Pelagic Ecosystem CO₂ Enrichment Study

EU Large-Scale-Facilities, University of Bergen, Norway

Experimental design:

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Experimental design

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2001/03 details

2005 details

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The mesocosm studies were conducted between 31 May - 25 June 2001, April 26 - May 28, 2003 and May 8 - June 15, 2005 at the Marine Biological Station of the University of Bergen in Espegrend, Norway. Nine polyethylene enclosures (2001: ~11 m³, 4.5 m water depth; 2003 and 2005: ~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 (2001) or 12 m (2003, 2005) depth adjacent to the raft. The enclosures were covered by gas-tight tents made of ETFE foil (Foiltec, Germany), which allowed for 95% light transmission of the complete spectrum of sunlight. In 2003 and 2005, 0.6 m³ of freshwater was added and mixed into the upper 5 m of the mesocosms to stratify the water column and avoid re-introduction of sedimented material into the surface layer. Sediment traps were positioned at the bottom of (2003) or below (2005) the low-salinity surface layer.

The atmospheric and seawater pCO₂ were manipulated through aeration with CO₂-enriched or CO₂-free air to achieve 3 different CO₂ levels in triplicate, with initial CO₂ concentrations as given above (see Approach). After 3-4 days of CO₂ adjustment target CO₂ values in water were reached (day 0) and CO₂ aeration of the water column was stopped. Aeration of the air underneath the tents at initial CO₂ levels was continued throughout the experiment.

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 25:1 yielding initial concentrations of approximately 15 µmol L^-1 NO₃ and 0.6 µmol L^-1 PO₄ in 2001 and 2005. In 2003, it was intended to initiate a diatom bloom. Initial nutrient concentrations were 9 µmol L^-1 NO₃, 0.5 µmol L^-1 PO₄ and 12 µmol L^-1 Si(OH)₄. Throughout the study, the upper 5m layer was gently mixed by means of an airlift (2001; for details see Egge & Asknes, 1992) and aquarium pumps (2003 and 2005).

View of station with mesocosm rack (click to enlarge)

View of an enclosure from above (with plankton net, click to enlarge)

The experimental design (click to enlarge)

Sketch of a tent for the enclosures (click to enlarge)