ON THE DOMINANCE OF FILAMENTOUS CYANOBACTERIA IN SHALLOW, TURBID LAKES

The phytoplankton community of eutrophic shallow lakes is often domina ted by filamentous cyanobacteria of the family Oscillatoriaceae. In th is paper we follow two independent approaches to show that this situat ion is likely to be one of two alternative stable states of the algal community. First we analyze patterns of cyanobacterial dominance obser ved in the field, and show that these patterns imply that the algal co mmunity is a hysteretic system with two alternative equilibria. Then, we construct a simple competition model to show that hysteresis should in fact be expected from differences in physiology between cyanobacte ria and algae. The basic mechanism is that cyanobacteria are the super ior competitors under conditions of low light, but also promote such c onditions, as they can cause a higher turbidity per unit of phosphorus than other algae. This mechanism of hysteresis offers an explanation for the resistance of cyanobacteria dominance in shallow lakes to rest oration efforts by means of nutrient reduction.