Year after year, the strong trend is confirmed: the St. Lawrence is increasingly upset by the impacts of the climate crisis, according to what emerges from the situation report of the estuary and the gulf presented Wednesday by the scientists from Fisheries and Oceans Canada (DFO).
“Almost all the indicators are in the red,” summarizes Peter Galbraith, physical oceanography researcher for the federal ministry. “The St. Lawrence has never been so stressed in terms of environmental pressures. »
He and his colleagues also took stock of various issues revealing the health of this immense marine ecosystem: water temperature, the quantity of dissolved oxygen and acidity.
According to Mr. Galbraith, the surface waters of the estuary and Gulf of St. Lawrence experienced a real “heat wave” last year, with temperatures reaching a “record” level. compared to data collected over three decades. These were, for example, 4.8°C higher than average in September and 5°C higher in October in the estuary.
The volume of the cold intermediate layer, which lies beneath surface waters, reached its fourth lowest volume since 1985 last year, while deep waters, which experienced a “slight cooling”, held steady. at historically high temperatures.
The deep waters of the Gulf come from offshore. They travel for a few years in the hollow of the St. Lawrence, rise closer to the surface in the Tadoussac region, then flow downstream. Their temperature depends closely on the mixture of oceanic water masses that enter the mouth of the gulf. However, in recent years, the warm waters of the Gulf Stream, coming from the south, occupy an increasingly large fraction of this mixture, to the detriment of the cold waters of the Labrador Current, coming from the north.
Less oxygen
The ice cover, for its part, has followed a significant shrinking trend in recent years, despite certain fluctuations. He reached a total of 6 km3 during last winter, the record low cover since 1969. This reduction in ice cover could add an additional threat to the survival of belugas. The vast majority of this population moves east and to the Gulf in winter, where they seek out areas largely covered by ice. This could allow them to shelter from storms and strong winter waves, which can cause large energy costs.
In addition to experiencing significant global warming, the waters of the St. Lawrence are becoming depleted of oxygen and continue to acidify, indicated biologist Marjolaine Blais. “In the space of 50 years, oxygen concentrations have decreased by 70%” at a depth of 300 meters in the estuary. “This is a fairly alarming observation,” according to the scientist.
In the deep waters located in the Rimouski sector, for example, we find ourselves in a situation of “severe hypoxia”, with an oxygen concentration of around 10% since 2022.
Several species cannot live in such marine environments, such as Atlantic cod, halibut or northern shrimp. The oxygen dissolved in the waters of the St. Lawrence is in fact an essential element for all underwater life, from the smallest invertebrates to the largest fish, including all the species that serve as prey for marine mammals, including several endangered species.
Unpredictable future
The acidity of the waters has increased by 60% in 15 years in the estuary, data from Fisheries and Oceans Canada reveal. This inevitable phenomenon, caused in large part by the absorption by marine environments of CO2 emissions from the combustion of fossil fuels, constitutes an increasingly serious threat for the entire food chain.
The drop in oxygen concentration and the increase in acidity “are environmental pressures that add to increases in temperature,” underlined Ms. Blais.
However, it is difficult to predict the impacts of these upheavals on the biodiversity of the St. Lawrence, according to MPO experts. “There are significant changes with a trend that is not encouraging, but at the same time, with warming, there are species that are doing very well,” underlined researcher Hugues Benoit. This is the case for lobster, an important species for the fishing sector, but also for fish such as bluefin tuna.
Other species are expected to see their range shrink in the coming years. This is the case of the northern shrimp. The researchers’ findings are also unequivocal: “The warming of deep waters and the increase in predation by redfish appear to be important factors contributing to the decline of shrimp. These ecosystem conditions are not expected to improve in the short to medium term,” we read in the federal assessment of the situation.