Stable isotope analysis of river food webs and carbon cycling

I used natural variation in stable isotope ratios of carbon and nitrogen to examine energy flow and trophic structure in stream and river ecosystems. Algal stable carbon isotope ratios (delta13C) were highly variable within the South Fork Eel River watershed in northern California, as commonly observed in lotic ecosystems. I analyzed factors known to influence algal delta13C in ocean and lake ecosystems, and found that much of the variation in algal delta13C was explained by variation in dissolved CO2 through effects of CO2 on both delta13C CO2 and algal fractionation of stable carbon isotopes. Dissolved CO2 levels, in turn, were determined by multiple factors including stream productivity, geomorphology and discharge. These analyses advance our understanding of carbon cycling in streams, and provide an empirical foundation for use of stable carbon isotope techniques to evaluate trophic interactions in lotic ecosystems. I then used understanding of controls of algal delta13C to examine energy flow in a river food web at multiple scales. Detailed study of the South Fork Eel River showed strong trophic linkages between all habitats in the river, but only weak connections to tributary streams and forests. Finally, I used a literature synthesis and original research to understand patterns and controls of energy flow in streams and rivers. For forested watersheds, these analyses showed clear transitions from terrestrial to instream algal sources of production for lotic food webs in streams larger than 10 km2 watershed area. These results suggest that the contribution of terrestrial versus algal production to lotic food webs is determined by both algal production, and the composition of consumer assemblages. By understanding controls of stable isotope ratios in lotic ecosystems, my research defines the spatial scales of energy flow in forested watersheds. Such information is essential to our understanding of population, community, and ecosystem dynamics in watersheds, as well as our predictive ability in assessing human impacts on river systems.