How waters feeding the eastern American continental slope affect oxygen concentrations in the St. Lawrence Estuary

Abstract:

Oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary have decreased by 50% over the past century, reaching hypoxic levels. Part of this decrease is due to biogeochemical changes such as eutrophication, and part is due to a reduced proportion, in the slope waters feeding the deep waters of the estuary, of high-oxygenated Labrador Current Waters (LCW) in favor of low-oxygenated North Atlantic Central Waters (NACW). Previous estimates of the LWC:NACW ratio in the deep waters of the estuary were derived from T-S diagrams, but this method does not take into account the diapycnal mixing that is known to occur on continental slopes. Here, we use a mixing model (an extended multi-parameter analysis - eOMP) to reconstruct the relative contributions of LCW and NACW to the deep waters of the system, based on time series of physical and biogochemical observations made in the St. Lawrence Estuarine System in the 1970s and from the late 1990s to 2018. We find that the causes of the oxygen decrease varied over the last 5 decades. From the 1970s to the late 1990s, the oxygen decrease was mainly driven by biogeochemical changes through an increase in microbial oxygen utilization in the St. Lawrence Estuary and to lower oxygen concentrations in LCW and NACW. Since 2008, the oxygen decrease has been mainly driven by changes in the circulation patterns in the western North Atlantic that resulted in a reduced relative contribution of LCW to the slope waters, reaching a historical minimum in 2016. We find that the variability in the LCW:NACW ratio is mainly controlled by the Scotian shelf-break current, an extension of the Labrador Current. These results highlight the primary role of the Labrador Current in determining the oxygen concentration and other water properties of the St. Lawrence Estuarine System as well as on the western North Atlantic continental shelf and slope, and the need to better understand how the LCW and NACW mix to form the slope waters.