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 prop osed as one explanation of why diversity is often highest at intermediate l evels of disturbance. We used a model phytoplankton responses to environmen tal 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 value s 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 a pplied to (or encourage the re-examination of) many previous studies.