Plankton and marine invertebrates of the Saguenay fjord
Plankton includes all the microscopic organisms that live in suspension in the water column. Two categories may be distinguished. The phytoplankton category is plant type and consists of small detached plants that depend on solar energy for growth. The zooplankton category consists of small animal organisms that feed on animal or vegetable plankton.
The Saguenay fjord is an ecosystem with very little phytoplankton. These organisms are much more abundant in salt water. In the fjord, the layer of fresh water at the surface considerably limits their growth. Specifically, the daily production rate in the Saguenay fjord is 165 mg C/m2 (expressed as mg of carbon per square meter). The level may reach up to 6,460 mg C/m2 in the surface water around the Saint Lawrence estuary.
Plankton growth is a phenomenon that depends on high turbidity and it allows very little light to pass. The thin photic layer that allows light to penetrate is what mostly slows down the proliferation of these plant planktonic organisms. The phytoplankton found mainly consists of freshwater species, a few of which can tolerate water salinity. The dominant species in the brackish water are asterionella formosa, melosira ambigua, melosira islandica and tabellaria fenestrata (Côté, 2002, personal communication).
Even though the Saguenay fjord produces little plankton, it gets a large contribution from the estuary. When the tides sweep icy water into the fjord, a considerable amount of plankton is carried in just above the 20 meter sill at the mouth of the Saguenay River. This phenomenon is amplified by the fact that this is the highest concentration of suspended organisms in this zone. There are here dense planktonic aggregations over an area 100 kilometers long and from 1 to 7 kilometers wide in the Tadoussac/Grandes-Bergeronnes region. These aggregations consist of significantly large groups of small shellfish originating from two zooplankton groups. One group is the euphausid species that is also called "krill". The second group is the small copepod species of the calanus type.
These concentrated organism colonies can sometimes reach layers as much as 100 meters deep. They are displaced to this location by the joint action of hydrodynamic circulation and the cyclic rising of deep downstream water from the Saint Lawrence River. These colonies do not develop or grow locally but originate from phytoplankton produced throughout the Gulf of Saint Lawrence. Their concentration results from the deep current directed upstream of the Laurentian channel that forms a dead end at the mouth of the Saguenay River. The krill is then trapped inside this narrow zone and their concentration is amplified with the euphausids that flee daylight (negative phototropism).
The aggregation of planktonic food in the Tadoussac/Grandes-Bergeronnes zone of the Saint Lawrence River attracts a large number of small fish that are in turn bait for other fish and larger organisms. This abundance also explains the presence of several whale species that are fond of krill.
The richly laden waters penetrating the depths of the fjord bring in a considerable amount of zooplankton that the whales feed on. The zooplanktonic composition is particular and amongst the 83 species listed, some have an affinity for the arctic.
The Saguenay fjord is the habitat of a wide variety of benthic marine organisms that include starfish, sea urchins, marine worms, bivalve mollusks and gastropods. There are about 410 species of invertebrates and the majority, as those of the estuary, have a northern affinity. However, the fjord has a greater proportion of arctic distribution species, 18 of which are exclusive to the fjord. These latter are vestiges of the past when they were more widely distributed. Today, certain physical and thermal conditions required for their survival have disappeared.
The profusion of these marine invertebrate organisms in the depths of the Saguenay fjord contributes to the wealth of this magnificent and unique ecosystem in the world.