Nutrient transport pathways during winter in the Lower St. Lawrence Estuary

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Introduction
Oceans and coastal seas at high-latitudes are relatively under-sampled due to their isolation and inhospitable weather.Although ambitious multidisciplinary campaigns have been undertaken in the Canadian Arctic aboard the icebreaker CCGS Amundsen, such as CASES (Fortier et al., 2008), CFL (Barber et al., 2010) and Arctic SOLAS (Papakyriakou andStern, 2011-2012), few scientific campaigns have been carried out in the St. Lawrence Estuary and Gulf during the sea-ice season (Figure 1).During this season, the CCGS Amundsen carries out icebreaking and ship escorting operations.In 2018, the Quebec Maritime Network initiated the Odyssée winter field program with the Canadian Coast Guard.This collaboration allowed a science team to sample alongside the Coast Guard's normal icebreaker operations.These new observations were temporally-limited but provided the first winter turbulence measurements.They also 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).
Our measurements were partly motivated by difficulties in modeling the physical characteristics of the water column (e.g., Smith et al., 2006a), 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).We were unsure what to expect from our winter mixing measurements, but the low nutrient consumption provided a better representation of the nutrient transport mechanisms than possible from summer observations.The accepted view is that nutrients in the surface layer of the Lower St-Lawrence Estuary originate from an upwelling at the HLC (Ingram, 1975(Ingram, , 1983;;Cyr et al., 2015) and/or entrainment of deep nutrient-rich water from the estuarine circulation as fluvial waters from the USLE flows to sea (e.g., Steven, 1971).Another source of nutrients is the horizontal advection of fluvial waters that drain the Great Lakes between Canada and the USA -notably the urbanised catchment upstream of Quebec City (Hudon et al., 2017).
Published field observations have typically focused on the nutrient transport from vertical mixing dynamics within the LSLE, as opposed to fluvial nutrient loads from upstream.Cyr et al. (2015) quantified with direct turbulence measurements the nitrate supply at the HLC by tidal-upwelling in late summer.Their measured nitrate vertical fluxes ranged between 0.2 and 3.5 µmol m −2 s −1 (95% bootstrapped confidence intervals).These estimates represent the world highest reported vertical fluxes since they are an order of magnitude higher than those reported for the Mauritanian upwelling region (Schafstall et al., 2010).Cyr et al. (2015) also estimated the nitrate enrichment elsewhere in the LSLE from shear-induced mixing at the base of the nitricline via the estuarine circulation.They estimated a total of 33 and 400 mol s −1 of nitrate being transported into the LSLE's surface layer by these two vertical mixing processes in late summer.Previous moored observations have shown that internal tides are still present at the HLC during winter (Smith et al., 2006b).However, the mixing and vertical nitrate fluxes associated with internal waves and tidal-upwelling processes have not been measured during winter.
Biogeochemical box-models have been used to evaluate the importance of vertical mixing processes in supplying nutrients into the LSLE's surface (e.g., Savenkoff et al., 2001;Jutras et al., 2020).These box-models consider the fluvial nutrients advected into the LSLE and assume that the estuarine circulation of the LSLE can be idealized using a few homogeneous layers in a steady-state.Savenkoff et al. (2001) created an inverse box-model using four layers that also separated the entire LSLE and Gulf region into eight different zones.
Vertical mixing processes brought 685 mol s −1 near the surface of the LSLE -more than 5 times than fluvial waters (Figures 6 and 10 of Savenkoff et al., 2001).Jutras et al. (2020) revisited the nutrient loads with a three-layer model representative of the LSLE's summer conditions.Their vertical flux of dissolved nitrogen in the surface (1050 mol s −1 ) was three times larger than the contributions from fluvial sources.Both box-modelling studies estimated vertical fluxes of nitrogen (nitrate) that were much larger than those obtained directly by Cyr et al. (2015) in the field.Neither considered winter conditions.Below, we quantify the nutrient transport from diverse pathways, such as fluvial advection and vertical mixing in the Estuary.We evaluate their relative importance for creating a nutrient inventory in the upper water column during winter, which sets-up the subsequent spring bloom.We also extend the analysis to other seasons and revisit the importance of fluvial inputs for supplying nutrients into the St-Lawrence Estuary throughout the year.The ∼400 m deep Laurentian Channel rises sharply at the HLC (Figure 1b) and denotes the upstream extent of the LSLE where the shallow USLE ends.At the HLC, the strong barotropic tide (up to 5 m amplitude) interacts with the ≈ 80-m sill and generates intense tidal upwelling.This interaction generates internal waves at tidal and higher frequencies, breaking lee waves, Kelvin-Helmholtz instabilities, and internal hydraulic jumps (Saucier and Chassé, 2000).The HLC was first hypothesized by Ingram (1975Ingram ( , 1983) to be a turbulent mixing hot spot that supplies nutrients for primary production in the LSLE.Others considered the importance of the estuarine circulation in entraining nitrate-rich waters below the mixed surface layer into the surface layer (Steven, 1971(Steven, , 1974;;Sinclair et al., 1976).
Throughout most of the year, the LSLE's circulation is characterized by a surface layer outflowing above a milder inflow in the intermediate layer, and even weaker inflow in the deep waters below about 150-m (see Figure 2 of Sinclair et al., 1976).The surface outflow, therefore, becomes saltier as it entrains deeper water from the intermediate layer.The magnitude of this nutrient transport process on primary production across the whole LSLE and Gulf system is debated.
Seawards, the LSLE progressively widens until reaching the Gulf that begins at Pointedes-Monts (Figure 1a).At the most seaward, eastern extent of the St-Lawrence system, Cabot Strait is where warmer and saliter waters from the Atlantic enter near the bottom.These waters are formed by the warm subtropical waters transported north by the Gulf stream, which mix offshore with cold water transported south by Labrador Current (e.g., Lauzier and Trites, 1958;Gilbert et al., 2005).These deep waters move slowly upstream because of the estuarine circulation.It takes about 3 to 4 years for water at Cabot Strait to reach the HLC (Gilbert, 2004).

Field measurements
The paper focuses on the field observations collected during the Odyssée winter program launched in 2018.To put these measurements into the annual cycle context, we also present observations from existing monitoring surveys during the ice-free months.Namely, Fisheries and Oceans Canada monitor the biogeochemical and physical conditions in the St. Lawrence Estuary and Gulf through their Atlantic Zone Monitoring Program (AZMP).
Another monitoring program, the St. Lawrence Ecosystem Health Research and Observation Network, provided additional nitrate observations for fall 2017 in the USLE.We provide an overview of these additional data sources in Table 1.

Odyssée winter campaign of 2018
We visited 15 stations during the inaugural Odyssée program (Figure 1), which spanned from February 8 to 23 2018 (Feb 8-15 2018).We named the stations using prefixes U, L and G to designate those in the USLE, LSLE, and Gulf.These prefixes were each followed by the number of kilometres downstream from Quebec City, Tadoussac, and Pointe-des-Monts.
Several sampling operations were undertaken at each station.We focus, however, on the physical observations from the conductivity, temperature, and depth profiler (CTD), nutrient concentrations derived from in-situ water samples.We also collected turbulence microstructure profiles in the LSLE and the Gulf since the USLE was too shallow for turbulence sampling (Figure 1).
At each station, a SBE9 CTD profiler (Seabird Electronics) was mounted to a rosette that was operated through the ship's moon-pool.Another pumped CTD (SBE19plus, Seabird Electronics) was regularly deployed from the vessel's side shortly after the SBE9.The SBE19-CTD's purpose was mainly to measure Photosynthetically Active Radiation (PAR) in the upper 16-m of the water column.We generally collected in-situ water samples at 10 m (i.e., 2 m beneath the ship's haul), 25 m, 50 m, 100, 150 m, 250 m and bottom, water depth permitting.These sampling depths were adjusted at shallower stations within the USLE and LSLE.At the station closest to Quebec City (U37) in 18 m depth, a single water sample was collected at 14 m depth from the rosette.Salinity was calculated from the temperature and conductivity sensors, which we present in practical salinity units (psu) herein.
All water samples obtained from the CTD-Rosette were filtered through a 0.7 µm GF/F filter using acid-washed syringes and Swinnex.The samples were analyzed immediately on board the vessel to derive nutrient concentrations.Concentrations of NO3-+NO2-, NO2-, PO4-and Si(OH)4 were determined using a colorimetric method adapted from Hansen and Koroleff (2007) with a Bran and Luebbe Autoanalyzer III.We calculated NO3-concentrations by differencing NO2-from the NO3-and +NO2-readings.The analytical detection limit was 0.015 µmol L −1 for NO3-+NO2-, 0.0015 µmol L −1 for NO2-, 0.02 µmol L −1 for PO4-, and 0.016 µmol L −1 for Si(OH)4.The samples yielded nitrate, nitrite, phosphate and silicate concentrations at different depths for each station visited.
At nine of the stations located between the Saguenay Fjord and Cabot Strait (Figure 1), we collected temperature, conductivity (salinity), and turbulence profiles with a VMP-500 manufactured by Rockland Scientific Ltd.We operated the Vertical Microstructure Profiler (VMP) from the ship's front deck.The VMP was fitted with two airfoil shear probes, two fast-response thermistors (FP07, GE Thermometics), one micro-conductivity sensor and pressure and it sampled at 512 Hz.The VMP was also equipped with a high-accuracy temperature (SBE-3F) and conductivity (SBE-4C) sensors from Seabird Electronics, which sampled at 64 Hz.These measurements enabled calculating the vertical salinity and density gradients using the pressure sensor on board the VMP.
Because of the Coast Guard's operations, the VMP and CTD-Rosette sampling occurred at different phases of the tides (Figure 2).With the VMP, we were unable to cover a complete semi-diurnal tidal cycle at any station.We obtained the best temporal coverage of the tidal cycle at station G163.Fourteen VMP profiles were collected during flood tide  (Figure 2).We attempted to cover another tidal cycle at station G294 on 19 February, but an ice-breaking request at the Magdalen Islands halted sampling after collecting seven profiles during ebb tide.This station was revisited on 21 February 2018 during the ebbing tides.For all other stations, the VMP collected two to three consecutive profiles (Figure 2).

Historical monitoring surveys
Fisheries and Oceans Canada run the Atlantic Zone Monitoring Program (AZMP) multiple times each year (Therriault et al., 1998;Blais et al., 2019;Galbraith et al., 2019).Their monitoring consists of surveys in March, June, August, and November, which cover the Gulf and the LSLE (Figure 1a).Here, we present nitrate, temperature and salinities measured in summer and fall preceding our boreal winter campaign (Table 1).For completeness, we also present nitrate and salinity observations from the fall monitoring survey of the St. Lawrence Ecosystem Health Research and Observation Network.
The AZMP's ship-based fall and summer surveys provided standard CTD profiles, along with nutrient concentrations at similar depths to our winter campaign, in addition to samples closer to the surface at 2.5 and 5 m depth.The summer nitrate measurements were collected from two different AZMP cruises (Table 1).The first cruise, during June, collected profiles in the downstream reaches of the USLE and near the HLC.The second cruise was in August and focused on the LSLE and the Gulf.The summer measurements are provided to contrast with our winter nutrient observations.We use the fall measurements to estimate the nitrate inventory generated between the fall monitoring survey and our winter observations within the LSLE.
We also present observations from the winter heli-survey conducted in mid-March 2018, a few weeks after our winter program in February 2018 (Table 1).Like most years, the 2018 AZMP's winter survey was conducted by helicopter.Therefore, sampling was limited to CTD profiles (pumped Sea-Bird Scientific SBE 19plus V2).Surface water samples were also filtered, frozen, and analyzed later for nutrient concentrations.This campaign surveyed stations within the Saguenay Fjord in addition to the St. Lawrence in mid-March (Figure 1b).

Turbulence analysis
We combined the VMP's turbulence profiles with the nutrients measurements to obtain vertical fluxes along the St. Lawrence during winter via where z is the height above the free surface.The mixing rates K were derived from the VMP's measurements, while the vertical background concentration gradients ∂N /∂z were derived for nitrate from the VMP profiles via a nitrate-salinity relationship developed by analysing the in-situ water sampled by the rosette ( §3.1.2).
The most commonly-used model for estimating K was proposed by Osborn (1980) for shear-induced mixing: and requires estimating the rate of dissipation of turbulent kinetic energy and the background buoyancy frequency N = (−g/ρ)(∂ ρ/∂z).A constant mixing efficiency Γ = 0.2 is often assumed, despite mounting evidence that it varies (e.g., Monismith et al., 2018).Several parametric models have been proposed (e.g., Ivey et al., 2018), and debated (e.g., Gregg et al., 2018), to relate Γ with external parameters.
Within the St. Lawrence, the temperature gradients were generally gravitationally unstable, i.e., cold water overlaying warmer water.These unstable temperature gradients were stabilized by the salinity gradients.In these situations, double-diffusive convection (DDC) is possible.However, our observations lacked the presence of distinctive large (∼ several meters high) steps that are typically suggestive of DDC.Even when DDC dominates in weakly sheared flows, equation 2 can be used to estimate K by increasing Γ ∼ 1 (see Hieronymus and Carpenter, 2016;Polyakov et al., 2019).In these situations, buoyancy is the main source of mixing.Our turbulence levels were much higher than those reported by Polyakov et al. (2019) and the strong tides are more conducive to shear-induced mixing than quiescent DDC mixing.We thus assume the custom value of Γ = 0.2 for shear-induced mixing.Our chosen Γ is consistent with field observations at low /(νN 2 ) (e.g., Holleman et al., 2016;Monismith et al., 2018).Here, ν represents the kinematic viscosity of seawater, while the ratio /(νN 2 ) is proportional to ratio of largest and smallest turbulent overturns in a stratified fluid.During our field campaign, /(νN 2 ) were 95% of the time less than 500.
To obtain the mixing rate K, we estimated using the methods described by Bluteau et al. (2016).Each profile was split into 4096 samples (8 s) that overlapped by 50% before computing the velocity gradient spectra.The VMP's profiling speed was derived from its pressure sensor to convert the spectra between the frequency and wavenumber domain.We applied the multivariate technique of Goodman et al. (2006) to remove motion-induced contamination from these spectra, which were then integrated over the viscous subranges to obtain .The VMP typically profiled at around 0.65 m s −1 , thus providing turbulence estimates at a resolution of about 2.5 m given the 50% overlap when splitting the cast.Turbulence estimates near the end and the beginning of a cast were discarded because of the VMP's deceleration and acceleration, respectively.We also discarded estimates within 25 m of the surface because of the turbulence induced by the ship.To derive the density gradients ∂ ρ/∂z, we relied on the high-accuracy temperature and conductivity sensors (SBE-3F and SBE-4C) aboard the VMP.We first low-pass filtered these signals with a Butterworth filter using a cutoff period of 8 s.We then centred-differenced these smoothed profiles before averaging them over the same segments used for getting -yielding the mean vertical gradients necessary for using equations 1 and 2.

Proxy nutrients concentrations
We developed a nitrate-salinity relationship from the rosette's in-situ water samples to estimate the vertical nutrient gradients from the VMP's measured salinity (Figure 3).The nutrient relationship is based only on nitrate concentrations.These are generally lower than silicate and, therefore, more likely to limit primary production in the LSLE (Tremblay et al., 1997;Jutras et al., 2020).When limited amounts of nutrients are being consumed (or generated), mixing and advection processes govern the spatial distribution of nutrients.Their concentrations vary linearly with salinity, as reflected by our winter observations in Figure 3f.A concave nitrate-salinity curve, such as observed during summer and fall 2017, indicates nitrate was being consumed along the USLE (gray points in Figure 3d,e).These trends in the nitrate-salinity diagrams are supported by incubation experiments that quantify the nitrate consumption (Villeneuve, 2020).Nitrate consumption in the lower reaches of the USLE and in the LSLE was more than an order of magnitude higher in summer than during winter.Nitrate consumption downstream of Quebec City was less than 0.50 nmol m −3 s −1 and about half as much in the LSLE (Figure 16 of Villeneuve, 2020).During winter, a nitrate-salinity relationship is thus justified to estimate nitrate from the VMP profiler's salinity measurements.
The temperature-salinity diagram suggests two, possibly three, significant water masses across the region (Figure 3a).The first water mass is nutrient-rich waters from the USLE that mix in the LSLE before eventually mixing with nutrient-poor surface water downstream in the Gulf (Figure 3d).This mixing resulted in nitrate N concentrations decreasing with salinity for S < 31.2 psu.For higher salinities, S > 31.9 psu, which includes samples deeper than 50-m in the LSLE and almost all samples in the Gulf, nitrates increased proportionally with salinity (Figure 3f).This vertical nutrient distribution resembles the expected waters ex- posed to the open ocean, which we associate with the region's second water mass.Evidence of a third water mass is featured between 31.2 and 32 psu for nitrate, phosphate, silicate, and in particular, the temperature-salinity diagram of LSLE stations (Figure 3).The salinity of 31.2 psu corresponds to a local subsurface maximum in water temperatures at station L0, where Saguenay Fjord enters the St.-Lawrence, and downstream at L34.The heli-survey a month later confirms that the Saguenay Fjord discharged fresher water near this station (Fig-ure 3a).The surface water samples at the head of the Fjord had much lower phosphate (∼ 0.2 mmol m −3 ) and nitrate (∼ 10 mmol m −3 ), than water with comparable salinity in the USLE (Figure 3).This third water mass created a more rapid decrease in nitrate between the 31.2 to 32 psu salinity range before increasing again due to mixing with Gulf waters within the LSLE (Figure 3f).
Nitrate concentrations depended on salinity but also on the location along the St. Lawrence.
We therefore created three separate nutrient-salinity relationships to estimate nutrient concentrations from salinity measured by the VMP: (3) The first relationship, applicable for S < 31.2 psu, reflects the nutrient-rich water in the USLE mingling with saltier water downstream.The third relationship included all samples with S > 31.9 psu.It was applied to the turbulence profiles within the Gulf irrespective of the VMP's measured salinity in this region.The second relationship links the other two using a quadratic fit to the samples outside of the Gulf within the range 31.2 < S ≤ 31.9.This relation reflects contributions from the Saguenay Fjord, which was applied solely to turbulence profiles collected in the LSLE.Outside of the 31.2< S ≤ 31.9 range, we applied the first or third relationship over their applicable salinity ranges.Typically, the first relationship was applied to surface waters in the LSLE that originated from the nutrient-rich USLE upstream.In contrast, the third relationship was used for deeper waters entering from the Gulf.
A mean relative error of 5.5% was obtained for the predicted N after applying equation 3 to all the 64 samples in Figure 3f.The poorest agreement is with the low surface nitrate concentrations measured in the Gulf, particularly the furthest downstream near Cabot Strait (station G540 in Figure 3f).tion U37) were used in (4).The sampled water was almost fresh with salinities below 0.5 psu and dissolved nitrate concentrations of 26.43 mmol m −3 .The application of equation 4 to the historical measurements provided long-term statistics on the fluvial nitrate loads entering the USLE.

Vertical mixing contributions of nitrate
To the surface area associated with tidal-upwelling at the HLC, we rely on the same techniques as Cyr et al. (2015).This process creates cooler surface temperatures during summer (see Figure 13 of Cyr et al., 2015), and likely coincides with the winter polynia (see Figure 10 of Galbraith et al., 2019).Cyr et al. (2015) used climatological sea-surface temperatures at 1.1 km resolution measured with AVHRR remote sensing to establish the summer upwelling area.The 25 year-long record, spanning between 1986 and 2010, was timeaveraged for May through to October when this upwelling process cools the surface.They estimated this surface area to be approximately 100 km 2 , although 15% of the coldest pixels in the LSLE were equivalent to 230 km 2 (Cyr et al., 2015).For our box-analysis, we use the surface area at the 75-m isobath.At this depth, the entire LSLE covers about 6000 km 2 , whereas the HLC is about 200 km 2 for the HLC (Figure 4a).We thus exaggerate the impact of the HLC's tidal-upwelling on transporting nitrate into the surface layer.We apply the vertical nitrate fluxes of the HLC to this area, while the average vertical nitrate fluxes obtained outside of the HLC was applied to the remaining 97% of the area.

Winter conditions
The temperature and salinity transects in Figure 4 show similar features to those monitored during winter over the last 20 years (Galbraith, 2006;Galbraith et al., 2019).The USLE is progressively less saline towards Quebec City because of the freshwater input from the St. Lawrence and Great Lakes watersheds.In the Gulf, surface water temperatures are close to freezing, while they are well above freezing at the HLC (Figure 3b and Figure 4a).
The polynya in the LSLE is a permanent feature due to tidal upwelling and mixing at the HLC, which results in warmer, more saline water at the surface (Galbraith et al., 2002(Galbraith et al., , 2012)).
When we sampled at L0 during flood tide (Figure 2), the salinity was relatively homogeneous across the depth.The water was nonetheless 2 psu more saline than near-surface waters (i.e., 15 m) measured at stations both (Figure 4b).The higher salinities at the HLC, station L0, cannot be attributed to inputs from the Saguenay Fjord.This waterway provides a freshwater source, confirmed by the AZMP's annual helicopter survey a few weeks later (Figure 3b).
Therefore, we attribute the relatively high salinities near the HLC to tidal upwelling and mixing that characterize this region throughout the year (Ingram, 1983;Galbraith, 2006;Cyr et al., 2015).
The origin of the LSLE's nitrate is illustrated in Figure 4 with the grey circles overlaid on the temperature and salinity transects.Nitrate concentrations in the Gulf increased with depth (and thus salinity), typical of an open-ocean environment.Further upstream, in the LSLE, nitrate concentrations were relatively high near the surface but decreased to reach a subsurface minimum at the 50-m deep sample.These samples were near or slightly more saline than 31.2psu, the corresponding break between nutrient-rich fluvial waters and the relatively nutrient-poor water downstream seen in the nitrate-salinity diagram (Figure 3a).At the HLC, nitrate concentrations in the upper 50-m were also lower than water both upstream and downstream at comparable depths (Figure 4a).The presence of low nitrate and high salinity water, especially in the upper 50-m, further supports that water was being tidallyupwelled at the HLC.The nitrate concentrations further upstream, in the USLE, progressively increase with decreasing salinity.This trend reflects the mixing of nitrate-rich fluvial waters with relatively nutrient-poor estuarine water downstream.

Seasonal nitrate variations
Our winter nitrate observations share similar spatial distribution to the fall, despite higher concentrations during winter in the upper 100-m of the water column (Figure 5).In winter, near-surface nitrate concentrations progressively decreased from 11.8 mmol m −3 at a distance of ≈ 500 km to 6.4 mmol m −3 at 1000 km (Figure 5b).In fall, concentrations were as low as 1 mmol m −3 in the Gulf's surface mixed-layer (Figure 5c).In comparison, they were even lower during summer with values as low as 0.1 mmol m −3 in the upper 20-  We revisit the notion of tidal-upwelling and mixing processes, especially at the HLC, acting as the nutrient pump for the LSLE.The low consumption during winter provides a better representation of the physical mechanisms transporting nitrate into the LSLE than in summer.Summer observations invariably track both physical and biogeochemical processes.
Studies have reached variable conclusions about the importance of tidal-upwelling and mixing at the HLC in supplying nutrients to the LSLE, and ultimately the Gulf (see section 1).
All of these studies concluded that vertical mixing processes dominate the supply of nutrients in the LSLE (e.g., Steven, 1974;Sinclair et al., 1976;Savenkoff et al., 2001;Cyr et al., 2015;Jutras et al., 2020;Greisman and Ingram, 1977).The biogeochemical box-model of Jutras et al. (2020) predicted vertical nitrate loads of 1050 mol s −1 for the entire LSLE, while that of Savenkoff et al. (2001) predicted 685 mol s −1 .To reach these high values, the turbulent nitrate fluxes at the HLC would need to be two to four times larger than the summer values observed by Cyr et al. (2015).
These studies contrast with our winter observations since the fluvial nitrate loads dominated the supply of nutrients into the LSLE (Figure 8c).This conclusion remains true even if the winter vertical nitrate fluxes at the HLC had been as high as those measured during summer.The summer fluxes were some of the highest reported in the world (see Table 1 of Cyr et al., 2015).During winter, the LSLE's intermediate layer coincides with our observed nitrate subsurface minima.So the estuarine circulation entrains nitrate-rich fluvial waters into the nitrate-poor subsurface layer (Figure 8c).Our direct turbulence observations suggest that the intermediate layer received less nitrate from the bottom than from the fluvial outflow above.These observations are consistent with the notion that the estuarine circulation in the LSLE causes relatively weak entrainment of deep bottom waters into the intermediate layer.
The nitrate-salinity diagram (Figure 3a) and their transects (Figure 5) also highlight that a large portion of the nitrate in the upper LSLE originates from fluvial sources entering from the USLE.
Our winter observations contrast with the observations of Cyr et al. (2015) collected in late summer/early fall.When they collected their measurements, fluvial nitrate loads were about 90 mol s −1 , near the 5th percentile of the 17-year averages during late summer (Figure 6b).This period of the year also has the weakest fluvial loads during the annual cycle.Their fluvial nitrate loads were much lower than the supply from vertical mixing processes (Fig-

Figure 1 .
Figure 1.(a) Map of the Estuary and Gulf of St. Lawrence along with the stations visited with VMP profiles indicated by red squares and CTD profiles with black ×.Station L96 coincides with a long-term monitoring station of Fisheries and Oceans Canada's Atlantic Zone Monitoring Program (AZMP), Rimouski station.(b) Enlargement of the magenta inset in (a) to illustrate the HLC region and two Saguenay stations surveyed by helicopter in March 2018.

Figure 2 .
Figure 2. Observed tidal amplitudes at Rimouski (station #2985) and Quebec City (station #3248) maintained by Fisheries and Oceans Canada.The circles (•) and diamonds ( ) denote, respectively, when the VMP and CTD-Rosette profiled at each station designated with the colorbar.The smaller triangle ( ) denote the SBE19-CTD, which provides near-surface measurements given the rosette was deployed from the moonpool.Tidal levels near station G163 (Grande-Vallée) precede those at Rimouski by about 15 min, while those recorded at Tadoussac (station L0) lag Rimouski by another 30 min.

Figure 3 .
Figure 3. Winter 2018 observations of (a) Potential temperature (at atmospheric pressure), (b) phosphate, and (c) silicate illustrated against salinity.The nitrate concentrations for (d) the summer 2017, (e) fall 2017, and (f) winter 2018 are also shown.The symbols in panels b-f indicate the sample's depth.The open diamonds are near the surface (depth<20-m), while the closed triangles and squares are samples at 25 m and 50 m, respectively.The closed circles represent deeper samples at or below 100 m.Density contours in σ T [kg m −3 ] (a) were computed at atmospheric pressure.Concentrations measured from the most upstream station U37 were 26.43 mmol m −3 for nitrate, 0.53 mmol m −3 for phosphate, and 39.8 mmol m −3 for silicate.This bottle at 10-m depth corresponded to salinity of ≈0.5 psu.T f is the freezing temperature of seawater in (a).

Figure 4 .
Figure 4. (a) Temperature and (b) salinity measured along the St. Lawrence during the field campaign.The grey circles are scaled with the nitrate concentrations of the rosette water samples in Figure 3a.The extent of the HLC and the LSLE are denoted in (a) by the cyan and green boxes, respectively.Tidal-upwelling processes are confined to the HLC.Elsewhere in the LSLE, vertical mixing at the base is caused by the estuarine mixing.The thick white lines in (b) delineate the salinity limits of 31.2 and 31.9 psu used for developing the nitrate-salinity relationships (equation 3).Water above the upper line (S = 31.2psu) originated from the USLE.Water below the lower line (S = 32 psu) originated from the Gulf.
to JGR-Oceans m of the water column (Figure 5d).Nitrate concentrations in the deep layer ( 200 m) were slightly less in winter than in fall and summer.Throughout the Gulf and the LSLE, deep nitrate concentrations remained relatively homogeneous at about 25 mmol m −3 for all seasons (200-900 km, Figure 5b-d).

Figure 5 .Figure 6 .
Figure 5. (a) Nutrient inventory generated in the upper 75-m of the LSLE's water column.(b) Nitrate concentrations during our winter field campaign, (c) fall 2017 and (d) summer 2017 monitoring surveys.The thick cyan lines in (b) denote the same water masses as in Figure4.Water in between these two lines coincides with the subsurface nitrate minimum at station L0 and downstream.

Table 1 .
Overview of measurement campaigns.All campaigns included CTD profiling with pumped and bottles † Surface bottles collected water at a depth of ∼ 1m.‡ The Vertical Microstructure Profiler (VMP) carries its own Seabird CTD.