FOOD WEB RESPONSE TO LONG-TERM EXPERIMENTAL ENRICHMENT OF A DETRITUS-BASED STREAM ECOSYSTEM by

Nutrient enrichment of freshwater ecosystems is occurring on a global scale with significant effects on their structure and function. However, our current understanding of these effects is limited because of the paucity of long-term experimental manipulations in detritus-based food webs. This study assessed the effects of enrichment in a detritus-based headwater stream. Using a paired watershed design, macroinvertebrate abundance, biomass, and production were compared in a treatment and reference stream during a five-year continuous enrichment. I examined whether the effects of nutrient enrichment varied between primary consumers and predators and whether consumer body size mediated consumer responses. To determine how changes in community structure affected nutrient fluxes, I also quantified the nutrient assimilation and excretion rate of a dominant primary consumer, Pycnopsyche spp. (Trichoptera). I also tested whether enrichment altered subsidies to riparian consumers via increased aquatic insect emergence from the nutrient-enriched stream. During the fourth and fifth year of enrichment, nutrient enrichment stimulated primary consumer, but not predator, production and biomass. The increased dominance of large-bodie