AN EXPERIMENTAL TEST OF THE INTERMEDIATE DISTURBANCE HYPOTHESIS USINGCULTURES OF MARINE-PHYTOPLANKTON

Connell's intermediate disturbance hypothesis (IDH) predicts that spec ies richness and diversity within a community are maximal at intermedi ate frequencies and intensities of disturbances. The predictions of th e IDH were tested experimentally by semicontinuous multispecies cultur es of marine phytoplankton. Disturbances consisted of dilutions at fix ed intervals. The disturbances eliminated parts of the experimental po pulations but supplied fresh nutrients. Intervals between disturbances ranged from 1 to 14 d. The magnitude of dilutions ranged from 30 to 9 9% removal. The IDH was fully supported when the results were expresse d as species numbers or as Shannon's diversity index based on biomass. On the other hand, the IDH was only partially supported when the resu lts were expressed as Shannon's diversity index based on cell numbers.