MANAGING 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 may lead to widespread environmental and ecological consequences. DOM is known to act as a master variable regulating various biogeochemical and ecological 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 to adjacent water bodies are surprisingly scarce. In particular, no systematic and coherent understanding has been formed regarding whether human activities have altered DOM in adequately systematic ways for the alterations to be unambiguously stated and quantitatively defined. We synthesized previous findings on anthropogenic alterations in the sources, ages, reactivity, and molecular compositions of DOM in streams and rivers across geographic regions. Our results highlight difficulties in linking between human activities and observed changes in freshwater DOM because of confounding effects from climatic, hydrological, and watershed variations. The mechanisms underlying anthropogenic changes in DOM are speculated, and challenges for incorporating DOM into management considerations are also identified. Our synthesis takes the essential first step towards establishing a framework for modeling, predicting and managing terrestrially-derived DOM in human-impacted streams and rivers.