Modeling of dissolved oxygen levels in the bottom waters of the Lower St. Lawrence Estuary: Coupling of benthic and pelagic processes

Abstract

Recent measurements of dissolved oxygen (DO) along the Laurentian Channel in Eastern Canada revealed the presence of hypoxic waters in the bottom 50 m of the water column. At hypoxic oxygen levels, many fish species cannot survive or reproduce, and the microbial life community undergoes significant modifications. The cumulative effect of a substantial sediment oxygen demand along the Lower St. Lawrence Estuary (LSLE) is proposed as the possible cause of this DO depletion. To verify this hypothesis, a laterally integrated, two-dimensional model of the DO distribution was implemented for the bottom waters of the Laurentian Channel along a transect of stations sampled in July 2003. The fluid transport was parameterized in a simple advection–diffusion finite element grid where sedimentation of organic matter (OM) feeds the processes that lead to O 2 depletion in the deep waters. Two major types of OM are considered in this study: a fast-reacting marine component that originates from autochthonous material produced in surface waters, and a more refractory terrestrial component originating from continental river discharges. To counterbalance the OM oxygen sink, the deep, landward mean circulation continuously brings O 2-rich waters from the Atlantic. Both the DO and the early diagenesis model parameters were calibrated using field data collected between 1985 and 2003. Our physical parameter sensitivity study reveals that vertical diffusion from the oxygenated upper water column has the greatest impact on deep DO concentrations. The diagenetic model reproduces the oxygen penetration depths and fluxes very well along the Gulf of St. Lawrence portion of the Channel but overestimates the sediment oxygen demand in the LSLE. We propose that the sediment oxygen demands calculated from DO gradients, measured by voltammetric micro-electrodes, across the sediment–water interface of cores retrieved in the Lower St. Lawrence Estuary are underestimated.