Hypoxia in the bottom water of the
St. Lawrence Estuary:
Is this ecosystem on borrowed time?
Stelly Lefort, Y. Gratton, A. Mucci, I. Dadou & D. Gilbert
McGill University & GEOTOP
Department of Earth & Planetary Sciences
PROBLEM
Hypoxic Systems Around the World
Oxygen concentrations (µmol/kg) at 200 m depth
Hypoxic system [O2] < 60 µmol/kg ~ 62.5 mmol/m3
Adapted from Diaz and Rosenberg (2008) and Falkowski et al. (2011)
PROBLEM
Hypoxia in the St. Lawrence System
Spatial distribution of dissolved oxygen concentration in the deep water
Near-bottom oxygen concentration in 2004-2005
20
21
23
24
25
22
19
18
17
16
Cabot
Gilbert et al. (2007)
PROBLEM
Hypoxia in the St. Lawrence System
Density and dissolved oxygen concentration measured in July 2010
along the central axis of the Laurentian Channel
Gulf of St. Lawrence
Depth (m)
LSLE
Density (kg m-3)
usurface
Depth (m)
permanent pycnocline
ubottom
Oxygen (mmol m-3)
Seaward distance from STN 25 (km)
PROBLEM
Temporal changes in the source water properties
Cabot
In the LSLE
At Cabot Strait
Hypoxic
Waters
62.5 Severe hypoxia threshold
Gilbert et al. (2005)
PROBLEM
Hypoxia in the St. Lawrence System
Density and dissolved oxygen concentration measured in July 2010
along the central axis of the Laurentian Channel
Gulf of St. Lawrence
Depth (m)
LSLE
Depth (m)
Density (kg m-3)
Oxygen (mmol
m-3) the vertical distribution of O
Explain
2
Identify
the driving
Seaward
distance mechanisms
from STN 25 (km)of hypoxia
PREVIOUS MODEL DESCRIPTION
A simple advective-diffusive conceptual model
Modified from Benoit et al. (2006)
NEW MODEL DESCRIPTION
Improvements of the simple advective-diffusive conceptual model
1. A realistic bathymetry: generation of an upward vertical advection
2. A pelagic sink
MODEL OUTPUTS
Model Results & Observations
STN 25
Adapted from Benoit et al. (2006)
Flat bathymetry & Respiration in sediments only
[O2] (mmol m-3)
Previous model
Cabot
Lefort et al. (accepted)
Respiration in sediments & water column
[O2] (mmol m-3)
New model
Climatology from 2002 to 2010
[O2] (mmol m-3)
Observations
IDENTIFYING THE DRIVING MECHANISMS
The oxygen minimum: A physical explanation
DiffV > AdvH
Previous model
DiffV > AdvH
DiffV < AdvH
New model (1/2)
u3
New model (2/2)
u2
u1
u1 < u2 < u3
SUMMARY OF FINDINGS
Which processes control the oxygen distribution in the deep
waters and what are the causes of the persistent hypoxia
in the LSLE?
• The oxygen minimum is mostly generated by physical processes
as the oxygen tongue is induced by changes in bathymetry.
• Hypoxia in the LSLE results from a combination of physical and
biogeochemical processes, but oxygen concentration is much more
sensitive to variations in physical than biogeochemical processes.
• Water properties at the continental shelf edge in the North Atlantic
Ocean are mostly responsible for the establishment of hypoxia in the
LSLE.
CONCLUSIONS
Is the St. Lawrence ecosystem on borrowed time?
Organic Carbon Flux
Bottom Water Temperature &
Increase
source water [O2]
Decrease
+
Respiration Rate
+
+
Persistent and Severe Hypoxia
Lefort S., Y. Gratton, A. Mucci, I. Dadou and D. Gilbert (accepted) Hypoxia in the
Lower St. Lawrence Estuary: How physics controls spatial patterns, JGR-Oceans.