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In 2000, a field study in two shallow, polytrophic lakes (Langer See and Melangsee) in eastern Germany re-
vealed an equilibrium state assemblage of Cyanoprokaryota in late summer. During 4 successive weeks in
Langer See Planktothrix agardhii (Gom.) Anagn. et Kom., Aphanizomenon gracile (Lemmerm.) Lemmerm. and
Pseudanabaena limnetica Lemmerm. were more than 80% of the standing biomass of phytoplankton, and their
cumulative biovolume was around 33 mm³ l − 1 (( ± 3.2 SD). In Melangsee, the very small Limnothrix species L.
amphigranulata (Van Goor) Meffert was the most common species, accompanied by Pseudanabaena limnetica
and Planktothrix agardhii. For 3 weeks, their cumulative biovolume was about 23 mm³ l− 1 (( ± 3.4 SD), which
represented 75 – 82% of total biovolume. The dominant species all belong to the functional group S1 defined
by Reynolds (1997), except for A. gracile, which we suggest to be included in group SN. In both lakes mean
light intensities ranged between 2.2 and 8.3 E m − 2d − 1. Overall species spectra were very similar in both lakes,
but dominance by Limnothrix and by Planktothrix in the respective lakes is observed repeatedly. The success
of these species is discussed in the context of the habitat properties in August/September. Summer mixing events
represented no disturbances in the sense of Connell (1978), since they do not interrupt the species dominance. More
frequent mixing events and higher concentrations of dissolved nitrogen occurred in Langer See than in the more
shallow, but wind protected Melangsee. In Langer See light deficient conditions were intensified by an increasing
biomass of P. agardhii, and this species probably benefited from nutrient input by more frequent resuspension.
The light deficiency also affected the diversity, expressed as Shannon–Wiener Index (H), which was reduced more
in lake Langer See (H = 0.51) than in Melangsee (0.74) during steady state periods. Recognizing the important
effects of mixing, we suggest an additional variable to describe habitat properties: the number of full mixing days
as a proportion of total days of observation should help to discriminate between shallow habitats with intermittent
mixing events, and those with more regularly mixing in summer period.
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