Natural isotopic composition of carbon (δ13 C) correlates with colony size in the planktonic cyanobacterium Gloeotrichia echinulata

To assess variability in carbon isotope signatures (d13C) between and within populations under natural conditions, with a particular emphasis on colony size, we repeatedly collected planktonic colonies of a freshwater cyanobacterium Gloeotrichia echinulata in two lakes, Pyhaj a rvi (southwest Finland) and Erken (southeast Sweden). Despite substantial differences in the average d13C signature of Gloeotrichia between lakes (26.9% in Pyhaj arvi and 220.7% in Erken), a similar, systematic increase in d13C with colony size was observed in both lakes (of 2-3% in Pyhaj arvi and 3-5% in Erken). This suggests declining isotope fractionation with increasing colony size, probably related to diffusion limitation of carbon availability. Temporal variation explained a minor fraction of total subsample variability (range d13C ,4% in Pyhaj a rvi and ,6% in Erken). Isotopic 13C fractionation in Gloeotrichia was likely affected both by carbon source and by colony size.