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The action of tides on density-driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski and Québec City. Timevarying fields of water level, currents and density were computed under typical summer conditions using a three-dimensional hydrostatic coastal ocean model that incorporates a second order turbulence closure submodel. These results are compared with current meter records and other observations. The model and the observations reveal buoyancy effects produced by tidal forcing. The semi-diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing internal tides radiating seaward. Relatively dense intermediate waters rise from below 75-m depth to the near surface over the sills, setting up gravity currents on the inner slopes. Internal hydraulic controls develop over the outer sills; during flood, surface flow separation occurs at the entrances of the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early during ebb flow (restratification), the surface layer deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts the formation of seaward internal jumps over the outer sills, which were confirmed from acoustic reflection observations. As the internal Froude number increases further, flow separation migrates up to sill height. As a result of these transitions, internal bores emanate from the head region one to two hours before low water. We find that the mixing of oceanic and surface waters near the sills is driven by the vertical shear produced during ebb in the channel south of Ilet Rouge, the shear produced in the bottom gravity flood currents, and, to a lesser extent, the processes over the sills. résumé L'action des marées sur l'écoulement associé à la flottabilité, les ondes de gravité internes et le mélange a été étudiée pour l'estuaire du Saint-Laurent entre Rimouski et la ville de Québec. Les variations du niveau d'eau, des courants et de la densité ont été calculées dans des conditions estivales typiques en utilisant un modèle hydrostatique tridimensionnel pour les eaux côtières incorporant un sous-modèle de fermeture turbulente au second ordre. *Corresponding author: saucierf@dfo-mpo.gc.ca 506 / François J. Saucier and Joël Chassé Ces résultats ont été comparés avec des enregistrements de courantomètres et d'autres observations. Le modèle et les observations nous révèlent des effets de flottabilité produits par la marée. La marée semi-diurne élève les isopycnes au-dessus des seuils à la tête du chenal Laurentien et du banc des Anglais, produisant des marées internes qui se propagent vers l'océan. Des eaux intermédiaires relativement denses, se trouvant sous 75 m de profondeur, remontent prés de la surface au-dessus des seuils, produisant des courants de gravité sur les pentes internes. Des contrôles hydrauliques internes se développent au-dessus des seuils extérieurs; durant le flot, une séparation de l'écoulement de surface a lieu à l'ent...

A bio-physical semi-Langrangian model was developed to follow the drift and abundance of lobster larvae from hatching to settlement as post-larvae. The geographic domain encompasses Northumberland Strait and the areas surrounding Prince Edward Island in the southern Gulf of St. Lawrence. The model was run for ten larval seasons, 1983-87 and 1997-2001, representing two periods of egg production. The model was forced with tides, winds, heat fluxes, and marine and freshwater fluxes at open boundaries. Biological inputs were location, date, and density of larvae at hatching, development rates, a time window and a minimum bottom temperature required for successful settlement, and two scenarios of daily mortality. Net drift was west to east but stronger on the north than the south side of Prince Edward Island. The hatch was greater in the second 5-yr period, but the spatial patterns of settlement were similar. For both mortality scenarios, the same five of the 24 larval source areas were important in providing settlers. Horizontal shear of larval distribution indicates fishing communities were dependent on the hatch from fishing grounds of multiple upstream communities. Variation in daily and annual post-larval settlement was greatest north of Prince Edward Island. From the 24 source areas there was an eightfold range in the fraction of larvae surviving to settlement, with advection into deep water an important cause of mortality. Four to five possible fishery management areas were identified using multidimensional scaling to group sink areas having shared source areas. We hypothesize leaky discontinuity in connectivity between these groups.