Aerobic Respiration and Hypoxia in the Lower St. Lawrence Estuary: Stable Isotope Ratios of Dissolved Oxygen Constrain Oxygen Sink Partitioning

We measured the concentration and the stable isotope ratios of dissolved oxygen in the water column in the Estuary and Gulf of St. Lawrence to determine the relative importance of pelagic and benthic dissolved oxygen respiration to the development of hypoxic deep waters. The progressive landward decrease of dissolved oxygen in the bottom waters along the axis of the Laurentian Channel (LC) is accompanied by an increase in the ¹⁸O: ¹⁶O ratio, as would be expected from O-isotope fractionation associated with bacterial oxygen respiration. The apparent O-isotope effect, $\varepsilon _{\text{O}-\text{app}}$ , of 10.8‰ reveals that community O-isotope fractionation is significantly smaller than if bacterial respiration occurred solely in the water column. Our observation can best be explained by a contribution of benthic O₂ consumption occurring with a strongly reduced O-isotope effect at the scale of sediment-water exchange ( $\varepsilon _{\text{O}-\text{sed}}$ ∼ 7‰). The value for $\varepsilon _{\text{O}-\text{sed}}$ was estimated from benthic O₂ exchange simulations using a one-dimensional diffusion-reaction O-isotope model. Adopting this $\varepsilon _{\text{O}-\text{sed}}$ value, and given the observed community O-isotope fractionation, we calculate that approximately two thirds of the ecosystem respiration occurs within the sediment, in reasonable agreement with direct respiration measurements. Based on the difference between dissolved oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary and in the water that enters the LC at Cabot Strait, we estimate an average respiration rate of 5500 mmol O₂ m⁻² yr⁻¹ for the 100-m—thick layer of bottom water along the LC, 3540 mmol O₂ m⁻² yr⁻¹ of which is attributed to bacterial benthic respiration.