The relationship between biodiversity and ecosystem function has emerged as a vital concept in conservation biology but has been little studied in benthic systems. Previous research has focused on how producer diversity affects functioning, but often without consideration of the environmental factors. Here we assess the effect of consumer diversity on algal accrual, an essential ecosystem process, under different benthic substrate “contexts”. Species richness and substrate were manipulated in a mesocosm experiment using a 2X3 (substrate type X species richness) factorial design. Substrate was either sand or gravel, and 3 consumer species (amphipods, water boatmen, and snails) were maintained at 0, 1 and 3 species across 54 mesocosms. Densities were established such that total species biomass was held constant across species treatments. 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). Diversity effects on algal accrual were context dependent. An interaction occurred 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.