Is excretion by grazing caddisflies a source of nutrients to N-limited streams?

Excretion of wastes by fishes and zooplankton can be an important mechanism of nutrient regeneration, but the importance of nutrient excretion by stream insects is largely unknown.  We measured excretion of nitrogen and phosphorous by four abundant genera of grazing caddisflies (Dicosmoecus, Neophylax, Gumaga, and Glossosoma) in the South Fork Eel River watershed (Mendocino, Co., CA, USA) during the summer of 2005 and 2006.  We placed 4 to 180 insects in sealed bags for 1-2 hours and determined concentrations of NH4+ and SRP before and after incubations.  Per biomass excretion rates of NH4+ were variable among species, but similar for single species among sites.  Per-biomass excretion of SRP was more variable within single species.  For example, excretion of SRP by Gumaga nigricula  was undetectable in one stream but comparable to other species at a different site.  Excretion rates will be compared to nutrient composition of insects’ food and tissues to determine if stoichiometric imbalances can explain variation within and among species.   Estimated per area rates of NH4+ excretion suggest these animals could be significantly increasing N turnover rates in productive streams where populations have high biomass.