help to determine the (response from) changes in the global, continental, and/or regional-scale Hg emissions (or other watershed loads of Hg such as land-use changes or discharges). Note that an important benefit of improved Hg monitoring programs would be the availability of high quality data to test and validate models. These data would help support the development and application of models as research tools to better understand the dynamics and cycling of Hg in complex environments. Improved and wellvalidated models could subsequently be used as management tools to predict the response of airsheds and watershed ecosystems to changes that might occur in emissions of Hg or other changes that might alter the transport or bioavailability of Hg (e.g., changes in atmospheric deposition, climate change, land disturbance). To achieve this objective we propose an integrated airshed/watershed Hg monitoring program. We propose that within an ecoregion detailed sampling at intensive study sites (intensive sites) and less intensive sampling at a larger number of clustered sites (cluster sites) would be conducted. To evaluate Hg response in airsheds we propose a series of air quality Hg intensive sites. At these intensive sites detailed measurements of atmospheric Hg speciation and deposition would be made together with supporting measurements of atmospheric chemistry and meteorology. Several air quality Hg intensive sites exist and could be used as templates for this approach. We also propose measurements of total ecosystem deposition at the air quality Hg intensive sites. Researchers have suggested that throughfall plus litterfall might be used as a cost-effective surrogate for total Hg deposition to forest ecosystems. While this approach needs further research, we believe it holds considerable promise and

might ultimately be implemented at cluster sites. We strongly endorse that continued use of the Mercury Deposition Network (MDN). The MDN is a North American network in which wet Hg deposition is measured using standard protocols. The MDN is the only national framework that currently exists to monitor changes in Hg deposition. The MDN needs continued support and should be expanded to improve spatial coverage. For watersheds, we recommend that an intensive watershed monitoring program be initiated to measure changes in the chemistry and flux of Hg species in streamwater over the long-term. Rather than implementing a new watershed monitoring program, we recommend that a Hg monitoring component be added to existing watershed networks (i.e., the NSF LTER program, USGS WEBB program). Existing programs have the advantage of monitoring infrastructure and expertise that is already in place and a record of ancillary measurements, which would be critical to the interpretation of ecosystems response to changes in Hg deposition. At the cluster-level, we recommend that a forest floor or surface soil monitoring program be implemented to evaluate the response of soil to changes in atmospheric Hg deposition.