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

Abstract

Persistent hypoxic bottom waters have developed in the Lower St Lawrence Estuary (LSLE) and have impacted fish and benthic species distributions. Minimum dissolved oxygen concentrations decreased from ∼ 125 µmol L−1 (38 % saturation) in the 1930s to ∼ 65 µmol L−1 (21 % saturation) in 1984. Minimum dissolved oxygen concentrations remained at hypoxic levels (< 62.5 µM = 2 mg L−1 or 20 % saturation) between 1984 and 2019, but in 2020, they suddenly decreased to ∼ 35 µmol L−1. Concurrently, bottom-water temperatures in the LSLE have increased progressively from ∼ 3 ∘C in the 1930s to nearly 7 ∘C in 2021. The main driver of deoxygenation and warming in the bottom waters of the Gulf of St Lawrence and St Lawrence Estuary is a change in the circulation pattern in the western North Atlantic, more specifically a decrease in the relative contribution of younger, well-oxygenated and cold Labrador Current Waters to the waters of the Laurentian Channel, a deep valley that extends from the continental shelf edge, through Cabot Strait, the gulf and to the head of the LSLE. Hence, the warmer, oxygen-depleted North Atlantic Central Waters carried by the Gulf Stream now make up nearly 100 % of the waters entering the Laurentian Channel. The areal extent of the hypoxic zone in the LSLE has varied since 1993 when it was first estimated at 1300 km2. In 2021, it reached 9400 km2, extending well into the western Gulf of St Lawrence. Severely hypoxic waters are now also found at the end of the two deep channels that branch out from the Laurentian Channel, namely, the Esquiman Channel and Anticosti Channel.