DISSOLVED ORGANIC MATTER IN LAKE SUPERIOR TRIBUTARIES: AN INVESTIGATION OF CARBON CONCENTRATION, OPTICAL PROPERTIES, AND PHOTODEGRADABILITY

Exposure of dissolved organic matter (DOM) to sunlight has been shown to re-mineralize dissolved organic carbon (DOC) and to alter the composition and bioavailability of DOM in Lake Superior. This study investigates the role of land use and season on the photodegradability of DOM from five Lake Superior tributaries using organic carbon analysis (via high temperature combustion) and proxies from UV-Visible absorption spectrophotometry. Base flow samples from June and August 2010 were irradiated with natural sunlight. Irradiated samples as compared to dark controls show: 1. a general loss in specific UV-absorption (SUVA 254), an indicator of aromaticity; 2. a general increase in the ratio of absorbance at 250 nm to absorbance at 365 nm (E2/E3), an indicator which varies inversely with molecular size; and 3. decreases in DOC with more pronounced decreases in June DOC than in August samples. Upon irradiation, snow melt samples from March 2011 show similar changes in UV-Visible proxies but do not show measurable changes in DOC concentration. Season (base flow vs. snow melt) appears to have a stronger impact on DOM photochemistry than does sample location for this set of streams, indicating that the effects of land use need to be measured across seasons rather than just at base flow.