Light, nutrients and predators can constrain primary producers and consumers; however, the ecological effects of these factors have rarely been tested simultaneously in open systems. To partly address this knowledge gap, primary producer biomass and consumer abundance and biomass were quantified on nutrient-diffusing substrata along a light gradient created by four riparian reserve treatments (forested controls, 30-m wide riparian reserve, 10-m reserve, clear-cut) each replicated twice in headwater streams of southwestern British Columbia in 2001 and 2002. Predation pressure, as estimated by trout abundance varied among streams, and was treated as a covariate, with nutrient enrichment and riparian reserve width as fixed effects. Primary producer and consumer biomass were limited by light flux as determined by reserve width, and nutrients, especially phosphorus. In 2002, nutrient effects on algal biomass were conditional upon reserve width: chlorophyll a biomass was higher on P- and N+P-amended pots compared to controls, but only in the 10-m reserve and clear-cut treatments or when light flux was greater than 75 μmol m−2 s−1. The effects of fish predators on primary consumers were also conditional upon reserve width, with strong predator effects occurring in the 10-m and clear-cut riparian treatments. The interactive effects of light flux, nutrient supply and predator abundance in constraining lower trophic levels may be common in nature. My results, therefore, suggest interactive effects were manisfested above a specific threshold condition. The interactive effects of light flux, nutrient supply and predator abundance in constraining lower trophic levels may be common in nature. Results from my study support this hypothesis and suggest that variation in light flux was an important condition dictating the strength of nutrient limitation and redation pressure in these headwater streams.