Dissolved Organic Matter in Impacted Streams and Rivers: Challenges and Future Research Directions

Human activities can alter the quantity and quality of dissolved organic matter (DOM) exported from lands to streams and rivers, which can lead to widespread environmental and ecological consequences. DOM is known to act as a master variable regulating important biogeochemical processes, such as protecting aquatic biota from UV, influencing states and transports of ecotoxins and trace metal pollutants, and serving as basal substrate and energy sources for heterotrophic food webs. Therefore, effective management decisions should include DOM monitoring and characterizations, while relevant data to inform how to monitor and regulate watershed exports of DOM, remain surprisingly scarce. In particular, it is still unclear whether human activities change DOM in systematic fashions that the changes can be unambiguously stated and quantitatively defined. We synthesized previous findings on anthropogenic alterations in the amount, source, and qualityof DOM in streams and rivers across geographic regions. While highlighting difficulties in unambiguously linking human activities and freshwater DOM characteristics, we observed that human activities can lead to opposite changes in DOM characteristics, depending on hydrological variation and watershed size. This synthesis represents the first step towards establishing a framework to predict and manage terrestrially derived DOM in human impacted streams and rivers.