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An approach combining laboratory experiments and numerical modelling was used to investigate the behaviour of a floc population during an idealized tidal cycle. The experiment was conducted on suspended sediments at a concentration of 93 mg l −1 collected in the field. It was based on a jar test device to reproduce tidal-induced turbulence and coupled with a CCD camera system and image postprocessing software to monitor floc size distribution. At the same time, a 0D size-class based aggregation/fragmentation model (FLOCMOD) was developed to simulate changes in the floc population over the tidal cycle. Experimental results revealed strong variability of the behaviour of microfloc and macrofloc populations with respect to the varying shear rates observed in situ. In particular, the major dependency of floc sizes on the Kolmogorov microscale was confirmed. Timescale differences were also observed for aggregation and fragmentation processes which led to asymmetrical floc behaviour despite symmetrical tidal forcing. Model results, i.e. average diameter, maximum diameter and floc size distribution, were in good agreement with experimental data with an RMS error between observed and computed average diameters of below 25 μm over the tidal cycle. FLOCMOD was optimized in terms of the time step, number of size classes and size range: only seven classes ranging from 50 to 643 μm associated with a dynamically-adaptable time step were needed to correctly reproduce experimental results, characterized by an RMS error of less than 5 μm with respect to the reference case (100 classes from 4 to 1500 μm). Sensitivity analyses were performed on major parameters or processes: initial floc size distribution, primary particle size, fractal dimension and fragmentation function (binary, ternary, erosion or collision-induced fragmentation). Results showed that initial floc size distribution played a role only during the first aggregation stage. Low variability of the fractal dimension did not significantly modify model results, while larger differences were observed when the primary particle size was changed, especially towards the largest sizes (10 μm). Nevertheless, these two structural parameters had a strong impact on the calculated mean settling velocity with differences of 0.2 mm s −1 compared with the reference case. Different fragmentation functions were shown to significantly modify model results, except for collision-induced shear stress. In particular, combining floc erosion with binary breakup in the shear fragmentation term enabled us to reproduce bimodal distributions, patterns that are typically observed in situ.

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)