| Summary: | The partial pressure of carbon dioxide (pCO 2 ), calculated from pH and total alkalinity measurements, was monitored together with chlorophyll a and bacterioplankton biomass in shallow coastal water located inside and outside a giant kelp bed ( Macrocystis pyrifera ) situated in the Kerguelen Archipelago, Southern Ocean. In spite of large changes over a short time-scale, pCO 2 variations over the year are large and exhibit a seasonal pattern in which the different stages of the annual biological turnover are well marked. The overall pattern of pCO 2 variations is related to biological activity (development of both photosynthesis and respiration) during almost the whole year. However, physical and thermodynamical constraints exert a strong influence on pCO 2 at meso time-scale (10?days) and/or when biological activity is weak. Macrocystis acts to maintain pCO 2 below saturation almost the whole year and large undersaturations (pCO 2 as low as 20?µatm) were observed within the kelp bed. Furthermore, primary production of Macrocystis covers a period of 8 ~ 9?months a year from winter to late summer and the kelp bed seems to favour the spring phytoplanktonic bloom. The buffer factor ß indicates that, outside the kelp bed, inorganic carbon dynamics are mainly influenced by air-sea exchange and photosynthesis without calcification. Inside the kelp bed, ß suggests calcification by the epiphytic community.
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