Program

The competitive fitness of phytoplankton in nutrient-limited oceans varies in part with each species' critical limiting nutrient concentration or R*.  For phytoplankton, R* values represent the steady-state balance between the supply of a limiting nutrient to the cell as governed by saturation uptake kinetics and demand for that nutrient as governed by growth.  Other factors being equal, species with low critical limiting nutrient concentrations may outcompete those with higher R* values in nutrient-limited waters.  As the concentration of CO₂ in the ocean's euphotic zones affects nutrient demand in various species of marine phytoplankton, CO₂ may also affect those cells' critical limiting nutrient concentrations, their competitive fitness, and, potentially, phytoplankton community composition.  The effects of CO₂ on critical limiting nutrient concentrations were examined in marine diatoms by scaling R* values to CO₂-driven variations in cellular N and P quotas.  As N and P quotas in diatoms generally decline with increasing CO₂, diatoms are predicted to become more competitive with respect to limiting nutrients (lower R* values) as CO₂ rises.  However, while N and P quotas decline most rapidly as CO₂ increases from approximately 5 to 12 µM (150 to 350 µatm), they decrease more slowly or not at all at higher concentrations of CO₂.  The nutrient resource competitiveness of many species of diatoms is therefore unlikely to improve as the concentration of CO₂ in the ocean's surface waters increases over the next century.