Research

Objectives

In October 2019, a mesocosm experiment took place at the CretaCosmos mesocosm facility of the Hellenic Center for Marine Research in Crete, Greece. The experiment involved members of the CMBR infrastructure (from the IMBBC and IO Institutes of HCMR and from the Biology Department of the University of Crete), as well as foreign scientists who participated through the EU H2020 Infrastructure Network AQUACOSM.

But what is a mesocosm and what are they used for?
They are an experimental tool, i.e. devices that allow experimentation in large volumes, in other words they make it possible to study aspects of the marine environment at the ecosystem level.

The marine environment is under intense anthropogenic pressures, including the addition of nutrients, organic and/or inorganic, which can lead, among other things, to a decrease in higher predator populations, thus affecting entire food webs and burdening marine ecosystems. Such a problem involving many variables and organisms from many food levels can only be studied in large experimental structures, such as the mesocosms.

In this experiment (CΟmpex: Competition and Coexistence Patterns in Microbial Plankton) the relationships of competition and coexistence created between plankton organisms, with and without the addition of nutrients, were investigated. Nine mesocosms (of 3 cubic meters each) were filled with seawater transported from the Cretan Sea to the facilities of HCMR (Figure 1). The experiment included 3 experimental conditions, in triplicate: 1) addition of nutrient, inorganic and organic, 2) additional removal of the upper predator/meso-zooplankton, and 3) control-mesocosms, without any intervention.

Who took part and what was studied during the 15-day experiment?
The experiment involved 27 scientists from 6 countries, who measured, on a daily basis, the concentration of all nutrients, organic and inorganic, dissolved and particulate, the absorption of inorganic phosphorus, as well as a variety of biological variables, such as the abundance, biomass and diversity of all plankton groups, from bacteria to copepods, but also the primary and secondary productivities. Numerous microbial ecology techniques have been used from flow cytometry, fluorescence and inverted microscopy to molecular techniques and isotopes.

And the results of this research?
They were many and interesting. The presence and absence of the higher predator changed the food web and the relationships between the populations. There was competition for nutrients among mainly osmotrophic plankton populations. Overall, important conclusions have been drawn, which, on the one hand, answer questions about the marine microbial web of the Eastern Mediterranean but, on the other hand, create new hypotheses and questions that need to be further explored. Numerous publications are expected to be produced, several of which are currently being finalized. There was also an opportunity for cooperation between CMBR infrastructure actors, which will lead to future joint actions.