Theory predicts that species diversity can enhance stability of community-level biomass while simultaneously decreasing population-level stability. Enrichment can theoretically destabilize communities but effects may become weaker with increasing diversity because of the inclusion of consumer-resistant prey. Few experiments using direct manipulations of species diversity have tested these predictions. We used laboratory-based aquatic food webs to examine the effects of species composition, diversity and enrichment on temporal variability of population- and community-level biomass. We found weak effects of enrichment on population- and community-level stability. However, diversity enhanced community-level stability while species composition had no influence. In contrast, composition effects outweighed diversity effects when stability was measured at the population level. We found no negative effects of diversity on population-level stability, in opposition to theory. Our results indicate that diversity can enhance stability in multitrophic systems, but effects vary with the scale of biological organization at which stability is measured.