Periphyton nitrate uptake and denitrification, not phytoplankton assimilation, accounted for ~90% and 73% of the depletion of ~35 μg NO3- N liter-1 added as whole-epilimnion enrichments of ammonium nitrate to Castle Lake, California, in midsummer 1980 and 1981. The importance of benthic processes is suggested by similar rates of depletion for nitrate and ammonium in the whole-lake experiments; these rates are inconsistent with previous microcosm studies of phytoplankton that showed strong preferential uptake of ammonium and low uptake rates of nitrate. The importance of the benthic pathways was confirmed by an NH415NO3 mesocosm experiment, which simulated the whole-epilimnion experiments. Less than 9% of the 15N transformed was associated with water-column pools; the remainder was incorporated into epipelic periphyton and sediments (56%) or attributed to 15N2-15N2O loss via denitrification. Historical analysis of nitrate depletion in the epilimnion during spring suggests that periphyton may outcompete phytoplankton for water-column nutrients.