Effects of grazing caddisflies on nutrient uptake in streams in a northern California watershed

Recent studies suggest that consumer-resource interactions within an ecological community are influenced by nutrient demands of individuals at different trophic levels.  In particular, stoichiometric imbalance between grazers and algae within stream ecosystems may strongly influence biogeochemical cycling of nitrogen and phosphorus.  In this study, we considered the influence of grazing and grazer stoichiometry on dissolved inorganic nitrogen (DIN) and PO₄ uptake by epilithic biofilms.  Representative rocks were selected from two sites of varying catchment area and light conditions, and incubated in re-circulating chambers.  DIN and PO₄ were added to each chamber and uptake was estimated by change in nutrient concentration over time.  Nutrient uptake was measured in each chamber with and without the addition of 5 (ambient density) grazing caddisflies (Neophylax reckeri) collected from the lower light site.  Biofilms were analyzed for organic matter, chlorophyll a, and % carbon, nitrogen and phosphorus.  Shifts in nitrogen and phosphorus uptake rates were observed between sites, as was a shift in the effect of grazer presence.  Grazers significantly reduced nitrogen uptake in the low light site, but not in the high light site, and significantly reduced PO₄ uptake in the high light site but not in the low light site.  This pattern may be explained by changes in the degree or direction of stoichiometric imbalance beween Neophylax and its resource, changes in patterns of nutrient demand by biofilms, or changes in community composition of algae and bacteria contained in the biofilm.