Summer Transports of Nutrients in the Gulf of St. Lawrence Estimated by Inverse Modelling

Abstract

In this paper, we use an inverse methodology to parameterize box models of the Estuary and Gulf of St. Lawrence (GSL) to study carbon, oxygen and nutrient exchanges and transports during the summer period. Three models were developed and compared: a purely physical model A1 and two models including biogeochemical processes (new production, particle sedimentation, remineralization, and burial): a ‘high-flux’ model A2 and a ‘low-flux’ model A3, named in reference to shallow particle flux measurements. Even though the best fits for oxygen, phosphate, and nitrate mass balances were obtained with the low-flux model A3, our results show that a model based only on advection and diffusion fluxes gives a good representation of all the nutrient patterns in summer. However, the use of nutrient and other biogeochemical processes improved the water, heat, and salt balances relative to using only physical data. Moreover, the use of biogeochemical processes allows model A3 more flexibility to reproduce oxygen and nutrient patterns in the deep layer. Without the biogeochemical processes, the deep circulation along the Laurentian Channel and the upward fluxes into the upper layers were reduced below expected values due to the incapacity of the model to balance deep inflows of nutrients with surface outflows through Cabot Strait. The low-flux model A3 estimated higher velocities in the deep layer since the deep nutrient inflows could then be balanced by biological removal in the upper layers. The resulting flow pattern indicated that the St. Lawrence system can be characterized by two main regions: the western region (Estuary, Gaspé Current and Northwestern Gulf), where high biological production is associated with a cyclonic and upwelling circulation, and the eastern region (Laurentian Channel Central, Jacques Cartier Passage, Esquiman Channel and Cabot Strait), where low production is associated with an anticyclonic and downwelling circulation. The Southern Gulf or Magdalen Shallows is mainly influenced by the Gaspé Current, which transports nutrients and production towards Cabot Strait.