Peter S Galbraith

and 3 more

Jean-Luc Shaw

and 1 more

Seasonal and inter-annual variability of transport in the Strait of Belle Isle (SBI) were examined using 14 years of moored ADCP data. Tidal currents were largely along strait and homogeneous with depth. Transports were inwards on average, lowest (−1.0±0.8 dSv) from April to July, and highest (−4.1±1.1 dSv) from September to January. Monthly scale outflow towards the Labrador Shelf was more than twice as common between April and August than between September and March. Averaged seasonal transports were generally lower but within one standard deviation of previously published modelled values. The volume of winter Labrador Shelf water (LShW) entering the Gulf of St. Lawrence (GSL) was computed by transport integration and compares well with integrated volumes that meet LShW temperature-salinity criteria during an annual March survey of the GSL. Integrating over the whole year showed that on average 590±351 km3 enters the GSL after the March surveys are conducted, or 36±19% of the total LShW volume. Annual volumes of LShW calculated from transport suggest that inflow through the SBI accounts for 12–18% of the GSL winter surface mixed layer. Cross-strait current shear may have affected transport integration values, but SST data suggests this bias was limited to the summer. Corrections were empirically derived to account for cross strait shear in our transport calculation. The corrected time series suggests dominant summer transport may have been flowing towards the Labrador Shelf (0.4 ± 0.6 dSv).

Cynthia Bluteau

and 4 more

The St. Lawrence Estuary connects the Great Lakes with the Atlantic Ocean. During summer, the Estuary’s surface layer receives its nutrient supply from vertical mixing processes caused by the estuarine circulation and tidal-upwelling at the Head of the Laurentian Channel (HLC). There has been few oceanographic process studies during winter when ice forms and flows on the surface. Winter monitoring is typically confined to vertical profiles of salinity and temperature and near-surface water samples collected from a helicopter. In 2018, however, the Canadian Coast Guard approved a science team to sample in tandem with its icebreaking and ship escorting operations. This opportunistic sampling provided the first winter turbulence observations, which covered the largest spatial extent ever measured during any season within the St. Lawrence Estuary and Gulf. The nitrate enrichment from tidal mixing resulted in an upward nitrate flux of about 30 nmol m$^{-2}$s$^{-1}$, comparable to summer values obtained at the same tidal phase. Further downstream, deep nutrient-rich water from the Gulf was mixed into the subsurface nutrient-poor layer at a rate more than an order of magnitude smaller than at the HLC. These fluxes were compared to the nutrient load of the upstream St. Lawrence River. Contrary to previous assumptions, fluvial nitrate inputs are the most significant source of nitrate in the Estuary. Nitrate loads from vertical mixing processes would only exceed those from fluvial sources at the end of summer when fluvial inputs reach their annual minimum.