The effects of consumers on ecosystem processes are often mediated by abiotic factors that vary both spatially and temporally.  In highly dynamic systems such as prairie streams, understanding these contingencies is particularly critical for understanding consequences of species losses or invasions.  We conducted eight intensive experiments using field exclosures and experimental streams to test the effects of a grazing minnow (Phoxinus erythrogaster) on primary productivity and nutrient retention.  Results from these experiments varied, including positive, negative and no effects of grazing minnows on primary productivity.  Negative effects on primary productivity coincided with high ambient nutrient concentrations in mesocosms and low flow conditions in field exclosures.  Positive effects of grazers on primary production occurred in experimental streams with low ambient nutrient concentrations.  In some cases there was a change in ecosystem structure (filamentous algal cover) that did not correspond to changes in rates of production.  Our results suggest a comprehensive understanding of nutrient dynamics and grazer functional responses are necessary to predict the relative influence of grazers on ecosystem processes.