A comparison of methods to determine algal d¹³C in freshwater

Jérôme Marty and Dolors Planas
Limnol. Oceanogr. Methods (2008) 6:51-63

To accurately assess the flux of mass and energy to higher trophic levels in a food web using stable isotopes, the isotopic signature of basal sources is required. When studying aquatic food webs, it is difficult to obtain a signature for algae due to challenges associated with isolating small organisms from a bulk sample. In this study, we compared freshwater algal d¹³C values obtained using five approaches from the literature. Results indicated that the signatures derived from a primary consumer such as Daphnia sp., from particulate organic carbon with a correction for algal biomass and from isolated algal samples were comparable. By contrast, algal d¹³C values based on the signature of carbon dioxide and algal carbon fractionation were significantly lower than those of the other approaches. The inconsistent values produced by this method were likely due to problems in determining fractionation values based on current models and were potentially related to bicarbonate uptake by algae.