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We measured the concentration and the stable isotope ratios of dissolved oxygen in the water column in the Estuary and Gulf of St. Lawrence to determine the relative importance of pelagic and benthic dissolved oxygen respiration to the development of hypoxic deep waters. The progressive landward decrease of dissolved oxygen in the bottom waters along the axis of the Laurentian Channel (LC) is accompanied by an increase in the 18 O : 16 O ratio, as would be expected from O-isotope fractionation associated with bacterial oxygen respiration. The apparent O-isotope effect, e O-app , of 10.8% reveals that community O-isotope fractionation is significantly smaller than if bacterial respiration occurred solely in the water column. Our observation can best be explained by a contribution of benthic O 2 consumption occurring with a strongly reduced O-isotope effect at the scale of sediment-water exchange (e O-sed , 7%). The value for e O-sed was estimated from benthic O 2 exchange simulations using a one-dimensional diffusion-reaction O-isotope model. Adopting this e O-sed value, and given the observed community O-isotope fractionation, we calculate that approximately two thirds of the ecosystem respiration occurs within the sediment, in reasonable agreement with direct respiration measurements. Based on the difference between dissolved oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary and in the water that enters the LC at Cabot Strait, we estimate an average respiration rate of 5500 mmol O 2 m 22 yr 21 for the 100-m-thick layer of bottom water along the LC, 3540 mmol O 2 m 22 yr 21 of which is attributed to bacterial benthic respiration.The Laurentian Channel (LC) is a 1200-km-long and more than 300-m-deep submarine valley that originates on the Atlantic continental shelf off Nova Scotia and ends near the mouth of the Saguenay Fjord (Fig. 1). The deep and slow landward flow in the deep waters brings oxygenrich water from the Atlantic Ocean into the Gulf of St. Lawrence (Gilbert et al. 2005). The deep water is separated from the oxygenated surface and the cold intermediate subsurface layer (Gilbert and Pettigrew 1997) by a strong density gradient that inhibits vertical mixing of oxygen-rich surface waters with oxygen-poor bottom water (Fig. 2). Even during winter, water-column convection does not reach beyond 150 m in depth (Galbraith 2006). Thus, isolated from the atmosphere, the bottom water loses oxygen gradually through organic matter respiration as it flows landward along the LC.The bottom water in the Lower St. Lawrence Estuary (LSLE) at the western end of the LC is hypoxic, with dissolved oxygen (O 2 ) concentrations as low as 55 mmol L 21 (Gilbert et al. 2005). The O 2 concentration in the LSLE bottom water has decreased by 50% since 1930 (Gilbert et al. 2005), corresponding to an average depletion of approximately 1 mmol L 21 yr 21 . Gilbert et al. (2005) attributed one half to two thirds of the oxygen depletion to changes in the properties of the deep-water mass that enters the ...

Micropaleontological and geochemical analyses of a sediment core collected in the Laurentian Trough of the Gulf of St. Lawrence were carried out to reconstruct temporal variations in pelagic productivity and benthic environmental conditions. Dinoflagellate cyst assemblages reveal relatively stable pelagic productivity over the last two centuries. Similarly, geochemical (organic C, C org : N) and isotopic (d 13 C org , d 15 N) data reveal that organic matter fluxes to the seafloor have been relatively constant over the same period. In contrast, significant changes are recorded in the benthic foraminifer assemblages. A sediment surface peak in the abundance of Cassidulina laevigata and Brizalina subaenariensis is consistent with the recent record of oxygen depletion in the bottom water. A decrease in the relative abundance of Nonionellina labradorica, concomitant with a relatively higher occurrence of Oridorsalis umbonatus in the upper part of the core, reflects a significant warming of the bottom water. Changes in bottom-water properties are further constrained by a negative trend of the d 18 O in Bulimina exilis carbonate shells over the last century, corresponding to a warming of about 2uC. These results strongly suggest that the recent oxygen depletion in the bottom waters of the Gulf of St. Lawrence is due to changes in water masses that have led to increased bottom-water temperatures and, to some extent, a resultant increase in organic matter respiration rates.Eutrophication is often identified as the main cause of bottom-water hypoxia (, 2 mg L 21 or 62.5 mmol L 21 ) in coastal environments (Cloern 2001). In the Lower St. Lawrence Estuary (LSLE), direct measurements have documented a progressive depletion of dissolved oxygen (DO) in bottom waters (. 250 m), from , 125 to 60 mmol L 21 over the last 80 yr (Gilbert et al. 2005). These observations are consistent with results of micropaleontological and geochemical analyses of sediment cores that show clear evidence of bottom-water oxygen depletion since the 1960s (Thibodeau et al. 2006). Whereas one half to two thirds of the oxygen depletion has been ascribed to a change in ocean circulation in the northwest Atlantic and, thus, variations in the water properties (DO, temperature, salinity) of the bottom waters that enter the Gulf and St. Lawrence through Cabot Strait (Gilbert et al. 2005), the remainder has been speculatively associated with increased organic carbon (C org ) fluxes to the seafloor, possibly due to anthropogenic eutrophication (Thibodeau et al. 2006). Thibodeau et al. (2006) and Gilbert et al. (2007) have shown that increases in sales of agricultural fertilizers, especially nitrogen fertilizers, in the St. Lawrence River drainage basin are concurrent with the depletion of bottom-water oxygen concentrations in the LSLE. Finally, an increase in bottom-water temperatures (Gilbert et al. 2005), leading to increased respiration rates, could also explain the recent decrease of DO concentrations.To determine if eutrophication and the trend ...

Primary productivity is limited by the availability of nitrogen (N) in most of the coastal Arctic, as a large portion of N is released by the spring freshet and completely consumed during the following summer. Thus, understanding the fate of riverine nitrogen is critical to identify the link between dissolved nitrogen dynamic and coastal primary productivity to foresee upcoming changes in the Arctic seas, such as increase riverine discharge and permafrost thaw. Here we provide a field-based study of nitrogen dynamic over the Laptev Sea shelf based on isotope geochemistry. We demonstrate that while most of the nitrate found under the surface freshwater layer is of remineralized origin, some of the nitrate originates from atmospheric input and was probably transported at depth by the mixing of brine-enriched denser water during sea ice formation. Moreover, our results suggest that riverine dissolved organic nitrogen (DON) represents up to 6 times the total riverine release of nitrate and that about 62 to 76% of the DON is removed within the shelf waters. This is a crucial information regarding the near-future impact of climate change on primary productivity in the Eurasian coastal Arctic.Plain Language Summary Climate change will enhance the release of organic nitrogen to the Arctic via increased river runoff and permafrost thawing. Here we show that more than half of this nitrogen can be used directly, or after recycling, by marine organisms and thus should be taken into consideration when investigating the global primary productivity of the Arctic coastal ecosystem.