13C dynamics in benthic algae:: Effects of light, phosphorus, and biomass development

We performed three experiments in indoor streams and one experiment in a natural stream to investigate the effects of growth factors on delta C-13 levels in benthic microalgae. In the indoor streams, algae grown under conditions of high light and high phosphorus had delta C-13 values that were 16 parts per thousand higher than those in algae grown under conditions of low light and low phosphorus. Light effects were much stronger than phosphorus effects. The effects of both factors increased in strength as algal biomass accrued, and by the end of the experiments, algal delta C-13 and biomass were highly correlated. In the natural stream, algae exposed to direct sunlight were enriched 15 parts per thousand over shaded algae, corroborating the strong effect of light in the indoor streams. Growth factors such as light and nutrients probably reduce discrimination against C-13 (raising delta C-13 values) in benthic microalgae by causing CO2 depletion both within individual cells and within the assemblage matrix. However, because the most marked fractionation occurred in older and thicker assemblages, CO2 depletion within the assemblage matrix appeared to be more important than depletion within individual cells. In the absence of carbon-concentrating mechanisms, elevated delta C-13 suggests that inorganic carbon may limit the growth of benthic algae. The extensive range of delta C-13 values (-14% to -36%) created by light and nutrient manipulations in this study easily encompassed the mean delta C-13 values of both C-3 and C-4 terrestrial plants, indicating the challenge aquatic ecologists face in identifying carbon sources for higher trophic levels when light and nutrient conditions vary.