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Cyanobacteria associations in shallow polytrophic lakes: influence of environmental factors
A 2 year field study in two shallow, polytrophic lakes (Langer See and Melangsee) in Brandenburg revealed the importance of environmental factors in controlling the population dynamics of various cyanobacterial species which do not form layers or aggregations. The species belonged to the toxin producing genera Cylindrospermopsis, Planktothrix, Aphanizomenon and Anabaena, and to the nontoxic members of Limnothrix and Pseudanabaena. In German waters, Cylindrospermopsis raciborskii is a neophyte, and exhibits a comparatively low temperature requirement of about 17 °C for germination and growth. The nutrient concentrations were comparable in both the lakes, and did not change measurably during the summer succession. The dissolved nitrogen fractions were well below 50 μg l –1 with total depletion of nitrate. Soluble reactive phosphate concentrations never fell below 3 μg l –1 throughout the summer season. The light attenuation factors (K d ) of 3–5.8 m –1 indicate high self-shading. However, under calm weather conditions, the cyanobacteria were not concentrated in near surface layers and the species composition was relatively uniform in the vertical. In mid-summer, the diazotrophic Nostocales were replaced by Oscillatoriales. Reduced light availability caused by mixing and self-shading is considered to be the main factor causing the summer decline of Nostocales. By means of two weather stations, more frequent, deeper and longer lasting mixing events could be observed in Langer See than in the more shallow, but wind protected Melangsee. The success of Planktothrix agardhii may be due to the more turbulent mixing regime in Langer See. The algal association, S of Reynolds (Reynolds, C.S., 1997), should be modified, since the different species grouped together in S have different responses to turbulent mixing.
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