The most recent data on the oxygen concentration in the deep waters of the St.Lawrence Estuary have shown “a precipitous and surprising fall”, learned The duty. The observation is such that it represents a direct threat to the marine life of this ecosystem increasingly weakened by the impacts of the climate crisis.
“Oxygen concentrations are 50% lower than they were less than three years ago. It’s a precipitous and surprising fall, because the speed at which it goes down is exceptional. This is unheard of since the 1930 data. The levels are located significantly lower than those of severe hypoxia, ”drops Professor Emeritus Alfonso Mucci, of the University’s Department of Earth and Planetary Sciences. McGill.
The latter took part in the research work carried out this year as part of three oceanographic expeditions on the research vessel Coriolis II between the months of August and October.
Specialists from the Rimouski Institute of Marine Sciences (ISMER), Concordia University and the Quebec Maritime Network also participated in this research.
Dissolved oxygen in the waters of the St. Lawrence is an essential element for all underwater life, from the smallest invertebrates to the largest of fish, including all the species that serve as prey for marine mammals, including several species. threatened.
However, the data collected in 2021 in the deep waters of the maritime estuary (which goes from Tadoussac to Anticosti), that is to say at depths of 250 meters and more, show the lowest concentration ever calculated: “less than 10 % ”Of what is found in surface water that is in exchange with the atmosphere. By comparison, from 2003 to 2019, the concentration was around 20%, a threshold that already represents severe hypoxia. This rate of 20% was also barely half of what was found in 1930.
A threat
When we drop below 10%, “several species cannot survive and if the concentrations continue to drop, the waters could become completely oxygen-deprived”, that is to say “anoxic”, specifies a summary of the results transmitted. to To have to. In this case, “all the macrofauna [poissons et faune benthique, tels les crustacés et mollusques] would disappear ”.
Worse, water so to speak devoid of this essential ingredient for maintaining marine ecosystems could even “become enriched in toxic heavy metals and dissolved sulphides”, underlines the document. “When the water becomes anoxic, bacteria use other oxidants to oxidize organic matter, namely iron oxides and manganese oxides. These oxides are very good absorbents for heavy metals, such as cadmium, and other toxic elements, such as arsenic. When these oxides dissolve, they will therefore release these elements into the water column, ”explains Alfonso Mucci.
Bacterial activity also risks producing sulphides, “which are very toxic to living things.” So all the wildlife will disappear. This is the extreme case, but nothing lives in these toxic waters, ”adds Mr. Mucci. If such a scenario were to occur, he is seriously worried about its consequences. “We could see impacts in the Saguenay – St. Lawrence Marine Park. I don’t think whales would be very attracted to sulphurous waters, because there would be no planktonic proliferation, which supports all the great marine fauna found in the Tadoussac region. “
It is in the Tadoussac region that the deep waters of the estuary, that is, those that could be loaded with toxic heavy metals and sulphides, rise to the surface. This area is located at the head of the Laurentian Channel, a deep submarine valley that carries over 1,400 kilometers of water from the North Atlantic that enters the Gulf of St. Lawrence through the Cabot Strait.
Climate crisis
A good part of the decline observed in the oxygen level in the depths of the estuary is also found on the side of this water supply of the Laurentian Channel, which has changed over the years, mainly under the influence of climatic upheavals. . “For 20 years, there has been a reduction in the contribution of the Labrador Current, ie cold and well oxygenated waters. They are replaced by warmer waters, which come from the northwest Atlantic Ocean, with less oxygen. And this oxygen is consumed as the bottom water passes upstream, to Tadoussac, ”explains Alfonso Mucci.
This phenomenon worries the professor of molecular ecotoxicology in coastal environments at ISMER, Émilien Pelletier. “It’s not going to get better. It’s going to continue, because it’s part of a larger movement of rising ocean water temperatures. We are therefore heading towards an increasingly important impoverishment in the estuary, which implies a change in the ecosystems. “
Same story with the researcher at Fisheries and Oceans Canada Denis Chabot, specialist in particular in the study of the impacts of variables such as temperature and dissolved oxygen on fish. “Hypoxia is never good news. As soon as we drop below the 30% oxygen concentration mark, we notice a drop in biodiversity which is accentuated when the rate decreases. There are also impacts such as the drop in the growth rate of certain species of fish, which leads to a drop in productivity and affects the performance of fisheries, ”he underlines. He cites as an example the case of cod, which cannot live in such waters. Other species such as shrimp or turbo could also suffer in the coming years.
It’s not going to get better. It’s going to continue, because it’s part of a larger movement of rising ocean water temperatures.
What is happening in the St. Lawrence is not unique, since other regions of the world experience, sometimes on a seasonal basis, a sharp drop in oxygen in the water, such as the Gulf of Mexico. In some cases, scientists even speak of “dead zones”. The case of the estuary could also be revealing of what awaits other important marine ecosystems, underlines Alfonso Mucci. “It’s a bit like the canary in the coal mine. It is also of great interest to Americans who study the Gulf of Maine, which is affected by the same phenomenon of changing ocean circulation. “