Who ate the Phytoplankton? A mesocosm experiment at LMU/SLS about the impact of major zooplankton grazer groups on phytoplankton

This year we could again offer Transnational Access (TA) in the framework of AQUACOSM-plus at LMU/SLS. Among the TA users were group leaders and trainees from different countries, including France, Italy and Slovenia. Dr. Ziga Ogorelec from Slovenia, however, was not only a TA user at SLS with his very interesting project; with his experiment we could successfully test our light weight mobile mesocosm system at the same time. The aim of Zigas's study was to disentangle the top-down effects of individual zooplankton species or groups on phytoplankton. Specifically, he wanted to disentangle the role of cyclopoid copepods, calanoid copepods, and daphniids in affecting phytoplankton biomass and composition. In addition, he wanted to evaluate the strength of cascading effects of invertebrate predators, like Chaoborus sp. in the pelagic environment.

The top-down effects of zooplankton on phytoplankton are of crucial importance for the function of lake food webs and the large amount of research conducted up to date has led to an ongoing debate. However, the role of individual zooplankton species or even higher taxa is still unclear. Due to the mixed effects of many zooplankton groups from field studies and due to difficulties of including the entire phytoplankton community in lab experiments, the role of individual zooplankton species is difficult to disentangle. Mesocosms are therefore providing a tool to establish experimental systems allowing both, a controlled zooplankton community composition but also natural environmental conditions and phytoplankton communities.

Ziga installed in total 35 mesocosms using our new construction, which was perfectly suited for his purposes. This summer we struggled a lot with unforeseen extreme weather events and in situ studies were difficult to conduct.

However, our design defied the difficult weather conditions well and we were able to make improvements to our construction to prepare it for the joint research activity in Romania next year. The experiment was successfully completed, and we were able to collect very interesting data. Furthermore, we could establish a new promising cooperation with Ziga!