State of health of the St. Lawrence
For over two centuries, the St. Lawrence has sustained extensive damage caused by industrial activities, agriculture, urbanization and maritime traffic. Climate change, which is already perceptible, adds a new dimension to this reality. The subject is complex and analysis demands a degree of caution as researchers continue their work. The state of health of the St. Lawrence is illustrated in the following table.
The changing nature of chemical pollution
In the 1980s and 90s, concerns mainly revolved around substances like lead, mercury and cadmium as well as organic molecules such as PAHs, DDT (insecticide), PCBs and Mirex (pesticide). These substances are especially prevalent in the pulp and paper, aluminum and agricultural industries. Under the St. Lawrence Vision 2000 Action Plan, over 100 factories located along the St. Lawrence reduced their toxic effluent by 90% since 1988. Good news like this have positive consequences on the health of the St. Lawrence, but the war on pollution is far from won.
In the Fluvial Section of the St. Lawrence, a number of sites around Montréal and Île d’Orléans are now open for swimming. Fish in this sector rarely exceed the levels of mercury (and other contaminants) tolerated by Health Canada, and mercury and PCB levels in the sediment have reached the “minor effect” and “no effect” thresholds, respectively. However, other sectors of the St. Lawrence, especially in proximity to urban zones, remain heavily polluted, and limiting one’s consumption of fish caught from the river is still advised.
Significant improvements are also noted for the St. Lawrence Estuary and the Gulf of St. Lawrence, as well as for the Saguenay. Generally speaking, quantities of heavy metals (mercury, cadmium, lead, zinc, copper) have fallen significantly. Regulation efforts and the elimination of these metals from certain manufacturing processes has paid off. Today, after reaching worrisome levels in the 1970s, mercury levels measured in northern shrimp in the Saguenay are conform with Health Canada standards. Remaining mercury pollution is atmospheric in nature, and chiefly stems from coal burning in the Great Lakes region and the American Midwest. Also, PCB and DDT concentrations in Estuary and Gulf sediment have plummeted, as have PAHs in the Fjord. However, recent data for living organisms are lacking. In fact, although PAHs are metabolized by numerous species including mammals, they are not easily detectable. Fish products are showing contaminant levels within the limits established for commercialization. The population of northern gannets on Île Bonaventure, formerly threatened by high levels of DDT in their eggs, has today reached 50,000 pairs.
Even if the influx of toxins has come to a halt or has abated considerably, they are still present in the system, being deposited on the seabed and gradually integrated into the sediment. These toxins are regularly resuspended, however, by natural water exchange as well as dredging required for the maintenance and development of the St. Lawrence Seaway. TBT, a toxic antifouling paint now banned and taken off the market, is still found on the hulls of many commercial vessels. This product is highly toxic for a number of aquatic species including marine mammals. Organobromines are amongst the latest substances whose toxicity was discovered well after their wide-scale deployment in the environment. Used to improve the fire resistance of plastic materials, these products are found in a multitude of everyday products: computer and television housings, electrical and electronic components, fabrics, cushioning material, etc. Even if their effects on the environment are still poorly understood, it is already known that some of them are highly persistent and are increasing dramatically in living organisms: in St. Lawrence belugas, concentrations are increasing exponentially, doubling every three years. A regulation to restrict or even ban their use is currently under development.
Municipal sewerage and wastewater from boats also represent sources of pollution. According to a recently published study, marine mammals seem to be increasingly falling victim to diseases typically affecting domestic animals, a trend possibly related to these water transfers. On both shores of the river, the quality of water treatment plants varies widely and is at times rudimentary. All of these issues are complex, regulations are evolving and enforcing them takes time.
A system that suffocates
In addition to chemical pollution, the decrease of oxygen in the deep waters of the Estuary has also become a source of concern: oxygen levels in these waters seem to have fallen by 50% in less than 20 years. The phenomenon of eutrophication is best known in lakes and rivers, but does occur in numerous bays and small estuaries that are exposed to excessive organic contamination and especially to nutrients (nitrogen, phosphorous, etc.) from agriculture, intensive livestock production and urban sewage. This phenomenon results in a proliferation of plant matter, algae and organic waste that stimulates bacteria and ultimately leads to the asphyxiation of marine life. In the case of the Lower Estuary, excessive and rapid development of microalgae might result in oxygen depletion in deep waters, whose rate of renewal is quite slow. An excessive drop in oxygen at the bottom of the Estuary could directly lead to profound modifications in the benthic ecosystem, including reduced biodiversity, a shift to species more tolerant of low oxygen levels, and displacement to better aired sites of fish and crustaceans, which are more active than worms or echinoderms and are in constant need of well oxygenated water. Today the problem even extends as far as Sept-Îles in the Gulf of St. Lawrence.
A changing climate
Over the last few years, the global warming of air temperatures and the modification of ocean currents have had an impact on the water masses of the St. Lawrence. More specifically, changes have been observed in terms of the water masses’ salinity, temperature, fresh water inflow, ice cover, and stratification and circulation structures. Researchers are trying to understand how the species living in the St. Lawrence are coping. These climate changes might translate into significant transformations at the bottom of the food chain, which would then have ramifications all the way up to the largest predators, including whales.
Introduction of invasive species
Non-native aquatic species have been plaguing Canadian waters for centuries, but the rhythm of invasions observed today is unprecedented. Lacking natural predators, the more aggressive species spread rapidly. Modifying habitats, competing with or preying on indigenous species, and introducing pathogens, they can disturb the ecosystem to the point of making it inhospitable for native species. Up to 86 invasive species have been identified to date in the St. Lawrence, the majority of which were first introduced in the Great Lakes. Exotic invasive species of greatest concern in fresh water include the zebra mussel, round goby, Chinese mitten crab, tench, and the water chestnut. In the marine environment, some forty invasive species have been identified, including the European green crab and a green alga known as the oyster thief or dead man’s fingers. The majority of these invasive species originate in the ballast water of commercial vessels. Ever since Canada adopted in 1989 guidelines for the control of ballast water in ships travelling to the Great Lakes, the number of accounts of non-native aquatic species introduced into the Great Lakes – St. Lawrence watershed and attributable to shipping has fallen by half. The optional guidelines became mandatory in 2006, which should help further reduce the risks of introductions.
Added to chemical pollution and climate change are the adverse effects of other human activities such as maritime shipping (noise pollution, collision risks, dredging) and fishing (overfishing, destructive practices). Will the St. Lawrence and its inhabitants be capable of accommodating this human presence? We will have to learn to tread more lightly in this fragile environment in order to enjoy its bounty for years to come.