Interactions between hydrology, dissolved organic matter, and algal populations in the headwaters of Boulder Creek

There were two main goals of the research presented in this thesis. The first of these goals was to link the biogeochemistry of redox active dissolved organic matter (DOM) to changes in the hydrology and primary productivity of an aquatic ecosystem. In this context, the role of biogeochemical hotspots in controlling the instream quantity and quality of DOM at different times of year in the Green Lakes Valley was investigated. Specifically, the importance as biogeochemical hotspots of (1) the hyporheic zone in an alpine wetland and (2) an alpine lake were examined using a combination of field experiments and the development of reactive transport models. Results suggest that fulvic acids are involved in rapid electron-transfer reactions in and near a headwater stream channel. Furthermore it was shown that the production and decay of three pools of DOM in an alpine lake are dependent on seasonal changes in the hydrology and biological productivity of the system. The second goal was to investigate factors that influence the growth of Didymosphenia geminata. D. geminata is a stalk-forming freshwater diatom historically found in oligotrophic lakes and streams that has recently become a nuisance species in many lotic systems worldwide. It was found that flood events may act as a control on the growth of D. geminata potentially via increased scour of the streambed.