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In recent years Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju a potentially toxic freshwater cyanobacterium of the order Nostocales initially classified as a tropical species, has invaded the temperate region and has established in lakes in northern Germany. The formation of dormant cells (akinetes) is an important part of the life cycle which enables Nostocales species to survive harsh conditions like the winter in the temperate zone. In this thesis, my aim is to throw more light on the life cycle of the invasive cyanobacterium Cylindrospermopsis raciborskii and also to compare it to native Nostocales species of the genus Aphanizomenon using derived field and laboratory data. This would help in the understanding of the invasion of C. raciborskii and spread and how it can be contained. Abiotic factors influencing the development and growth of photoautotrophic planktonic organisms in nature are examined and their variation with the years is discussed. A comprehensive look at C. raciborskii’s akinetes in the sediment and the filaments and akinetes in the water column is done. The timing and temperature condition of akinete germination, recruitment, formation in the pelagic and importation into the sediment is determined. An assessment of the role of abiotic factors on the dynamics and abundance of filaments is done with a statistical model used to isolate which abiotic factors are most influential on the population growth of C. raciborskii. Akinetes distribution on the bed of the lake and the effect of loss processes on their abundance are looked into. Cylindrospermopsis is compared to native species Aphanizomenon spp in terms of the seasonal dynamics and the germination condition of their akinetes. The main findings in this thesis were that the seasonal dynamics of C. raciborskii had a similar trend but different timing of each event in the four years of the study. The variabilities were due to the different timing of the abiotic conditions. But in all temperature was identified as the most important factor influencing the timing of most events with germination of akinetes occurring at 13°C which is lower than so far reported in literature Subsequent increment in temperature to at least 23°C was found to be essential for the development of high populations of C. raciborskii in the water column. With a drop of temperature in autumn the filaments are stimulated in to producing akinetes which settle out into the sediment and remain there until the next spring when favourable conditions force them to germinate. They might also be consumed before germination by non identified grazer or losses due to microbial infections are discussed. Overall, the native Nostocales species of the genus Aphanizomenon have a selective advantage over the invaded species C. raciborskii under low temperatures and temperature fluctuations in spring as observed from the field and laboratory studies.
Das Vorkommen von Cyanobakterien, Anatoxin a und Microcystin wurde in 10 Gewässern erfaßt. Die Steuerung ihrer Dynamik wurde in 3 polymiktischen Seen untersucht und Effekte von Mixis mit Sedimentresuspension in Enclosures fokusiert. Die Gewässer waren durch Oscillatoriales, entweder Planktothrix agardhii oder Limnothrix spp. dominiert und wiesen hohe Biovolumina von Nostocales im Frühsommer auf. Hohe Lichtintensität und niedrige Stickstoff-Konzentrationen förderten die Entwicklung der Nostocales. Während des Sommers nahm die Lichtintensität aufgrund des steigenden Biovolumens der Cyanobakterien ab, was zum Rückgang der Nostocales und Dominanzen von Oscillatoriales führte. Durch Sedimentresuspension wurden Nährstoffpulse erzeugt, die zur Dominanz von P. agardhii führten. Bei Stagnation der Wassersäule waren die Nährstoffkonzentrationen niedriger und Limnothrix spp. gelangte zur Dominanz. Anatoxin a konnten auf Anabaena spp. zurückgeführt werden, Microcystine auf P. agardhii. Abnehmende Lichtintensität und/oder zunehmende Stickstoff-Konzentrationen förderten die Microcystin Produktion oder das Wachstum toxischer Stämmen.