Evidence for light-environment control of carbon isotope fractionation by benthic microalgal communities

Assemblages of filamentous algae and diatoms were grown on glass plates suspended in the water column in Tampa Bay, Florida (USA). Carbon isotope ratios (δ13C) of the ac cu mulated OM (OM) were analyzed to compare the effects of aqueous CO₂ concen tration, OM accumulation rate, algal type, and photosynthetically active radiation (PAR) on community-level carbon isotope fractionation between aqueous CO₂ and organic carbon. Concentrations of nutrients, PAR, and colored dissolved OM exhibited spatiotemporal variation during 6 experiments conducted over 2 mo of variable rainfall and terrestrial runoff. Fractionation ranged from 6.8 to 13.7‰ and was positively correlated with PAR, resulting in higher δ13C values for OM in low-light conditions. Fractionation was lower and OM accumulation rates were higher during the 3 early experiments, which had high nutrient concentrations as a result of terrestrial runoff. OM accumulation rates and CO₂ concentrations did not independently correlate with fractionation. The dominant factors influencing fractionation were PAR and spatial coverage by pennate diatoms, which together explained 56% of variability in fractionation. These results indicate that PAR significantly influences microalgal fractionation in estuaries, supporting the concept that the light environment may contribute to the widely observed phenomenon of higher δ13C values in benthic algae relative to phytoplankton.