Technical Note: A mobile sea-going mesocosm system – new opportunities for ocean change research

Abstract

Abstract. One of the great challenges in ocean change research is to understand and forecast the effects of environmental changes on pelagic communities and the associated impacts on biogeochemical cycling. Mesocosms, experimental enclosures designed to approximate natural conditions, and in which environmental factors can be manipulated and closely monitored, provide a powerful tool to close the gap between small-scale laboratory experiments and observational and correlative approaches applied in field surveys. Existing pelagic mesocosm systems are stationary and/or restricted to well-protected waters. To allow mesocosm experimentation in a range of hydrographic conditions and in areas considered most sensitive to ocean change, we developed a mobile sea-going mesocosm facility, the Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS). The KOSMOS platform, which can be transported and deployed by mid-sized research vessels, is designed for operation in moored and free-floating mode under low to moderate wave conditions (up to 2.5 m wave heights). It encloses a water column 2 m in diameter and 15 to 25 m deep (∼50–75 m3 in volume) without disrupting the vertical structure or disturbing the enclosed plankton community. Several new developments in mesocosm design and operation were implemented to (i) minimize differences in starting conditions between mesocosms, (ii) allow for extended experimental duration, (iii) precisely determine the mesocosm volume, (iv) determine air–sea gas exchange, and (v) perform mass balance calculations. After multiple test runs in the Baltic Sea, which resulted in continuous improvement of the design and handling, the KOSMOS platform successfully completed its first full-scale experiment in the high Arctic off Svalbard (78°56.2′ N, 11°53.6′ E) in June/July 2010. The study, which was conducted in the framework of the European Project on Ocean Acidification (EPOCA), focused on the effects of ocean acidification on a natural plankton community and its impacts on biogeochemical cycling and air–sea exchange of climate-relevant gases. This manuscript describes the mesocosm hardware, its deployment and handling, CO2 manipulation, sampling and cleaning, including some further modifications conducted based on the experiences gained during this study.

Marine CDOM accumulation during a coastal Arctic mesocosm experiment: No response to elevated pCO2 levels

“…In a closed mesocosm experiment, one could expect that processes such as in situ production and microbial alteration would play a central role in a dynamics of CDOM. We believe that processes of CDOM photobleaching that are certainly relevant in the ocean surface layer did not contribute to a change in CDOM in mesocosms because of the very limited transmittance of UV light by the mesocosm walls and roofs [Riebesell et al, 2013b;Schulz et al, 2013].…”

“…A detailed description of the experimental setup and sampling procedures is given in Riebesell et al [2013b], Schulz et al [2013], and Czerny et al [2013a]. Briefly, the KOSMOS are enclosures (17 m long and 2 m wide) that are attached to hard floating frames and made of 0.5-1 mm thick thermoplastic polyurethane, which absorbs almost 100% of the incoming UV radiation, while it is transparent to PAR [Riebesell et al, 2013b;Schulz et al, 2013].…”

“…pCO 2 inside the mesocosms was gradually increased between days t-1 and t4 by adding CO 2 enriched seawater. Added CO 2 -enriched water was injected and evenly dispersed throughout the water column between days t4 and t8 using a device known as the "spider" [Riebesell et al, 2013b]. The range of 8 pCO 2 levels averaged for the course of experiment was between 177 ppm and 1084 ppm [Silyakova et al, 2013].…”

Ocean Acidification-Induced Restructuring of the Plankton Food Web Can Influence the Degradation of Sinking Particles

“…We deployed nine "Kiel Off-Shore Mesocosms for Ocean Simulations" (KOSMOS, M1-M9), each consisting of a floatation frame and a mounted bag (2 m diameter) made of 1 mm thick thermoplastic polyurethane foil (Riebesell et al, 2013). Once deployed, the initially folded bags were lowered to a depth of 13 m and thereby filled with approximately 35 m 3 of seawater.…”

“…While divers regularly cleaned the outside, a cleaning ring was used to wipe the inside of the walls every week (Riebesell et al, 2013). This procedure prohibits wall growth, except for the flange ring connecting the mesocosm to the sediment trap and the funnel of the sediment trap, which are too narrow to be cleaned with the ring.…”