Stable carbon isotope biogeochemistry of lakes along a trophic gradient

The stable carbon (C) isotope variability of dissolved inorganic and organic C (DIC and DOC), particulate organic carbon (POC), glucose and polar-lipid derived fatty acids (PLFAs) was studied in a survey of 22 North American oligotrophic to eutrophic lakes. The λ13C of different PLFAs were used as proxy for phytoplankton producers and bacterial consumers. Lake CO₂ was primarily determined by autochthonous production (phytoplankton biomass), especially in eutrophic lakes, and governed the λ13C of DIC. All organic-carbon pools showed overall higher isotopic variability in eutrophic lakes ( Combining double low line 11) compared to oligo-mesotrophic lakes ( Combining double low line 11) because of the high variability in λ13C at the base of the food web (both autochthonous and allochthonous carbon). Phytoplankton λ13C was negatively related to lake CO₂ over all lakes and positively related to phytoplankton biomass in eutrophic lakes, which was also reflected in a large range in photosynthetic isotope fractionation (ϵCO₂-phyto, 8-25‰). The carbon isotope ratio of allochthonous carbon in oligo-mesotrophic lakes was rather constant, while it varied in eutrophic lakes because of maize cultivation in the watershed.