We use the results from eight of the Earth System Models (ESMs) made available for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change to analyze the projected changes in biogeochemical conditions over the next 50 years in the northwest Atlantic. We looked at the projected changes using the Representative Concentration Pathway 8.5 scenario in the 100-400 m depth range over a large region and at more specific locations to assess the relevance of using these outputs to force a regional climate downscaling model of the Gulf of St. Lawrence. The projected trends for dissolved oxygen (decrease), pH (decrease), and nitrate (variable although negative in general) represent a continuation of the recently observed trends in the area. For primary production, no firm conclusions can be drawn because of large differences in the trends from one model to another. The consistency of the trends near the regional model lateral boundaries leads us to conclude that the ESM trends can be used to set up future boundary conditions to evaluate regional impacts of climate change although the uncertainty of the results for the Scotian Shelf will be greater than for the Gulf of St. Lawrence. RÉSUMÉ [Traduit par la rédaction] Nous utilisons les résultats de huit modèles du système terrestre qui ont servi à la rédaction du Cinquième Rapport d'évaluation du Groupe d'experts intergouvernemental sur l'évolution du climat, afin d'analyser les changements prévus des conditions biogéochimiques au cours des 50 prochaines années, dans le nord-ouest de l'Atlantique. Nous examinons les changements prévus fondés sur le scénario 8.5 des profils représentatifs d'évolution de concentration (RCP), et ce, entre 100 et 400 m de profondeur, sur une grande région et à des sites précis, afin de déterminer la pertinence d'utiliser ces résultats pour piloter un modèle régional de réduction d'échelle du climat pour le golfe du Saint-Laurent. Les tendances prévues pour l'oxygène dissous (diminution), le pH (diminution) et le nitrate (variables mais généralement négatives) représentent une continuation des tendances récemment observées dans cette région. Dans le cas des tendances de la production primaire, nous ne pouvons tirer aucune conclusion robuste, puisque de grandes différences existent d'un modèle à l'autre. La cohérence des tendances près des frontières latérales du modèle régional nous porte à conclure que les tendances provenant des modèles du système terrestre peuvent servir à configurer des conditions limites futures pour évaluer les impacts régionaux des changements climatiques, bien que l'incertitude des résultats associés au plateau Néo-Écossais sera supérieure à celle du golfe du Saint-Laurent.

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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.

As the second most important greenhouse gas with continuously increasing atmospheric concentrations, methane has attracted much attention during the last several decades (IPCC, 2013). The ocean is recognized as a natural source of atmospheric methane, however, estimates of oceanic emissions vary by more than an order of magnitude (0.4-18 Tg CH 4 year −1 ) (

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