A 30 model of the Atlantic shelf has been developed and applied to study shelf dynamics and evolution of hydrology. The model takes the combined effects of tide, wind, river discharges and surface heat flux into account. Time scales vary from 1 day to several years. Following a brief description of the model and forcing variables, the behaviour of the Loire and Gironde plumes is described, first for winter and spring, then over a period of several years, under realistic forcing. The results show that plume evolution depends on the high variability of river runoff and winds. Model simulations performed over 7 years (1990 to 1996) have highlighted several features of these plumes: 1) In early winter and periods of high river runoff, plumes usually spread northwards and along shore. 2) During winter, vertical stratification is weak on the shelf. 3) Near the bottom, low-salinity water spreading hardly varies, reaching about the same extent each year. The low-salinity waters are located in the north of each estuary, often not reaching the 50 m isobath. 4) When river discharges are reduced and prevailing winds are from the northwest , the northward spreading of plumes may be stopped. In that case, plumes may be driven offshore or southwards. This path change usually occurs in spring. Salinity gradients become weaker under mixing and spreading effects. The low-salinity strip along the shore seldom builds up again, and the shelf circulation of water masses becomes mainly wind-driven. 0 Elsevier, Paris river plume I modelling I shelf dynamics I Bay of Biscay RCsumC-ModClisation 3D de 1'Cvolution saisonnike des panaches de la Loire et de la Gironde. La dynamique du plateau continental Atlantique et l'tvolution de l'hydrologie sont 6tudiCrs par un modele mathematique 3-D. Les effets combines de la maree, du vent, des apports fluviaux et des Cchanges thermiques entre l'octan et l'atmosphere sont pris en compte. Les echelles de temps varient de la journee a plusieurs annees. Apres une description rapide du modele et des parametres forCants, on etudie l'kolution des panaches fluviaux de la Loire et de la Gironde durant l'hiver et le printemps et leur variabilite interannuelle. Celle-ci est directement like aux variations des debits et des vents. Les simulations des annees prectdentes (1990 a 1996) ont mis en evidence plusieurs caracteristiques des panaches : (1) En periode de true (debut de l'hiver), les pan,aches se propagent vers le Nord et restent collts a la c&e. (2) En hiver, les stratifications sont faibles. (3) Pres du fond, l'influence des panaches est moins variable et relativement constante chaque annee. Les dessalures se situent au nord de chaque estuaire et ne dtpassent quasiment jamais l'isobathe 50 m. (4) Au printemps quand les debits faiblissent et que le vent est de secteur NW, la progression des panaches vers le Nord est stoppee et ceux-ci sont d&i&s vers le large ou repousses vers le Sud. Les gradients de salinite faiblissent sous l'effet du melange ou de l'etalement. La reconstitution ulterieure dune bande &...

A synthesis of 9 years of hydrographic measurements, acquired during approximately 25 cruises (1992-2000) on the French continental shelf of the Bay of Biscay, is presented. The main focus is upon salinity distribution, as it is little known in this area. The whole of the data set covers 11 months of the year, with some sampling fields repeated several times a year, for multiple years. This timeseries allows temperature and salinity distributions to be studied together with their seasonal, interannual and mesoscale variability. The seasonal trend in temperature can be described in three stages, which are in agreement with past studies. Thermal stratification occurs between May and mid-September in a layer not, vert, similar50 m in thickness, whereas the water column remains homogenous from January to the beginning of April. The mid-September-December period corresponds to the progressive destruction of the thermocline. In contrast, the salinity distribution displays two main seasonal patterns. From March to June, the haline stratification is strong; this is in response to high river runoff and relatively low vertical mixing. In autumn, stratification decreases because of low river discharge and wind-induced vertical mixing. Surface salinity increases by up to 1 psu inshore of the 100-m isobath, from spring to end of summer. The salinity gradient runs cross-shore in spring and along-shore in summer: this is due to the mean wind direction during the previous 6 months (SW from September to March, NW from March to September). Nevertheless, this seasonal signal could be biased by the high inter-annual variability. Indeed, the monthly extent of low surface salinity (S<35) varied between years, this is driven by river runoff over the previous 3-6 months and short-term wind events (not, vert, similar10 days), particularly when they are upwelling-favourable. Forcing at these timescales are responsible also for mesoscale structures, such as lower salinity lenses and wind-induced coastal upwelling off Southern Brittany. In the deeper layers the inter-annual variability of a denser water structure named (the "Cold Pool") is also investigated. Consequently, the inter-annual, seasonal and mesoscale variability in salinity patterns are caused by (i) river runoff from the Loire and Gironde, that reduces surface salinity locally; and (ii) the wind that influences the location of low salinity water masses. Mesoscale structures and the observed links between inter-annual and seasonal variability, raise problems in relation to the interpretation of in situ data, making it impossible to establish the actual mean distribution. There is a pressing need within the Bay of Biscay, for sampling at higher spatial and temporal resolution.

Thermostratification and seasonal light increase are generally considered the first causes of phytoplankton spring blooms in temperate waters. The objective of this study is to confirm the existence of winter phytoplankton blooms, responsible for the early exhaustion of phosphate, within the Gironde plume waters (southeast Bay of Biscay), and to understand what may initiate them so early. Two cruises, BIOMET 2 and BIOMET 3, were carried out respectively in early (8 to 21 January) and late winter 1998 (25 February to 11 March). An increase of phytoplankton biomass (chl a) between the 2 cruises and non-conservative nutrients observed in late winter confirm an early phytoplankton growth both in the Gironde plume and adjacent oceanic waters. Among factors that possibly initiate these blooms, light availability seems to be the best candidate. First, the status of available light for phytoplankton changed dramatically between the 2 cruises when comparing the instantaneous depth-averaged irradiance (E m ) received by phytoplankton with the light saturation parameter E k determined from P versus E curves. Light was limiting for phytoplankton growth in early winter according to systematically lower values of E m than E k in the daytime. However, light was not limiting during a large part of the day in late winter since E m was above E k during 40% of the daylight.

A complex set of hydrodynamical structures co-exist over the Bay of Biscay continental shelf. These structures, which can spread over a limited spatial range (10s of km) and last for periods ranging from days to months, influence the rates and nature of biological production. They need to be accurately described before the mechanisms linking hydrodynamics to production can be unraveled. Such description generally requires not only 3D observations of hydrological fields (temperature, salinity, density), but also a description of how these fields vary with time. This is hardly achievable with current means of field observation but can be mimicked by hydrodynamic model simulations. Using simulations from a 3D hydrodynamic model, we developed a methodology that allows for the description of hydrological landscapes, i.e. a description of the 3D fields and of their changes with time. The method was applied to the structures present over the Bay of Biscay shelf in spring. Eight hydrological landscapes were identified: high estuary, low estuary, coastal, river plume, near coastal, central shelf, open shelf and northwestern shelf. With this technique, we show that structures that have been qualitatively described in the literature can be identified objectively and that some new structures can be detected. All 8 hydrological landscapes are found every year, but their spatial extent can vary greatly from year to year. This variation for all landscapes except for the northwestern shelf can be summarised by a single index, which is strongly related to the runoff from the main rivers along the French coast.

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