PHYTOPLANKTON SUCCESSION AND DIVERSITY IN A WARM MONOMICTIC, RELATIVELY SHALLOW LAKE - LAKE VOLVI, MACEDONIA, GREECE

In Lake Volvi, phytoplankton dominance was examined in relation to the main phases of the fluctuating physical state of the water column and nutrient levels. Four stages of algal succession were identified. The successional pattern was, in general, similar from year to year. External disturbances such as increased wind mixing and high floods had the effect of setting the succession back to an earlier stage. Nanoplanktic diatoms and flagellates dominated during the first stages of succession. The succession was running from r-selected species in early stages to K-strategists in summer and towards a mixed community in the terminal stage. Diversity was expressed by the indices of Gleason and Shannon (based on physical units and biomass). Interpretation of species diversity took into account the differential variations of its two components, the number of species and the evenness of their distribution. Diversity was more affected by evenness than by number of species. Diversity increased in late stages of succession corresponding to the complexity of the community (high number of species, high evenness). Physical disturbances influenced positively the diversity. The maximal diversity appeared in the transition periods between the compositional changes generated by disturbances and the true successional development. Disturbances may generate a more uniform distribution of diversity throughout the whole water column. The role of physical disturbances in increasing the phytoplankton diversity may be demonstrated from comparisons of diversity - evenness - species number of the same stages under different meteorological conditions.