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 & GEOTOPMcGill University & GEOTOP

Department of Earth & Planetary SciencesDepartment of Earth & Planetary Sciences

Adapted from Diaz and Rosenberg (2008) and Falkowski et al. (2011)

PROBLEMPROBLEM

Hypoxic Systems Around the World

Hypoxic system [O2] < 60 µmol/kg ~ 62.5 mmol/m3

Oxygen concentrations (µmol/kg) at 200 m depth

Near-bottom oxygen concentration in 2004-2005

Spatial distribution of dissolved oxygen concentration in the deep water

2524

2322

2120 19

18

17

16

Cabot

PROBLEMPROBLEM

Gilbert et al. (2007)

Hypoxia in the St. Lawrence System

Density and dissolved oxygen concentration measured in July 2010 along the central axis of the Laurentian Channel

Density (kg m-3)

Oxygen (mmol m-3)

Seaward distance from STN 25 (km)

Dep

th (

m)

Dep

th (

m)

permanent pycnocline

ubottom

usurface

Hypoxia in the St. Lawrence System

LSLE Gulf of St. Lawrence

PROBLEMPROBLEM

LSLE

Cabot

Gilbert et al. (2005)

In the LSLE At Cabot Strait

Hypoxic Waters

62.5 Severe hypoxia threshold

PROBLEMPROBLEM

Temporal changes in the source water properties

Density and dissolved oxygen concentration measured in July 2010 along the central axis of the Laurentian Channel

Density (kg m-3)

Oxygen (mmol m-3)

Seaward distance from STN 25 (km)

Dep

th (

m)

Dep

th (

m)

Hypoxia in the St. Lawrence System

LSLE Gulf of St. Lawrence

PROBLEMPROBLEM

Explain the vertical distribution of O2

Identify the driving mechanisms of hypoxia

PREVIOUS MODEL DESCRIPTIONPREVIOUS MODEL DESCRIPTION

Modified from Benoit et al. (2006)

A simple advective-diffusive conceptual model

NEW MODEL DESCRIPTIONNEW MODEL DESCRIPTION

1. A realistic bathymetry: generation of an upward vertical advection2. A pelagic sink

Improvements of the simple advective-diffusive conceptual model

MODEL OUTPUTSMODEL OUTPUTS

STN 25 Cabot

Observations

Respiration in sediments & water columnNew model

Previous model Flat bathymetry & Respiration in sediments only

Model Results & Observations

Adapted from Benoit et al. (2006)

Lefort et al. (accepted)

Climatology from 2002 to 2010

[O2 ] (m

mo

l m-3)

[O2 ] (m

mo

l m-3)

[O2 ] (m

mo

l m-3)

DiffV > AdvH

u1

u2

u3

u1 < u2 < u3

IDENTIFYING THE DRIVING IDENTIFYING THE DRIVING MECHANISMSMECHANISMS

The oxygen minimum: A physical explanation

DiffV > AdvH

DiffV < AdvH

New model (1/2)

Previous model

New model (2/2)

SUMMARY OF FINDINGSSUMMARY OF FINDINGS

• The oxygen minimum is mostly generated by physical processes as the oxygen tongue is induced by changes in bathymetry.

Which processes control the oxygen distribution in the deep waters and what are the causes of the persistent hypoxia

in the LSLE?

• 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.

CONCLUSIONSCONCLUSIONS

Organic Carbon Flux Bottom Water TemperatureIncrease

& source water [O2] Decrease

Respiration Rate

Persistent and Severe Hypoxia

++

+

Is the St. Lawrence ecosystem on borrowed time?

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.