Using a phytoplankton growth model to predict the fractionation of stable carbon isotopes

In the preceding paper in this issue, a phytoplankton growth model based on an analogy with chemical kinetics (the CR model) was re-derived, and a comparison made with the growth rate of cultured phytoplankton assemblages extracted from temperate lakes. In this paper, further derivation of the CR model leads to the same model of carbon isotope fractionation used by Rau et al. (Mar. Ecol. Prog. Ser., 133, 275–285, 1996). Both the CR and Rau et al. models are compatible with the observation that isotope fractionation during phytoplankton growth, εp, is proportional to the growth rate, μ, divided by the extracellular carbon concentration, C. The CR model is then used to explain the observation that the initial slope of εp divided by μ/C is a negative linear function of the cellular carbon to surface area ratio of the phytoplankton cell. The CR model applies the same model framework to predict both phytoplankton growth and stable isotope fractionation. The ability of the CR model to predict two measurable phenomena increases its usefulness, and also our confidence in its predictive capabilities.