A seventy-two-year record of diminishing deep-water oxygen in the St. Lawrence estuary: The northwest Atlantic connection

Abstract

Oxygen concentrations in the bottom waters of the Lower St. Lawrence estuary (LSLE) decreased from 125 mol L Ϫ1 (37.7% saturation) in the 1930s to an average of 65 mol L Ϫ1 (20.7% saturation) for the 1984-2003 period. A concurrent 1.65ЊC warming of the bottom water from the 1930s to the 1980s suggests that changes in the relative proportions of cold, fresh, oxygen-rich Labrador Current Water (LCW) and warm, salty, oxygen-poor North Atlantic Central Water (NACW) in the water mass entering the Laurentian Channel probably played a role in the oxygen depletion. We estimate that about one half to two thirds of the oxygen loss in the bottom waters of the LSLE can be attributed to a decreased proportion of LCW. This leaves between one third and one half of the oxygen decrease to be explained by causes other than changes in water mass composition. An increase in the along-channel oxygen gradient from Cabot Strait to the LSLE over the past decades, combined with data from sediment cores, suggests that increased sediment oxygen demand may be partly responsible for the remainder of the oxygen decline. In July 2003, approximately 1,300 km 2 of seafloor in the LSLE was bathed in hypoxic water (Ͻ62.5 mol L Ϫ1).Severe hypoxia is a condition that occurs in the water column when oxygen (O 2 ) falls below the 2 mg L

Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

“…Downstream from Québec, tidal mixing of C, N, and P-rich fresh water with saltwater stimulates microbial production and induces the flocculation of terrigenous dissolved and colloidal organic matter. A significant fraction of the organic material brought from upstream eventually settles in the deep Laurentian channel (Annane et al 2015) where rising organic content of sediment and dropping O 2 concentrations have been reported (Gilbert et al 2005;Thibodeau et al 2006). In turn, mineralization of organic matter into CO 2 contributes to estuarine acidification (Mucci et al 2011).…”

“…Microbial degradation of organic matter generated by coastal eutrophication also amplifies acidification (Wallace et al 2014). As with many coastal waters of the world's ocean, the bottom layers of the St. Lawrence Estuary have shown declining oxygen concentrations (Gilbert et al 2005) and concurrent rising acidification (Mucci et al 2011).…”

Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

“…The warm and relatively fresh surface layer (0 to 30 m) overlies the cold intermediate layer or CIL (30-150 m deep; S P = 32.0 to 32.6) that is formed by advection of the GSL's wintertime surface mixed layer (Galbraith, 2006). Below the CIL, a warmer (2 to 6 • C) and saltier (S P = 33 to 35) bottom layer (> 150 m deep), originating from the mixing of western-central Atlantic and Labrador shelf waters that intrude at depth primarily through Cabot Strait, flows sluggishly landward (∼ 0.5 cm s −1 ; Bugden, 1988) toward the head region of the Laurentian Channel (Saucier et al, 2003;Gilbert et al, 2005).…”

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

“…Waters below the CIL temperature minimum, which lies roughly at a depth of 60-100 m, have high and almost constant nutrient concentrations [Steven, 1974]. The vertical exchanges between the nutrientrich layer and the surface layer are however generally low, because of the weak turbulent ventilation that is also responsible for hypoxic conditions in the deep LSLE [Gilbert et al, 2005;Bourgault et al, 2012].…”