Limnol. Oceanogr., 44(4), 1999, 1114–1119

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 search for the causal explanation for species diversity and for its maintenance in natural systems is one of the key questions in modern ecology. There is a contrast between the obvious variety of species existing in natural systems and the competitive exclusion principle (Hardin 1960), which predicts that competition selects for the fittest species and leads to the exclusion of all the others. In aquatic ecology, this phenomenon is widely known as Hutchinson’s paradox of the plankton (Hutchinson 1961). A wealth of community theories have been developed to address this paradox. Two types of approach dominate the theoretical development. The most widespread approach assumes the existence of stable equilibria in natural systems. Under the equilibrium assumption, the maintenance of diversity is explained by factors such as niche differentiation or natural enemies. These factors tend to reduce competition so that exclusion does not occur. On the other hand, approaches have been developed that remove the steady-state assumption and focus instead on transient dynamics and stochastic descriptions (Chesson and Case 1986). Hutchinson (1961) suggested that environmental changes could prevent phytoplankton diversity from reaching a state of stable equilibrium if those changes are frequent enough to reverse competitive hierarchies before exclusion occurs (Sommer et al. 1993). According to the intermediate disturbance hypothesis 1 Present address: Center for Ecological Research, Kyoto University Kamitanakami Hirano-cho 509-3, Otsu, 520-2113, Japan.