Hypoxia in the bottom water of the St. Lawrence Estuary: Is this ecosystem on borrowed time?

1. Hypoxia in the bottom water of the St. Lawrence Estuary: Is this ecosystem on borrowed time? StellyLefort, Y. Gratton, A. Mucci, I. Dadou & D. Gilbert McGill University & GEOTOP Department of Earth & Planetary Sciences

2. 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)

3. Problem Hypoxia in the St. Lawrence System Spatial distribution of dissolved oxygen concentration in the deep water Near-bottom oxygen concentration in 2004-2005 Cabot 21 23 16 22 19 24 20 17 18 25 Gilbert et al. (2007)

4. Problem Hypoxia in the St. Lawrence System Density and dissolved oxygen concentration measured in July 2010 along the central axis of the Laurentian Channel LSLE Gulf of St. Lawrence Depth (m) Density (kg m-3) usurface permanent pycnocline Depth (m) ubottom Oxygen (mmol m-3) Seaward distance from STN 25 (km)

5. Problem Temporal changes in the source water properties LSLE Cabot In the LSLE At Cabot Strait Hypoxic Waters Severe hypoxia threshold 62.5 Gilbert et al. (2005)

6. Problem Hypoxia in the St. Lawrence System Density and dissolved oxygen concentration measured in July 2010 along the central axis of the Laurentian Channel LSLE Gulf of St. Lawrence Depth (m) Density (kg m-3) ? Depth (m) Oxygen (mmol m-3) • Explain the vertical distribution of O2 • Identify the driving mechanisms of hypoxia Seaward distance from STN 25 (km)

7. Previous model description A simple advective-diffusive conceptual model Modified from Benoit et al. (2006)

8. New model description Improvements of the simple advective-diffusive conceptual model • A realistic bathymetry: generation of an upward vertical advection • A pelagic sink

9. Model outputs Model Results & Observations STN 25 Cabot Adapted from Benoit et al. (2006) [O2] (mmol m-3) Previous model Flat bathymetry & Respiration in sediments only Lefort et al. (accepted) [O2] (mmol m-3) New model Respiration in sediments & water column Climatology from 2002 to 2010 [O2] (mmol m-3) Observations

10. Identifying the driving mechanisms The oxygen minimum: A physical explanation DiffV > AdvH Previous model DiffV > AdvH DiffV < AdvH New model (1/2) u3 u2 u1 u1 < u2 < u3 New model (2/2)

11. 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.

12. 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.