Temporal and spatial evolution of bottom-water hypoxia in the St Lawrence estuarine system

[摘要] Persistent hypoxic bottom waters have developed in theLower St Lawrence Estuary (LSLE) and have impacted fish and benthic speciesdistributions. Minimum dissolved oxygen concentrations decreased from ∼  125  µ mol L −1 (38 % saturation) in the 1930s to ∼  65  µ mol L −1 (21 % saturation) in 1984.Minimum dissolved oxygen concentrations remained at hypoxic levels (  62.5  µ M  =  2 mg L −1 or 20 % saturation) between 1984 and 2019, butin 2020, they suddenly decreased to ∼  35  µ mol L −1 .Concurrently, bottom-water temperatures in the LSLE have increasedprogressively from ∼  3  ∘ C in the 1930s to nearly7  ∘ C in 2021. The main driver of deoxygenation and warming in thebottom waters of the Gulf of St Lawrence and St Lawrence Estuary is a change in thecirculation pattern in the western North Atlantic, more specifically adecrease in the relative contribution of younger, well-oxygenated and coldLabrador Current Waters to the waters of the Laurentian Channel, a deepvalley that extends from the continental shelf edge, through Cabot Strait,the gulf and to the head of the LSLE. Hence, the warmer, oxygen-depletedNorth Atlantic Central Waters carried by the Gulf Stream now make up nearly100 % of the waters entering the Laurentian Channel. The areal extent ofthe hypoxic zone in the LSLE has varied since 1993 when it was firstestimated at 1300 km 2 . In 2021, it reached 9400 km 2 ,extending well into the western Gulf of St Lawrence. Severely hypoxicwaters are now also found at the end of the two deep channels that branchout from the Laurentian Channel, namely, the Esquiman Channel and Anticosti Channel.

[学科分类] 大气科学