There is considerable support in the ecological literature for the theory that phytoplankton communities experience non-equilibrium conditions. Therefore, it is possible that not all species of an algal community are deficient in the same nutrient, challenging the belief held by some aquatic scientists and lake managers that phosphorus is the universal nutrient limiting phytoplankton growth in lakes. Simultaneous additions of NH4+and PO43−were necessary to stimulate community photosynthesis – uptake of14CO2– during summer stratification in a number of northern Rocky Mountain lakes and reservoirs, whereas adding one nutrient had no significant effect on community photosynthesis. However, size-fractionated bioassays showed that additions of nitrogen alone stimulated photosynthesis of specific size classes of phytoplankton in these systems. The authors' data have the following implications for lake management: (1) phosphorus should not be assumed to be the only nutrient controlling photosynthesis; (2) simultaneous enrichment of nitrogen and phosphorus may be necessary to measurably increase community photosynthesis, especially in nutrient dilute systems; and (3) adding NH4+may cause changes in phytoplankton community structure without changing community primary production.