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. 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. Photomodifica-tion 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.