Effects of N: P ratio and total nutrient concentration on stream periphyton community structure, biomass, and elemental composition

Abstract

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 algal 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.