COMPETITIVE EXCLUSION AMONG THREE PLANKTONIC BLUE-GREEN ALGAL SPECIES1,2

Although domination of 1 species in an algal community has often been attributed to toxic effects, nutrient exhaustion can also lead to competitive exclusion among algal species. The bacteria-associated blue-green algae Microcystis aeruginosa, Nostoc muscorum, and Phormidium foveolarum were cultivated in Zehnder-Gorham's medium No. 11 individually and in their combinations. The growth rates of Microcystis and of Nostoc were reduced by the presence of Phormidium. This detrimental effect was apparently not due to formation of any toxic or allelopathic matter. It was caused by the reduction of iron and trace metal concentrations in the medium by Phormidium to levels too low for normal growth of Microcystis and Nostoc. Work with the axenic forms of the 3 algal species also indicated a lowering of iron and trace element concentrations by Phormidium to levels not sufficient for sustained growth of Microcystis and Nostoc. Thus, bacteria played no significant role in this example of competitive exclusion.