Taxon-specific variation in the stable isotopic signatures (delta13C and delta15N) of lake phytoplankton

Summary 1. The variability in the stable isotope signatures of carbon and nitrogen (δ13C and δ15N) in different phytoplankton taxa was studied in one mesotrophic and three eutrophic lakes in south-west Finland. The lakes were sampled on nine to 16 occasions over 2–4 years and most of the time were dominated by cyanobacteria and diatoms. A total of 151 taxon-specific subsamples covering 18 different phytoplankton taxa could be isolated by filtration through a series of sieves and by flotation/sedimentation, followed by microscopical identification and screening for purity. 2. Substantial and systematic differences between phytoplankton taxa, seasons and lakes were observed for both δ13C and δ15N. The values of δ13C ranged from −34.4‰ to −5.9‰ and were lowest in chrysophytes (−34.4‰ to −31.3‰) and diatoms (−30.6‰ to −26.6‰). Cyanobacteria were most variable (−32.4‰ to −5.9‰), including particularly high values in the nostocalean cyanobacterium Gloeotrichia echinulata (−14.4‰ to −5.9‰). For δ13C, the taxon-specific amplitude of temporal changes within a lake was usually