CHARACTER VARIATION AMONG PLANKTIC DIATOM S IN BERLIN-BRANDENBURG WATERS

Stable, diacritical features are necessary for the description of morp hospecies. However, many planktic species of different waters show a w ide range of phenetical plasticity and are therefore difficult to iden tify. In the case of diatom frustules, character variation can be disc ussed on two levels - within and between individuals. A character vari ation between individuals is exemplified by the number of shortened st riae on the entire valve face of Cyclotella radiosa, which continuousl y increases with the circumference of the valve. Within individuals di scontinuous moiphological changes may also occur, since frustules are units of cell wall structures successively developed over generations (e.g. heterovalvare cells of the taxon of the Cyclotella kuetzingiana/ ocellata/comensis-group. Frustules of C. radiosa as well as frustules of C. kuerzingiana/ocellata/comensis have different lengths of ribs on the epi- and on the hypotheca. This phenomenon of different lengths o f the ribs within a single frustule is discussed with respect to cell wall silification. The character variability of diatoms related to cha nges in cell wall structures of other algal groups is discussed (Chlor ococcales and Desmidiales). The occurrence of the planktic diatom spec ies is indicated for 11 investigated waters in the Berlin-Brandenburg area of Germany during the observation period from 1990 to 1993.