Abstract
The goal of this paper is to give a detailed description of the coupled physical-biogeochemical model of the Gulf of St. Lawrence that includes dissolved oxygen and carbonate system components, as well as a detailed analysis of the riverine contribution for different nitrogen and carbonate system components. A particular attention was paid to the representation of the microbial loop in order to maintain the appropriate level of the different biogeochemical components within the system over long term simulations. The skill of the model is demonstrated using in situ data, satellite data and estimated fluxes from different studies based on observational data. The model reproduces the main features of the system such as the phytoplankton bloom, hypoxic areas, pH and calcium carbonate saturation states. The model also reproduces well the estimated transport of nitrate from one region to the other. We revisited previous estimates of the riverine nutrient contribution to surface nitrate in the Lower St. Lawrence Estuary using the model. We also explain the mechanisms that lead to high ammonium concentrations, low dissolved oxygen, and undersaturated calcium carbonate conditions on the Magdalen Shallows.
Highlights
The model results are in accordance with these estimates, with 5 to 20% of the primary production reaching the sediments in the deep channels of the Gulf of St. Lawrence (GSL) (Figure 9B), the highest values being found in the downstream part of the Lower St. Lawrence Estuary (LSLE)
Changes to LF05 were made in order to maintain the biogeochemical characteristics over multiyear runs
This is essential if we want to use the model to study the inter-annual variability and make climate projections. These important changes included the addition of nitrification, modification to the particulate organic matter sinking and fragmentation rates, and instantaneous remineralisation of part of the organic matter that reaches the sediments
Summary
Two main factors are responsible for these gradients: (1) the proportion of Labrador Current and North Atlantic Central waters in the mixture that enters the Laurentian Channel, since these water masses have different chemical properties, and (2) the sinking and remineralization of organic matter produced in the surface layer by primary producers and supplied by the rivers (Gilbert et al, 2005; Claret et al, 2018; Lavoie et al, 2019) These processes lead to loss of oxygen and acidification in the deep water of the LSLE where ventilation is weak (Gilbert et al, 2005; Mucci et al, 2011). The very large contribution in December 2010 (51%) is anomalous (the December average is 36%) and results from strong easterly winds that led to high water levels in the LSLE and reduced upwelling
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