The Lower St. Lawrence Estuary is running out of steam
The deoxygenation of coastal waters has been a phenomenon observed on a global scale since the middle of the 20th century. However, such a significant and rapid change has never been observed for an environmental variable of such great importance in coastal and estuarine marine ecosystems. Alfonso Mucci, Gwénaëlle Chaillou, Mathilde Jutras explain to us why this phenomenon takes place.
ANALYSIS – On the surface, nothing is apparent… but, at depth, the waters of the St. Lawrence run out of steam! The lack of oxygen is increasing and the de-oxygenation is irremediable.
Our research covers a range of projects in marine geochemistry, including the causes and consequences of hypoxia (low oxygen) in the deep waters of the Estuary and Gulf of St. Lawrence.
The Colossal St. Lawrence
< p>The St. Lawrence is the largest estuary (a coastal body of water where fresh and salt water mix and empties into an ocean) on Earth. It connects the Great Lakes to the Atlantic Ocean, draining nearly 25% of the world's freshwater reserves. Its average flow in Quebec is about 12 000 m³ per second, the second highest flow of fresh water on the North American continent after that of the Mississippi.
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Its estuary begins at the eastern tip of Île d'Orléans, east of Quebec City. This is where we find the first traces of salt water. It then extends over 400 km to Pointe-des-Monts, where it widens and becomes the Gulf of St. Lawrence, an inland sea that is connected to the Atlantic Ocean by the Cabot and Belle straits. -Isle.
The estuary is separated into two segments. On the one hand, the fluvial estuary, which extends from Île d'Orléans to Tadoussac, near the mouth of the Saguenay Fjord. It is relatively narrow (2 to 24 km) and shallow, generally less than 30 meters. There is a strong horizontal salinity gradient (gradual increase in the amount of dissolved salt in the water) and the water column is well mixed or weakly stratified. A water column is stratified when water masses of different densities (determined by temperature and salt content) are superimposed. These masses of water cannot easily mix without a significant input of energy.
On the other hand, the maritime estuary of the St. Lawrence, which extends from Tadoussac to Pointe-des-Monts, is much wider (30 to 50 km) and deeper, since at its center, a 1 240 km long underwater valley plunges more than 300 m deep; this is the Laurentian Channel.
And it is in the depths of this channel that oxygen is lacking.
The system and the estuary of the St. Lawrence. (Alfonso Mucci), Provided by the author
Three layers of water
To properly understand the causes of de-oxygenation, it should be noted that the water column in the Lower Estuary and the Gulf is made up of three layers: a surface layer, which is not very dense, relatively warm and brackish (located between 0 at 30 m) which flows towards the Atlantic; a cold intermediate layer, located between 30-150 m depth with a salinity of approximately 32 g/kg (the water of a lake or a river typically contains less than 0.01 g/kg of dissolved salts while seawater contains nearly 35 g/kg), which forms in the winter in the Gulf and goes up the estuary.
Finally, below 150 m, there is dense deep water, warmer (between 2° to 7°C) and salty (salinity between 33 to 35 g/kg). They form on the continental slope near Cape Breton, Nova Scotia, by a mixture of cold, well-oxygenated waters from the Labrador Current and warmer, less oxygenated waters from the Mid-Western Atlantic Current. /p>
These dense waters line the bottom of the Laurentian Channel and slowly migrate towards Tadoussac, with a limited amount of dissolved oxygen. This quantity decreases during its transit since bacteria, mainly, gradually consume oxygen.
Stratification of the different water masses in the Estuary, the Gulf of St. Lawrence and the Laurentian Channel. (Alfonso Mucci), Supplied by the author
History of deep-sea de-oxygenation
In 2003, a first series of measurements revealed the presence of low concentrations of dissolved oxygen in the deep waters of the lower estuary, near Rimouski. The concentrations were below the threshold value for severe hypoxia (62.5 µmol/L or 20 % saturation, or 20 % of what the dissolved oxygen concentration should be if the water was in equilibrium with the atmosphere). Below this threshold, several species of fish, such as cod, cannot survive for long.
In addition, the structure and activity of benthic communities, such as molluscs and shrimp, which live deep near the bottom, are greatly modified. Concentrations of 60 µmol/L (i.e. 18 % saturation) lined the bottom of the channel and this hypoxic zone extended over approximately 1 300 km2, from Tadoussac to Pointe-des-Mont.
< p>A compilation of historical data and data acquired between 2003 and 2021 reveals that dissolved oxygen concentrations in the deep waters of the estuary have decreased considerably over the last century, from around 135 µmol/L in 1934 to 60 µmol /L between 1985 and 2010. In 2021, however, the measured concentrations drop to 35 µmol/L, which is 2 times lower than the threshold for severe hypoxia, and almost 2 times lower than just two years earlier.< /p>
Moreover, the area bathed by these low concentrations of oxygen now extends to the Gulf, tripling the area of the hypoxic zone in just 20 years!
Minimum dissolved oxygen concentrations in the Lower St. Lawrence Estuary at depths greater than 250 m. These data are recorded at a station near Rimouski. (Alfonso Mucci), Provided by the author
What are the causes?
While dissolved oxygen concentrations are decreasing, deep water body temperatures are increasing dramatically, from 3°C to over 6°C over the past century. These historical changes in temperature and oxygen concentration are supported by micropaleontological (counting and identification of microorganism fossils) and geochemical analyzes of sediments collected from the bottom of the Laurentian Channel. And this trend extends well beyond the 20th century.
An analysis of the chemical and physical variables of the deep waters also reveals a change in the relative proportions of the waters coming from the Labrador and Gulf Stream currents. feeding the Laurentian Channel. The proportion of warm, less oxygenated waters from the Gulf Stream is increasing, at the expense of colder, more oxygenated waters from the Labrador Current.
Therefore, a lower contribution from the Labrador Current results in warmer and less oxygen-rich waters. While in 1930, 72% of deep water came from the Labrador Current, this proportion has fallen to less than 20%.
Distribution of dissolved oxygen concentrations (in µmol/L and in % oxygen saturation) at the end of August 2021, from the river estuary upstream of Tadoussac, to the entrance to the Gulf. The map shows the location of the various stations visited during the oceanographic expedition. (Gwénaëlle Chaillou), Supplied by the author
A global phenomenon
The deoxygenation of coastal waters has been observed on a global scale since the middle of the 20th century. In 1995, about 200 sites were listed. By 2008, that number had more than doubled!
Such a large and rapid change has never been observed for an environmental variable of such great importance in coastal and estuarine marine ecosystems. In most cases, however, the lack of oxygen is temporary. When the water column is shallow and seasonally stratified, as in the Gulf of Mexico, episodes of hypoxia, or even anoxia (total absence of dissolved oxygen), do not persist throughout the year.< /p>
The dissolved oxygen is renewed there during episodes of autumn or winter ventilation. These vents occur when the water column is cooled by the atmosphere and the surface water density becomes high enough to cause complete mixing of the water column, bringing oxygen to the bottom.
However, in the St. Lawrence Estuary, hypoxia is persistent because the water column is deep and highly stratified all year round. The deep waters that migrate from the Atlantic to Tadoussac are isolated from the atmosphere for 4 to 7 years and the dissolved oxygen is gradually consumed there throughout the transit.
It remains to be seen whether the warming of Atlantic waters will persist, threatening even more the ecosystems of the St. Lawrence.
We would like to thank the contribution of Joannie Cool who recently completed a master's degree on the dynamics of oxygen in the sediments of the St. Lawrence estuary and who ensured the acquisition of conclusive data for a few years.
Alfonso Mucci, Emeritus Professor of Geochemistry and Oceanography, McGill University; Gwénaëlle Chaillou, professor of marine chemistry at the Institut des sciences de la mer de Rimouski (ISMER-UQAR), University of Quebec at Rimouski (UQAR), and Mathilde Jutras, PhD candidate, physical and biogeochemical oceanography, McGill University
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