Role of nutrient limitation and competition in controlling the populations of Asterionella formosa and Microcystis aeruginosa in semicontinuous culture1

The nutrient uptake and growth kinetics of the freshwater diatom Asterionella formosa and the blue‐green alga Microcystis aeruginosa under P or Si limitation were determined in batch cultures at 20°C. Asterionella had a higher maximum growth rate and a lower half‐saturation constant for growth than Microcystis under P limitation. The maximum uptake velocity for P was higher for Asterionella than for Microcystis while the half‐saturation constants for P uptake were similar for the two species. The Monod model of growth predicted a switch from Si limitation to P limitation for Asterionella at Si:P of about 93 (±15). In experiments with the two species grown together at various Si:P ratios, Asterionella dominated steady state semicontinuous cultures when both species were P‐limited, Si:P > 100. Microcystis became dominant when Asterionella was Si‐limited, Si:P < 100. The competition experiments showed the importance of the concentration of nutrients, as well as the ratio of the nutrients and rate of supply, in controlling the cell densities of the algal species.