Algal, invertebrate, and fish production in headwater streams along a forested gradient

As urban populations increase globally, the conversion of natural land cover to anthropogenic land use is on the rise. Diffuse perturbations that accompany such land use changes are considered a chief threat to headwater streams, which provide habitat for diverse faunas of macroinvertebrates, fish, and amphibians and are important sources of sediments, nutrients, and organic matter for downstream systems. Although environmental assessments of stream ecosystems generally measure responses to these perturbations as shifts in community structures, functional rather than structural properties may provide a better indicator of land use impacts since measures of ecosystem function can reveal more about the mechanisms that alter running water ecosystems. The primary objective of this thesis was to quantify stream productivity along a gradient of forest cover and assess bottom-up and top-down mechanisms for observed patterns in stream productivity. In the first chapter, I quantified algal, invertebrate, and fish biomass, the single best correlate of productivity, at 38 first-third order streams along a forest cover gradient at 55 spatial scales and determined that subcatchment forest cover was consistently the best predictor of biomass at all trophic levels in terms of the variation explained by forest cover and the unexplained residual variance. When considering the independent effects of forest cover on biomass at three coarse scales, subcatchment forest cover was the only significant predictor of biomass, suggesting that the common practice of maintaining riparian buffers is an ineffective management practice for protecting stream function from catchment-wide impacts. In the second chapter, I developed two time-saving approaches for processing benthic invertebrate samples that reduced laboratory processing time by 37% and 82% and provided production estimates that were strongly related to production estimates obtained from standard processing techniques. These time-saving approaches were used in chapter 3 so that I could measure algal, invertebrate, and fish production at 12 sites along a gradient of subcatchment forest cover, which I determined to be the most influential spatial scale on biomass in chapter 1. Total nitrogen was a significant bottom-up force that explained 34-83% of the variation in stream productivity at all three trophic levels. However, consumer production to consumption ratios indicated that predators and grazers can potentially deplete their food sources, and that top-down forces are therefore also important contributors to observed patterns in productivity and biomass along the forested gradient