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

Recent measurements of dissolved oxygen (DO) along the Laurentian Trough revealed the presence of hypoxic waters in the bottom 50 meters of the water column of the Lower St. Lawrence Estuary (LSLE). In addition to the change in the oceanic regime on the continental shelf at the mouth of the Gulf of St. Lawrence proposed by others, a large sediment oxygen demand along the LSLE though to contribute to this DO depletion. To verify the latter hypothesis, I developed a laterally integrated, two-dimensional model of the DO distribution for the bottom waters of the Laurentian Trough. The fluid transport is parameterized in a simple advection-diffusion finite-element grid where the sedimentation of organic matter feeds the mineralization processes that lead to O2 depletion in the deep waters. Using realistic parameters obtained from field data, the diagenetic model reproduces the measured sediment oxygen demands (SODs) along the Gulf of St. Lawrence portion of the trough but overestimates them in the lower estuary. Since our modeled estuary DO levels are comparable to the measured DO values when a large SOD is applied, we suggest that the oxygen fluxes, calculated from the DO gradients measured with micro-electrodes across the sediment-water interface of cores recovered in the LSLE, are underestimated.