Summer transports of nutrients in the Gulf of St. Lawrence estimated by inverse, modelling

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 nutr ient exchanges and transports during the summer period. Three models were d eveloped and compared: a purely physical model Al and two models including biogeochemical processes (new production, particle sedimentation, remineral ization, and burial): a 'high-flux' model A2 and a 'low-flux' model A3, nam ed 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 advectio n and diffusion fluxes gives a good representation of all the nutrient patt erns in summer. However, the use of nutrient and other biogeochemical proce sses improved the water, heat, and salt balances relative to using only phy sical data. Moreover, the use of biogeochemical processes allows model A3 m ore flexibility to reproduce oxygen and nutrient patterns in the deep layer . Without the biogeochemical processes, the deep circulation along the Laur entian Channel and the upward fluxes into the upper layers were reduced bel ow expected values due to the incapacity of the model to balance deep inflo ws of nutrients with surface outflows through Cabot Strait. The low-flux mo del A3 estimated higher velocities in the deep layer since the deep nutrien t inflows could then be balanced by biological removal in the upper layers. The resulting flow pattern indicated that the St. Lawrence system can be c haracterized by two main regions: the western region (Estuary, Gaspe Curren t and Northwestern Gulf), where high biological production is associated wi th a cyclonic and upwelling circulation, and the eastern region (Laurentian Channel Central, Jacques Cartier Passage, Esquiman Channel and Cabot Strai t), where low production is associated with an anticyclonic and downwelling circulation. The Southern Gulf or Magdalen Shallows is mainly influenced b y the Gaspe Current, which transports nutrients and production towards Cabo t Strait.