41 2 University of East Anglia, School of Environmental Sciences, Norwich NR4 7TJ, U.K. (Bakker, D: D.Bakker@uea.ac.uk ) THE CHALLENGES OF MODELLING ECOSYSTEM RESPONSE TO OCEAN ACIDIFICATION. M5 Blackford, Jerry The chemistry of ocean acidification is relatively predictable but the precise response of the many sensitive ecological and biogeochemical processes remains speculative. Experiments have revealed diverse responses for different species although few species have been studied. Over the same timescale as acidification, global warming will alter the physical drivers of the marine system, prediction therefore requires an integrated multi-driver approach. Modelling provides our only predictive tool but is limited by contentious parameterisations, a lack of evaluation data and complexity. Given these uncertainties, it is important not to be overly deterministic with models, however models do have an important role. These include: predicting carbonate system parameters and carbonate saturation states under different emission scenarios, assessing the sensitivity to parameter uncertainty and model structure, producing probabilistic ensemble scenarios rather than deterministic assessments, testing if a given response is by itself capable of altering key system properties, assessing if an acidification response might be affected by changing climate or other drivers, asking how do process responses identified in (mono-)culture type studies express within a virtual complex ecosystem and finally as provocateurs of debate and experimentation. To illustrate these approaches, results from 3D simulations of the North West European Shelf will show predictions for the carbonate system, an assessment of the vulnerability of the nitrogen cycle and benthic coupling due to the inhibition of nitrification, an illustration of how phytoplankton species specific carbon uptake rates might provoke changing community structure and an examination of the sensitivity of acidification response to changing climate. Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, U.K. (jcb@pml.ac.uk). KIEL CO 2 MANIPULATION EXPERIMENTAL FACILITY (KICO2) T5 Bleich, Markus 1 , Frank Melzner 2 , Claas Hiebenthal 2 , Helgi Mempel 2 , Kai Schulz 2 , Ulf Riebesell 2 , Martin Wahl 2 , Frank Sommer 2 , Ulrich Sommer 2 , Armin Form 2 , Uwe Piatkowski 2 , Reinhold Hanel 2 , Dieter Piepenburg 3 , Michael Spindler 3 , Anton Eisenhauer 2 , Andrea Franke 2 , Volker Mller 5 , Gunnar A. Baumert 4 , and Catriona Clemmesen 2 We will present the technical details and our first experiences with a new CO 2 !manipulation facility, set up at IFM-GEOMAR in March 2008. The system adds pure CO 2 to pressurized air to create 5 different CO 2 !air mixtures (380-4,000 ppm of CO 2 ), which are distributed into 6 different temperature constant rooms at gas flow rates of up to 1000 l/h per room and per CO 2 concentration. Precise addition of CO 2 is achieved via computer controlled valve systems (red-y smart series) that respond to variations in gas flow according to the gas demand by the experiments. CO 2 flow is detected by pressure and temperature independent thermal measurement of mass. High precision of final CO 2 values is achieved within 1-100% of maximal gas flow rate. Actual values and set points are continuously monitored and documented online via PC interface. The facility is completed by a security and alarm system. The gas mixtures are used to equilibrate different types of experimental setups: (1) large flow-through Baltic Sea systems for benthic invertebrate long-term incubations, (2) small flow-through systems for short- and long-term incubations of Baltic- and North Sea invertebrates, (3) small-scale larval incubation systems for simultaneous manipulation of CO 2 and temperature. All types of experimental setups will be described in detail and quantitative information of crucial parameters (e.g. water and gas flow rates, TA, DIC, pH, experimental animal biomasses, and nitrogenous waste accumulation) will be given and discussed. This contribution is an effort towards establishing guidelines for highly controlled and reproducible CO 2 manipulation experiments.