Description: 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 single species 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.

Description: Due to its naturally low carbonate saturation states the Arctic Ocean is considered particularly vulnerable to ocean acidification. If CO2 emissions continue to rise at current rates, half of the Arctic Ocean will be undersaturated with respect to calcium carbonate and, therefore, corrosive for calcareous organisms within the next three to four decades. While recent studies have demonstrated sensitivities of some Arctic species to ocean acidification, no information is presently available on community- and ecosystem-level responses. As a first attempt to closing this gap, an off-shore mesocosm system (KOSMOS) developed at IFM-GEOMAR was deployed in the Kongsfjord off Spitsbergen - about 1000 nautical miles south of the North Pole - in June/July 2010 and was used to conduct a pelagic CO2 enrichment experiment. IFM-GEOMAR, which provided the logistics for this experiment, received support from the Greenpeace vessel M/S ESPERANZA, transporting the mesocosms and assisting during deployment and recovery. In nine 15mlong mesocosms, each enclosing about 50m3 of seawater, stepwise addition of CO2 saturated seawater was applied to achieve CO2 concentrations ranging from ca. 180 to 1400 atm. Half way through the experiment inorganic nutrients (5, 2.5, and 0.32 mol L􀀀1 nitrate, silicate, and phosphate, respectively) were added to the enclosed, nutrient-poor waters. In the framework of the EU integrated project EPOCA a team of 35 scientists from 12 institutes monitored the mesocosms over a period of 35 days. In total 45 parameters were measured daily and over 15000 samples analysed to cover aspects ranging from viral, bacterial, phytoplankton and zooplankton abundances, compositions, biomasses, and productivities, carbon and nutrient dynamics and stoichiometry, vertical particle fluxes, to the production of climate relevant gases and air/sea gas exchange. In bringing together a wide range of scientific expertise this study provides a comprehensive data set on pelagic ecosystem and biogeochemical responses to ocean acidification in Arctic waters.

Description: Data presented and discussed here were collected continuously during April/May 1975 in the Bornholm Basin of the Baltic Sea. Sedimentation rates of particulate matter were recorded with 5 multisample sediment traps from different depths in the water column at 2 positions 170 km apart. Current meter data collected during the same period and depths indicated that the positions remained hydrographically distinct during the investigation. Particulate matter from the euphotic zone including diatom cells formed the bulk of the material collected by all traps. This flux of organic particles to the bottom was unimpeded by the strong density stratification present in the water column. The upper traps always collected less material than lower ones. This paradox has been ascribed to diminishing current speeds with depth, concomitant with an increase in sinking rates of phytoplankton and phytodetritus. Both factors influence the sampling efficiency of sediment traps, which are thought to have underestimated actual sedimentation rates here. A time lag of 2 to 3 weeks in bloom development seemed responsible for the characteristic differences between the two positions. The phase of major sedimentation at one position covered about 18 days, and a distinct sequence in the composition of the material collected by the 6 glasses of each trap indicated phases of a progressively deteriorating phytoplankton population in the water column contributing the particulate material. A total of 6.2 g C m-2 in 34 days was recorded at this station. Apart from a trap situated in an oxygen deficient layer which collected 0.44 g C m-2 of zooplankton corpses, zooplankton mortality was overestimated by the traps. Large-scale sedimencation of “fresh” organic matter produced by the spring bloom is probably a regular feature in areas with low over-wintering zooplankton populations and, as such, possibly has a direct stimulatory effect on growth and reproduction of the benthos.