KOSMOS (Kiel Off-Shore Mesocosms for Ocean Simulations)

Description of the infrastructure: The Kiel Off-Shore Mesocosms for Ocean Simulations (KOSMOS) are a seagoing mobile platform for mesocosm experiments. They can be deployed from any mid-sized research vessel. KOSMOS comprises 9 free-floating units, each consisting of a floatation frame and a flexible bag 2 m in diameter. After deployment in the sea, the bag unfolds by weights pulling down its lower end to 20 m depth, thereby enclosing an undisturbed water column of 55 m3. The bag is then closed at the bottom by a full-diameter sediment trap. Regular cleaning of the mesocosm walls prevents wall growth and thus enables long-term experiments that capture plankton seasonal succession. The enclosed water encompasses the entire plankton community from bacteria and viruses up to the level of fish larvae and small pelagic fish. Sampling of sedimented matter in combination with measurements of dissolved and suspended matter and air-sea gas exchange can be used for budget calculations of major elements (C, N, P, Si). Potential experimental perturbations include the addition of inorganic nutrients or organic compounds, carbonate chemistry manipulation (i.e. simulating ocean acidification or alkalisation), manipulation of mixed-layer depth, simulation of deep-water upwelling, species exclusion and addition of invasive species.

Sampling devices include depth-integrated and discrete water samplers, nets, gas tight samplers, and sediment samplers. Various sensors continuously record physical and chemical characteristics of the enclosed water along depth profiles. Underwater optical instruments allow for non-invasive in situ quantification of plankton community composition. The KOSMOS facility was successfully employed in long-term experiments in different climate zones, ranging from the high Arctic, temperate waters in the Baltic and North Sea, upwelling ecosystems off Peru, to oligotrophic waters off Gran Canaria and Hawai’i. These studies, which focussed on the effects of ocean acidification, deoxygenation and simulated upwelling on plankton dynamics and biogeochemical cycling, involved researchers from a wide range of scientific fields, including marine and atmospheric chemistry, molecular and evolutionary biology, marine ecology and biological oceanography, aquaculture, fish biology, and biogeochemistry. Results of the KOSMOS experiments have been used in various ecosystem and biogeochemical modelling activities.