The effect of in-line lakes on stream dissolved organic matter (DOM) in oligotrophic mountain systems

DOM is the most abundant form of organic matter across aquatic ecosystems. Chemical and physical properties of dissolved organic matter (DOM) play a major role in aquatic ecosystem functioning and DOM bioavailability. However, the processing and production of DOM in different aquatic ecosystems (i.e. streams and lakes) may have opposite affects on DOM concentration and characteristics. Understanding how interactions of aquatic ecosystems can alter dissolved organic matter characteristics, sinks and sources is essential to understanding carbon dynamics at watershed scales. The main objective of this research was to elucidate the hydrologic and biogeochemical controls on DOM dynamics in stream-lake mountain networks. We hypothesized that lakes would decrease temporal variability (i.e. buffer) and alter DOM bioavailability to downstream reaches relative to upstream reaches. We tested these hypotheses by evaluating DOM temporal trends and annual export in seven-paired lake inflows and outflows. We then evaluated how Ultraviolet (UV) exposure affected DOM characteristics during spring snowmelt and summer baseflow conditions, as well as UV exposure alters DOM bioavailability and nutrient limitation during baseflow conditions in four lake inflows and outflows. Given that water residence increases UV exposure, we hypothesized that lake outflow DOM would be more photorecalcitrant than lake inflows. We further hypothesized that UV exposure would increase DOM quality, resulting in increased heterotrophic processing, and thus increased nutrient demand. Results indicate that lakes can buffer stream temporal variability by acting as a DOM sink during snowmelt and a DOM source during baseflow conditions. Additionally, lake outflow DOM photodegradation was similar to lake inflows during snowmelt (p=0.66) but was 2X more photorecalcitrant than lake inflow DOM during baseflow conditions (p=0.03). These changes were strongly related to water residence time. During baseflow, light exposure increased both inflow and outflow DOM bioavailability (p= 0.059 and 0.024, respectively) and nutrient limitation (p= 0.03 and 0.09, respectively). Combined, these results indicate that water residence time in subalpine stream-lake systems is important to reduce DOM temporal variability and increase DOM degradation and processing, providing temporal stability of DOM and biologically available DOM to lake outflows and potentially increasing both carbon and nutrient uptake.