MEPS 495:77-90 (2014)  -  DOI: https://doi.org/10.3354/meps10559

ABSTRACT: 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 accumulated OM (OM) were analyzed to compare the effects of aqueous CO₂ concentration, 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.

KEY WORDS: Photosynthetic fractionation · Benthic algae · Basal resource · Light attenuation · Estuary