We found a match
Your institution may have access to this item. Find your institution then sign in to continue.
- Title
Temporal Changes in the Causes of the Observed Oxygen Decline in the St. Lawrence Estuary.
- Authors
Jutras, M.; Dufour, C. O.; Mucci, A.; Cyr, F.; Gilbert, D.
- Abstract
Oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary have decreased by 50% over the past century. The drivers of this decrease are investigated by applying an extended Optimum Multiparameter analysis to a time series of physical and biogeochemical observations of the St. Lawrence Estuarine System in the 1970s and from late 1990s to 2018. This method reconstructs the relative contributions of the two major water masses feeding the system, the Labrador Current Waters (LCW) and the North Atlantic Central Waters (NACW), as well as oxygen utilization, and accounts for diapycnal mixing. The causes of the oxygen decline varied over the last 5 decades. Between the 1970s and late 1990s, the decrease was mainly driven by biogeochemical changes through an increase in microbial oxygen utilization in the St. Lawrence Estuary in response to warmer temperatures and eutrophication and lower oxygen concentrations in LCW and NACW. Between 2008 and 2018, the decrease was mainly driven by circulation changes in the western North Atlantic associated with a reduced inflow of high‐oxygenated LCW to the deep waters of the system in favor of low‐oxygenated NACW, reaching a historical minimum in 2016. The LCW:NACW ratio is strongly correlated with the volume transport of 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 and on the western North Atlantic continental shelf and slope. Plain Language Summary: Over the past century, the oxygen concentrations of the deep waters of the Lower St. Lawrence Estuary have declined dramatically, with dire consequences to marine life and the fishing industry. Two processes are thought to be responsible for this decline. First, nutrient and organic matter discharge from urban and agricultural activities in its drainage basin promote algae blooms, which are decomposed by bacteria that consume oxygen in the deep waters, a mechanism called eutrophication. Second, the oxygen‐rich Labrador waters feeding the deep waters of the estuary have been progressively replaced by oxygen‐poor Gulf Stream waters. Using observations and a water mass reconstruction method, we determined to what extent each of the above processes contributed to the oxygen decline over the last 5 decades. Between the 1970s and early 1990s, the oxygen decline was mostly due to biogeochemical changes (including eutrophication). From 2008 to 2018, the decline was mostly due to a reduction in the amount of Labrador waters feeding the system. Understanding how circulation in the Northwest Atlantic Ocean influences oxygen concentrations along the coast is crucial, given that circulation patterns vary naturally with time and could be affected by climate change. Key Points: The causes of the observed decrease in oxygen concentrations at the head of the Lower St. Lawrence Estuary varied over the last 50 yearsThe oxygen decline is driven by biogeochemical changes over 1970–1995, and by circulation changes in the North Atlantic over 2008–2018Inflow of oxygen‐rich Labrador Current Waters to the Estuary correlates with the strength (or transport) of the Scotian Shelf break current
- Subjects
SAINT Lawrence River Estuary (Quebec); QUEBEC (Province); NORTH Atlantic Ocean; ESTUARIES; OXYGEN in water; OCEAN circulation; SCOTIAN Shelf; CONTINENTAL shelf; ESTUARINE eutrophication
- Publication
Journal of Geophysical Research. Oceans, 2020, Vol 125, Issue 12, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016577