Temporal and spatial patterns of DOM photodegradation in stream-lake subalpine systems: Implications for heterotrophic processing

We evaluate how subalpine mountain lakes alter dissolved organic matter (DOM) photodegradation in fluvial networks. Given that lake water retention increases the time that DOM is exposed to ultraviolet (UV) radiation, we hypothesized that lake outflow DOM will be photorecalcitrant, and therefore, photodegradation will be lower than in lake inflows. We further hypothesized that UV exposure would increase DOM quality, resulting in increased heterotrophic processing. During 2008 snowmelt and summer baseflow, we evaluated the effect of natural light on DOM photodegradation of four lake inflows and outflows. Photodegradation was measured as the loss of ability to absorb light at 254nm. We then assessed how streamwater photodegradation of one lake inflow and outflow affects heterotrophic processing through the use of a laboratory DOC bioassay and field incubation. Our results indicate that during snowmelt, inflow and outflow photodegradation was similar (p=0.66). However, during summer baseflow conditions outflow photodegradation was approximately half that of the inflow (p=0.03). Heterotrophic processing experiment results indicate that light increases DOM bioavailability in both inflows and outflows (p=0.059 and 0.024, respectively). Our results indicate that lakes decrease the capacity of stream DOM to absorb light during baseflow conditions, however in both streams light exposure increased DOM bioavailability.