Recent theory suggests that biodiversity and productivity are both constrained by resource supply rates and ratios, and that resource stoichiometry is the key to understand the relationship between biodiversity and productivity. We experimentally tested this theory using pelagic metacommunities. We amended existing predictions by explicitly considering evenness as biodiversity aspect and consumer instead of competitive control of algal biomass. The metacommunities received different phosphorus (P)-supply, the three patches within each metacommunity differed in nitrogen (N)-supply to create different N:P ratios (2, 16, 128). All patches were inoculated with a phytoplankton assemblage consisting of 5 species, half of the metacommunities received 2 ciliate species as consumers. At the level of the entire metacommunity, algal biomass increased whereas species richness and evenness decreased with increasing P-supply. Without consumers, resource use efficiency (RUE, realized biomass per unit P) increased with increasing richness and evenness. Consumer presence reduced overall biomass and richness and precluded a correlation between RUE and biodiversity. At the patch level, local evenness correlated to higher RUE at both imbalanced N:P ratios (2 or 128) but not at balanced N:P. In conclusion, overall P-supply constrained realized biomass and altered diversity, whereas resource stoichiometry shaped the relationship between biodiversity and RUE.