Physical and biogeochemical drivers of deoxygenation in the Gulf and Lower St. Lawrence Estuary

Minimum dissolved oxygen concentrations in the bottom waters of the Lower St. Lawrence Estuary have decreased by 75% over the past century, reaching 34 µmol/kg in 2021, well below the threshold of severe hypoxia (∼62 µmol/kg). At the same time, the areal extent of the hypoxic zone has increased by more than 7-fold since the 1990s. These changes have dire consequences on marine fauna, including on economically important species such ascod, halibut and shrimp. In this thesis, in addition to describing the most recent decline, I investigate the main causes of the deoxygenation of the bottom waters of the Gulf and Lower St. Lawrence Estuary over the last five decades and examine their underlying biogeochemical and physical mechanisms.Using observations collected between the 1970s and 2018, I find that the causes of deoxygenation of the bottom-waters of the Gulf and Lower St. Lawrence Estuary varied over time. From the 1970s to the late 1990s, a local increase in oxygen consumption rates due to human-induced eutrophication dominated the oxygen decline. A box model of summer nutrient cycling in the Lower St. Lawrence Estuary confirms that eutrophication is driven bya stronger nitrogen discharge from the tributaries to the system. The effect of this discharge is damped by the upwelling of nutrient-rich waters at the head of the Lower St. Lawrence Estuary. From 2000 to 2018, changes in the ocean circulation patterns in the western North Atlantic Ocean dominated the oxygen decline, resulting in an increased contribution from the warm, oxygen-poor Gulf Stream Waters to the deep waters of the Gulf of St. Lawrence,at the expense of the previously dominant cold, oxygen-rich Labrador Current Waters. In 2020, the system even reached a symbolic level, with little to no Labrador Current Waters entering the system. These changes in circulation result from an increased eastward retroflection of the Labrador Current at the tip of the Grand Banks of Newfoundland. Using Lagrangian virtual particle tracking experiments performed with the outputs of an ocean reanalysis, I show that the retroflection is stronger when large-scale circulation adjustments in the western North Atlantic Ocean accelerate the western subpolar gyre and the Labrador Current as well as shift the Gulf Stream northward, partly driven by a northward shift of the zero-wind-stress-curl line in the western North Atlantic Ocean. In summary, the causes of deoxygenation in the bottom waters of the Gulf and Lower St. Lawrence Estuary are both local and remote, as they relate to both riverine nutrient and organic matter discharge as well as large-scale oceanic and atmospheric variability

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