Temporal and spatial evolution of bottom-water hypoxia in the St Lawrence estuarine system
<p>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 <span class="inline-formula">∼</span> 125 <span class="inlin...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-02-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/20/839/2023/bg-20-839-2023.pdf |
| Summary: | <p>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
<span class="inline-formula">∼</span> 125 <span class="inline-formula">µ</span>mol L<span class="inline-formula"><sup>−1</sup></span> (38 % saturation) in the 1930s to
<span class="inline-formula">∼</span> 65 <span class="inline-formula">µ</span>mol L<span class="inline-formula"><sup>−1</sup></span> (21 % saturation) in 1984.
Minimum dissolved oxygen concentrations remained at hypoxic levels (<span class="inline-formula"><i><</i></span> 62.5 <span class="inline-formula">µ</span>M <span class="inline-formula">=</span> 2 mg L<span class="inline-formula"><sup>−1</sup></span> or 20 % saturation) between 1984 and 2019, but
in 2020, they suddenly decreased to <span class="inline-formula">∼</span> 35 <span class="inline-formula">µ</span>mol L<span class="inline-formula"><sup>−1</sup></span>.
Concurrently, bottom-water temperatures in the LSLE have increased
progressively from <span class="inline-formula">∼</span> 3 <span class="inline-formula"><sup>∘</sup></span>C in the 1930s to nearly
7 <span class="inline-formula"><sup>∘</sup></span>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 km<span class="inline-formula"><sup>2</sup></span>. In 2021, it reached 9400 km<span class="inline-formula"><sup>2</sup></span>,
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.</p> |
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| ISSN: | 1726-4170 1726-4189 |