Temporal Changes in the Causes of the Observed Oxygen Decline in the St. Lawrence Estuary

Abstract

AbstractOxygen 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.