The input and fate of dissolved organic matter (DOM) can have important consequences for coastal zone productivity in large lakes and oceans. Chromophoric DOM (CDOM) is often delivered to coastal zones from rivers and streams and affects light penetration in a water column. CDOM can protect biota from damaging ultraviolet (UV) light by acting as sunscreen, resulting in increased ecosystem productivity. Alternatively, CDOM can decrease ecosystem productivity by absorbing light needed for photosynthesis and forming photoreaction products that are harmful to coastal zone biota. Increased urbanization of watersheds and seasonal differences in weather patterns change the delivery pathways, reactivity, input, and energy flow of DOM (and its CDOM component) into aquatic systems. This study investigated the effects of watershed and season on the concentrations and potential photodegradation of stream-derived DOM in Lake Superior tributaries, chosen to be geographically and geologically similar but differing in land use. Organic carbon analysis, UV-Visible spectrophotometry, and terrestrial (land use) analysis were used to investigate differences among samples and sample treatments. The major differences in DOM concentration and photochemical response appeared seasonal rather than site specific, with snow-melt samples showing stronger and more consistent changes in UV-Visible parameters while base-flow samples showed stronger and more consistent losses in DOC.
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Acknowledgments The authors wish to thank Stephanie Guildford and Thomas Pevan (at UMD-LLO) for microscopy assistance, Stephanie Guildford and Josef Werne (U.Pitt) for useful advice and comments on earlier drafts of this work, and Jay Austin (UMD-LLO) for the calculations of light transmission and absorption through a hemisphere. This work was funded by a grant from the Vice Chancellor for Academic Administration and the Dean of the Swenson College of Science and Engineering, University of Minnesota Duluth.
- Dissolved organic carbon
- Lake Superior
- UV-Visible spectrophotometry