A plankton trap 

The energy needs of the 100-tonne blue whale are enormous: it eats some two to four tons of krill a day. It, therefore, searches for areas containing huge concentrations of these crustaceans, like the 1- to 7-km wide, 25-m deep, 100-km long shoal off Les Escoumins, discovered in 1982 with the aid of a depth sounder by a Fisheries and Oceans Canada team led by Yvan Simard.

Greater biological activity takes place in the ocean zones situated near the poles rather than the tropics. The climate in the polar regions causes the movement of water masses which brings cold water up to the surface and, along with it, minerals that have accumulated in deeper water. This enriched surface water, exposed to sunlight, creates conditions ideal to the proliferation of phytoplankton — microscopic algae at the base of the aquatic food web.

This phenomenon is particularly present in the upper part of the St. Lawrence’s Lower Estuary, at the "head" of the Laurentian Channel, between Tadoussac and Grandes-Bergeronnes. In this area, a layer of cold water, located at a depth of between 40 and 150 m and rich in minerals, is pulled by tidal currents toward the surface and shallow regions of the Upper Estuary.

These tidal currents, which can travel up to 10 km/h, produce intense physical phenomena such as internal waves, fronts and turbulence. The surface waters are cooled and enriched with mineral salts as they mix with deeper waters. Then, strong currents rapidly carry these waters downstream toward the Lower Estuary and the Gulf. The area around the head of the Laurentian Channel is, therefore, an important source of nutrient salts essential to the biological productivity of the St. Lawrence, although the physical conditions that characterize this area are not conducive to significant local production of phytoplankton and zooplankton. The downstream portion of the Lower Estuary and the Gulf are areas of optimal plankton productivity.

A dead end rich in food

So how do we explain the enormous concentrations of krill and copepod at the head of the Channel? According to researchers with the Maurice Lamontagne Institute, these organisms are carried all the way from the Gulf! Adult zooplankton generally migrate vertically between surface waters, where they feed at night, and deep waters, where they hide from predators during the day. During the long summer days, they spend much more time in deep water than at the surface. Surface waters flow toward the Atlantic while the Channel’s deep waters are slowly pumped upstream. This deep current therefore gradually transports the adult zooplankton toward the head of the Channel, a topographical dead end, where it is caught by the upwelling movement of the deep water. Fairly good swimmers, they fight to stay in deep waters during the day, which results in a process of accumulation. In this way, by the end of this voyage that lasts from one to two years, they form large concentrations like the one detected by Simard’s team in 1982. These adults lay eggs which are discharged along with a portion of the nutrient salts toward the Gulf, and the cycle is renewed.

• A warm (0 to 14° C), relatively fresh (25 to 31 ‰) surface layer (0 m to 25 m), that is light and floats on the cold intermediate layer.
• A salty (32 to 33 ‰) cold (-1 to 2° C), intermediate layer (from 25-40 m to 150 m) and more dense than the surface layer. Rich in nutrient salts, it is formed locally during the winter by means of a convection mechanism and by the release of salts as a result of ice formation. In the Gulf, up to 30% of this layer is fed by cold water from Labrador.
• A relatively cold (2 to 5° C), salty (33 to 35 ‰) bottom layer (150 m to 450 m) that is low in oxygen. Its strong salinity gives it high density which provides support for the intermediate layer. It arrives from the Atlantic Continental Shelf and takes approximately 5 to 7 years to migrate to the head of the Channel.

It is known that aggregations of zooplankton attract fish and whales. A lot less is known about the abundance and distribution of the various species of fish, as well as the feeding ecology of the cetaceans of the St. Lawrence Estuary. It appears that two species of fish, the capelin and Atlantic herring, are especially abundant. Large concentrations of capelin are found in the Saguenay and its mouth almost year round. These fish are undoubtedly a major food source for cetaceans. Other species, like the American sand lance and the rainbow smelt, are also found in the Estuary, but little is known about them.

Organisms living at the head of the Laurentian Channel know how to exploit the resources generated by the complex hydrodynamic, chemical and biological processes which scientists began to better understand some 30 years ago. Advances in technology are enabling oceanographers to refine their understanding of the dynamics and circulation of water masses of the St. Lawrence. Their findings are opening new doors to scientists studying whales and their prey.

Special thanks to François Saucier and Jean-Claude Therriault, MLI, Fisheries and Oceans Canada