On the Dominance of Filamentous Blue-Green Algae in Shallow Lakes

The algal community of eutrophic shallow lakes is often dominated by filamentous cyanobacteria. In this paper we follow two independent approaches to show that this state should be an alternative equilibrium over a range of conditions to the state dominated by green algae and diatoms. First, we analyze the patterns of blue-green dominance observed in the field, and show that in view of the dynamical systems theory, those patterns imply that the algal community must be a hysteretic system. Then, we construct a minimal competition model to show that hysteresis should, in fact, be expected from differences in physiology between cyanobacteria and algae measured in the laboratory. The basic mechanism behind this is that cyanobacteria are the superior competitors under conditions of low light, but also promote such conditions as they cause a higher turbidity per unit of phosphorus than other algae. This theory of hysteresis explains why blue-green dominance is so resistant to restoration efforts by means of nutrient reduction. It also suggests that a shock therapy, such as temporary flushing, can suffice to get rid of blue-greens.

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