f Cys

isubki, Q

Available online 8 October 2011

Keywords:CDOMParticles

ophoric dissolved organic matter (CDOM) and particles were obtained in May

Marine Chemistry 128-129 (2012) 4456

Contents lists available at SciVerse ScienceDirect

Marine Ch

l se1.1. Overview of absorption properties in marine waters

Chromophoric dissolved organic matter (CDOM), phytoplankton,and non-algal particles (NAP) are major ultra-violet (UV) and visiblelight absorbing constituents in the ocean. Independent variability ofthese optically active constituents complicates remote sensing of chlo-rophyll a concentration (chl a) and hence the estimation of primaryproductivity from ocean color imagery (e.g. Antoine et al., 1996;Behrenfeld and Falkowski, 1997). Using standard empirical algorithmsto retrieve chl a often creates non-trivial uncertainties stemming from

by improving space-based estimates of biomass (Siegel et al., 2005),primary productivity (Behrenfeld et al., 2005), and the abundance ofCDOM and NAP (Siegel et al., 2002). More recently, Blanger et al.(2008) have built an empirical ocean color algorithm to retrieve thecontribution of CDOM to total absorption in coastal waters whereCDOM and NAP are more abundant and variable than in open oceansdue to stronger land inuences. The performance of this algorithm canbe improved by tuning with local optical data from the region of inter-est. In many estuarine and coastal waters, CDOM absorption in surfacewaters displays linear anti-correlations with salinity (Bowers andinconsistent correspondence between chl a aof CDOM and NAP (Siegel et al., 2005). Attempvelop semi-analytical remote-sensing algorith

Corresponding author. Tel.: +1 418 723 1986x1767E-mail address: huixiang_xie@uqar.qc.ca (H. Xie).

1 Present address: Qubec-Ocan, Facult des scienQubec, Qubec, Canada.

0304-4203/$ see front matter 2011 Elsevier B.V. Alldoi:10.1016/j.marchem.2011.10.001color imagery of open oceans into individual optical components (e.g.Garver and Siegel, 1997;Maritorena et al., 2002; Lee et al., 2002), there-1. IntroductionOptical propertiesAbsorptionMolecular weightWater mass tracerEstuarine mixingEstuaryFjordstream and with depth and showed an inverse relationship to tidal cycles. Phytoplankton absorption in sur-face water of the SLE increased seaward while non-algal particle absorption trended oppositely; bothvariables declined with depth. Surface water CDOM absorption surpassed particle absorption in the SLEwhile phytoplankton absorption dominated in the NWG. Elevated non-algal and CDOM absorption werefound in the turbidity maximum zone near the head of the SLE. Enriched CDOM absorption also occurredin the bottom water of the lower SLE and NWG. The spectral slope ratio of CDOM absorption, dened asthe ratio of the spectral slope between 275 and 295 nm to that between 350 and 400 nm, was conrmedto be an indicator of the source and molecular weight of CDOM. This surrogate functionality, however, failedfor absorption spectra exhibiting shoulders in short ultraviolet wavelengths observed in deep waters of theSLE and NWG. CDOM absorption mainly displayed conservative mixing behavior in both the SLE and the Sag-uenay Fjord. CDOM was employed to trace the source identity of the Fjord's deepwater. It was found that themarine end member of the Fjord's deepwater possessed a salinity of 32.92 and a temperature of ca. 1 C andoriginated from the intermediate cold layer of the lower SLE. The marine end member contributed 94% of thedeepwater by volume while freshwater mainly own from the Saguenay River supplied the remaining 6%.Implications of our results for remote sensing-based assessments of primary productivity, surface water cir-culation, and water column photochemistry in the SLE are also discussed.

2011 Elsevier B.V. All rights reserved.Received in revised form 1 October 2011Accepted 3 October 20112007 in the St. Lawrence estuary (SLE, Canada), the northwestern Gulf of St. Lawrence (NWG), and theSaguenay Fjord (CDOM only), the main tributary of the SLE. CDOM absorption generally decreased down-Article history:Received 15 March 2011

Absorption spectra of chromThe dynamics of absorption coefcients oestuarine system: Biogeochemical and ph

Huixiang Xie a,, Cyril Aubry a,1, Simon Blanger b, Gua Institut des sciences de la mer de Rimouski, Universit du Qubec Rimouski, Rimouski, Qb Dpartement de biologie, chimie et gographie, Universit du Qubec Rimouski, Rimous

a b s t r a c ta r t i c l e i n f o

j ourna l homepage: www.end the optical propertiests have beenmade to de-ms to decompose ocean

; fax: +1 418 724 1842.

ces et gnie, Universit Laval,

rights reserved.DOM and particles in the St. Lawrenceical implications

heng Song a

ec, Canada G5L 3A1ubec, Canada G5L 3A1

emistry

v ie r .com/ locate /marchemBrett, 2008 and references therein), prompting the notion of trackingsalinity elds through remote sensing-based observation of CDOM(Bowers and Brett, 2008; Sasaki et al., 2008). Particles, however, inter-fere with this application. To properly apply and interpret ocean colordata, therefore, critically relies on our knowledge of the relative compo-sitions of diverse light absorbers.

The optical properties of CDOM and NAP affect not only the applica-tion and interpretation of ocean color imagery but also the functioning

of marine ecosystems. Because of its strong absorption at the UVwave-lengths (280400 nm), CDOMmitigates the detrimental effects of solarUV radiation onmarine organisms (Zepp, 2003). On the other hand, thecompetition of CDOM andNAP for visible radiationmay reduce primaryproduction in coastal waters laden with these non-algal constituents(Mei et al., 2010). More recently, CDOM has been employed to traceglobal ocean biogeochemistry (e.g. the oxidation of sinking organicma-terials) and water mass circulation and mixing processes (Nelson et al.2007, 2010; Yamashita and Tanoue, 2009). In coastal waters CDOM isoften a dominant fraction of dissolved organic matter (DOM) andhence plays crucial roles in major biogeochemical cycles (Blough andDel Vecchio, 2002). The changes in optical properties of CDOM acrossland-ocean transitional zones are often indicative of shift of the originof CDOM (marine vs. terrestrial) and/or of processing of CDOM by var-ious physicochemical processes, such as photobleaching and variationsin pH and ionic strength (Minor et al., 2006; Helms et al., 2008).

1.2. The St. Lawrence estuarine system

The St. Lawrence estuary (SLE) and the Gulf of St. Lawrence (GSL),referred to as the St. Lawrence estuarine system (SLES) herein, is asemi-closed water body with connections to the northwest Atlantic

winter and extending from ~50 to 150 m, and (3) a warmer (36 C)and saltier (3435) landward-owing bottom layer (>150 m deep)with a mixture of Labrador Current and North Atlantic Central waters.The SLES boasts various hydrographic and oceanographic features,including runoff plumes from major tributaries such as the SaguenayFjord, upwelling near the head of the LSLE, and a year-round turbiditymaximum zone near the upper limit of the SLE (d'Anglejan, 1990;Koutitonsky and Bugden, 1991). The turbidity maximum zone extendsfor ca. 120 km downstream of le d'Orlans (d'Anglejan and Smith,1973) and ismaintained by a complex tidally forced, density-driven cir-culation in combination with the resuspension of bottom sediments(Silverberg and Sundby, 1979).

The St. Lawrence River delivers to the GSL 1.52106 t yr1 of dis-solved organic carbon and 0.31106 t yr1 of particulate organic car-bon (Pocklington and Tan, 1987). DOM and particulate organic matter(POM) in the USLE are predominantly of terrigenous origin (Tremblayand Gagn, 2009). POM in surface waters of the LSLE primarily origi-nates from terrestrial runoff in spring and autochthonous productionin summer (Gobeil, 2006). POM in the bottom water of the LSLE isconsidered to have a stronger terrigenous character than its surfacewater counterpart (Gobeil, 2006). The origin of DOM in the LSLE isless clear but its humic fraction is mostly terrigenous in both the sur-

45H. Xie et al. / Marine Chemistry 128-129 (2012) 4456Ocean (Fig. 1). The St. Lawrence drains a basin area of approximately~1.3106 km2 and ranks as the second largest river system in NorthAmerica with a freshwater discharge of ~375 km3 yr1 at QuebecCity (El-Sabh and Silverberg, 1990). The St. Lawrence estuary is tradi-tionally divided into the upper estuary (USLE), situated between led'Orlans and the mouth of the Saguenay Fjord, and the lower estuary(LSLE) stretching from there to Pointe-des-Monts with the GSL far-ther downstream. The USLE averages only ~60 m deep, sharply con-trasting the deep Laurentian Channel (250500 m deep) thatextends from the head of the LSLE to the eastern Canadian continentalmargin (El-Sabh, 1988). The USLE possesses a strong surface salinitygradient encompassing freshwater near Quebec City to brackishwater of salinity ~23 near the mouth of the Fjord. Vertically, theUSLE is well mixed near its headwater from Quebec City to slightlydownstream of le d'Orlans but becomes partially stratied fartherseaward (Painchaud and Therriault, 1989; Gobeil, 2006). Throughoutmost of the year, the water column in the LSLE and the GSL is highlystratied and characterized by three distinct layers (Dickie and Trites,1983): (1) a thin (b50 m), seaward-owing surface layer having atemperature of 210 C and a salinity of 2532, (2) an intermediatecold (12 C) and saline (31.533) layer, formed at the surface inFig. 1. Study area and sampling stations. See Table 1 forface and deep waters (Tremblay and Gagn, 2009). The dynamics ofPOM and DOM in the GSL is roughly synchronized with in situ prima-ry production (Pickard et al., 2000; Romero et al., 2000), suggestingthat in situ biological processes are the principal organic matter pro-duction pathways in this region. Increased autochthonous and/or ter-restrial input of organic materials are one of the hypotheses toexplicate the persistent, year-round hypoxia in the bottom 50 m ofthe water column in the central LSLE (Gilbert et al., 2005).

The Saguenay Fjord, draining a basin area of 8.8104 km2, is themost important tributary of the SLES due to its large freshwater dis-charge (~41 km3 yr1) and the unique geographic location of its out-ow which intersects the upper and lower St. Lawrence estuary. Thisnarrow (14 km), long (100 km) submerged valley is separatedfrom the SLE by a 20 m deep sill located at the mouth of the Fjord.A second sill, ~80 m deep and 18 km upstream from the rst, dividesthe Fjord into the outer (up to 250 m deep) and inner (up to 275 mdeep) basin with the latter occupying two-thirds of the Fjord (Schaferet al., 1990 and references therein). Cold, dense water from the LSLEis displaced over the sills into the deep basins by ood tides on timescales of only days to weeks (Siebert et al., 1979; Blanger, 2003),maintaining well oxygenated bottom waters and promotingcoordinates and total water depth for each station.

three times the standard deviation ofve replicate blankmeasurementsusing Nanopure water, was estimated to be 0.020.01 m1 over 250700 nm. Following the concept proposed by Helms et al. (2008),a spectral slope ratio (SR) for each acdom spectrum, dened as the ratioof the spectral slope between 275 and 295 nm to that between 350and 400 nm, was calculated to characterize CDOM. Spectral slopeswere estimated using nonlinear exponential tting as suggested byStedmon and Markager (2003) and Twardowski et al. (2004).

The absorption spectrum of particles retained on the lter, ap()(m1), was measured following the transmittance-reectance meth-od by Tassan and Ferrari (2002). The measurement was made be-tween 350 and 800 nm with 1-nm increments using the PerkinElmer spectrometer tted with a 50-mm integrating sphere. Prior tooptical measurements, each frozen lter was warmed to room tem-perature and hydrated with 12 mL of an aqueous sodium chloridesolution. The salinity of the sodium chloride solution was close tothat of the ltered seawater to minimize cell lysis. Hydratedparticle-free GF/F lters served as blanks that were subtracted togive ap(). A further correction on ap() was made by subtractingthe average of the ap() values between 750 and 800 nm from theap() values at b750 nm, assuming negligible absorption by parti-cles in the near infrared (Babin and Stramski, 2002). The absorptionspectrum of non-algal particles, anap() (m1), was determined

46 H. Xie et al. / Marine Chemistry 128-129 (2012) 4456considerable vertical stratication during most of the year. A strongpycnocline separates the thin (b5 m thick) freshwater surface layerfrom the thick, cold (0.52 C) and saline (salinity30) deep layer.The brownish surface layer is highly enriched with terrigenous DOM(Tremblay and Gagn, 2009).

Optically, surface waters in the SLES have been classied into oce-anic water-dominated Case 1 water in the GSL and freshwater-dominated Case 2 water in the SLE (Babin et al., 1993). Previouswork conducted in this region examined the spatial variability ofthe specic absorption coefcient of phytoplankton in summer(Babin et al., 1993), the effect of pigment packaging on phytoplank-ton absorption in spring (Roy et al., 2008), and the surface waterCDOM absorption and uorescence properties in summer (Nieke etal., 1997). The latter study found that the CDOM absorption and uo-rescence in the surface waters of the SLES are linearly correlated andthat both properties inversely correspond to salinity. However, theseprior studies lack subsurface data, simultaneous CDOM and particlemeasurements, and sufcient spatial coverage in some cases (e.g.Roy et al., 2008), thereby limiting our ability to adequately elucidatethe dynamics and the relative importance of CDOM and particle ab-sorption in the SLES. Such information is crucial for probing biogeo-chemical cycles and remote-sensing applications in this region.Besides, the potential of CDOM as a physical tracer in the SLES hasbeen little explored despite an intense interest in water mass circula-tions in this area, such as deepwater formation in the Saguenay Fjord(e.g. Therriault and Lackroix, 1975; Siebert et al., 1979; Blanger,2003). The present study concurrently determined the spectral ab-sorption coefcients of both dissolved and particulate materials inthe whole water column and expanded the sampling area by includ-ing the Saguenay Fjord. It also for the rst time quantied the molec-ular weight of CDOM in this region. Using these more comprehensivedata, we aim to better understand the dynamics of CDOM and particleabsorption and their implications for biogeochemical cycling of theseconstituents in the SLES; to examine the relative importance of vari-ous light absorbers and its remote-sensing applications in the studyarea; and to explore the possibility of using CDOM as a tracer to iden-tify the origin and quantify the mass composition of the deepwater inthe Saguenay Fjord.

2. Methods

2.1. Sampling

Samples were collected aboard the R/V Coriolis II from 3 to 11 May2007. Eleven stations were distributed along the main longitudinalaxis of the SLES from the upstream limit of the SLE near Quebec Cityto the northwestern GSL (NWG) (Fig. 1). The ship also visited fourstations on an along-channel transect in the inner basin of the Sague-nay Fjord (Fig. 1). Depending on the total water depth and the inten-sity of vertical stratication of the sampling station, one or twodepths were sampled in the USLE, ve depths in the LSLE and theNWG, and four or ve depths in the Fjord (Table 1). A time-series sta-tion (station TS in Fig. 1), slightly outside the mouth of the Fjord, wasestablished to study the effects of tidal movement and river runofffrom the Fjord on the optical properties of the water column. Samplesat the time-series station were taken about every 2 h at three depths(Table 1), starting at 23:40 on 9 May and terminating at 19:35 on 10May (east standard time, EST).

Bulk water was taken using 12-L Niskin bottles attached to a stan-dard conductivitytemperaturedepth (CTD) rosette. Samples forCDOM absorption and molecular weight (MW) measurements weregravity-ltered using sterile Pall AcroPak 500 capsules sequentially con-taining 0.8-mand 0.2-mpolyethersulfonemembrane lters. The cap-sules were connected to the Niskin bottles' spigot with clean silicontubing. Prior to sample collection, the capsules were thoroughly rinsed

with Nanopure water and then with sample water. Filtered water wastransferred into acid-cleaned 60-mL clear-glass bottles and stored inthe dark at ~4 C until analysis within two weeks of sample collection.For particle absorption measurement, a water volume of a few millili-ters to 2 L, depending on particle load, was ltered onto a pre-combusted 25-mm GF/F glass ber lter (Whatman). Filters werethen kept at80 C in darkness for up to 1 month before analysis.

2.2. Analysis

All measurements were made in an onshore laboratory at Rimouski.CDOM absorbance spectra were recorded at room temperature from250 to 800 nm at 1 nm increments using a dual beam UVvisible spec-trometer (Perkin Elmer, Lambda 35) tted with 10 cm quartz cells andreferenced to Nanopure water. A baseline correction was applied bysubtracting the absorbance value averaged over an interval of 5 nmaround 685 nm from all the spectral values (Babin et al., 2003). CDOMabsorption coefcient at wavelength , acdom() (m1), was calculatedas 2.303 times the absorbance divided by the cell's light path length inmeters. The lower detection limit of acdom measurement, dened as

Table 1Sampling locations and depths. All stations were sampled for CDOM absorption and mo-lecular weight measurements but only stations starting with SLwere sampled for parti-cle absorption measurement. Keys: USLE = upper St. Lawrence estuary; LSLE = lowerSt. Lawrence estuary; NWG= northwestern Gulf of St. Lawrence.

Region Station Position Total depth(m)

Sampling depth(m)

Latitude(N)

Longitude(W)

USLE SL1 46.74 71.30 58 2SL2 47.14 70.67 15 2SL3 47.28 70.54 22 2, 10SL4 47.52 70.16 65 2, 55SL5 47.83 69.82 77 2, 20, 65TS 48.13 69.62 38 2, 10, 30

LSLE SL6 48.29 69.33 322 2, 20, 50, 150, 250SL7 48.48 69.04 320 2, 20, 50, 150, 250, 305SL8 48.70 68.67 350 2, 20, 50, 150, 250, 332SL9 48.92 67.96 300 2, 20, 50, 150, 250SL10 49.18 67.17 328 2, 20, 50, 150, 250, 315

NWG SL11 49.42 64.75 378 2, 20, 50, 150, 250, 365SaguenayFjord

SAG5 48.41 70.83 90 2, 10, 50, 70SAG15 48.36 70.70 225 2, 10, 50, 150, 210SAG30 48.36 70.40 260 2, 50, 150, 250SAGET 48.32 70.31 255 2, 10, 50, 150, 245after pigment bleaching with methanol (Kishino et al., 1985) and

Seabird (model SBE9plus) CTD proler. The CTD salinity data werevalidated with salinity values of discrete rosette samples analyzed

47H. Xie et al. / Marine Chemistry 128-129 (2012) 4456using a Portasal (model 8410A) salinometer.

3. Results

3.1. General hydrography

The monthly mean freshwater runoff of the St. Lawrence River atQuebec City was 12570.7 m3 s1 in May 2007, 18% lower than thatin April and 15% higher than the annual mean runoff for the sameyear according to data published by St. Lawrence Global Observatory(http://slgo.ca/en/runoffs/data/tables.html). Surface water (2 mdeep) salinity increased seaward from essentially zero at the headof the USLE (Stn SL1) to 31.85 at the downstream limit of the LSLE(Stn SL10) while temperature decreased from 9.49 to 1.53 C acrossthe same area. Stn SL11 in the NWG was likely located at the outeredge or within the meanders of the fresher and warmer Gasp current(Sheng, 2001) and thus had a slightly lower salinity (31.62) and highertemperature (1.80 C) than Stn SL10 farther upstream. In the SaguenayFjord, surface salinity and temperature both increased with down-stream distance from Stn SAG5 but exhibited rather narrow ranges ofvariation (salinity: 4.517.14; temperature: 3.324.64 C).

The water column of the USLE was fairly well mixed near its headand became increasingly stratied toward its downstream limit at themouth of the Fjord (Fig. 2). The water column was stratied through-out the LSLE, the NWG, and the Fjord with the latter having the shal-lowest and steepest pycnocline (Fig. 2). The surface mixed layerdepth, based on the 0.05 kg m3 density difference criterion, wasb1 m in the Fjord, b10 m at Stns SL1SL9 and 20 m at Stn SL10 inthe SLE, and 12 m at Stn SL11 in the NWG. The core of the intermedi-ate cold layer in the LSLE and NWG (Fig. 2, Stns SL7 and SL11) wassituated between 36 and 64 m at the time of survey and its lowerlimit, dened as the depth with a temperature of 2 C, between 77subjected to the same blank and baseline corrections as for ap(). Wealso tried sodium hypochlorite (Ferrari and Tassan, 1999) but found itto be less effective than methanol to extract pigments. The phyto-plankton absorption coefcient, aph() (m1), was estimated as thedifference between ap() and anap().

The molecular weight (MW) of CDOM was determined usinghigh-pressure size exclusion chromatography (HPSEC) according tothe method reported by Chin et al. (1994) and modied by Lou andXie (2006). Briey, samples (100 L) were injected at controlled tem-perature (20 C) into a Waters Protein-Pak 125 column carrying amobile phase of 0.1 mol L1 sodium chloride aqueous solution buff-ered with phosphate to pH 6.8. The absorbance of eluents wasdetected at 254 nm with a Waters 2487 dual- absorbance detector.Peak integration was performed using Waters Breeze GPC software.The void volume and total permeation volume of the column weredetermined using blue dextran and acetone, respectively. The systemwas calibrated with acetone (MW: 58) and sodium polystyrene sulfo-nate standards (PPS, Polysciences, MA) with molecular weights of 1.8,4.6, 8, 18, and 35 kDa. Data from the PPS 1.8 kDa were excluded be-cause it displayed wide multiple peaks, as also reported by Zhou etal. (2000). Calculation of number- and weight-average MW followedpublished formulae (Chin et al., 1994). The ratio of weight-averageMW (Mw) to number-average MW (Mn) gives polydispersity, a mea-sure of the heterogeneity of CDOM (Chin et al., 1994). The systemwasperiodically checked with the International Humic Substances Socie-ty's standard humic substance: the Suwannee River fulvic acid(SRFA). Our values (Mn: 117966 Da; Mw: 200067 Da) agreedwithin 7% of those obtained by Zhou et al. (2000) using the HPSECtechnique.

Vertical proles of temperature and salinity were acquired with aand 132 m.3.2. Distributions of CDOM optical properties and molecular weight

The relatively coarse horizontal and vertical sampling resolutions(Fig. 1 and Table 1), in conjunction with the nonsynoptic nature ofour data, make it inappropriate to describe the distributions ofCDOM properties by contour mapping. We thus use typical prolesand statistic results to characterize the main distributional featuresof these properties. CDOM absorption coefcient typically decreasedwith increasing wavelength in the UV and visible regimes and formost samples the trend was roughly exponential (Fig. 3A). However,shoulders over 255300 nm were conspicuous in acdom spectra ofsamples collected from the bottomwater of Stn SL10 and from depthsbelow 50 m at Stn SL11 (Fig. 3B). Ranges, means, and standard devi-ations of acdom, SR, and Mw, along with salinity ranges, are shown inTable 2. Briey, CDOM was much more abundant in the Fjord thanin the SLES. Mw exceeded 1 kDa in the USLE and in the top 10-mlayer of the Fjord but was mostly b1 kDa in the LSLE, in subsurfacewater (50 m) of the Fjord, and at Stn TS. The lowest Mw valueswere found in the bottom water of the LSLE (~0.50 kDa). acdom andMw generally decreased from upstream to downstream in both theSLES and the Fjord and from surface to bottom in all three studyareas. SR roughly showed inverse patterns. Variabilities in acdom, SR,and Mw well corresponded to those in salinity, shrinking with depthin the SLES and at Stn TS and diminishing from the intermediatelayer toward both the surface and bottom in the Fjord. Variations inSR and Mw were usually smaller than those in acdom.

Vertical proles of CDOM optical properties and Mw revealed nerstructures. acdom in stratiedwaters exhibited little variation in the sur-face mixed layer, a swift drop within the pycnocline, and a gradual de-crease further downward (Fig. 4).When approaching the bottom, acodmeither remained constant (Stns SL7, SL10 and SL11) or slightly increasedwith depth (Stns SL6, SL8, and SL9) (data not shown). Mw also de-creased with depth but with gentler gradients, especially within thepycnocline (Fig. 4). SR steadily increased with depth in the downstreamsection of the USLE having signicant vertical stratication (Fig. 4, StnSL5). The vertical distribution of SR in the LSLE and the NWG was char-acterized by an increase in SR with depth in the upper water layer fol-lowed by a reversed trend toward deeper depths (Fig. 4, Stns SL7 andStn SL11). This reversal occurred mostly below 150 m in the LSLE(Fig. 4, Stn SL7) but at a much shallower depth (50 m) at Stn SL11 inthe NWG (Fig. 4). Notably, the SR values in bottom waters of Stn SL10(data not shown) and Stn SL11 (Fig. 4) are lower than those at the sur-face, a phenomenon that was absent on other occasions. SR in the Fjordaugmented with depth within the upper layer (b50 m deep) butremained rather constant below the pycnocline (Fig. 4, Stn SAG30).

Detailed time and depth variations of salinity and CDOM opticalproperties at Stn TS are displayed in Fig. 5. acdom decreased withdepth throughout the 20-h sampling period with the diminutionbeing faster in the upper layer than in the lower layer (Fig. 5B). Thispattern well reected the mainly salinity-driven stratication of thewater column (Fig. 5A). The temporal progression of acdom inverselycorresponded to that of salinity which in the upper layer principallyfollowed the semidiurnal tidal cycle in this area (El-Sabh, 1988). Con-sequently, acdom in the upper layer (2 and 10 m in Fig. 5B) reachedmaxima at the low tides (01:35 and 13:40 EST) and a minimum atthe high tide (07:35 EST). Tidal actions merely affected the bottomlayer (30 m in Fig. 5A, B). The low salinity and high acdom at 09:44EST, which were inconsistent with the tidal cycle, were due to astronger inuence of the runoff plume from the Fjord as visually con-rmed from the ship. Evidently, this intensied runoff inuence atthe sampling site was ephemeral and restricted to a thin surfacelayer that did not extend to 10 m deep (Fig. 5A, B). Overall, verticaland temporal distributions of SR at Stn TS were in line with those ofsalinity but inverse to those of acdom (Fig. 5 C). Yet, the variation inSR was relatively small both vertically (maximum difference of 0.35

at 13:40 EST) and temporally (maximum difference of 0.34 at 2 m).

Fig. 2. Typical vertical proles of temperature, salinity, and sigma-t in the upper St. LawrenceSt. Lawrence (Stn SL11), and Saguenay Fjord (Stn SAG30).

Fig. 3. Typical CDOM absorption spectra (A) and those showing noticeable shoulders inthe short ultraviolet region (B).

48 H. Xie et al. / Marine Chemistry 128-129 (2012) 44563.3. Relationships among CDOM optical properties, molecular weight,and salinity

estuary (Stns SL2 and SL5), lower St. Lawrence estuary (Stn SL7), northwestern Gulf ofacdom in each study area was strongly inversely correlated with sa-linity (Fig. 6 and Table 2). Intercepts and slopes of the tted lines,however, vary widely, with the Fjord having the largest interceptand steepest slope followed sequentially by Stn TS and the SLES. Acloser examination of the SLES data reveals that (1) measured acdomvalues at depths150 m are systematically higher (ANOVA, p0)than those predicted from the 2-m mixing line (Fig. 6; measuredacdom(350)/predicted acdom(350)=6.55.2); (2) measured acdomvalues at shallower depths are not signicantly different (ANOVA,p>0.62) from those predicted from the 2-m mixing line (Fig. 6; mea-sured acdom(350)/predicted acdom(350)=1.030.13); (3) acdom waslower at Stn SL1 at the head of the estuary than at Stn SL2 slightly down-stream (acdom (350): 6.80 vs. 7.13 m1) despite the lower headwatersalinity (0.00 vs. 0.713).

SR in the SLES increased with salinity for samples collected fromdepths65 m; SR for most samples from depths150 m, however,strayed far from this trend, exhibiting much lower values thanexpected and a large variability (0.761.57) within a narrow salinityrange (33.0834.79) (Fig. 7). It is noteworthy that data points forthe upper 65 m layer closely follow the 2-m conservative mixingline constructed according to the approach of Stedmon and Markager(2003). SR in the Fjord also increased with salinity but was constantlylower than that in the SLES. SR values at Stn TS were in line with thosein the Fjord at relatively low salinities and with those in the SLES athigher salinities (Fig. 7).

Regardless of sampling areas and depths, Mw generally decreasedwith salinity, faster at low (b8) and high salinities (>27) and slowerat intermediate salinities (827) (Fig. 8A). Mw was consistentlyhigher in the Fjord than in the SLES at salinityb21 but convergedwith the latter and Mw for Stn TS at higher salinities (Fig. 8A). Thepolydispersitysalinity plots for the SLES and Fjord exhibited an up-ward trend at low salinities, a broad plateau at intermediate salinities,

and a downward trend at high salinities (Fig. 8B). The polydispersityvalues of the Fjord were comparable to those of the SLES at salinityb6but greater than the latter at higher salinities. Polydispersity data col-lected from Stn TS are rather scattered and are often above those fromthe SLES and Fjord at similar salinities (Fig. 8B).

The compositeMwSR plot shows thatMw decreased with increas-ing SR irrespective of sampling locations and depths excluding thebottom layer (150 m) in the LSLE and NWG (Fig. 9). Like SR vs. salin-ity (Fig. 7), Mw for most of the bottom-layer samples was lower thanexpected from SR, the deviation aggravating rapidly with decreasingSR. A single exponentially decay function with a positive Y-axis inter-cept well describes the rest of the dataset.

3.4. Particle absorption

Total particle absorption (ap) in surface water of the SLES was high-est at Stn SL2, leveled off seaward, and ascended toward the lower limit

of the SLE and the NWG (Fig. 10). Particle absorption spectra in theNWG and LSLE manifested important phytoplanktonic contributions(Fig. 11). aph(440) in surface water contributed 7389% of ap(440) inthe NWG and its neighboring LSLE, 1242% further upstream in theLSLE, and 017% in the USLE (Fig. 10). anap(440) thus dominated overaph(440) across the SLES excluding the NWG and the downstreamLSLE where anap(440) was low but signicant. Vertically, anap(440)and aph(440) in the NWG and LSLE decreased rapidly near surface anddisplayed much less variations at depth. The contribution of aph(440)to ap(440) diminished from 80% within the upper 10 m to 40% at150 m and became fairly constant further downward (Fig. 12). Surfacewater in the NWG displayed characteristic chlorophyll absorption

Table 2Ranges, means, and standard deviations (s.d.) of CDOM absorption coefcients at 350 and 412 nm, spectral slope coefcient ratios, and weight-average molecular weights. acdom(412) is shown due to its relevance to space-based ocean color imaging. Key: SLES = St. Lawrence estuarine system.

Region/Station Depth (m) Salinityrange

acdom(350) (m1)Range (mean, s.d.)

acdom(412) (m1)Range (mean, s.d.)

SRRange (mean, s.d.)

Mw (kDa)Range (mean, s.d.)

SLES 2 0.0031.62 0.497.13 (3.02, 2.37) 0.162.46 (1.06, 0.81) 0.871.31 (1.02, 0.14) 0.641.84 (1.14, 0.40)1065 14.0032.54 0.414.04 (1.03, 0.94) 0.141.48 (0.39, 0.34) 0.941.58 (1.24, 0.19) 0.581.20 (0.78, 0.18)150 33.0834.79 0.220.53 (0.36, 0.10) 0.070.27 (0.16, 0.06) 0.761.57 (1.27, 0.23) 0.430.61 (0.50, 0.05)

Saguenay Fjord 2 4.517.14 13.415.6 (14.6, 1.02) 4.945.80 (5.40, 0.39) 0.820.84 (0.83, 0.01) 1.562.05 (1.74, 0.23)1070 19.8829.59 1.698.03 (3.79, 2.60) 0.642.98 (1.42, 0.96) 0.861.02 (0.95, 0.07) 0.931.35 (1.10, 0.18)150 30.7131.17 1.311.55 (1.42, 0.08) 0.470.56 (0.53, 0.04) 0.961.06 (1.00, 0.04) 0.840.98 (0.89, 0.06)

Stn TS 2 21.5430.50 0.853.94 (2.09, 0.94) 0.321.52 (0.79, 0.36) 0.911.28 (1.01, 0.10) 0.661.13 (0.87, 0.15)10 28.7431.90 0.551.88 (1.15, 0.45) 0.200.69 (0.44, 0.18) 0.961.22 (1.10, 0.08) 0.661.14 (0.86, 0.14)30 31.1332.24 0.451.46 (0.71, 0.31) 0.160.58 (0.27, 0.13) 1.071.35 (1.23, 0.08) 0.560.96 (0.71, 0.14)

49H. Xie et al. / Marine Chemistry 128-129 (2012) 4456Fig. 4. Examples of vertical proles of CDOM absorption coefcient at 350 nm (acdom(350)), spectral slope (SR), weight-average molecular weight (Mw), and polydispersity(PD) in the upper St. Lawrence estuary (Stn SL5), lower St. Lawrence estuary (Stn SL7),northwestern Gulf of St. Lawrence (Stn SL11), and Saguenay Fjord (Stn SAG30).Fig. 5. Temporal evolution and vertical distribution of salinity (A), CDOM absorptioncoefcient at 350 nm (acdom(350)) (B), and spectral slope (SR) (C) at the time-seriesstation occupied between 9 and 10 May.

4. Discussion

4.1. Estuarine mixing behavior of CDOM and particles

The strong linear correlations between acdom and salinity observedin the SLES (above 65 m) and Saguenay Fjord (Fig. 6 and Table 2)imply either a conservative mixing behavior of CDOM or a balance

Fig. 6. CDOM absorption coefcient at 350 nm (acdom(350)) versus salinity. Keys: SL =the St. Lawrence estuarine system, SAG = Fjord, TS = time-series station. Black, blue,and pink solid lines are best ts of the St. Lawrence, Fjord, and time-series data, respec-tively. The St. Lawrence regression line excludes data from Stn SL1 and depths150 m.Fitted parameters are shown in Table 3. Black broken line is the SL's 2-m mixing line,

50 H. Xie et al. / Marine Chemistry 128-129 (2012) 4456peaks around 675 and 440 nm that greatly abated at depth and essen-tially disappeared near the bottom (Fig. 11). aph(440) at 2 m increasedexponentially with salinity (Fig. 13). Excluding Stn SL1, anap(440) eitherat 2 m only or for all depths combined decreased exponentially with sa-linity. anap(440) approached constant values at sufciently high salin-ities (Fig. 13).

If we dene total non-water absorption (at-w) as the sum of acdom,aph, and anap, then the contribution of aph(440) to at-w(440) was es-sentially negligible in the USLE and near the head of the LSLE(03.5%), small but signicant in the middle section of the LSLE(Stns SL7SL9, 624%), and elevated in the downstream LSLE andNWG (~63%) (Fig. 14). The contribution of anap(440) to at-w(440) in-creased quickly from 16% at the head of the USLE to 58% in the turbid-ity maximum zone and declined progressively seaward over the

taking Stn SL2 (salinity: 0.72; acdom(350): 7.13 m1) and SL10 (salinity: 31.85; acdom(350): 0.49 m1) as the fresh and marine end-members, respectively.rest of the USLE (4013%). It remained low and relatively constant(92%) across the entire LSLE but rebounded to 15% in the NWG.acdom(440) accounted for 59% of at-w(440) throughout the sampledtransect excepting the turbidity maximum zone (Stn SL2), the down-stream LSLE (Stn 10), and the NWG (Stn SL11).

Fig. 7. Spectral slope ratio (SR) versus salinity. Solid line is the 2-m conservative mixingline connecting Stn SL2 near the head of the St. Lawrence estuary to Stn SL10 in thenorthwestern Gulf of St. Lawrence. Keys are the same as those in Fig. 6.Fig. 8.Weight-average molecular weight (Mw) (A) and polydispersity (B) versus salin-ity. All sampled depths are included. Keys are the same as those in Fig. 6.of production and loss of CDOM across the freshwater-saltwater tran-sition zones in the two environments. However, the agreement be-tween the measured and modeled SR (Fig. 7), a more characteristicproperty of CDOM (Helms et al., 2008), reinforces the argument ofconservative mixing rather than of balanced sources and sinks. Notethat conservative mixing of CDOM occurs not only horizontally but

Fig. 9. Weight-average molecular weight (Mw) versus absorption spectral slope ratio(SR). All data points but SL's with depths150 m were tted to the equation ofY=793.6exp(7.93X)+0.66 (R2=0.807, N=75, pb0.0001). Keys are the sameas those in Fig. 6.

also vertically since subsurface CDOM data also follow the mixinglines (Figs. 6 and 7). This suggests downward transport of CDOM dur-ing its seaward transit despite intense vertical stratications prevail-ing in the LSLE, NWG, and Fjord (see Section 3.1). In line with this twodimensional CDOM transfer scheme, Tremblay and Gagn (2009)have revealed that dissolved humic substances are mostly terrige-nous in both the surface and deep waters of the LSLE. Linear relation-ships between acdom and salinity in the SLES' surface water (25 mdeep) have also been reported for the summer season (Nieke et al., irradiation (Zhang et al., 2006), months-long surface water renewal

Fig. 10. Absorption coefcients (440 nm) of total particulate matter (ap), non-algal par-ticles (anap), and phytoplankton (aph) and the ratio of aph to ap in surface water (2 m)along the main axis of the St. Lawrence estuarine system.

Fig. 12. Vertical proles of absorption coefcients (440 nm) of non-algal particles(anap) and phytoplankton (aph) and the ratio of aph to total particulate matter (ap) atStn SL11 in the northwestern Gulf of St. Lawrence.

51H. Xie et al. / Marine Chemistry 128-129 (2012) 44561997; Zhang et al., 2006). These previous observations, however, dis-covered slope breaks at salinity ~29 that were postulated as an indi-cation of differing chemical compositions of CDOM between the SLEand GSL (Nieke et al., 1997). The present study demonstrates thatthis type of slope discontinuity is not a year-round phenomenon.

The conservative CDOM mixing behavior observed in the presentstudy thus indicates that occulation, particle adsorption, and photo-chemical and microbial degradations were insignicant CDOM sinksin the sampled area and season. The trivial role of physical CDOM re-moval is somewhat surprising given the high loads of suspended par-ticles in and near the turbidity maximum zone (Xie et al., 2009),which should facilitate CDOM occulation and adsorption (Uher etal., 2001; Shank et al., 2005; Guo et al., 2007b). The inconsequentialeffect of photochemical CDOM loss also appears unexpected consid-ering relatively easily measurable photobleaching under laboratoryFig. 11. Examples of particle absorption spectra in the lower St, Lawrence etimes (El-Sabh and Silverberg, 1990), and the strong downstreamvertical stratication (Fig. 2). Photobleaching could, however, havebeen suppressed due to higher freshwater ushing rates in earlyspring (Therriault et al., 1990), strong competition for UV radiationby NAP in the USLE (Fig. 14), and lower winter-to-early-spring solarradiations that are further diminished by extensive ice cover duringthe winter period (El-Sabh, 1988). The lack of detectable photo-bleaching in the SLES is not unprecedented. Bowers and Brett(2008) observed conservative CDOM mixing in the Clyde Sea, S.W.Scotland that is also characterized by intense water column stratica-tion and long ushing times of ~4 months. Physical dispersion canthus be an overarching control on the distribution of CDOM evenunder weak mixing and sluggish water renewal conditions. Indeed,Stedmon and Markager (2003) have suggested that some previouslyreported CDOM photobleaching events in coastal and shelf watersstuary (Stn SL7) and the northwestern Gulf of St. Lawrence (Stn SL11).

Fig. 13. Absorption coefcients (440 nm) of non-algal particles (anap) and phytoplank-ton (aph) versus salinity. Surface (2 m) anap data are tted to the equation ofY=2.36exp(0.15X)+0.0046 (R2=0.998, N=10, pb0.0001) (solid line) and sur-

5 2

52 H. Xie et al. / Marine Chemistry 128-129 (2012) 4456as inferred from the variability of the spectral slope can actually beexplained by conservative mixing. The insignicant photobleachingfound in the present study also coincides with the modeled estimatesof CDOM photomineralization in the estuarine section of the SLES,which can only account for 1.12.2% of the annual dissolved organiccarbon discharge from the St. Lawrence River (Zhang and Xie, inpress). It is noteworthy that the conservative behavior of acdom andSR does not imply that other CDOM properties are not impacted byestuarine processes. For instance, the trends of polydispersity againstsalinity in the SLES and Fjord (Fig. 8B) are incoherent with thedilution of freshwater CDOM of relatively high heterogeneity by ma-rine CDOM of lower heterogeneity. Further studies are required toelucidate the processes dictating the polydispersity during estuarinemixing.

The relatively weaker acdom-salinity correlation found at Stn TScan be attributed to multiple CDOM end members at this locationwhere three major water masses with contrasting CDOM abundancesconverge, i.e. the St. Lawrence River water, the Saguenay River water,and the North Atlantic seawater. The concentration of CDOM and itsvariability at Stn TS are thus controlled by the relative compositionof the three water masses, which is inuenced by riverine freshwateruxes, tidal actions, and directional uctuations of the Fjord's plume.That the variations of acdom and SR at Stn TS are bracketed by those ofthe SLES and Fjord (Figs. 6 and 7) supports the three-end member no-

face aph data to the equation of Y=5.8410 exp(0.27X) (R =0.848, N=11,pb0.0001) (dashed line). All-depth anap data are tted to the equation of Y=2.38exp(0.17X)+0.019 (R2=0.971, N=42, pb0.0001). anap at Stn SL1 (zero salinity) is ex-cluded from data tting.tion. This is also consistent with the elevated polydispersity values atStn TS relative to those along the main axes of the SLES and Fjord(Fig. 8B).

Fig. 14. Percentage contributions of phytoplankton absorption (aph), CDOM absorption(acdom), and non-algal particle absorption (anap) to total non-water absorption (at-w=aph+acdom+anap) at 440 nm in surface water (2 m) along the main axis of theSt. Lawrence estuarine system.In contrast to acdom, the highly non-linear relationships betweenparticle absorption coefcients, anap and aph, and salinity (Fig. 13)are indicative of non-conservative behavior of these variables. Thesharp increase in anap from Stns 1 to 2 (Fig. 10) is due largely to sed-iment resuspension in the turbidity maximum zone (see Section 1.2).The exponential decrease in surface water anap seaward from Stn SL2should primarily result from particle settling and biological degrada-tion. Note that we use Stn SL2, instead of Stn SL1, to describe the anapmixing behavior downstream of the turbidity maximum zone sinceriver-borne suspended particles owing into this turbidity zoneare subjected to substantial physical, chemical and biological alter-ations and long-term internal cycling via onshoreoffshore exchange(d'Anglejan, 1990). Rapid degradations of both terrigenous and ma-rine POM have been observed in the SLES (Tremblay and Gagn,2009; Bourgoin and Tremblay, 2010), in line with the quick down-ward decreases in anap and aph within the upper layer of the watercolumn (Fig. 12). The decline of the ratio of aph to ap with depth sug-gests faster degradation of fresh phytoplanktonic materials duringparticle sinking, corroborating the nding of preferential microbialreworking of nitrogen-rich POM (Bourgoin and Tremblay, 2010).The increase in surface water aph with salinity is consistent with pro-gressively increasing vernal primary productivity from the USLE toLSLE to GSL (Gobeil, 2006; Mei et al., 2010).

4.2. In situ production of CDOM

Although conservative mixing dominates the dynamics of acdom inthe SLES, certain features of the acdom distribution do suggest in situCDOM production. The higher acdom and lower SR values at Stn SL2(salinity 0.72) compared to its upstream neighbor Stn SL1 (salinityzero) counter the general land-to-sea trends of these two opticalproperties (Figs. 6 and 7). Elevated acdom values at Stn SL2 have alsobeen found in the autumn and winter seasons (H. Xie, unpublisheddata). Given the comparable salinity and pH (Zhang et al., 2008) be-tween the two stations, the increase in acdom at Stn SL2 indicateslocal CDOM production associated with high particulate loads in theturbidity maximum zone. This newly produced CDOM is of more ter-rigenous character than CDOM farther upstream since lower SR im-plies a stronger linkage to terrestrial origin (Helms et al., 2008).DOM leaching from soil- or sediment-derived suspended particles(Guo et al., 2007a), accelerated by intense turbulent mixing andphotodissolution (Kieber et al., 2006; Mayer et al., 2006), is likelythe chief CDOM production pathway in the turbidity maximumzone. More detailed studies are required to quantify the signicanceof this CDOM source in the SLES. As turbidity maxima are widely pre-sent in global estuaries, our study points to the need of an evaluationof the role of turbidity zones in CDOM and DOM cycling in other estu-arine environments.

The mechanisms for the formation of the short-wavelength shoul-ders in the acdom spectra (Fig. 3B) are unclear. Similar shoulders havebeen observed in acdom spectra of Pacic surface waters by Yamashitaand Tanoue (2009), who ascribed this feature to in situ production ofprotein-like materials or other bio-components by biological process-es. Phototransformations of biogenic dissolved and particulate mate-rials may also produce CDOM (Kieber et al., 1997; Mayer et al., 2009).If in situ CDOM production is primarily responsible for the acdomshoulders observed in the present study, this CDOM production is un-likely related to photochemical processes, since it occurs in deepwa-ter where no UV radiation can reach. Biological CDOM production isthen more likely. The vernal primary productivity and hence down-ward ux of planktonic detrital materials in the NWG and down-stream LSLE are typically far higher than the more inner section ofthe SLE (Mei et al., 2010). This explains why the production of newCDOM was observed only at the most marine locations Stns SL10and SL11 (Fig. 3B). It should be noted that certain inorganic com-

pounds, such as nitrate and nitrite, also absorb UV radiation. The

53H. Xie et al. / Marine Chemistry 128-129 (2012) 4456peak absorption wavelengths for nitrate (300 nm) and nitrite(350 nm) are, however, far longer than the maximum absorptionwavelengths for the shoulders we found.

Higher than expected acdom values in most bottom water samples(150 m deep) from the entire LSLE and the NWG (Fig. 6) implywidespread in situ CDOM production, diffusion of CDOM from sedi-ments, or a marine CDOM end member that is more elevated thanthat in the surface water. The bottomwater of the Laurentian Channeloriginates from a mixture of approximately half Labrador Currentwater and half North Atlantic Central water (Gilbert et al., 2005).According to Nelson et al. (2007), acdom(325) in subsurface waters(>100 m deep) of the North Atlantic continental shelf and openocean rarely exceeds 0.2 m1, which is far below acdom(325) in thebottom water of the LSLE and NWG (range: 0.360.76 m1; mean:0.52 m1). Local production in the water column or diffusion fromunderlying sediments should therefore be responsible for the higher-than-expected acdom in the bottomwater. The sedimentary diffusion ar-gument is supported by increasing acdom with depth near the bottomobserved at several stations in the LSLE (see Section 3.2). It is also con-sistent with gradually rising acdom(350) in the bottommost sampledwater from Stn SL11 (0.23 m1) upstream to Stn SL6 (0.49 m1)since the bottom water would continuously accumulate CDOM diffus-ing from the sediments during its landward transit. Benthic microbialreworking of POM (Bourgoin and Tremblay, 2010) could be a potentialsource of the sedimentary CDOM. Notably, the dominant component ofCDOM from the sediments should be lowMWmaterials (Fig. 8), plausi-bly low MW fulvic acids as revealed by a previous solid-phase extrac-tion study (Tremblay and Gagn, 2009).

Evidence in the literature exists that CDOM is produced andmaintained at high levels within marine sediments (Blough and DelVecchio, 2002 and references therein). The present study, alongwith some previously published data (e.g. Boss et al., 2001; Burdigeet al., 2004), qualitatively indicates that sediments represent a sourceof CDOM to the water column in coastal environments in addition toriverine input and in situ biological production. Based on the enrich-ment of CDOM in sedimentary pore water DOM and the signicanceof sedimentary dissolved organic carbon uxes relative to terrestrialinputs in coastal environments, Burdige et al. (2004) suggested thatthe coastal sedimentary CDOM source could be signicant comparedto riverine transport. Quantitative surveys of the sedimentary CDOMcontribution in coastal environments, including the SLES, are neededto verify this proposition.

4.3. SR as an indicator of CDOM characteristics

Helms et al. (2008) proposed the use of SR as indicators of source,molecular weight, and photochemical and microbial alterations ofCDOM. Several prominent ndings of these authors are: (1) terrestrialCDOM is characterized by low SR while marine CDOM by high SR;(2) photobleaching increases SR while microbial processing decreasesit; (3) SR is linearly anti-correlatedwithMw. The present study conrmsthat SR is a sensitive parameter to differentiate terrestrial from marineCDOM and that SR is related toMw. We also have evidence that photo-bleaching pushes SR upward (Y. Zhang and H. Xie, unpublished data).Our study, however, draws attention that caution should be exercisedto employ SR as a CDOM source indicator. The low SR values, in somecases even lower than the riverine endmembers, for the bottomwatersof the Laurentian Channel (Fig. 7) can be erroneously interpreted as in-trusions of terrestrial CDOM into deep depths without inspecting theacdom spectral shapes. Clearly, these excessively low SR values resultfrom the shoulders covering thewavelengthswithin which the spectralslopes between 275 and 295 nm were calculated. As discussed above,these shoulders are more likely associated with locally produced bio-molecules rather than terrestrial runoff (see Section 4.2). A secondpoint to be noted is that the exponential relationship between Mw and

SR found in the present study (Fig. 9) differs from the linear relationshipidentied by Helms et al. (2008). This discrepancy could arise from thedifferent Mw ranges examined by the two studies. The relationship ofHelms et al. (2008) was constructed from samples with Mw rangingfrom 1 to 3 kDa while ours was based on Mw from 0.4 to 2 kDawith the majority of the samples having Mwb1 kDa (Fig. 9). Like theSRsalinity relationship (Fig. 7), irregular acdom spectral shapes alsolead to violations of the normalMwSR relationship (Fig. 9).

4.4. Implications for ocean color imaging

Satellite or aircraft-based ocean color imaging is extensively usedto assess ocean primary productivity (e.g. Behrenfeld et al., 2005) andmonitor ocean circulation (Bowers and Brett, 2008) and CDOM-driven photochemical uxes (Blanger et al., 2008; Fichot and Miller,2010) through the retrieval of aph and acdom, respectively. High tur-bidity (anap), elevated CDOM (acdom) (Fig. 10), and strong mixing dy-namics greatly suppress primary productivity in the USLE. This, addedby its relatively small size, renders the USLE to be of little relevancefor space-based assessment of phytoplankton production. Primaryproductivity in the LSLE is considerably higher as a consequence of in-creased bottom nutrients upwelling, water clarity, and stratication.Nevertheless, the excessive dominance of acdom over aph in this region(Fig. 14) precludes the possibility of remote sensing-based estimationof chlorophyll concentrations using existing empirical ocean coloralgorithms. In the NWG and the mouth of the LSLE, aph prevailsover acdom and anap (Fig. 14), conrming previous ndings that thispart of the SLES is a Case 1 water system (Babin et al., 1993). Howev-er, the signicant presence of CDOM and NAP in this region requiresthe use of semi-analytical algorithms capable of separating aph fromacdom and anap, such as the GSM (Garver and Siegel, 1997; Maritorenaet al., 2002). We tested the applicability of the GSM to the SLES by re-trieving the sum of acdom(443) and anap(443) from the merged L3monthly composite of SeaWiFS and MODIS-Aqua using the GSM(Fig. S1). The satellite data were provided by the NASA MEaSUREsOcean Color Product Evaluation Project (http://wiki.icess.ucsb.edu/measures/index.php/main_page). The retrieved May 2007 meanvalue (0.340.16 m1) agrees well with the in situ value (0.320.12 m1) for the LSLE and NWG. The GSM, however, severely under-estimates the aggregate acdom(443) and anap(443) value (satellite:0.360.08 m1; in situ: 1.170.6 m1) in the USLE due to high tur-bidity, which completely incapacitates the GSM retrieval upstream ofmid-way of the USLE (Fig. S1). On an average basis, the present studythus validates the applicability of the GSM to the LSLE and GSL in thespring season but invalidates its suitability to the USLE. As surfacewater turbidity in summer and autumn is much lower than in spring(Xie et al., 2009), the validity of the GSM may be extended to thesetwo seasons, covering at least the same sections of the SLES as thosein spring. Winter is excluded from discussion due to extensive icecover at this time of the year. Given that the LSLE is typically of Case2 water character with CDOM accounting for up to 86% of the non-water total absorption (Fig. 14), the GSM, which has been principallyapplied to open-ocean (Case 1) waters, should also be useful forcoastal waters in general without excessive turbidity (i.e. CDOM-dominated Case 2 water).

The excellent linear relationship between acdom and salinity in theSLES allows tracking salinity elds from space if acdom can be re-trieved from the bulk absorption coefcient. Blanger et al. (2008)have recently developed an ocean color algorithm to retrieve the con-tribution of acdom to the total absorption coefcient (at). Combiningthis algorithm with any validated inversion models for at (IOCCG,2006) permits acquiring the absolute value of acdom, as has been dem-onstrated by Blanger (2006) for the coastal Beaufort Sea. Such algo-rithms may, however, necessitate regional tuning with in situ databefore their applications to the SLES since the spectral inherent opti-cal properties controlling the performance of these algorithms often

differ from one environment to another. Data from the present

study are instrumental for this purpose. The regionally tuned algo-rithms may also be used to separate the solar irradiances absorbedby particles from those by CDOM in the water column of the SLES,thereby permitting to decipher the relative importance of particlevs. CDOM photochemistry (Xie and Zariou, 2009).

4.5. CDOM as a tracer of deepwater formation in the Saguenay Fjord

Although there is no doubt that the deepwater in the SaguenayFjord originates from the SLE and is modied by the Saguenay Riverwater (Schafer et al., 1990; Blanger, 2003), it remains debatable orunclear regarding the source water identity in the water column ofthe estuary and the relative composition (salt vs. fresh) of the Fjord'sdeepwater. For example, Therriault and Lackroix (1975) ascribed thedeepwater in the Fjord to the penetration of the surface water of theestuary based on the similarity in physicochemical characteristics be-tween the two water bodies and on the presence of elevated subsur-face chlorophyll concentrations in the Fjord. In a more extensive

the Saguenay Fjord. Atypical CDOM absorption spectra may, how-ever, invalidate this surrogate functionality.

4. The conservative mixing behavior of CDOM in the SLES and theSaguenay Fjord provides a unique tool to identify the source andwater mass composition of the Fjord's deepwater. The marineend member of the Fjord's deepwater was traced to the intermedi-ate cold layer of the LSLE and comprised 94% of its volume.

5. The GSM ocean color algorithm can be potentially applied to theCDOM-dominated LSLE but is not suitable for the more turbidUSLE. The strong linear relationship between CDOM absorptionand salinity in the surface water of the SLES also offers possibilityfor space-based monitoring of the surface salinity eld in thisregion.

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.marchem.2011.10.001.

Acknowledgments

Rmy Villeneuve performed part of the CDOM absorption analysis.

54 H. Xie et al. / Marine Chemistry 128-129 (2012) 4456temperaturesalinity proling study, Siebert et al. (1979) concludedthat the Fjord's deepwater carries the same thermohaline signatureas that of the intermediate cold layer in the LSLE rather than the sur-face water.

The highly linear acdomsalinity relationships with strikingly differ-ent slopes found for the Fjord and estuary (Fig. 6, Table 3) enable usto characterize the Fjord's deepwater using an approach fundamentallydifferent from those previously employed. This approach begins withextrapolating the two regression lines that form an intercepting point(Fig. 15). The salinity (32.92) and acdom(350) (0.31 m1) at the inter-cepting point represent those of the marine end member of the Fjord'sdeepwater. Taking into account the analytical error of acdom measure-ment (0.02 m1), the salinity of the marine end member falls in therange from 32.79 to 33.03. A comparison between the salinity and tem-perature vertical proles collected in the seaward side of the Fjord'smouth (Stn 6) reveals that salinity 32.92 corresponds to temperature0.94 C (range: 0.491.28 C) and depth 97 m (range: 90103 m).These temperature and depth values are characteristic of the intermedi-ate cold layer of the water column in the LSLE, thereby corroboratingthe nding by Siebert et al. (1979).

Knowing the marine endmember allows us to quantitatively assessthe water mass composition of the Fjord's deepwater. The volume frac-tion of themarine endmember, i.e. the intermediate cold layerwater inthe LSLE, can be estimated as the measured mean salinity of the Fjord'sdeepwater (30.95, below 150 m) divided by the salinity of the marineend member (32.92). The calculation gives 94.0% (range: 93.794.4%),leaving the remaining 6.0% (range: 5.66.3%) to be accounted for bythe Saguenay River end member (salinity: zero and acdom(350):

Table 3Results of linear least-squares regression of CDOM absorption coefcients against sa-linity. A and Y0 are tted slope and intercept, respectively. S.E. stands for standarderror. Key: SLES = St. Lawrence estuarine system.

Region/Station

Depth (m) AS.E. Y0S.E. R2 N p

Fitting equation: acdom(350)=Asalinity+Y0SLES 2 0.2110.0058 7.100.13 0.994 10a b0.0001

65 0.2030.0034 7.000.094 0.993 26a b0.0001SaguenayFjord

All depths 0.5360.011 17.960.27 0.994 17 b0.0001

Stn TS All depths 0.3480.016 11.790.49 0.933 35 b0.0001

Fitting equation: acdom(412)=Asalinity+Y0SLES 2 0.07320.0024 2.480.057 0.991 10a b0.0001

65 0.07050.0015 2.450.042 0.989 26a b0.0001SaguenayFjord

All depths 0.1970.0040 6.620.10 0.993 17 b0.0001

Stn TS All depths 0.1320.0065 4.480.20 0.924 35 b0.0001a Stn SL1 is excluded.17.96 m1). The Saguenay River water is carried down to the deepdepths through diffusion and turbulent mixing which is particularly in-tense in the outer basin (Blanger, 2003). Using acdom(350) instead ofsalinity or using the regressions between acdom(412) and salinity(Table 3) instead of those between acdom(350) and salinity gives essen-tially identical results. More eld studies are required to constrain theseasonal variability of themarine and freshwater endmembers. Furtherimprovements can bemadewithner spatial and vertical sampling res-olutions. In particular, more depths should be sampledwithin the inter-mediate cold layer of the LSLE.

5. Conclusions

Main conclusions from this study are summarized as follows:

1. CDOM absorption displayed conservative mixing behavior in boththe SLES and the Saguenay Fjord, excepting the bottom water ofthe LSLE and NWG where sedimentary CDOM input likely oc-curred. Particles-driven processes plausibly added CDOM to thewater column of the turbidity maximum zone in the USLE.

2. The dominant light absorber was CDOM in the SLE and phyto-plankton in the NWG and its adjacent landward waters. The tur-bidity maximum zone was characterized by very high non-algalparticle absorption that could surpass CDOM absorption.

3. The spectral slope ratio of CDOM absorption can be used as indica-tors of the origin and molecular weight of CDOM in the SLES and

Fig. 15. Best-t lines of acdom(350) vs. salinity in Fig. 6 for the St. Lawrence estuarinesystem and Saguenay Fjord (solid lines) and their extrapolations toward higher salin-ities (broken lines). The large lled circle represents the intercepting point of thetwo extrapolated lines. Arrows indicate coordinates of the intercepting point.We thank the chief scientist, colleagues, captain and crew of the R/V

phys. Res. 102, 1860718625.

55H. Xie et al. / Marine Chemistry 128-129 (2012) 4456Gilbert, D., Sundby, B., Gobeil, C., Mucci, A., Tremblay, G.H., 2005. A seventy-year recordof diminishing deep-water oxygen levels in the St. Lawrence estuarythe north-west Atlantic connection. Limnol. Oceanogr. 50, 16541666.

Gobeil, C., 2006. Biogeochemistry and chemical contamination in the St. Lawrence es-tuary. In: Wangersky, P.J. (Ed.), Handbook of Environmental Chemistry Part H.Springer-Verlag, Berlin, pp. 121147.

Guo, L., Ping, C.L., Macdonald, R.W., 2007a. Mobilization pathways of organic carbonfrom permafrost to arctic rivers in a changing climate. Geophys. Res. Lett. 34,L13603. doi:10.1029/2007GL030689.Coriolis II cruise. Reviewers' and editor's comments improved themanuscript. This work was supported by grants from the NaturalSciences and Engineering Research Council of Canada (NSERC) andthe Canada Foundation for Innovation (CFI). This is a contribution tothe research programs of the Institut des sciences de la mer deRimouski (ISMER).

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56 H. Xie et al. / Marine Chemistry 128-129 (2012) 4456

The dynamics of absorption coefficients of CDOM and particles in the St. Lawrenceestuarine system: Biogeochemical and physical implications1. Introduction1.1. Overview of absorption properties in marine waters1.2. The St. Lawrence estuarine system

2. Methods2.1. Sampling2.2. Analysis

3. Results3.1. General hydrography3.2. Distributions of CDOM optical properties and molecular weight3.3. Relationships among CDOM optical properties, molecular weight, and salinity3.4. Particle absorption

4. Discussion4.1. Estuarine mixing behavior of CDOM and particles4.2. In situ production of CDOM4.3. SR as an indicator of CDOM characteristics4.4. Implications for ocean color imaging4.5. CDOM as a tracer of deepwater formation in the Saguenay Fjord

5. ConclusionsAcknowledgmentsReferences