DYNAMICS OF COMPETITION AMONG MICROALGAE IN VARIABLE ENVIRONMENTS - EXPERIMENTAL TESTS OF ALTERNATIVE MODELS

Two species of green microalgae, Scenedesmus quadricauda var. longispina and Chlorella sp., were grown together in continuous cultures under conditions of phosphorus-limitation. This resource was supplied as a series of discrete periodic pulses, preventing achievement of a strict equilibrium. Two mathematical models, based on the equations of Monod and Droop were parameterized from earlier, single-species experiments. These models were then tested for their ability to predict competitive dynamics in these and similar competition cultures. Chlorella outcompeted Scenedesmus in all cultures, regardless of the period of phosphorus pulses. This outcome was predicted qualitatively by both models, but the Droop model gave much more accurate estimates of the rate at which competitive exclusion proceeded. The Droop model also successfully predicted the trajectories of population density in bacteria-free competition cultures, and the rates of uptake of large phosphorus pulses applied to these cultures. This study suggests that competitive trends can be rapid in both equilibrium and nonequilibrium habitats. The traits that confer competitive ability in equilibrium habitats can also confer competitive ability in nonequilibrium habitats, which may limit the potential tradeoffs between species along gradients of environmental variability. Lack of equilibrium may nevertheless increase diversity if convergence of competitive strategies and life histories occurs, a possibility suggested by the competition models used in this study, and by other competition models.