The measured annual increasein the concentration of carbon dioxide (CO2) in the atmosphere is only60 percent of the annual emissions from fossil fuels. The other 40percent is absorbed by the seas and oceans. Yann Bozec investigated howthe North Sea fulfils this task.
Up until now, little was knownabout the concentrations and transport cycle of CO2 in the North Sea.This lack of data was rectified with four expeditions, each ofone-month duration, with the oceanographic research vessel 'Pelagia'from the Royal Netherlands Institute for Sea Research (NIOZ). Perexpedition the researchers made a vertical water profile at 97locations. Travelling between locations they also measured the levelsof CO2, nutrients (phosphate, nitrogen and silicate), and the amount ofalgal growth. This resulted in the most extensive and accurate data setever for a coastal sea.
Bozec used these data to calculate howmuch carbon dioxide the North Sea absorbs from the atmosphere each yearand then stores in the deep North Atlantic Ocean. This storage capacitywas found to be almost three times as high as the average for all ofthe world's seas. When the data over the entire North Sea were pooled,this gave an annual net uptake of carbon dioxide of no less than 8.5million tonnes of carbon per year. If this performance is theoreticallyextrapolated to all of the world's coastal seas - of which the NorthSea constitutes just 2 percent - then these would together account forabout 20 percent of the carbon dioxide uptake for all of world's seas,even though they only account for 7 percent of the sea surface.Therefore coastal seas absorb CO2 far more efficiently than open oceans.
Seawatercan absorb carbon dioxide in two different ways. The 'physical pump'works as a result of the CO2 concentration in the atmosphere beinghigher than that in the seawater. As nature always tries to restore thebalance, carbon dioxide flows from the atmosphere into the surface ofthe seawater. In the winter, the surface water in the cold polar seascools down and becomes slightly heavier. As a result of this it sinksunder its own weight into the deep sea.
The second way is byalgae fixing carbon dioxide under the influence of sunlight(photosynthesis). This mechanism is important in the North Sea. Algaegrow in the surface water and form their cell material by assimilatingcarbon dioxide from the seawater. This reduces the carbon dioxideconcentration in the seawater thereby allowing more carbon dioxide tobe absorbed from the atmosphere. The algae are further assimilated inthe entire food web. As a result of this water that flows out of theNorth Sea is much richer in organisms (and carbon-containing wasteproducts from these) than the inflowing, blue oceanic water.
Yann Bozec's research was funded by NWO.
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