Advances in knowledge about the state of the St. Lawrence: results of acidification and hypoxia work
Ailing deep waters
The deep waters of the Lower St. Lawrence Estuary have undergone major changes in recent decades. Lower oxygen levels, also known as hypoxia, and higher acidity are a threat to marine species. Fisheries and Oceans Canada researchers explain the importance of having a better understanding of this phenomenon and its potential effects on the organisms that live in these waters.
Although acidification is not a recent global phenomenon, its magnitude in the St. Lawrence is disproportionate to that observed in the open ocean, according to Denis Gilbert and Michel Starr, researchers with the Maurice Lamontagne Institute (MLI) at Fisheries and Oceans Canada.
For the past 10 years, Fisheries and Oceans Canada has been studying ocean acidification in order to identify the most vulnerable marine areas and to gain a better understanding of the potential impact on marine organisms. "In the span of 75 years, the pH of the deep waters in the estuary has dropped by 0.2 to 0.3 units, stabilizing at around 7.6 to 7.7. By comparison, oceans have lost only 0.1 unit. To put this into perspective, a sharp decrease in the pH 55 million years ago led to a mass extinction of marine species," said MLI oceanographer Michel Starr, pointing out that the pH scale ranges from 0 (acidic) to 14 (basic) and that a pH of 7 is considered neutral.
Published in September 2011 in the journal Atmosphere-Ocean and hailed as one of the top 10 discoveries of the year by the magazine Québec Science, their discovery was no accident. Far from it. The two men worked with a team of scientists from McGill University and the Institut des sciences de la mer de Rimouski to paint this bleak pathological picture. Some would say this was a critical diagnosis.
Even though about 30% of the carbon dioxide (CO2) released into the air by humans was absorbed by the oceans over the course of two centuries, it is not enough to explain this accelerated acidification of the deep waters of the St. Lawrence Estuary. This phenomenon is intricately tied to the lack of oxygen (hypoxia) at depth.
When the current moves in...
Today, two culprits share the blame: bacterial respiration and ocean current circulation.
"At depths of 170 to 335 metres, bacteria break down available organic matter, which is more abundant near inhabited coastlines. During this breakdown process, they breathe, ingest oxygen (O2) and produce CO2. Depleted in oxygen, the water layer is gradually enriched with CO2. Once dissolved in water, this gas produces a reaction that releases hydrogen ions responsible for acidification," explains Michel Starr.
Aside from bacterial respiration, another explanation for the acidification and hypoxia of the deep waters of the St. Lawrence Estuary has to do with changes in the circulation of ocean waters entering the Laurentian Channel from the Northwest Atlantic.
"Warming of the deep waters of the St. Lawrence Estuary over the past 80 years suggests that the proportion of warm, oxygen-depleted salt water from the Gulf Stream has increased at the expense of water from the Labrador Current, which is colder, more oxygen-rich and less salty. We are proud of having demonstrated that deep-water hypoxia was caused by this phenomenon. Most studies had previously associated it with human activity," indicates Denis Gilbert, an ocean climate researcher with the MLI. Since then, a similar phenomenon has been discovered on Canada's West Coast.
In the Limnology and Oceanography article published in 2005, Denis Gilbert and his colleagues estimate that 50 to 67% of the oxygen lost in deep waters is attributable to this change in the mixing of waters. Bacterial respiration, which increases with higher water temperatures, accounts for the rest.
"When oxygen becomes scarce at depth, it is not unlike climbing a high mountain. It becomes difficult for you to breathe and move. It is no different for marine species," explains Denis Gilbert.
Cod, a commercial species in the St. Lawrence, restricts its diet and reduces its movements as soon as oxygen saturation falls below 70% (this percentage represents the amount of dissolved oxygen relative to the maximum oxygen concentration in sea water at a given temperature and salinity).
Citing the work of his MLI colleagues, Denis Gilbert indicates that "cod mortality occurs when the oxygen level is below 28%. And this level has already dropped to 20% in some areas of the St. Lawrence Estuary! To oxygenate properly, fish travel east where water is more oxygen-rich or swim to a shallower depth."
Over the past 10 years, the international scientific community has been working with Fisheries and Oceans Canada researchers to assess the impacts of acidification on marine life, and for good reason. In acidic waters, organisms such as shellfish struggle to form their shells or hard parts from calcium carbonate. Why? Because of lower carbonate due to the abundance of hydrogen ions in acidic waters.
These conditions slow the growth of these calcified species, a staple in the ocean food chain, and lower their chances of survival. What are the potential repercussions for fish and the fishing industry?" According to a 2007 U.S. study, water acidification could affect 73% of commercially harvested organisms either directly, for calcified species, or indirectly, for organisms that feed on calcified species. The economic impact could be considerable," says Michel Starr.
So could the impact on humans. According to the United Nations Environment Programme, more than 2.6 billion people rely on seafood as an essential part of their diets.
A model for others?
Michel Starr says that "the pH levels observed in the deep waters of the estuary will only be reached globally in the oceans after a hundred years. Here is an opportunity for us to observe the ecosystem and assess its ability to succeed or fail at adapting. In this sense, the estuary is a good model for studying the impacts of ocean acidification."
And so, the two Fisheries and Oceans Canada scientists continue their fieldwork, which primarily involves sampling campaigns aboard large vessels equipped with a lab. A CTD (Conductivity, Temperature, Depth) rosette—a round frame with 12 to 24 bottles attached—is immersed in various locations at sea to collect water from multiple depths in order to analyze the physicochemical properties of the water samples, including the oxygen levels.
Although Michel Starr takes on-site pH measurements, he also plays an active role in conducting experiments with his colleagues to assess the effects of acidification on commercial species (reproduction, viability of offspring, etc.) and to scrutinize a number of key processes at the base of the food chain in the St. Lawrence ecosystem. Over the course of this multi-year study, the researchers simulate aquatic conditions in artificial pools where pH is controlled.
Other teams of scientists are studying the evolution of marine life in acidic waters. In this instance, however, they are sampling volcanic wells (where CO2 is released naturally). Their preliminary findings are clear: close to one third of species are missing from more acidic areas.
Will marine organisms adapt to the rapid, unprecedented acidification of their living environment? Will they be strong enough? The results of experiments conducted by Michel Starr and his colleagues should answer those questions.
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