processes with only DOC measurements is comparable to trying to understand the inorganic geochemistry of natural waters with only measurements of conductivity.

Question: What are the different pathways by which detrital organic carbon is respired in aquatic systems? How does the source of the detrital organic carbon determine the quality of the substrate?

Importance of Long-Range Transport of Constituents

Question: What are the mechanisms controlling interactions of watersheds with overlying airsheds?

The transport and deposition of atmospheric acidic constituents, SOx and NOx, affects the weathering rates of soils and nutrient conditions on land and in surface waters, with consequent effects on biological processes and ecosystems. For example, approximately one-third of the present nitrogen input to the Chesapeake Bay is estimated to arise from atmospheric deposition in the watershed. Another example is tropospheric ozone: ozone production in regional airsheds such as the Baltimore-Washington area involves photochemical reactions between NOx and hydrocarbons from anthropogenic and terrestrial sources to yield atmospheric ozone concentrations many miles from urban sources that exceed allowable limits.

Chlorinated organic substances such as PCBs (polychlorinated biphenyls), DDT, PAHs (polycyclic aromatic hydrocarbon), and dioxin are transported to aquatic systems primarily by the air route, and may have important effects on the organisms in these systems. Mercury is also transported atmospherically; however, the available data base for metals in freshwater ecosystems may be limited to recent years because of potential contamination of samples collected earlier.

The interactions of sulfur, nutrients, synthetic organic substances (including but not limited to pesticides), and many metals during transport through the vegetation canopy, across soil surfaces, through the highly reactive surficial zone of soils, and downward into aquifers is a challenging research area of environmental concern that requires an integration of chemical and hydrologic concepts and field measurements.

A Changing Climate

Question: What is the relative importance of direct physical changes compared to indirect biogeochemical changes in determining responses to changing climate?

The effect of global climate change on regional hydrology and, in turn, on vegetation and on wetlands is a very important concern requiring substantial attention. Changes in hydrologic regimes may alter the extent

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