Carbon and nitrogen dynamics in stream ecosystems

Carbon sources for streams and rivers are classified as autochthonous or allochthonous, depending on whether the primary producers reside within or outside the stream or river ecosystem. Within freshwater aquatic ecosystems, dissolved organic carbon (DOC) is the largest pool of organic material. Therefore, processes that produce, consume and transform DOC are important in the overall cycling of nutrients and energy. In most headwater streams, humic and fulvic substances of terrestrial (allochthonous) origin are the major constituents of the DOC pool. Humic and fulvic substances are typically recalcitrant and difficult to metabolize, so stream metabolism is more directly coupled to the consumption of labile DOC, mostly of autochthonous origin. The mineralization of this labile material also drives the transformation of nitrogen through both assimilatory and dissimilatory pathways. Humans produce more bioavailable N than that produced from natural N fixation, enriching ecosystems where productivity was historically limited by N availability. Much of this anthropogenic nitrogen enters aquatic ecosystems. Streams have been shown to be important areas for landscape-scale N uptake and transformation, especially headwater streams which can process N rapidly and efficiently.This dissertation describes carbon and nitrogen dynamics in different stream ecosystems. Chapter 1 focuses on migrating Pacific salmon ( Oncorhynchus spp.) as a source of organic carbon and nutrients to Alaskan streams. Chapter 2 evaluates nitrate (NO3) retention and transformation in desert stream ecosystems in the context of human impacts from urbanization and agriculture. Chapter 3 focuses on N dynamics in the hyporheic zone (surface water/ground water interface) of reference and human-altered streams. The overall goal of this dissertation is to better comprehend C and N dynamics in a range of stream ecosystems.