The paradox of the plankton: species competition and nutrient feedback sustain phytoplankton diversity

The diversity of phytoplankton species and their relationship to nutrient resources are examined using a coupled phytoplankton and nutrient model for a well-mixed box. The phytoplankton community either reaches a competitive exclusion state, where there is an optimal competitor, or the abundance of each phytoplankton species continually varies in the form of repeating oscillations or irregular chaotic changes. Oscillatory and chaotic solutions make up over half of the model solutions based upon sets of 1000 separate model integrations spanning large, moderate or small random changes in the half-saturation coefficient, Kji. The oscillatory or chaotic states allow a greater number of phytoplankton species to be sustained, even for their number to exceed the number of resources after additional species have been injected into the environment. The chaotic response, however, only occurs for particular model choices: when there is an explicit feedback between nutrient supply and ambient nutrient concentration, and when there are physiological differences among species, including cell quota and Kji. In relation to the surface ocean, the nutrient feedback can be viewed as mimicking the diffusive nutrient supply from the nutricline. Inter-species competition might then be important in generating chaos when this diffusive transfer is important, but less likely to be significant when other transport processes sustain surface nutrient concentrations.