Arctic landscapes are experiencing intense warming and modification of precipitation regimes with climate change. Permafrost disturbances and climate change impacts on hydrology of Arctic watersheds are likely to modify the quantity and composition of exported dissolved organic matter (DOM). In July 2007, intense rainfall and active layer thickening caused widespread active layer detachments at Cape Bounty, Melville Island (Canada). This study investigates the impacts of seasonal hydrology and permafrost disturbance on DOM composition exported from High Arctic headwater catchments. In 2012, streams were sampled from three disturbed catchments and one undisturbed catchment. The composition of DOM was characterized using absorbance and fluorescence spectroscopy. DOM was mostly exported during the spring freshet. Throughout this period, the undisturbed catchment exported humified DOM with high humic-like fluorescence that likely originated from runoff through shallow organic rich soil. In contrast, DOM exported from disturbed catchments was fresher, less humified with a high proportion of low molecular weight humic acid. We demonstrate that disturbed catchments delivered likely more labile DOM derived from either thawed permafrost or enhanced microbial activity. If this labile DOM comes from an ancient pool, as indicated by other studies at this site, disturbances may strengthen the permafrost carbon feedback on climate change.Key words: active layer detachments, dissolved organic matter, fluorescence, High Arctic, parallel factor analysis, PARAFAC.Résumé : Avec le changement climatique, les paysages arctiques connaissent un réchauffe-ment intense et une modification des régimes de précipitation. Les impacts des perturbations du pergélisol et du changement climatique sur l'hydrologie des bassins hydrographiques arctiques vont probablement modifier la quantité et la composition de matière organique dissoute (MOD) exportée. En juillet 2007, des averses intenses et l'épaississement de la couche active ont causé des décollements généralisés de la couche active à Cape Bounty, île Melville (Canada). Cette étude examine les impacts de l'hydrologie saisonnière et de la perturbation du pergélisol sur la composition de la MOD exportée des bassins versants d'amont du HautArctique. En 2012, on a échantillonné des cours d'eau de trois bassins versants perturbés et d'un bassin versant non perturbé. La composition de la MOD a été analysée en utilisant la spectroscopie à fluorescence et l'absorptiométrie. La MOD a surtout été exportée pendant la crue printanière. Pendant cette période, le bassin versant non perturbé a exporté de la MOD humifiée à haute fluorescence telle humique qui provenait probablement de ruissellement par sol riche organique mince. En revanche, la MOD exportée de bassins versants perturbés était plus fraîche, moins humifiée avec une haute proportion d'acide humique à Mots-clés : décollements de la couche active, matière organique dissoute, fluorescence, Haut-Arctique, analyse de facteurs parallèles...

Abstract. The circulation in the northwestern North Atlantic Ocean is highly complex, characterized by the confluence of two major western boundary current systems and several shelf currents. Here we present the first comprehensive analysis of transport paths and timescales for the northwestern North Atlantic shelf, which is useful for estimating ventilation rates, describing circulation and mixing, characterizing the composition of water masses with respect to different source regions, and elucidating rates and patterns of biogeochemical processing, species dispersal, and genetic connectivity. Our analysis uses dye and age tracers within a high-resolution circulation model of the region, divided into nine subregions, to diagnose retention times, transport pathways, and transit times. Retention times are shortest on the Scotian Shelf (∼ 3 months), where the inshore and shelf-break branches of the coastal current system result in high along-shelf transport to the southwest, and on the Grand Banks (∼ 3 months). Larger retention times are simulated in the Gulf of St. Lawrence (∼ 12 months) and the Gulf of Maine (∼ 6 months). Source water analysis shows that Scotian Shelf water is primarily comprised of waters from the Grand Banks and Gulf of St. Lawrence, with varying composition across the shelf. Contributions from the Gulf of St. Lawrence are larger at near-shore locations, whereas locations near the shelf break have larger contributions from the Grand Banks and slope waters. Waters from the deep slope have little connectivity with the shelf, because the shelf-break current inhibits transport across the shelf break. Grand Banks and Gulf of St. Lawrence waters are therefore dominant controls on biogeochemical properties, and on setting and sustaining planktonic communities on the Scotian Shelf.

The circulation in the northwestern North Atlantic Ocean is highly complex, characterized by the confluence of two major western boundary current systems and several shelf currents. Here we present the first comprehensive analysis of transport paths and timescales for the northwestern North Atlantic shelf, which is useful for estimating ventilation rates, describing circulation and mixing, characterizing the composition of water masses with respect to different source regions, and elucidating rates and patterns of biogeochemical processing, species dispersal, and genetic connectivity. Our analysis uses dye and age tracers within a highresolution circulation model of the region, divided into nine subregions, to diagnose retention times, transport pathways, and transit times. Retention times are shortest on the Scotian Shelf (∼ 3 months), where the inshore and shelf-break branches of the coastal current system result in high alongshelf transport to the southwest, and on the Grand Banks (∼ 3 months). Larger retention times are simulated in the Gulf of St. Lawrence (∼ 12 months) and the Gulf of Maine (∼ 6 months). Source water analysis shows that Scotian Shelf water is primarily comprised of waters from the Grand Banks and Gulf of St. Lawrence, with varying composition across the shelf. Contributions from the Gulf of St. Lawrence are larger at near-shore locations, whereas locations near the shelf break have larger contributions from the Grand Banks and slope waters. Waters from the deep slope have little connectivity with the shelf, because the shelf-break current inhibits transport across the shelf break. Grand Banks and Gulf of St. Lawrence waters are therefore dominant controls on biogeochemical properties, and on setting and sustaining planktonic communities on the Scotian Shelf.Published by Copernicus Publications on behalf of the European Geosciences Union.

Abstract. Continental shelves are thought to be affected disproportionately by climate change and are a large contributor to global air–sea carbon dioxide (CO2) fluxes. It is often reported that low-latitude shelves tend to act as net sources of CO2, whereas mid- and high-latitude shelves act as net sinks. Here, we combine a high-resolution regional model with surface water time series and repeat transect observations from the Scotian Shelf, a mid-latitude region in the northwest North Atlantic, to determine what processes are driving the temporal and spatial variability of partial pressure of CO2 (pCO2) on a seasonal scale. In contrast to the global trend, the Scotian Shelf acts as a net source. Surface pCO2 undergoes a strong seasonal cycle with an amplitude of ∼ 200–250 µatm. These changes are associated with both a strong biological drawdown of dissolved inorganic carbon (DIC) in spring (corresponding to a decrease in pCO2 of 100–200 µatm) and pronounced effects of temperature, which ranges from 0 ∘C in the winter to near 20 ∘C in the summer, resulting in an increase in pCO2 of ∼ 200–250 µatm. Throughout the summer, events with low surface water pCO2 occur associated with coastal upwelling. This effect of upwelling on pCO2 is also in contrast to the general assumption that upwelling increases surface pCO2 by delivering DIC-enriched water to the surface. Aside from these localized events, pCO2 is relatively uniform across the shelf. Our model agrees with regional observations, reproduces seasonal patterns of pCO2, and simulates annual outgassing of CO2 from the ocean of +1.7±0.2 mol C m−2 yr−1 for the Scotian Shelf, net uptake of CO2 by the ocean of -0.5±0.2 mol C m−2 yr−1 for the Gulf of Maine, and uptake by the ocean of -1.3±0.3 mol C m−2 yr−1 for the Grand Banks.

Abstract. Continental shelves are thought to be affected disproportionately by climate change and are a large contributor to global air-sea carbon dioxide (CO2) fluxes. It is often reported that low-latitude shelves tend to act as net sources of CO2 whereas mid- and high-latitude shelves act as net sinks. Here, we combine a high-resolution regional model with surface water time-series and repeat transect observations from the Scotian Shelf, a mid-latitude region in the northwest North Atlantic, to determine what processes are driving the temporal and spatial variability of partial pressure of CO2 (pCO2). In contrast to the global trend, the Scotian Shelf acts as a net source. Surface pCO2 undergoes a strong seasonal cycle associated with both a strong biological drawdown of Dissolved Inorganic Carbon (DIC) in spring, and pronounced effects of temperature, which ranges from 0 °C in the winter to near 20 °C in the summer. Throughout the summer, events with low surface-water pCO2 occur nearshore associated with coastal upwelling. This effect of upwelling on pCO2 is also in contrast to the general assumption that upwelling increases surface pCO2 by delivering DIC-enriched water to the surface. Aside from these localized events, pCO2 is relatively uniform across the shelf. Our model agrees with regional observations, reproduces seasonal patterns of pCO2, and simulates annual outgassing of CO2 from the ocean of +1.9 ± 0.2 mol C m−2 yr−1 for the Scotian Shelf, net neutral CO2 flux of −0.09 ± 0.16 mol C m−2 yr−1 for the Gulf of Maine and uptake by the ocean of −0.88 ± 0.4 mol C m−2 yr−1 for the Grand Banks.

A latitudinal pattern in coastal air‐sea CO2 flux has emerged where mid‐and high‐latitude shelves act as net sinks and low‐latitude shelves as net sources to the atmosphere. Regional studies, however, report the mid‐latitude Scotian Shelf (SS) at the eastern Canadian seaboard acts as a large source of CO2, contradicting several global syntheses. Here, we combine observations and a regional biogeochemical model to explain, for the first time, how this net outgassing of CO2 is sustained. We employ a novel approach using passive dye tracers to estimate how carbonate properties change along dominant transport pathways. We show that cold, carbon‐rich subpolar North Atlantic water is a dominant endmember that warms and combines with low alkalinity (TA), carbon‐deplete freshwater from the Gulf of St. Lawrence, becoming oversaturated with CO2 on the SS. Our approach explicitly considers the 3‐dimensional nature of coastal ocean transport processes and should be applied to other shelf regions.

The abundance, distribution, and size of marine species are linked to temperature and nutrient regimes and are profoundly affected by humans through exploitation and climate change. Yet little is known about long-term historical links between ocean environmental changes and resource abundance to provide context for current and potential future trends and inform conservation and management. We synthesize >4000 years of climate and marine ecosystem dynamics in a Northwest Atlantic region currently undergoing rapid changes, the Gulf of Maine and Scotian Shelf. This period spans the late Holocene cooling and recent warming and includes both Indigenous and European influence. We compare environmental records from instrumental, sedimentary, coral, and mollusk archives with ecological records from fossils, archaeological, historical, and modern data, and integrate future model projections of environmental and ecosystem changes. This multidisciplinary synthesis provides insight into multiple reference points and shifting baselines of environmental and ecosystem conditions, and projects a near-future departure from natural climate variability in 2028 for the Scotian Shelf and 2034 for the Gulf of Maine. Our work helps advancing integrative end-to-end modeling to improve the predictive capacity of ecosystem forecasts with climate change. Our results can be used to adjust marine conservation strategies and network planning and adapt ecosystem-based management with climate change.

The circulation in the northwestern North Atlantic Ocean is highly complex, characterized by the confluence of two major western boundary current systems and several shelf currents. Here we present the first comprehensive analysis of transport paths and timescales for the northwestern North Atlantic shelf, which is useful for estimating ventilation rates, describing circulation and mixing, characterizing the composition of water masses with respect to different source regions, and elucidating rates and patterns of biogeochemical processing, species dispersal, and genetic connectivity. Our analysis uses dye and age tracers within a highresolution circulation model of the region, divided into nine subregions, to diagnose retention times, transport pathways, and transit times. Retention times are shortest on the Scotian Shelf (∼ 3 months), where the inshore and shelf-break branches of the coastal current system result in high alongshelf transport to the southwest, and on the Grand Banks (∼ 3 months). Larger retention times are simulated in the Gulf of St. Lawrence (∼ 12 months) and the Gulf of Maine (∼ 6 months). Source water analysis shows that Scotian Shelf water is primarily comprised of waters from the Grand Banks and Gulf of St. Lawrence, with varying composition across the shelf. Contributions from the Gulf of St. Lawrence are larger at near-shore locations, whereas locations near the shelf break have larger contributions from the Grand Banks and slope waters. Waters from the deep slope have little connectivity with the shelf, because the shelf-break current inhibits transport across the shelf break. Grand Banks and Gulf of St. Lawrence waters are therefore dominant controls on biogeochemical properties, and on setting and sustaining planktonic communities on the Scotian Shelf.