According to Connell's intermediate disturbance hypothesis (IDH), diversity within a community is maximal at intermediate frequencies and intensities of disturbances. In order to test the IDH, disturbances of different frequencies and intensities were imposed on natural plankton communities in controlled field experiments. These disturbances consisted of an artificial deepening of the mixed layer, leading to the dilution of epilimnetic populations and to a higher level of nutrients. Intervals between disturbances ranged from 2 to 12 d. Different intensities of disturbance were caused by differences in the experimental mixing depth (150 and 225% of the original epilimnion depth). Investigation focused on the effect that disturbances had on the diversity of natural phytoplankton communities. Additionally, we were interested in determining the effect of grazing by zooplankton. The results of the field 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).

The influence of fluctuating light intensities on phytoplankton composition and diversity was investigated for 49 days under semi-continuous culture conditions with sufficient nutrient supply, using phytoplankton assemblages from Lake Biwa, Japan. Light conditions were either periodically changed from high intensity (100 µmol photons m s) to low intensity (20 µmol photons m s) at intervals of 1, 3, 6 and 12 days, or fixed to constant intensities (permanent high and low light levels). All treatments additionally experienced a day:night cycle of 16:8 h.Phytoplankton abundance increased and reached a saturation level on day 19 of the treatment with permanent high light, but increased continuously until the end of the experiment (day 49) in the treatment with permanent low light intensity. In treatments with periodically changing light intensities, the phytoplankton abundance reached saturation levels between these dates. Under phytoplankton abundance saturation, chlorophytes predominated in the treatment with permanent high light, while either cyanophytes or diatoms were abundant under permanent low light intensity. Treatments with changing light supply had chlorophyte- and cyanobacteria-dominated replicates as well as replicates with balanced proportions of both. Furthermore, species diversity, measured by the Shannon index, was low in cultures under permanent light intensity, while slow fluctuating light at the scale of 3 -12 days resulted in an increased diversity index.These results indicate that species composition and diversity of the phytoplankton were affected by the periodically changing light regime in the order of days, and suggest that temporal changes in weather conditions are a major impediment to competitive exclusion of phytoplankton species in nature.

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