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The depleting oxygen levels of the Saint Lawrence
The Saint Lawrence river is a large north-american river running approximately 3058 km from Lake Ontario to the Atlantic Ocean through the Gulf of Saint Lawrence. The river was first explored by the Norse in the 11th century, and by European mariners in the 15th century. Today, it is an important trade route from the Saint Lawrence Segway. The river is home to a wide array of fish and marine mammals, such as cod, salmon, halibut, and beluga whales, blue whales and humpback whales to name a few. However, for the past few decades, the oxygen levels of the river have been decreasing for multiple reasons, all of which threaten the wildlife it sustains.
The water of the Saint Lawrence River
The Saint Lawrence receives its water from two major currents: the Labrador current, a cold current in the North Atlantic Ocean which flows from the Arctic Ocean continuing south along the east coast of Nova Scotia (see blue), and the warm North Atlantic Central Water from the Gulf Stream (see red). The Labrador Current is a shallow water flow, cold and well oxygenated, while the North Atlantic Current flows in deeper waters, warm and poorly oxygenated.
In recent years, climate change has had significant effects on the world's currents and thus has affected the composition of the Saint Lawrence River. Less water is coming in from the Labrador current, and hence more is coming from the North Atlantic Current. This has had two major consequences: first of all, a decrease in oxygen levels was observed, due to a higher volume of less oxygenated water, secondly, the overall temperature of the river has increased by 1.65℃. This temperature change has accentuated the decreasing oxygen levels, as oxygen is less soluble in warm water than in cold water. Climate change has therefore had a small impact on the oxygen concentration of the Saint Lawrence river.
The extent of the problem
Climate change has not been the only factor affecting the oxygen levels on the river. Human based pollution has also had a major impact in the decrease, contributing to as much as ½ of the ecological causes of the problem. Municipal waste, fertilizers and manure runoffs have changed the water composition, adding nutrients to the water and thus allowing for an extreme bloom of plankton during the summer. However, when these incredible masses of plankton die, they fall to the river bed and consume the oxygen supply as they decompose. This phenomenon, as well as the current changes resulting from climate change have led to extremely poorly oxygenated areas, particularly in deep water. The hypoxiation of the Saint Lawrence started in the mid-1980s, and in just a few years, the oxygen concentration has decreased by more than half at depths greater than 250 meters. Researchers believe that at this rate, the deep waters of the Saint Lawrence will become anoxic, meaning they will no longer have any oxygen. This will have grave consequences on the wildlife of these areas.
The effects of hypoxia
So why is a low oxygen concentration a problem? Well fish need oxygen to survive, just like terrestrial animals. The major difference is that fish absorb the oxygen in the water through their gills, and as the oxygen concentration is much lower in water than in air, they need a very efficient absorbing mechanism. Most fish require at least a 30% saturation to survive in water. Some fish, like the cod, cannot tolerate living in low oxygen environments, meaning that a decrease in the oxygen concentration of its habitat will cause it to die. Areas in the Saint Lawrence have reached saturation levels below 15%. These “dead zones” are therefore unable to sustain any form of life. At this rate, major extinction of deep-water fish communities in the Saint Lawrence are expected. However, hypoxia does not only concern fish. Marine mammals, particularly the ones who feed on benthic fish, will have to change their feeding habits to make up for the lack of food in the Saint Lawrence. Thus, animals such as sperm whales, belugas, harbour porpoise and many more will be indirectly affected.
The increasing hypoxia in the Saint Lawrence river is a major ecological concern, as it drastically affects the biodiversity of the river. With the current rate of decrease, many fish communities are expected to go extinct in the region, leading to a change in predators' hunting grounds. How will that affect scuba divers around the Saint Lawrence? When you go diving, there won't be anything to see! Okay so that's a very minor problem when you consider the whole situation, but it's still worth mentioning.
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