Descriptive Physical Oceanography of Great-Entry Lagoon, Gulf of St. Lawrence

This study describes some aspects of the physical oceanography of Great-Entry lagoon (GEL), Canada, using sea level, current, salinity and temperature time series recorded in 1988 and 1989. GEL is part of a communicating two-lagoon system, each having one opening with the gulf of St. Lawrence. The length of GEL is 25 km, its surface area is 58 km2 and its mean depth is 3 m. A navigation channel divides the lagoon into a relatively deep basin (4–6 m) to the east and a shallow (1–3 m) region to the west leading to the second lagoon. Water in the lagoon is generally well mixed in the vertical but can remain horizontally stratified. Harmonic analysis reveals that GEL is a ‘ leaky ’ lagoon, that tides are mixed and predominantly semi-diurnal and that the tide progresses from the inlet to the adjacent lagoon in about 3 h. This sets up tidal currents reaching 0·5 m s−1 at the entrance and in the western section. In the basin however, tidal currents remain weak (<0·05 m s−1). Low-frequency currents have typical speeds of 0·05–0·1 m s−1. They respond to non-local sea-level fluctuations at the entrance and to local winds in the form of downwind currents in shallow areas and upwind currents at depth inside the basin. Atmospheric pressure plays a major role in the renewal of waters through an inverse barometer effect acting at time scales of 3 to 15 days. The low salinity signature of the St. Lawrence River freshet reaches the lagoon at the beginning of August. Using this natural tracer in conjunction with spectral analysis, a renewal time for waters in the deeper basin is estimated to lie in the range of 10 to 20 days.