Anthropogenic inputs of nitrogen (N) and phosphorus (P) create novel environmental conditions that alter biological organization and ecosystem functioning in freshwaters. My research objectives were: 1) identify levels of nutrient enrichment thresholds for stream periphyton nutrient ratios, individual algal and fish species abundances, and overall assemblage composition; 2) experimentally investigate interactions between P enrichment, an important grazing fish, and periphyton biomass and elemental composition; and 3) examine the influence of nitrogen enrichment on fish-mediated nutrient recycling effects on downstream ecosystem structure and function. Analyses of field collected data indicated that sharp declines in periphyton elemental composition, a synchronous decline in several algal species, and sharp declines in at least two benthic specialist fishes (Etheostoma spectabile and Campostoma anomalum) occurred when surface-water TP exceeded approximately 20 µg L⁻¹ and when TN was greater than 550 µg L⁻¹. A stream mesocosm experiment demonstrated that grazing by C. amomalum decreased benthic C:Chlorophyll a ratios, disrupted the continuous deposition of detritus and inorganic sediment, and as a result, increased periphyton P and N content. Variation in grazer effects on periphyton C:P and C:N ratios across the P enrichment treatments resulted in N:P ratios that were greater on grazed substrates in control and low P streams but grazer effects diminished with increasing P enrichment. A second experiment that measured nutrient recycling effects showed that fish-mediated nutrient recycling decreased periphyton C:N and increased C:P and N:P longitudinally in N enriched streams. Additionally, chlorophyll a increased significantly downstream of cages with fish. Comparisons of bacterial biomass production (BBP) and photosynthesis (PS) rates downstream of enclosures indicated that aerial BBP and PS rates and coupling of algal and bacterial production (COV_BBP-PS) were all higher downstream of enclosures with fish in high N:P streams. The combined results of these three studies suggest that there are serious biodiversity declines at low levels of nutrient enrichment, and loss of grazing fish species may have important consequences for stream ecosystem structure and function, as grazing fish plays an important role in modulating interactions between surface-water nutrients and benthic resources in streams.Ph.D.