Stoichiometry of consumer-driven nutrient recycling across nutrient regimes in streams.

Stoichiometric constraints within ecological interactions and their ecosystem consequences may depend on characteristics of the abiotic environment such as background nutrient levels. We assessed whether consumer identity, via differing body stoichiometry, could regulate periphyton stoichiometry across nutrient regimes in open systems. In 60 flow-through artificial streams, we factorially crossed dissolved inorganic nitrogen levels (elevated = 294 micog L(-1), ambient = 26 microg L(-1)) with dissolved inorganic phosphorus levels (DIP: elevated = 15 microg L(-1), ambient = 3 microg L(-1)) and consumer type [crayfish (body N : P = 18), snails (body N : P = 28) or a control]. At ambient DIP, periphyton in the crayfish treatment had a lower %P and a lower C : P than periphyton in the snail treatment suggesting that consumer identity, probably mediated by differing P-excretion, regulated periphyton P content. At high DIP, consumer identity no longer affected periphyton elemental composition. Therefore, the stoichiometry of consumer-driven nutrient recycling and consumer identity may be less important to ecosystem functioning in environments with elevated nutrient levels.