Tests of Resource Competition Theory Using Four Species of Lake Michigan Algae

The results of nutrient competition experiments performed for all the pairwise combinations of four species of freshwater algae often agreed with the predictions of a graphical model of resource competition. As predicted by theory, the species with the significantly lower resource requirement, as measured by R*, was the superior competitor when both species were limited by the same resource. Two species were observed to coexist only if either (1) each was limited by a different resource and met the theoretical criteria for coexistence or (2) the species were limited by the same resource and did not differ significantly in the their resource requirements. Single—species experiments were used to determine the functional dependence of the growth rate of each Lake Michigan diatom species on the concentration of limiting silicate or phosphate. The results, fit to the Monod (Michaelis—Menten) model, predicted that Fragilaria crotonensis and Asterionella formosa had identical silicate requirements, and that both were silicate competitors superior to Synedra filiformis and Tabellaria flocculosa. The phosphate requirements of the four species were very similar, with Synedra requiring significantly less phosphate than Tabellaria. Competition experiments demonstrated that Tabellaria was competitively displaced by the three other species for all silicate—to—phosphate ratios, as predicted. Asterionella and Fragilaria were apparently competitive equal, coexisting for all silicate—to—phosphate ratios used, suggesting evolution of similar resource requirements by these two commonly dominant species. Asterionella and Synedra coexisted for intermediate silicate—to—phosphate ratios, as did Synedra and Fragilaria.