The partial pressure of carbon dioxide (pCO2), 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, pCO2 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 pCO2 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 pCO2 at meso time-scale (10?days) and/or when biological activity is weak. Macrocystis acts to maintain pCO2 below saturation almost the whole year and large undersaturations (pCO2 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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