The effects of vertical mixing on a phytoplankton community: a modelling approach to the intermediate disturbance hypothesis

1. Connell's (1978) intermediate disturbance hypothesis (IDH) has been proposed as one explanation of why diversity is often highest at intermediate levels of disturbance. We used a model phytoplankton responses to environmental change (PROTECH) to investigate the validity of this hypothesis. 2. In a simulated phytoplankton assemblage of eight species, we found that the relationship between the increased intensity of a single forcing event and diversity was described by a positively skewed curve. 3. A progressive increase in forcing frequency introduced a sharp decrease in diversity at a threshold frequency. However, the highest diversity values were found at an intermediate frequency of disturbance. 4. We described the shape of this breakpoint response as like a 'cliff' and reconcile it with multiple stable-point theory. It is argued that the IDH should possibly be represented by this 'cliff' relationship, which may be applied to (or encourage the re-examination of) many previous studies.