Over recent decades, dissolved organic carbon concentrations (DOC) in lakes and streams have increased through the northern hemisphere, leading to concerns about changes in water quality and about the productivity of these aquatic ecosystems. Researching the causes of increasing DOC concentrations in streams and lakes is complicated because these observations have been made across many different forest types, different air pollution histories, and shifting atmospheric carbon dioxide and climate. A Northern Research Station scientist and her partners used a novel technique, fluorescence spectroscopy, in conjunction with data from two decades-long watershed studies to explore a hypothesis for the cause of increased DOC. The combination of novel methodology and data from the Bear Brook Watershed in Maine and the Fernow Experimental Forest in West Virginia allowed the research team to isolate the effects of soil acidification and to evaluate the sources of DOC at the whole ecosystem and decadal scales. Researchers found that watershed acidification significantly decreased streamwater DOC, with less of the DOC coming from terrestrial sources and more from in-stream microbial sources. Thus, cleaner air (decreasing sulfur deposition) does appear to be increasing the flux of terrestrial DOC to surface waters. Knowing this will be of use in designing water quality treatments to ensure quality drinking water from forested watersheds in the future.