Dissolved Organic Matter in the Great Lakes: Role and Nature of Allochthonous Material Véronique P. Hiriart-Baer 1,* , Ngan Diep 2 , and Ralph E.H. Smith 3 1 Aquatic 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 2 Ontario Ministry of the Environment Great Lakes Unit of the Water Monitoring and Reporting Section West Wing, 125 Resources Rd. Etobicoke, Ontario M9P 3V6 3 University 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 fluo- rophore, 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. Expo- sure 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 high- est in lake water samples and was inversely related to DOM average molecular weight. Photomodifica- tion of DOM resulted in higher bacterial activity although these differences were not significantly differ- ent. 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 micro- bially derived DOM in the Great Lakes aquatic ecosystems. INDEX WORDS: DOM, fluorescence, absorption, BOD, Great Lakes. J. Great Lakes Res. 34:383–394 Internat. Assoc. Great Lakes Res., 2008 383 * Corresponding author. E-mail: Veronique.Hiriart-Baer@ec.gc.ca INTRODUCTION Coastal regions of the Laurentian Great Lakes re- ceive allochthonous dissolved organic matter (DOM) in large quantities from surrounding catch- ments, with additional, autochthonous DOM gener- ated by in situ lake processes (e.g., Laird and Scavia 1990). DOM is a critical factor in the attenu- ation of ultraviolet radiation (UVR) in aquatic sys- tems, and thus plays a key role in protecting aquatic organisms from harmful UVR (e.g., Laurion et al. 2000, Morris et al. 1995, Scully and Lean 1994, Yan et al. 1996). Changes in DOM quantity and quality in lakes may therefore have as much rele- vance to aquatic ecosystems as changes in stratos- pheric ozone due to climate change (Williamson et al. 1996). The effectiveness of DOM in attenuating