Abstract. Most species of filamentous freshwater algae can occasionally be observed growing in floating clusters. The phenomenon seems to be characteristic for filamentous algae in shallow habitats. Dominant species may vary and show different periodicities. General characteristics can be observed in the development and dynamics of these floating clusters, called flab in the Dutch language. Occurring in shallow and often even ephemeral water bodies, flab shows a pronounced stratification. The first development of algae is usually at the bottom. After a period of submerged growth, formation of oxygen bubbles leads to an ascent of the algal mass. During flotation, these algae are exposed to extreme conditions, especially of radiation, oxygen content, pH, and nutrient depletion. These conditions are greatly influenced by the weather and thus show rapid changes.
Optimum growth in uni-algal cultures of Tribonema minus (Wille) Hazen, a dominant alga in blanketing algal mats in the early spring, was obtained at temperatures between 15-25°C and light intensities above 181!Einstein m -2 s -I. Uni-algal cultures of Spirogyra singularis Nordstedt, dominant in algal mats during summer, grew well between 2o-25"C and the highest light intensities tested(62 I! Einstein m -2 s -I). In mixed cultures T. minus outgrows S. singularis below 131!Einstein m-2 s-I and 15"c. At higher light intensities and temperatures levels S. singularis dominates, which agreeswith field observations. The growth reduction of T. minus at these higher levels is discussed.
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