A dynamic model of the cellular carbon to chlorophyll ratio applied to a batch culture and a continental shelf ecosystem

A novel parameterization of the dynamical relationship between cellular carbon (C) and chlorophyll (Chl) is developed using a Chl synthesis term that includes the physiological status of the cell and the effect of packaging of pigments within cells. The geometric derivation highlights the non‐linear relationship between Chl content and absorption due to the package effect. When parameterized for a generic 3 µm radius phytoplankton cell, the model reproduces the magnitude and daily variations of C : Chl and C : nitrogen ratios of the diatom Skeletonema costatum in published laboratory experiments. The parameterization is then applied in a three‐dimensional biogeochemical model containing three phytoplankton classes in the coastal waters off southeast Tasmania, Australia, which demonstrates the behavior of the dynamic Chl parameterization over a range of light‐ and nutrient‐limiting environments for phytoplankton of different sizes and growth rates. The model produces C : Chl ratios of ∼ 12–20 (weight : weight) and ∼ 60–80 for phytoplankton communities dominated by fast‐growing small and fast‐growing large cells, respectively, close to the ratios of 17 and 76 observed at two sampling stations during periods with diatom‐ and flagellate‐dominated communities. Throughout the simulation, community C : Chl ratios generally vary between 12 and 200, which is similar to the range observed globally. In the new parameterization, C : Chl ratios are most influenced by the package effect for light‐limited, slow‐growing large microalgae, with physiological processes becoming important for smaller, nutrient‐limited, fast‐growing microalgae.