We evaluated how benthic algal assemblages that vary in composition, richness, and other diversity metrics remove NO₃-N from the water column of a mountain stream. Ecological theory and empirical studies suggest that ecosystem process rates should increase as richness increases because of niche separation or activity of dominant taxa. Accordingly, we predicted that algal assemblages with highest richness would show the highest rates of NO₃-N uptake. To test this prediction, we transplanted 225 rocks representing 3 patch types (green, yellow, and brown) that differed macroscopically in algal composition from a lake outflow stream to a lake inflow stream where an experimental release of ¹⁵N-NO₃ was ongoing. We measured ¹⁵N uptake in each patch type during the stable isotope release. Benthic algal richness varied from 28 genera in the green patch type and 26 genera in the yellow patch type to 22 genera in the brown patch type. Without accounting for differences in chlorophyll a content, NO₃-N uptake (2.1–3.3 × 10⁻⁴/d) was highest in the green patch type, lowest (0.3–0.6 × 10⁻⁴/d) in the yellow patch type, and intermediate (1.2–1.5 × 10⁻⁴/d) in the brown patch type. NO₃-N uptake normalized to chlorophyll a increased in concert with algal richness in the 3 patch types. This result supports the hypothesis that increased assemblage diversity leads to higher rates of community processes. Aside from diversity differences per se, lower rates of NO₃-N uptake in the brown patch type might be the consequence of differences in functional characteristics of the taxa present. Approximately 29% of algal biovolume in the brown patch type consisted of taxa capable of N₂-fixation, a result that suggests that algae in this patch type might be capable of meeting N needs via N₂-fixation rather than via removal from the water column.