Linking chlorophyll-nutrient dynamics to the Redfield N : C ratio with a model of optimal phytoplankton growth

The Redfield N:C ratio is a fundamental quantity in marine biogeochemistry because it is a key determinant of the efficiency of the biological carbon pump, yet no convincing explanations have been put forward for its remarkable constancy over much of the world ocean. Phytoplankton growth models have so far been unable to account for the different relationships between growth rate and N:C ratio under nutrient and light limitation, and have not been able to predict the Redfield N:C ratio. A relatively simple model of coupled chlorophyll and nutrient dynamics is developed from the premise that phytoplankton maximize growth by optimally allocating nutrient and energy resources among competing metabolic requirements for nutrient uptake, light-harvesting, and growth. The model reconciles nutrient and light limitation and appears valid under both balanced and non-balanced growth conditions. The Redfield N:C ratio and its constancy are explained as a result of evolutionary pressure towards maximizing light-limited growth rates in relatively carbon-rich oceanic waters.