Top-down and bottom-up forces in an Appalachian stream: can consumers differentially mediate effects of nutrient loading on algae?

The effect of nutrient loading on the biotic integrity of streams is of great concern, given increased anthropogenic inputs common in agricultural and urban environments. Few studies have addressed the potential of top-down forces to regulate increased primary production that often results. In this study, I examined effects of nutrients on algae in response to both snails and macroconsumers, in a factorial design. Snails had the strongest regulatory impact on nutrient-stimulated algae, both with and without predators. In contrast, omnivorous and algivorous macroconsumers were not able to directly suppress increased growth. However, reductions of invertebrate biomass by both omnivorous and insectivorous macroconsumers indirectly increased algae, regardless of nutrient addition. Results provide evidence for the existence of trophic cascades in lotic systems, even in complex food webs where consumers feed at more than one trophic level, and illustrate the importance of grazer identity in the mediation of nutrient effects on algae.