Turbulent nitrate fluxes in the <scp>L</scp>ower <scp>S</scp>t. <scp>L</scp>awrence <scp>E</scp>stuary, <scp>C</scp>anada

Cyr, Frédéric; Bourgault, D.; Galbraith, Peter S.; Gosselin, Michel

doi:10.1002/2014jc010272

Cited by 27 publications

(57 citation statements)

References 86 publications

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“…The discrepancy can be attributed to oxygen replenishment from the surface through turbulent mixing. A simple calculation reveals that the discrepancy can be explained by a turbulence coecient O(10 −5 ) m 2 s −1 (1.1 × 10 −5 m 2 s −1 ), in agreement with observational (Cyr et al, 2015) and reverse modeling (Savenko et al, 2001) studies.…”

Section: Methods For Sensitivity Analysissupporting

confidence: 79%

“…This is consistent with studies (Savenko et al, 2001;Coote and Yeats, 1979) that describe the LSLE as a 'nutrient pump' which sustains the primary productivity in the Gulf (Steven, 1971). Our estimate of nutrient export towards the Gulf is, however, 70 times larger than the Cyr et al (2015) estimate based on a balance of turbulent uxes.…”

Section: General Observationscontrasting

confidence: 76%

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Nutrient cycling in the Lower St. Lawrence Estuary: Response to environmental perturbations

Jutras

Mucci

Sundby

et al. 2020

Estuarine, Coastal and Shelf Science

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We present a simple linear three-box model of nutrient cycling in the Lower St. Lawrence Estuary (LSLE).A present-day nutrient budget is obtained for xed-nitrogen, phosphorus, and silica, from which the model's parameters are derived. The model is used to (i) test the sensitivity of each layer's nutrient concentration to perturbations in nutrient and water volume inputs, (ii) obtain the response time of the system to a new steady state following a perturbation, and (iii) estimate bottom-water oxygen consumption. We nd that most of the dissolved nutrients (70% of xed-nitrogen, 90% of phosphorus) that reach the surface waters in the Lower Estuary originate from the deep waters, implying that the anthropogenic eutrophication potential of the St.Lawrence River is moderate. Our nutrient budget suggests that the Lower St. Lawrence Estuary acts as a nutrient pump for the Gulf of St. Lawrence. Nitrate appears as the limiting nutrient to surface productivity in the LSLE. This model can be used to test the impact of natural or anthropogenic perturbations on nutrient and oxygen concentrations in the LSLE.

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Section: Methods For Sensitivity Analysissupporting

confidence: 79%

Section: General Observationscontrasting

confidence: 76%

Nutrient cycling in the Lower St. Lawrence Estuary: Response to environmental perturbations

Jutras

Mucci

Sundby

et al. 2020

Estuarine, Coastal and Shelf Science

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“…and shelf region (Schafstall et al, 2010). Vertical fluxes of nutrients driven by mixing processes are amongst the largest reported in literature (Cyr et al, 2015).…”

mentioning

confidence: 97%

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Rapp¹,

Schlösser²,

Barraqueta³

et al. 2018

Preprint

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Abstract. The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation and microbial community structure in large parts of the world's ocean, and thus plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g. Fe, manganese (Mn) and cobalt (Co)), with shelf sediments typically forming a key source. Over the last five decades, an expansion and intensification of OMZs has been observed and this trend is likely to proceed. However, it is unclear how trace metal (TM) distributions and transport are influenced by decreasing oxygen (O2) concentrations. Here we present dissolved (d; <&#8201;0.2&#8201;&#956;m) and leachable particulate (Lp; >&#8201;0.2&#8201;&#956;m) TM data collected at 7 stations along a 50&#8201;km transect in the Mauritanian shelf region. We observed enhanced concentrations of Fe, Co and Mn corresponding with low O2 concentrations (<&#8201;50&#8201;&#956;mol&#8201;kg&#8722;1), which were decoupled from major nutrients and nutrient-like and scavenged TMs (cadmium (Cd), lead (Pb), nickel (Ni) and copper (Cu)). Additionally, data from repeated station occupations indicated a direct link between dissolved and leachable particulate Fe, Co, Mn, and O2. An observed dFe decrease from 10 to 5&#8201;nmol&#8201;L&#8722;1 coincided with an O2 increase from 30 to 50&#8201;&#956;mol&#8201;kg&#8722;1 and with a concomitant decrease in turbidity. The changes in Fe (Co and Mn) were likely driven by variations in their release from sediment pore water, facilitated by lower O2 concentrations and longer residence time of the water mass on the shelf. Variations in organic matter remineralization and lithogenic inputs (atmospheric deposition or sediment resuspension) only played a minor role in redox-sensitive TM variability. Vertical dFe fluxes from O2-depleted subsurface to surface waters (0.08&#8211;13.5&#8201;&#956;mol&#8201;m&#8722;2&#8201;d&#8722;1) were driven by turbulent mixing and vertical advection, and were an order of magnitude larger than atmospheric deposition fluxes (0.63&#8211;1.43&#8201;&#956;mol&#8201;m&#8722;2&#8201;d&#8722;1). Benthic fluxes are therefore the dominant dFe supply to surface waters on the continental margins of the Mauritanian upwelling region. Overall, our results indicated that the projected future decrease in O2 concentrations in OMZs may result in increases in Fe, Mn and Co concentrations.

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“…These turbulence measurements covered the largest spatial extent of the St-Lawrence and Gulf during any season. Previous turbulence sampling were limited to a few specific sites at the head of the Laurentian Channel (HLC, Cyr et al, 2015), downstream near Rimouski in the Lower St. Lawrence Estuary (LSLE, Cyr et al, 2011;Bourgault et al, 2012;Cyr et al, 2015), and upstream of the HLC in the Upper St. Lawrence Estuary (USLE, Bourgault et al, 2008;Richards et al, 2013). The new winter measurements were partly motivated by difficulties in modeling the physical characteristics of the water column (e.g., Smith et al, 2006b), which in turn affect biogeochemical predictions given the reliance on properties such as water temperature and turbulent diffusivity (Mei et al, 2010;Sibert et al, 2011;Taucher and Oschlies, 2011).…”

Section: Introductionmentioning

confidence: 99%

Nutrient transport pathways in the Lower St. Lawrence Estuary: seasonal perspectives from winter observations

Bluteau

Galbraith

Bourgault

et al. 2021

Preprint

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Abstract. The St. Lawrence Estuary connects the Great Lakes with the Atlantic Ocean. The accepted view, based on summer conditions, is that the Estuary's surface layer receives its nutrient supply from vertical mixing processes. This mixing is caused by the estuarine circulation and tidal-upwelling at the Head of the Laurentian Channel (HLC). During winter when ice forms, historical process-based studies have been limited in scope. Winter monitoring has been typically confined to vertical profiles of salinity and temperature and near-surface water samples collected from a helicopter for nutrient analysis. In 2018, however, the Canadian Coast Guard approved a science team to sample in tandem with its icebreaking and ship escorting operations. This opportunistic sampling provided the first winter turbulence observations, which covered the largest spatial extent ever measured during any season within the St. Lawrence Estuary and Gulf. The nitrate enrichment from tidal mixing resulted in an upward nitrate flux of about 30 nmol m−2 s−1, comparable to summer values obtained at the same tidal phase. Further downstream, deep nutrient-rich water from the Gulf was mixed into the subsurface nutrient-poor layer at a rate more than an order of magnitude smaller than at the HLC. These fluxes were compared to the nutrient load of the upstream St. Lawrence River. Contrary to previous assumptions, fluvial nitrate inputs are the most significant source of nitrate in the Estuary. Nitrate loads from vertical mixing processes would only exceed those from fluvial sources at the end of summer when fluvial inputs reach their annual minimum.

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Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

Cited by 27 publications

References 86 publications

Nutrient cycling in the Lower St. Lawrence Estuary: Response to environmental perturbations

Nutrient cycling in the Lower St. Lawrence Estuary: Response to environmental perturbations

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Nutrient transport pathways in the Lower St. Lawrence Estuary: seasonal perspectives from winter observations

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