Historical climate change and ocean turbulence as selective agents for two key phytoplankton functional groups

Using a classical physiological model based on nutrient uptake kinetics, we explored the effect of turbulence on resource competition and succession between 2 phytoplankton functional groups on ecological and geological time scales. The 2 groups we considered are silica-precipitating diatoms and carbonate-precipitating coccolithophorids. Using published experimental laboratory data for parameterization, our model results suggest that diatoms dominate under highly turbulent regimes, while coccolithophorids tend to dominate under stable, nutrient-depleted conditions. We attribute the success of diatoms in highly dynamic systems to luxury uptake of nutrients afforded by the evolution of storage vacuoles. In contrast, coccolithophorids are more successful in resource-depleted waters, due to their lower minimum limiting-nutrient requirement (R-.). We examine how these differences in nutrient acquisition strategy potentially explain the long-term trends in the fortunes of these 2 taxa on geological time scales. The fossil record indicates that coccolithophorids rose to ecological prominence in the mid-Jurassic and reached an apex in the mid-Cretaceous, but have declined throughout the Cenozoic. In contrast, diatoms have risen rapidly in the late Cenozoic, especially from early-Miocene time to the present. Based on paleoclimate reconstructions, from Mesozoic times, we hypothesize that the relative success of the 2 functional groups reflects, in part, long-term changes in upper ocean turbulence and its influence on the temporal distribution of nutrients.