Nutrient‐determined dominance in multispecies chemostat cultures of diatoms1

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Abstract
Thalassiosira gravida was rapidly displaced from ammonium‐limited chemostats at moderate (D = 0.03·h−1) and low (D = 0.01·h−1) dilution rates by Skeletonema costatum or Chaetoceros septentrionalis. Reduced temperature or reduced light did not favor Thalassiosira. Chaetoceros displaced Skeletonema at D = 0.03·h−1 but the two species coexisted at D 0.01·h−1. A brief temperature increase triggered changes in population densities in chemostats with coexisting species, leading to an increase in the less abundant species. Reduced turbulence did not change the pattern of coexistence.Nutrient‐dependent growth‐rate kinetics for the three species were deduced from rates of displacement in competition experiments and from published uptake kinetic and chemical composition data. The kinetics deduced by the two methods were qualitatively similar. Nutrient kinetics apparently do not explain the survival of natural populations of T. gravida in the field. On the basis of these results, a nutrient‐based functional group of phytoplankton is defined to include S. costatum and C. septentrionalis and exclude T. gravida.