St. Lawrence River

Category: Inland Aquatic Biomes.

Geographic Location: North America.

Summary: This major river is a complex ecosystem with diverse habitats; it supports a variety of endemic species—as well as 60 million people.

The St. Lawrence River (locally known as the Fleuve Saint-Laurent) is located in northeastern North America, forming part of the border between the United States and Canada. It generally runs northeast from the outlet of Lake Ontario into the Gulf of St. Lawrence and the North Atlantic Ocean. The river is the main outlet of the five North American Great Lakes (Ontario, Erie, Huron, Michigan, and Superior), and several tributaries flow into the main stem as well. On an annual basis, 70 percent of the St. Lawrence River discharge at Quebec City, Canada, originates from the Great Lakes via Lake Ontario, 20 percent from the Ottawa River, and 10 percent from smaller tributaries.

Geography and Surrounds

The watershed of the St. Lawrence is 617,763 square miles (1.6 million square kilometers), of which 55 percent is forests (boreal and temperate deciduous), approximately 22 percent is urban area, 20 percent agricultural lands, and 2 percent other types of land cover. Two major physiographic divisions influence the water chemistry of the river: the Precambrian Shield (north and west of the river) and the Appalachian Highlands (south and east). The Precambrian Shield is relatively homogeneous and dominated by silicate rocks; the Appalachian Highlands is more complex, but Paleozoic rock dominates.

More than 60 million people live in the watershed of the river. Several cities (Chicago, Illinois; Cleveland, Ohio; and Detroit, Michigan, in the United States; and Toronto, Ontario; and Montreal, Quebec, in Canada) are among the largest built-up areas along the shores of the Great Lakes or the St. Lawrence. Especially in the fluvial section, natural physiography has been altered by human intervention, such as construction of hydroelectric works, creation of the St. Lawrence Seaway locks and canals, dredging of shipping channels, intensive shore modification, creation of the Expo 67 (held in 1967) artificial islands, and ongoing fill work.

Although it is highly disturbed, the St. Lawrence is still a relatively complex natural river with diverse habitats. It contains three fluvial lakes (Saint-François, Saint-Pierre, and Saint-Louis), a few island chains (such as the Hochelaga Archipelago), and several isolated large islands; these last include Anticosti Island at 3,059 square miles (7,923 square kilometers). The shores of the fluvial lakes form large wetlands, the most notable being Lake Saint-Pierre, a Ramsar World Heritage Site. Wetlands here—excluding aquatic plant communities—cover at least 83,027 acres (33,600 hectares); this is augmented by 21,315 acres (8,626 hectares) of low marsh, 46,016 acres (18,622 hectares) of high marsh, and 15,884 acres (6,428 hectares) of swamps.

The St. Lawrence can be seen as four separate sections: the fluvial, covering more than 149 miles (240 kilometers) from Cornwall, Ontario, to the outlet of Lake Saint-Pierre, Quebec; fluvial estuary, encompassing more than 99 miles (160 kilometers) from the eastern tip of Lake Saint-Pierre to the eastern tip of Île d’Orléans; the upper estuary and Saguenay River, totaling 93 miles (150 kilometers) from the eastern tip of Île d’Orléans to the mouth of the Saguenay River; and the lower estuary and Gulf of St. Lawrence, a semi-enclosed sea that together are more than 143 miles (230 kilometers) long. Salinity gradually increases from Quebec City at the eastern tip of Île d’Orléans to the outlet of the Saguenay River.

Vegetation

The four sections of the river have different shoreline plant zonation patterns. In the upper parts of the river, such as the fluvial section, the strong seasonal fluctuations in water levels is the main driver at the shoreline. Along the riverbank is a silver maple (Acer saccharinum) forest inland, with a willow stand at the shore, alongside wet meadows and/or marsh, and beds of aquatic vegetation.

Downstream, the other sections are more influenced by tides and water chemistry. Shoreline vegetation in the freshwater estuary (Grondines-Montmagny) is characterized by a decrease in diversity and the lesser extent of riparian forests, which are often reduced to a narrow band composed of tree willows, species particularly adapted to the wide water-level fluctuations.

In the saltwater estuary, cordgrass (Spartina spp.) and sedge (Carex) salt marshes clearly dominate. The riparian vegetation of the Gulf has a similar structure to that of the estuary, but sand ryegrass (Elymus mollis) and beachgrass (Ammophila breviligulata) communities are more widespread.

Biodiversity

For the phytoplankton, seasonal environmental changes are an important control on community composition: Diatoms (a major group of algae) and Cryptophyceae (freshwater algae) are abundant year round, but Chlorophyceae become more plentiful in the summer. Inside the river, the fluvial lakes are major habitats for the plankton. The dense beds of aquatic vegetation in the fluvial lakes are highly productive compared with those in other sections of the river.

For benthic (near-bottom-dwelling) invertebrates, the main drivers of community composition are depth, substrate size, and organic content of the water. Dominant groups of zooplankton inside the freshwater areas are rotifers (wheel animals), cyclopoid copepods (T-shaped, shrimp-like bodies), and small cladocera (water fleas). In the estuary section, one particularly productive habitat for the zooplankton is the confluence—itself a narrow freshwater fjord—of the St. Lawrence and Saguenay rivers, where saltwater upwells in the form of nutrient-rich deep-water flows from the Laurentian channel arise to stimulate plankton growth and support krill populations.

This abundance of krill attracts approximately 21 species of cetaceans and pinnipeds, or seals, to the estuary section. One the most emblematic species of the St. Lawrence is the beluga whale (Delphinapterus leucas), which is more commonly found in the Arctic. The beluga is the only whale that stays inside the estuary all year; all other species spend only a few summer months there. This beluga community, estimated in 2018 at 900 individuals locally, is listed as locally endangered due to overfishing and water pollution.

Fish diversity is relatively low compared with other great fluvial ecosystems, with 87 species of freshwater fish and 18 types of diadromous fish inhabiting the river. Fish endemism (those species found nowhere else) is also relatively low. Only the copper redhorse (Moxostoma hubbsi) and pygmy smelt (Osmerus spectrum) are true endemic species restricted to the river. Other fish species found in the St. Lawrence are the spring cisco (Coregonus spp.), chain pickerel (Esox niger), redfin pickerel (Esox americanus americanus), bridle shiner (Notropis bifrenatus), cutlip minnow (Exoglossum maxillingua), and eastern silvery minnow (Hybognathus regius).

Relatively few fish species are hunted on a commercial basis in the fluvial ecosystem. Among the species locally important to Native Americans of this region is the American eel (Anguilla rostrata). Like a number of other species, the eel population has declined, affected by river modifications such as hydroelectric dams and bridges that have disturbed their long migrations from the Atlantic Ocean to the Great Lakes.

Herpetofauna (reptile life) biodiversity is relatively low due to the northern nature of the river, although there are relatively abundant reptiles near Montreal, on the shores of the islands of the Hochelaga Archipelago, and in the wetlands of the fluvial lakes. In Canada, this section of the river is a national hot spot for herpetofauna biodiversity with several species of turtles and snakes. However, most of these species are considered endangered or at risk because of the human pressure exerted on their habitats.

Many important bird colonies are located on the shores of the river or use the St. Lawrence as a feeding ground. A colony of great blue heron (Ardea herodias) is located on Grande Île (Berthier-Sorel Archipelago, Lake Saint-Pierre), with more than 1,000 breeding pairs, perhaps the largest breeding colony for this species in the world. In the gulf, Bonaventure Island on the Gaspe Peninsula is home to the largest breeding outpost of northern gannets (Morus bassanus) in the world, with tens of thousands of breeding pairs. The St. Lawrence also is located on an important migration route for many bird species reproducing in the Arctic during summer. The St. Lawrence River wetlands provide resting and feeding areas for more than 1 million snow geese (Chen caerulescens) during their spring and fall migrations.

Environmental Threats

Regarding invasive species, the St. Lawrence is similar to other large rivers in human-modified environments. Several introduced species threaten the biodiversity and the functioning of the different subecosystems of the river. More than 163 aquatic species have been introduced into the Great Lakes during the past 200 years, and at least 85 are found inside the St. Lawrence, including the zebra mussel (Dreissena polymorpha), black spotted goby (Neogobius melanostomus), and Chinese mitten crab (Eriocheir sinensis). Examples from the plant kingdom include Eurasian watermilfoil (Myriophyllum spicatum), common frogbit (Hydrocharis morsus-ranae), water chestnut (Trapa natans), reed canary grass (Phalaris arundinaceae), and common reed (Phragmites australis ssp. australis).

Climate change poses some challenges to the river. If the water temperature rises too much and precipitation fluctuates too broadly, the river’s flow could be altered by changes to the aquatic chemistry balance of the St. Lawrence, an ecosystem already struggling with hypereutrophic (over-abundance of nutrient flow) conditions, low-oxygen zones, areas of high turbidity, and other challenges typical of developed rivers. Species such as the copper redhorse are categorized as endangered as a result of dam construction, habitat degradation, invasive species, water contamination, and water level fluctuation. Higher water levels and more intense storms could increase shoreline erosion and instream sedimentation and turbidity, damaging the balance of flora habitats at the foundation of the food web.

Pollution from major cities is of concern. Pathogens such as E. coli, other bacteria, and viruses associated with diseases and infections are common in the river. Notably, Montreal's sewage treatment system removes solid waste but releases the water into the river without disinfecting it. High bacterial counts close beaches along the river periodically. The city was responding to environmental concerns in 2022 by constructing an ozone disinfection unit to its water treatment station, although this project would necessitate periodic reduction in wastewater treatment capacity.

Bibliography

"Copper Redhorse (Moxostoma Hubbsi): Report on the Progress of Recovery Strategy Implementation for the Period 2012 to 2018." Government of Canada, 26 May 2022, www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/report-progress-recovery-document/copper-redhorse-2012-2018.html. Accessed 2 Sept. 2022.

DesGranges, Jean-Luc. “Biodiversity Portrait of the St. Lawrence.” Environment Canada. http://www.qc.ec.gc.ca/faune/biodiv/en/table‗contents.html.

Environment Canada. “St. Lawrence River.” http://ec.gc.ca/stl/default.asp?Lang=En&n=F46CF5F8-1.

Olson, Isaac. "Montreal Major Polluter of St. Lawrence as E. Coli Plumes Drift to Trois-Rivieres, Study Finds." CBC/Radio Canada, 11 Oct. 2021, www.cbc.ca/news/canada/montreal/e-coli-st-lawrence-river-bacteria-pollution-1.6205541. Accessed 2 Sept. 2022.

Thorp, James H., Gary A. Lamberti, and Andrew F. Casper. “St. Lawrence River.” In Field Guide to Rivers of North America, edited by Arthur C. Benke and Colbert E. Cushing. San Diego, CA: Academic Press, 2009.