Taxon‐specific variation in the stable isotopic signatures (δ13C and δ15N) of lake phytoplankton

Abstract

Summary1. 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 <1–8‰ (<1–4‰ for microalgae alone and <1–8‰ for cyanobacteria alone), whereas the amplitude among taxa within a water sample was up to 31‰.3. The values of δ15N ranged from −2.1‰ to 12.8‰ and were high in chrysophytes, dinophytes and diatoms, but low in the nitrogen‐fixing cyanobacteria Anabaena spp., Aphanizomenon spp. and G. echinulata (−2.1‰ to 1.6‰). Chroococcalean cyanobacteria ranged from −1.4‰ to 8.9‰. For δ15N, the taxon‐specific amplitude of temporal changes within a lake was 2–6‰, (2–6‰ for microalgae alone and 2–4‰ for cyanobacteria alone) and the amplitude among taxa within a water sample was up to 11‰.4. The isotopic signatures of phytoplankton changed systematically with their physical and chemical environment, most notably with the concentrations of nutrients, but correlations were non‐systematic and site‐specific.5. The substantial variability in the isotopic signatures of phytoplankton among taxa, seasons and lakes complicates the interpretation of isotopic signatures in lacustrine food webs. However, taxon‐specific values and seasonal patterns showed some consistency among years and may eventually be predictable.