Perform a mesocosm study in Lunz

AQUACOSM enables us to invite external researchers to collaborate with us on the research outlined below (see How to get involved, below). Depending on positive evaluation of your application, AQUACOSM covers your travel and accomodation for the time of the experiment,as well as local resources (lab space, consumables etc). Please don’t hesitate contacting us for questions or support with the proposal.


Your proposal can be related to the following research ideas, but you can also suggest your own research. Pls contact us with your research idea so we can jointly develop an exciting research project. Depending on your state of career, you can participate as a trainee or researcher, as part of a team or individual.


  • Mixotrophy & climate change – Mixotrophic protists have been identified as key bacterivores in lakes (and oceans), especially in oligotrophic systems, such as Lake Lunz.  The apparent prevalence of mixotrophic protists in the field contrasts our current understanding about their impact on microbial food web dynamics and carbon and nutrient cycles. Moreover, changes to aquatic environments due to global climate change, such as rising water temperatures and increasing input of terrigenous dissolved organic carbon, are likely to promote the growth of mixotrophic plankton.



1.) Rising water temperature promotes conditions favorable for mixotrophic bacterivores and pushes the microbial food web towards heterotrophy.

The metabolic theory of ecology predicts that heterotrophic, as compared to             photoautotrophic processes, respond more strongly to rising temperatures. This may effectively enhance the heterotrophic metabolism in mixotrophic organisms. This in turn might change the functional role of mixotrophs in aquatic food webs and associated carbon- and nutrient cycles.

We will study the impact of increased water temperature on the dynamics of mixotrophs. We will quantify changes of the relative abundances of mixotrophs to community composition and relate those to grazing impact (bacterivory), primary productivity and resource use efficiency. We will test the effect of temperature on the functional role of mixotrophs in the microbial food web (“producers or consumers”).


2.) Allochthonous C fuels mixotrophy.

It is assumed that only a minor fraction of allochthonous C is labile, and hence available for bacterial production. Nonetheless, a number of studies suggest that allochthonous C may significantly increase bacterial activity and ultimately alter the carbon cycling of lakes. Mixotrophs that ingest bacteria primarily to acquire limiting nutrients are assumed to excrete large proportions of ingested C. These exudates in turn support bacterial production. Increased bacterial production combined with the more efficient nutrient utilization of mixotrophs aggravates nutrient limitation of photoautotrophs.

We will quantify changes in bacterial activity & abundances and related those to abundances of mixotrophs, dissolved nutrients and seston stoichiometry. Short term incubations at different time points during the experiments will be used to elucidate the fate of added organic carbon in the microbial food web.



  • There are 40 experimental units available (320-L cattle tanks)
  • the “core” experiment will consist of 20 mesocosms in a full factorial design with 2 treatments and controls each replicated 5 times (Fig. 1)

Depending on number of applications and YOUR ideas, the experiment can be extended!



Figure 1 Schematic representation of the planned experiment. Sketch of the experimental design where cylinders represent the mesocosms with 5 replicates for each treatment combinations.


  • Light-dependent bacterivory – mixotrophic algae are light dependent consumers and are now recognized as key bacterivores. Recognizing this effect challenges teh classical concept of the microbial food web. For testing the structuring effect of light intensity on the microbial food web, natural plankton communities can be exposed to a range of light intensities, as we have done in a previous marine study.



Numeric responses of bacteria and Synechococcus (left) and their grazers (right) along an experimental light gradient. PE: mixotrophic picoeukaryotes; HF: het. flagellates. Ptacnik et al. SciRep 2016.


  • Role of connectivity for diversity in plankton communities – by manipulating frequency of inoculation events and/or the diversity of lakes used for inoculating mesocosms, we can test how diversity of plankton communities is affected by colonization events, and how diversity changes translate into differences in functioning.


Effects of connectivity (C) on taxon richness of bacteria and protists in a previous mesocosm experiment. Richness of +C treatments (y-axis) plotted against richness in -C treatments (x-axis). (Bengtsson, Eggers et al in prep.)





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