Seasonal changes of pCO(2) over a subantarctic Macrocystis kelp bed

The partial pressure of carbon dioxide (pCO(2)), calculated from pH and tot al alkalinity measurements. was monitored together with chlorophyll a and b acterioplankton biomass in shallow coastal water located inside and outside a giant kelp bed (Macrocystis pyrifera) situated in the Kerguelen Archipel ago, 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 whi ch 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 whol e year. However, physical and thermodynamical constraints exert a strong in fluence on pCO(2) at meso time-scale (10 days) and/or when biological activ ity is weak. Macrocystis acts to maintain pCO(2) below saturation almost th e whole year and large undersaturations (pCO(2) as low as 20 mu atm) were o bserved within the kelp bed. Furthermore, primary production of Macrocystis covers a period of 8 similar to 9 months a year from winter to late summer and the kelp bed seems to favour the spring phytoplanktonic bloom. The buf fer factor beta indicates that, outside the kelp bed, inorganic carbon dyna mics are mainly influenced by air-sea exchange and photosynthesis without c alcification. Inside the kelp bed. beta suggests calcification by the epiph ytic community.