20 Oct 2022
20 Oct 2022
Status: this preprint is open for discussion.

Temporal and spatial evolution of bottom-water hypoxia in the Estuary and Gulf of St. Lawrence

Mathilde Jutras1,4, Alfonso Mucci1,5, Gwenaëlle Chaillou2,4, William A. Nesbitt3, and Douglas W. R. Wallace3 Mathilde Jutras et al.
  • 1Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC, H3A OE8, Canada
  • 2Institut des Sciences de la Mer de Rimouski (ISMER) - Université du Québec à Rimouski, 300 Allée des Ursulines, Rimouski, QC, G5L 3A1, Canada
  • 3Department of Oceanography, Dalhousie University, Steele Ocean Sciences Building, 1355 Oxford St., PO Box 15000, Halifax, NS, B3H 4R2, Canada
  • 4Québec-Océan

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. 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 1930’s to nearly 7 °C in 2021. The main driver of deoxygenation and warming in the bottom waters of Gulf 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 9700 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 and Anticosti Channels.

Mathilde Jutras et al.

Status: open (until 04 Dec 2022)

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  • RC1: 'Comment on egusphere-2022-1090', Anonymous Referee #1, 10 Nov 2022 reply

Mathilde Jutras et al.

Mathilde Jutras et al.


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Short summary
The deep waters of the Lower St. Lawrence Estuary and Gulf have, in the last decades, experienced a strong decline in their oxygen concentration. Below 65 µmol L⁻¹, the waters are said to be hypoxic, with dire consequences on marine life. We show that the extent of the hypoxic zone increased by seven-fold in the last 20 years, reaching 9400 km2 in 2021. After a stable period at ~65 µmol L⁻¹ from 1984 to 2019, the oxygen level also suddenly decreased to ~35 μmol L⁻¹ in 2020.