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 populationand community-level responses of consumers to nutrient enrichment of detritus-based aquatic ecosystems. Nutrient enrichment of aquatic ecosystems is occurring worldwide as a result of human-induced changes to global nitrogen (N) and phosphorus (P) cycling (e.g., Bennett et al. 2001; Galloway et al. 2003). Fertilizer production and application, fossil fuel combustion, and suburban/urbanization have all contributed to increased mobilization of N and P and elevated concentrations of these elements in streams, lakes, and coastal marine environments (e.g., Caraco 1993; Vitousek et al. 1997; Carpenter et al. 1998). Such changes in nutrient availability can have strong indirect effects on invertebrate primary consumers because many basal food resources (i.e., primary producers and detritus) undergo large changes in quantity and quality as a result of nutrient en1 To whom correspondence should be addressed. Present address: El Verde Field Station, HC-05 Box 8974, Rio Grande, Puerto Rico 00745-9601 ([email protected]). 2 Present address: U.S. Environmental Protection Agency, National Exposure Research Laboratory, Cincinnati, Ohio 45268.