Disturbance characteristics determine the timescale of competitive exclusion in a phytoplankton model

The way in which disturbance shapes phytoplankton diversity is still subject to much debate. Various disturbance-diversity relationships have been proposed, with diversity increasing or decreasing with the intensity or frequency of disturbance, or peaking at intermediate levels. A key problem in this discussion is the use of different concepts of “disturbance”, which can encompass both positive and negative effects on phytoplankton. To investigate how different modes of disturbance affect phytoplankton diversity, we subject populations of modelled phytoplankton to different frequencies and intensities of three different disturbance modes: a positive, a negative and a combined positive and negative disturbance mode. A chemostat system, considered undisturbed, is used for comparison. We show that (1) disturbance increases phytoplankton coexistence, (2) the disturbance-diversity relationship depends on the type of disturbance and (3) the effect is transient and disturbances predominantly affect the timescale of competitive exclusion.