Food web response to long-term experimental enrichment of a detritus-based stream ecosystem
Davis, John Michael
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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-bodied primary consumers that were predator-resistant likely attenuated the positive nutrient effect on higher trophic levels. Consumer response to nutrient enrichment also varied with body size, but this body size effect varied with trophic level. Specifically, enrichment increased the abundance and biomass of large-bodied primary consumers, but not large-bodied predators. Nutrient enrichment also accelerated the rate that Pycnopsyche assimilated and excreted nitrogen and phosphorus. Because Pycnopsyche disproportionately increased their rate of phosphorus assimilation relative to nitrogen, Pycnopsyche facilitated phosphorus sequestration at the stream-level. Despite nutrient enrichment doubling aquatic emergence biomass, it did not increase the biomass or abundance of riparian spiders, likely because of the increased dominance of predator-resistant prey. Enrichment increased the relative abundance of Trichoptera and the individual body size of emerging adults; two groups of prey that are not readily eaten by spiders. Thus, shifts in the primary consumer composition reduced the positive effects of nutrient enrichment on instream and riparian predators. Because consumer body size was an important factor determining how nutrient enrichment affected this stream food web, such species-specific traits may be key determinants in predicting ecosystem-level responses to nutrient enrichment.