The effects of elevated atmospheric CO2 on freshwater periphyton in a temperate stream

This study examines the effects of elevated CO2 on the benthic biology of a temperate freshwater stream. We tested the hypotheses that elevated CO2 would increase periphyton biomass, alter elemental composition, and change community composition by increasing the frequency of algal taxa most limited by CO2 availability. Carbon dioxide was bubbled into reservoirs of stream water, increasing the ambient pCO2 by approximately 1100 ppm. The CO2-enriched water then flowed into artificial stream channels. Ceramic tiles were placed into the channels to allow for periphyton colonization. Dissolved inorganic carbon increased and pH decreased with added CO2. Measurements of biological parameters including periphyton biomass, algal C:N:P ratios, and community composition suggest that the periphyton were unaffected by the changes in stream water chemistry. We infer that rising atmospheric CO2 will impact stream water chemistry but that periphyton may not be the first to respond to these changes. Impacts to alkaline freshwater streams from elevated CO2 initially may be due to changes to terrestrial inputs that affect microbial decomposition and grazer activity, rather than through increases in periphyton carbon fixation. However, environmental characteristics of freshwater systems vary considerably, and additional studies are needed for accurate predictive modeling and monitoring of the effects of increasing atmospheric CO2 on freshwater streams.