Background/Question/Methods Although the relationship between biodiversity and ecosystem function has emerged as a vital concept in conservation biology, it has been little studied in benthic systems. Previous research has focused on how producer diversity affects ecosystem functioning, but often without considering the environmental context. Here we present the results of a mesocosm experiment designed to assess the effects of consumer diversity on algal accrual -- an essential ecosystem process -- under different “contexts” of benthic substrate. Species richness and substrate were manipulated using a 2X3 (substrate type X species richness) factorial design. Substrate was either sand or gravel, and 3 species of aquatic consumers (amphipods, water boatmen, and snails) were maintained at 0, 1 and 3 species treatments across 54 mesocosms. Species density was established such that total species biomass was held constant across treatments.

Results/Conclusions After 21 days, increasing consumer richness from one to three species significantly decreased periphytic biomass by 22%, periphytic chlorophyll by 25%, and suspended chlorophyll by 19% (ANOVA, P < 0.05). Consumer effects on algal species composition were examined via non-metric multi-dimensional scaling and showed the three consumers created distinctive algal assemblages (ANOVA, P < 0.05). However, the 3-species treatment did not differ from the snail treatment, suggesting this consumer contributed most to algal removal in the high richness treatment. Examining the proportion of algal removed (based on non-grazed controls), showed that consumer diversity effects were context dependent. There was an interaction between consumer richness and substrate type such that consumer richness reduced algae in sand, but not in gravel. In gravel, rather than reduce total algae, the combined consumer treatment actually had more. These results suggest algal removal rates within aquatic communities may change if poor land use practices result in increased sediment loading. Sediments alter the physical structure of aquatic environments, but these data suggest they may also alter the diversity–function relationship as well.