Veronique P. Hiriart-Baer¹, Ngan Diep² and Ralph E.H. Smith³, ¹Aquatic Ecosystem Management Research Division, Water Science and Technology Directorate, Science and Technology Branch, National Water Research Institute Environment Canada, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6; ²Ontario Ministry of the Environment, Great Lakes Unit of the Water Monitoring and Reporting Section, West Wing, 125 Resources Rd., Etobicoke, Ontario, M9P 3V6; ³University of Waterloo, Biology Department, 200 University Avenue W, Waterloo, Ontario, N2G 3L1

ABSTRACT: Dissolved organic matter (DOM) quality and the modifying influence of light on DOM bioavailability were investigated along a natural gradient of allochthonous influence in the lower Great Lakes. Using parallel factor analysis (PARAFAC), three DOM fluorophores were identified. One fluorophore, previously identified as peak C, was of allochthonous (component 1) origin and two previously uncharacterized fluorophores were identified as autochthonous (components 2 and 3). Component 1 was photoreactive and the dominant form in creek water samples while components 2 and 3 were dominant in Hamilton Harbour and lake water samples. Components 2 and 3 showed limited photoreactivity. Exposure to full spectrum irradiance decreased the average molecular weight of DOM (i.e., increased the absorbance ratio (a254:a365)) for all water samples. DOM bioavailability was lowest in creek and highest in lake water samples and was inversely related to DOM average molecular weight. Photomodification of DOM resulted in higher bacterial activity although these differences were not significantly different. This suggests that light plays a significant role in the cycling of terrestrially-derived DOM and to a certain extent autochthonous DOM, potentially increasing metabolism of both terrestrially and microbially derived DOM in the Great Lakes aquatic ecosystems.

KEYWORDS: DOM, fluorescence, absorption, BOD, Great Lakes.

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