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Spatio-temporal dynamics of Planktothrix rubescens and heterotrophic bacteria in Lake Zurich

This project was my master thesis which I conducted at the limnological station of the University of Zürich under the supervision of PD Dr. Thomas Posch and Prof. Jakob Pernthaler. The main objective was the establishment of an empirical dataset, describing the seasonality and potential coexistence patterns of the major freshwater heterotrophic bacterial groups and picocyanobacteria with the dominant toxic filamentous cyanobacteria Planktothrix rubescens in Lake Zürich. P. rubescens forms massive blooms during autumn in the metalimnion. Considering its toxicity and the fact that Lake Zürich represents the major drinking water source for the city of Zürich it is important to improve our knowledge on this cyanobacteria and its interaction with other inhabitants in the lake. This thesis contributed to this knowledge by focusing on the consequences of a toxic P. rubescens bloom on the prokaryotic community in Lake Zürich. Main results were the observation of a clear niche differentiation between P. rubescens and picocyanobacteria. In contrast to the often observed positive correlation between heterotrophic bacteria and primary production, we found a negative relationship between the autumnal P. rubescens bloom and bacterial biomass. This suggests that P. rubescens is an effective competitor for nutrients and organic carbon. However, we also found indications that some bacterial taxa (affiliated with Flexibacteraceae) might nevertheless profit from a P. rubescens bloom since they were only present in the layer of maximal P. rubescens densities (Van den Wyngaert et al. 2011).

Thanks to this master thesis I gained deeper insight into general limnological concepts and aquatic microbial ecology.

Acquired skills:

  • limnological field sampling techniques
  • microscope techniques
  • flow cytometry
  • molecular methods such as in situ hybridization.

The perfect geographical situation of the limnological station at the shores of Lake Zürich together with a highly enthusiastic team of limnologists motivated me to continue a research career in aquatic ecology.

This master thesis received the Swiss Hydrobiology-Limnology Award 2009.

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