1. Riebesell, U. 1, Allgaier, M. 2, Avgoustidi, V. 3, Bellerby, R. 4, Carbonnel, V. 5, Chou, L. 5, Delille, B. 6, Egge, J. 4, Engel, A. 7, Grossart, H.-P. 2, Huonnic, P. 8, Jansen, S. 7, Johannessen, T. 4, Joint, I. 9, Kringstad, S. 4, Lovdal, T. 4, Martin-Jézéquel, V. 8, Moros, C. 4, Mühling, M. 9, Nightingale, P.D. 9, Passow, U. 7, Rost, B. 7, Schulz, K. 1, Skjelvan, I. 4, Terbrüggen, A. 7, Trimborn, S. 7 1 Leibniz Institut für Meereswissenschaften IFM-GEOMAR, Kiel, Germany, 2 IGB, Neuglobsow, Germany, 3 University of East Anglia, Norwhich, U.K., 4 University of Bergen, Bergen, Norway, 5 Université Libre de Bruxelles, Bruxelles, Belgium, 6 Université de Liège, Liège, Belgium, 7 Alfred Wegener Institute, Bremerhaven, Germany, 8 Université de Nantes, Nantes, France, 9 Plymouth Marine Laboratory, Plymouth, U.K. Pelagic Ecosystem CO 2 Enrichment Study For further information about the Pe lagic Ecosystem CO 2 Enrichment ( PeECE ) Study see http://spectrum.ifm.uni-kiel.de/peece/index.htm http://spectrum.ifm.uni-kiel.de/peece/index.htm 1. Motivation The ocean has absorbed nearly 50% of fossil fuel CO 2 emissions between 1800 and 1994 (Sabine et al. 2004), and will absorb 90% of the CO 2 emitted to the atmosphere over the next millennium. CO 2 absorption leads to acidification of surface seawater (Fig. 1) and lowers its carbonate saturation state (Wolf-Gladrow et al. 1999). Because most of the CO 2 will initially be stored in the upper 200 m of the ocean, surface layer pelagic ecosystems are the first to be exposed to the “acid ocean”. While short-term bottle incubations and single species culture experiments have indicated strong effects of CO 2 -enrichment on various marine organisms (Riebesell 2004), there is almost nothing known about its effect on marine ecosystems and the possible consequences for biogeochemical cycling and ocean- atmosphere gas exchange. 3. Technical design The mesocosm studies were conducted between 31 May and 25 June 2001 and April 26- May 28, 2003 at the EU-Large Scale Facilities (LSF) in Bergen, Norway. Nine polyethylene enclosures (2001: ~11 m3, 4.5 m water depth; 2003: ~20 m3, 9.5 m water depth) were moored to a raft in the Raunefjorden, 60.3° N, 5.2° E (for more details see Williams & Egge, 1998). The enclosures were filled with unfiltered, nutrient-poor, post-bloom fjord water, which was pumped from 2 m depth adjacent to the raft. The enclosures were covered by gas-tight tents made of ETFE foil (Foiltec, Bremen, Germany), which allowed for 95% light transmission of the complete spectrum of sunlight. In 2003, 0.6 m 3 freshwater was added and mixed into the upper 4.5 m of the water column to stratify the water column and avoid re- introduction of sedimented material into the surface layer. Sediment traps were positioned at the bottom of the low-salinity surface layer. The atmospheric and seawater pCO 2 were manipulated to achieve 3 different CO 2 levels in triplicate, corresponding to approximately year 2100, assuming the IPCC's 'business as usual' scenario IS92a- (mesocosms 1-3), present (mesocosms 4-6) and glacial atmospheric CO 2 levels (mesocosms 7-9). Different CO 2 levels in seawater were achieved through aerating the water and fumigating the tents with air either enriched with pure CO 2 (pureness 2.7), natural air, or air depleted in CO 2. The latter was obtained by running natural air through a CO 2 absorber (Na 2 CO 3 platelets with a pH indicator). After 3-4 days of CO 2 adjustment the desired pCO 2 levels in water were reached (day 0) and the CO 2 aeration of the water column was stopped. 10m S=31.3 S=29.8 S=31.3 Sediment Trap 95% PAR 190 370 700 pCO 2 (ppmv) 5m References: 1 Sabine C.L. et al. 2004. The oceanic sink for anthropogenic CO 2. Science 305, 367-371 2 Wolf-Gladrow, D., Riebesell, U., Burkhardt, S., Bijma, J. (1999) Direct effects of CO 2 concentration on growth and isotopic composition of marine plankton. Tellus 51B, 461-476 3 Riebesell U. 2004. Effects of CO 2 enrichment on marine phytoplankton. J. Oceanogr. 60, 719-729 4 Williams P. J. le B., Egge, J.K. 1998. The management and behaviour of the mesocosms. Est Coast Shelf Sci 46 : 3-14 5 Egge, J. K., Aksnes, D.L.. 1992. Silicate as regulating nutrient in phytoplankton competition. Mar. Ecol. Prog. Ser. 83: 281-289. 2. Approach To assess the effect of CO 2 -enrichment and the related changes in seawater chemistry at the ecosystem level, mesocosm perturbation experiments were conducted at the Large- Scale-Facility of the University of Bergen, Norway. Pelagic Ecosystems in a High CO 2 Ocean : the Mesocosm Approach Fig. 1: Seawater pH and atmospheric pCO 2 between 1700 and 2100. Future conditions estimated assuming ‘business-as-usual‘ CO 2 emission scenario; IS92a, IPCC 1995. To initiate bloom development, nutrients were added on day 0 of the experiment. To promote a bloom of the coccolithophore Emiliania huxleyi, nitrate and phosphate were added in a ratio of 30:1 yielding initial concentrations of 15.5 µmol L -1 NO 3 and 0.5 µmol L -1 HPO 4 in 2001. In 2003, it was intended to initiate a diatom bloom. Initial nutrient concentrations were 9 µmol L -1 NO 3, 0.5 µmol L -1 HPO 4 and 12 µmol L -1 Si(OH) 4. After nutrient addition and throughout the study, the water was gently mixed by means of an airlift (for more details see Egge & Asknes, 1992), using the same air as for gassing the tents. The gassing of the tents was continued to keep the pCO 2 of the overlying atmosphere at a constant level. The mesocosm team Fig. 2. A.View of mesocosm facility at the University of Bergen, Norway showing floating raft with 9 mesocosms covered by gas-tight tents. Numbers indicate CO 2 levels in triplicate treatments. B.Schematic drawing of CO 2 control system. C.Side view of a mesocosm, indicating the air-lift mixing and sediment traps Air lift A BC Fig. 3: Chlorophyll a concentration (A) and seawater pCO 2 (B) over the course of the experiment. The peak of the bloom coincides with nutrient exhaustion (day 13). 0 2 4 1 3 5 6 pCO2 (ppm) Chlorophyll a (µg L -1 ) Year 2100 Present Glacial 4. Outcome The CO 2 perturbation experiments in 2001 and 2003 successfully followed the development and decline of phytoplankton blooms (Fig. 3A). Differences in the drawdown of pCO 2 due to photosynthetic carbon fixation between CO 2 treatments (Fig. 3B) can be explained by CO 2 -related changes in the seawater buffer capacity. Detailed results of the 2003 experiment are presented in companion posters by Avgoustidi et al., Bellerby et al., Egge et al., Grossart et al., Huonnic et al., Jansen, et al., Martin-Jézéquel et al., Mühling et al., Passow et al., and Riebesell et al. 750 ppm 375 ppm 190 ppm CO 2 in airflow