Diversity in planktonic communities: An experimental test of the intermediate disturbance hypothesis

According to Connell's intermediate disturbance hypothesis (LDH), diversity within a community is maximal at intermediate frequencies and intensities of disturbances. In order to test the IDH, disturbances of different freque ncies and intensities were imposed on natural plankton communities in contr olled field experiments. These disturbances consisted of an artificial deep ening of the mixed layer, leading to the dilution of epilimnetic population s and to a higher level of nutrients. Intervals between disturbances ranged from 2 to 12 d. Different intensities of disturbance were caused by differ ences in the experimental mixing depth (150 and 225% of the original epilim nion depth). Investigation focused on the effect that disturbances had on t he diversity of natural phytoplankton communities. Additionally we were int erested in determining the effect of grazing by zooplankton. The results of the held experiments show for the first time the applicability of the IDH to phytoplankton within complete planktonic communities. Diversity showed a clear maximum at the intermediate disturbance interval of 6 d. Similarly, species number peaked at intermediate interval length (6-10 d).