The effect of water column mixing on phytoplankton succession, diversity and similarity

The lake number was used to describe the mixing condition for three consecutive years (1992-1994) in Lake Tegel, Berlin, and compared to the successions of diatoms, dinoflagellates and cyanobacteria, the main phytoplankton groups in the lake, as well as to diversity and similarity indices. Using both diversity and similarity indices in juxtaposition provides an indication of the growth type of prevailing species (r- or K-strategists) and the degree of competition in the aqua-ecological system. A general pattern of these indices can be recognized as three phases: (i) high diversity-during spring, summer and autumn, interrupted by (ii) phases of low diversity during the late spring clear-water phase as the number of spring species plummeted, and (iii) during the late summer, climax populations of K-strategists. On a smaller time scale, similarity and diversity proved to react sensitively to disturbances at frequencies intermediate in relation to the generation times of the phytoplankton. This supports the 'intermediate disturbance hypothesis', as proposed for phytoplankton by Padisak et al. [(eds) Intermediate Disturbance Hypothesis in Phytoplankton Ecology. Kluwer Academic, 1993]. Diversity may remain quite high even for extended periods during summer climax situations, in conjunction with a high degree of similarity, if deeper mixing of the epilimnion occurs at time intervals of 2-3 weeks, as during the summer of 1993. This enables the prevalence of 'ruderal' species, together with some motile K-strategists who actively seek optimal depths for photosynthesis. During such summer situations described by frequent occurrences of lower lake numbers, the epilimnion of Lake Tegel is mixed deeply enough to support ruderals, but not too vigorously to counteract competitive advantages of motile species. Thus, vertical niche separation enhances diversity.