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

The effects of nutrient ratios on algal community structure and algal growt h have been examined extensively in lakes and marine environments, but rare ly 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, commun ity structure, and elemental composition of periphyton communities. Early i n the experiment, periphyton chlorophyll a and total algal biovolume were h igher for treatments where N was added (high total nutrient concentration) but were not affected by N:P ratio. This response is contrary to our predic tion that P would limit periphyton growth based on the high N:P ratio in th e source water and unamended periphyton mate. The relative abundance of nin e of eleven common algal taxa was affected by N:P ratio, total nutrient con centration, or both. Overall, algal community structure was more sensitive than bulk measures of periphyton abundance to changes in N:P ratio and tota l nutrient concentration. Periphyton %N and %P increased with the N and P c oncentration of stream water, and periphyton N:P tracked stream water N:P r atio. Responses in periphyton chemical composition to nutrients could affec t the food quality of periphyton for consumers.