The study of vertical particle dynamics in the highly turbid Gironde Estuary has shown intense cycles of sedimentation and resuspension at both diurnal and neap-spring time scales. Fluid mud, with suspended particulate matter (SPM) concentrations between 50 and 500 g liter ) preferentially reworked at the neap-spring time scale. Due to the alternation of sedimentation and resuspension periods, most of the sediment experienced oxic/anoxic oscillations throughout the neap-spring cycle. Fluid mud resuspension occurred without any observable incidence on the surface-water oxygenation. An increase in total alkalinity was found in the fluid mud, due to both anaerobic respiration and a carbonate dissolution coupled to aerobic respiratory CO 2 generation. This phenomenon significantly affected the inorganic carbon budget of the estuary, increasing the HCO input to the coastal ocean and reducing the CO 2 flux to the atmosphere. An Ϫ 3 accumulation of labile-dissolved organic carbon observed in the fluid mud suggests that these oscillations result in an acceleration of particulate organic matter (POM) decomposition. In the Gironde MTZ, a net loss of refractory land-derived POM occurs. This system acts as an efficient oxic/suboxic ''fluidized bed reactor,'' similar to mobile deltaic muds.Understanding processes affecting carbon distribution through estuaries is of major importance for a better assessment of the contribution of world rivers to the carbon budget of the coastal ocean. The influence of tides in macrotidal estuaries increases the residence times of both water and 1 Present address: Université de Liège, Mécanique des Fluides Géophysiques, Unité d'Océanographie Chimique, Institut de Physique (B5), B-4000 Sart Tilman, Belgium. AcknowledgmentsThis work has been funded by the European Commission in the framework of the Biogest project (ENV4-CT96-0213); this is a contribution of the Eloise Projects network. It was also supported by the URM 13 project, linking IFREMER and the University of Bordeaux I.-DGO-UMR 5805. Fieldwork was carried out onboard the RV Gwen Drez (IFREMER, France), the RVs Côte d'Aquitaine and Côte de la Manche (INSU, France), and the RV Belgica (Belgium); we thank captains and crews. A. De Resseguier (DGO) built the fluid mud sampler and was present on the field for sampling assistance. We are grateful to P. Castaing and A. Sottolichio (DGO) for help in using the OBS and to A. V. Borges (University of Liège)

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The arslenic (As) cycle in the marine environment is known to be sensitive to biological activity. Within the scope of the National Coisstal Oceanography Program, we undertook a specific study of the behaviour of this element in a water column on thle continental shelf of the Bay of Biscay off the Gironde estuary during two oceanographic cruises conducted in May 1994 and May 1995. Various chemical forms were measured: arsenite (ASP), arsenate (ASP), monomethylarsenic (MMA), dimethylarsenic (DMA) and dissolved total As after ultraviolet mineralization. The net flux of total As from the Gironde estuary was evaluated as well as its effect on the concentration in surface marine waters. The vertical profiles in the coastal zone during a period of active primary production confirmed a certain analogy between arsenates and phosphatles as well as the formation of As3 and DMA. The importance of organoarsenic species not directly accessible to the formation of volatile hydrides is demonstrated. Their presence modifies assessments, reducing the deficit of dissolved total As in the euphotic layer to just 5 %. For particles, the As content in phytoplankton was estimated at 6 pg.g-' compared to 20 to 30 pg$' in iron-and aluminium-rich terrigenous particles. These results do not invalidate the role of phytoplanktlon in the speciation of dissolved As but indicate that their involvement in vertical transfers was apparently not predominant in the zone under study. 0 Elsevier, Paris arsenic / phytoplanktoln I speciation / Gironde plume I Biscay Bay RCsumL -Comportement de l'arsenic sur le plateau continental au large de l'estuaire de la Gironde : rble du phytoplancton dans le flux vfertical en situation de bloom printanier. Le cycle de l'arsenic en milieu marin est bien connu pour sa sensibilitk g l'activite biologique. Dans le cadre du Programme National d'Oc&anographie C&i&e, nous avons CtudiC de man&e plus prCcise le comportement de cet ClCment dans la colonne d'eau sur le plateau continental du Golfe de Cascogne, au large de la Gironde. Deux campagnes oc&anographiques ont Ctk rCal:isCes en avril 1994 et mai 1995. Nous avons mesurC les diverses formes chimiques: ardnite (ASP), arstniate (ASP), mlDnomCthylarsenic (MMA), dimCthylarsenic (DMA) ainsi que 1'As total dissous aprhs minkralisation UV. Le flux net d'As total issu de la Gironde a 6tk CvaluC ainsi que son influence sur la concentration dans les eaux marines superficielles. Les profils verticaux en zone c&&e, pendant une p&ode active de production primaire, confirment une certaine analogie entre arsCniates et phosphates, ainsi que la formation d'As3 et de DMA. L'importance d'espbces OrganoarsCniCes inaccessibles directement B la formation d'hydrures volatils, est dCmontreie. Cette pr&sence influe sur les bilans et rCduit g 5 % seulement le dCficit en As total dissous dans * Correspondence and reprints Oceanologica Acta 0399 1784/98/02/0 Elsevier, Paris 325More free publications from Archimer