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

1Leibniz Institut für Meereswissenschaften IFM-GEOMAR, Kiel, Germany, 2IGB, Neuglobsow, Germany, 3University of East Anglia, Norwhich, U.K., 4University of Bergen, Bergen, Norway, 5Université Libre de Bruxelles,

Bruxelles, Belgium, 6Université de Liège, Liège, Belgium, 7Alfred Wegener Institute, Bremerhaven, Germany, 8Université de Nantes, Nantes, France, 9Plymouth Marine Laboratory, Plymouth, U.K.

                            Pelagic Ecosystem

CO2 Enrichment Study

For further information about the For further information about the PePelagic Ecosystem COlagic Ecosystem CO22 Enrichment ( Enrichment (PeECEPeECE) Study see) Study see

http://spectrum.ifm.uni-kiel.de/peece/index.htmhttp://spectrum.ifm.uni-kiel.de/peece/index.htm

1. Motivation

The ocean has absorbed nearly 50% of fossil fuel CO2 emissions between 1800 and 1994

(Sabine et al. 2004), and will absorb 90% of the CO2 emitted to the atmosphere over the

next millennium. CO2 absorption leads to acidification of surface seawater (Fig. 1) and

lowers its carbonate saturation state (Wolf-Gladrow et al. 1999). Because most of the CO2

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 CO2-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 m3 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 pCO2 were manipulated to achieve 3 different CO2 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 CO2 levels (mesocosms 7-9).

Different CO2 levels in seawater were achieved through aerating the water and fumigating the tents with

air either enriched with pure CO2 (pureness 2.7), natural air, or air depleted in CO2. The latter was

obtained by running natural air through a CO2 absorber (Na2CO3 platelets with a pH indicator). After 3-4

days of CO2 adjustment the desired pCO2 levels in water were reached (day 0) and the CO2 aeration of

the water column was stopped.

10m

S=31.3S=29.8

S=31.3

Sediment Trap

95% PAR95% PAR

190370 700

pCO2 (ppmv)

5m

References:

1 Sabine C.L. et al. 2004. The oceanic sink for anthropogenic CO2. Science 305, 367-3712 Wolf-Gladrow, D., Riebesell, U., Burkhardt, S., Bijma, J. (1999) Direct effects of CO2 concentration on growth and isotopic composition of marine plankton. Tellus 51B, 461-4763 Riebesell U. 2004. Effects of CO2 enrichment on marine phytoplankton. J. Oceanogr. 60, 719-7294 Williams P. J. le B., Egge, J.K. 1998. The management and behaviour of the mesocosms. Est Coast Shelf Sci 46 : 3-145 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 CO2-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 CO2 Ocean : the Mesocosm Approach

Fig. 1: Seawater pH and atmospheric pCO2 between 1700 and 2100. Future conditions estimated assuming ‘business-as-usual‘ CO2 emission scenario; IS92a, IPCC 1995.

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16 18

day

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 NO3 and 0.5

µmol L-1 HPO4 in 2001. In 2003, it was

intended to initiate a diatom bloom. Initial nutrient concentrations were 9 µmol L-1 NO3,

0.5 µmol L-1 HPO4 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 pCO2 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 CO2 levels in triplicate treatments.

B. Schematic drawing of CO2 control system.

C. Side view of a mesocosm, indicating the air-lift mixing and sediment traps

Air lift

A

B C

Fig. 3: Chlorophyll a concentration (A) and seawater pCO2 (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

pCO

2 (

ppm

)C

hlor

ophy

ll a

(µg

L-1)

Year 2100

Present

Glacial

4. Outcome

The CO2 perturbation experiments in 2001 and 2003 successfully followed the

development and decline of phytoplankton blooms (Fig. 3A). Differences in the drawdown of pCO2 due to photosynthetic carbon fixation between CO2 treatments (Fig. 3B) can be

explained by CO2-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 ppm375 ppm190 ppm

CO2 in airflow