| Rivers and their tributaries are the arteries of the planet, pumping freshwater to wetlands and lakes and out to sea. Understanding energy flow up trophic levels, nutrient cycling pathways, and relative importance of terrestrial and aquatic carbon sources supporting aquatic consumers in large river food webs is essential in planning for wildlife conservation, environmental protection, and floodplain management. The principal goal of my dissertation is to understand better the factors controlling the complexity of river food webs through time.;At a shorter time scale, I first look at how season and food availability affect fish in rivers. I employ bulk tissue stable isotope analysis to determine trophic position of fish in the field, over different seasons, and fish in the lab, under different amounts of nutrient stress.;Scientists continue to debate which factors control the relative importance of organic sources fueling food webs of large rivers. Resolution of this debate requires a new technique: identifying food sources and trophic position using traditional bulk-tissue stable isotope techniques is difficult because of spatiotemporal variability of carbon sources, mixing model problems with too few tracers, and unavailability of reliable basal signatures.;In the remaining chapters of the dissertation, I utilize a new technique, applying nitrogen and carbon stable isotope analysis of amino acids to samples to determine trophic position and carbon food sources over time. First, in Chapter 2, I demonstrate the utility these new methods in a controlled feeding experiment in the laboratory, determining fish trophic positions. I show that the new methods seem to offer more accuracy and precision in trophic position estimates when compared to more traditional methods of bulk tissue isotope analysis. With these new analytical methods, I propose multidimensional metrics for use with compound specific analyses of food webs, as well as other multidimensional community measures (e.g., fatty acids, ordinal traits) in Chapter 3. Then, I evaluate long-term historical changes in trophic position (chapter 4) and food sources (chapter 5) of fish museum specimens using amino acid stable isotope analyses of both the Mississippi and Ohio rivers. |
