Contrasting response of stream detritivores to long-term nutrient enrichment

We examined growth and production responses of two dominant stream detritivores (chironomids and Tallaperla spp. stoneflies) at opposite ends of the “slow-fast” life-history continuum and with distinct feeding characteristics (i.e., consumption of fine particulate organic matter vs. leaf litter) to a 2-yr experimental nutrient enrichment of a headwater stream. Enrichment had large positive effects (∼50% increase) on chironomid growth rates but no effects on those of Tallaperla spp. On an areal basis, enrichment had a large positive effect on chironomid production (∼183% increase) but no detectable effect on the production of Tallaperla spp. When production data were examined on a per gram food basis, enrichment had an apparent positive effect on the production of both chironomids and Tallaperla spp. Together, these results suggest that nutrient-induced changes to organic matter quality had consistent and substantial positive effects on short-lived chironomids, but effects were limited for longer-lived stoneflies. The lack of a system-wide effect on Tallaperla spp. may have been due to nutrient-induced reductions in leaf litter quantity, despite increases in litter quality. Our results indicate that species-specific characteristics such as life span and dominant food type may be important in determining population- and community-level responses of consumers to nutrient enrichment of detritus-based aquatic ecosystems.