The effects of nutrient ratios on algal community structure and algal growth have been examined extensively in lakes and marine environments, but rarely in streams. We manipulated stream water N:P ratio (65:1, 17:1, 4:1) and total nutrient concentration (low and high) in a factorial experiment using once-through streamside flumes and measured responses in abundance, community structure, and elemental composition of periphyton communities. Early in the experiment, periphyton chlorophyll a and total biovolume were higher for treatments where N was added (high total nutrient concentration) but were not affected by N:P ratio. This response is contrary to our prediction that P would limit periphyton growth based on the high N:P ratio in the source water and unamended periphyton mats. The relative abundance of nine of eleven common algal taxa was affected by N:P ratio, total nutrient concentration, or both. Overall, algal community structure was more sensitive than bulk measures of periphyton abundance to changes in N:P ratio and total nutrient concentration. Periphyton %N and %P increased with the N and P concentration of stream water, and periphyton N:P tracked stream water N:P ratio. Responses in periphyton chemical composition to nutrients could affect the food quality of periphyton for consumers.