first-order organic carbon budget in the st lawrence lower ...13... · organic carbon budget in st...

Estuarine, Coastal and Shelf Science (1991) 32,297-312

First-order Organic Carbon Budget in the St Lawrence Lower Estuary from 13C Data

Marc Lucotte, Claude Hillaire-Marcel and Patrick Louchouarn GEOTOP, Waivers& du Qudbec d! Montrkal, C.P. 8888, Succursale A, Montkal, P.Q., H3C 3P8, Canada

Received 21 March 1990 and in revisedform 14 September 1990

Keywords: carbon isotope; particulate organic matter; surface sediment; St Lawrence Estuary; carbon budget

Carbon isotope ratios and concentrations of suspended particulate matter and surface sediments of the Lower St Lawrence Estuary were used to determine the seasonal fluxes of particulate organic matter to the estuarine floor. Light carbon particles [ - 26.0%0 < G”C(PDB) < - 24%0] brought by the St Lawrence River during spring freshet are carried to the intermediate and deep watermasses of the Lower Estuary until July. In contrast, heavy loads of “C-enriched particles (6°C up to - 19.8%0) in the surface waters at the head of the Laurentian Channel are directly attributable to the early summer phytoplankton bloom or indirectly to the related zooplankton populations. Only small amounts of terrigenous particles ( - 25%~ < 6% C - 24%) from the turbid zone are still found in the early fall water column. All particles have a short residence time of a few days within the St Lawrence Estuary, limiting most seaward export. The surface sediments of the Lower Estuary have an average C-isotope composition of -22.4%0. Using a simple two-components 13C-mixing equation, this value represents 43 f 8% and 57 + 8% of terrigenous and marine particulate organic carbon, respectively. A first-order carbon budget was then derived from published sedimentation rates and primary production estimates. About 75% of the terrigenous particulate organic matter introduced in the Upper Estuary is deposited within the limits of the Lower Estuary. Simultaneously, the equivalent of less than 30% of the annual phytoplanktonic carbon production in the Lower Estuary reaches the estuarine floor, mostly under the form of copepod faecal pellets. Over 70% of the photosynthesized carbon is biodegraded and recycled in the water column.

Introduction

The sedimentation of organic matter on the floors of the continental margins plays a major role in the world carbon budget (Walsh et al., 1985; Broecker, 1982). In this context, estuarine environments appear to represent both efficient traps for river-borne particulate organic carbon (POC) (Showers & Angle, 1986; Fontugne & Jouanneau, 1987) and sinks for a significant fraction of the biogenic carbon metabolized in the highly productive coastal area (Tan & Strain, 1979a; Gearing et al., 1984). Although needed for establishing carbon budgets in transition environments, the relationship between the spatial and tem- poral distributions of terrigenous vs. marine organic material in the water column and the

0272-7714/91/030297 + 16 $03.00/O @ 1991 Academic Press Limited

298 M. Lucotte et al.

Manicouagan-Outardes

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Figure 1. St Lawrence Estuary. Sampling stations with box cores (0) and with gravity cores or grabs (x).

final composition of the sedimentary product is not well documented yet. In fact, complex biogeochemical reactions considerably alter suspended organic particles prior to their sedimentation on a seasonal basis (Billett et al., 1983; Mayer et al., 1985; Sancetta, 1989).

The sedimentary fluxes in the St Lawrence Lower Estuary have been the object of numerous studies, but these are often restricted to limited sampling sites and isolated periods of time, and are not correlated to the nature of the participating particulate organic matter. Up until now, the knowledge of the respective sources of terrestrial and marine particles had been circumscribed. The St Lawrence River is the main contributor of terrigenous material to the estuary (Pocklington, 1988). The Upper Estuary (Figure 1) then temporarily retains the suspended matter in its turbid zone or on its marginal tidal flats, and sustains only limited biological production (Lucotte, 1989a,b). The Lower Estuary (Figure 1) is characterized by a tidal upwelling (Gratton er al., 1988) which

Organic carbon budget in St Lawrence Estuary 299

promotes intense phytoplankton blooms in July and September (Thtrriault & Levasseur, 1985). The corresponding high secondary production (mainly copepods and salps) is spread out over the year (Syvitski et al., 1983; Rainville & Marcotte, 1985).

Stable carbon isotope ratios have been used to distinguish between the various sources of POC in the St Lawrence Upper Estuary. Estuarine transition is characterized by the progressive partial dilution of the seasonal upstream 613C pole with a heavier downstream component (Lucotte, 19896). In spring, the upstream pole is dominated by terrigenous POC (6’3ca - 26*6OA) carried down by the river with the spring freshet. After spring runoff, sedimentary exchanges between the intertidal flats and the nearby estuarine channels assume increasing control of the POC composition of the upstream pole (613C= -24.4%). The late August POC of the Lower Estuary has been described as isotopically distinct from that of the Upper Estuary. The bulk of POC is then characterized by a high 13C content (613C= - 20.8Oh) suggesting the dominant contribution of authigenic material (Tan & Strain, 19793,1983). The fate of the terrigenous POC from the Upper Estuary is still uncertain.

Seasonal estimates of particulate downward fluxes exist for only one reference station in the Laurentian Channel (near station H2; Figure 1) (Silverberg et al., 1985) measured by means of free-drifting sediment traps. These sedimentation rates decrease from w 45 mm year-’ in spring to m 1 mm year-’ in fall. These rates are in keeping with the annual sedimentation rates determined by radionuclide profiles (Silverberg et al., 1986) and by seismostratigraphic records (Syvitski 8z Praeg, 1989). Although carbon fluxes at the sediment-water interface could be deduced from the previous measurements (Silverberg et al., 1987), the respective contributions of the different sources of POC to these fluxes remain to be determined. In particular, the POC flux related to the abundant faecal pellet snow (Syvitski et al., 1983; Silverberg et al., 1985) must be quantified in relation with the living biomass in the water column. An estimate of the various proportions of allochthonous and autochthonous POC in the surface sediments of the St Lawrence Estuary was attempted by Tan and Strain (1979a) using carbon isotope ratios. However, no precise partition could be given because of uncertainties on 613C values of terrestrial and marine 613C end-members.

The primary aim of this research is to determine the seasonal nature of the particulate organic matter in suspension in the various watermasses of the estuary on a tight grid basis. The contribution of authigenic sources of POC, such as that from the summer plankton bloom, is evaluated with respect to the river-borne particle influx, in particular that of the spring freshet. The particulate organic matter partition in the water column is then juxtaposed to the average composition of the top centimetre of the sediments, integrating the main early diagenetic transformations of the buried organic matter (Silverberg et al., 1987). This comparison allows an estimate of the downward fluxes of terrigenous and marine organic particles, respectively. The integration of the data on the estuarine scale leads to the establishment of a first-order POC budget.

Materials and methods

Sampling A series of 28 stations along the north and south shallow platforms and the Laurentian Channel of the Lower Estuary were visited repeatedly during three cruises carried out on 15-17 May, l-9 July and 26-29 September 1988 (Figure 1). Seven additional stations (five at the downstream end of the Upper Estuary and two in the Saguenay Fjord,


Figure 1) were used as references for the terrigenous load to the Lower Estuary. The three sampling periods were chosen to coincide respectively with the spring freshet (Pocklington & Tan, 1987), the maximum summer phytoplanktonic bloom (Levasseur et al., 1984), and the low fall freshwater runoff (Pocklington & Tan, 1987). For the suspended particulate matter (SPM) study, bottle casts were taken in the core of each of the three superimposed watermasses identified by preliminary conductivity-temperature- depth profiles, namely the river-inffuenced warm surface watermass, the glacial intermediate watermass and the warmer Atlantic bottom watermass (Gratton et al., 1988). The surface samples were obtained between 1 and 3 m in depth, and were therefore free of floating microdebris. The intermediate water samples were collected around the temperature minimum (about 60 m in depth), well below the main surface pycnocline. Finally, the bottom water was sampled at depths ranging from 150 to 200 m, in the zone of minimum suspended particulate matter, avoiding contamination from the nepheloid layer.

SPM concentrations in the water column were gravimetrically obtained by on-board filtering of the water samples through preweighed 0.45pm Nuclepore filters. The results showed a good correlation with simultaneous nephelometric readings (New Turbidity Units) obtained with a model 2100A Hach turbidimeter: y (NTU) = 0.27~ (mg 1-l) + 0.25, r = 0.79 on average of 57 values in the 0.2 to 1 NTU range, and y = 0.55x + 0.15, T = 0.85, for 43 values in the 1 to 10 NTU range. The SPM samples to be analysed for POC content were obtained by filtering several litres of water through Whatman GF-C glass fibre filters, precombusted at 450 “C for 1 h. Live macrozooplankton organisms were removed from the filters when present. The filters were then immediately dried on-board with a vacuum desiccator.

Bottom sediments were sampled with a box corer smoothly operated in order to avoid sediment resuspension. The top 10 mm of sediments were then carefully subsampled and kept in the dark at 4 “C for ulterior freeze-drying and grinding. For most sampling sites, these surface samples represent an integrated picture of several years of sedimentation and early diagenesis, accumulation rates being of the order of a few millimetres per year in the Laurentian Channel (Silverberg et al., 1986; Syvitski & Praeg, 1989). Intense bioturbation generally results in the homogenization of these surface sediments (Silverberg et al., 1986; Syvitski et al., 1983). Only at the head of the Laurentian Channel, (station G2) where record high sedimentation rates prevail (up to 16mm year-‘, Silverberg et al., 1986), subsamples of the box core were taken at each 5 cm, down to a depth of 50 cm, in order to record better the heterogeneity of recent deposits. At stations where the box corer could not be operated because of the lack of a soft sedimentary surface (mostly in the Upper Estuary, see stations marked by an ‘ x ’ on Figure l), gravity cores or grab samples completed the sampling.

Laboratory analyses Size determinations with a model TA Coulter counter were performed on selected SPM samples. Particulate organic carbon from the SPM and surface sediments was converted to CO, by combustion of the samples at 450 “C for 1 h in sealed tubes containing pre- cornbusted CuO. Since carbonates represent less than 0.0496 of the total carbon content (dry weight) in Lower Estuary sediment (Bouchard, 1983), the POC measured can be referred to as total particulate carbon. Stable carbon isotope ratios of the produced CO, were determined with a VG-602 mass spectrometer. After the usual corrections for I70 and peak overlap (Hillaire-Marcel, 1976), the results are reported in the standard 613C (%o) notation relative to the Pee Dee Belemnite limestone standard (PDB). Measurements on


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Figure 2. Concentrations of suspended particulate matter (mg 1-l) in July 1988. (a) South shore, (b) Laurentian Channel, (c) north shore. WI Surface; q , intermediate; 0, bottom.

five aliquots of SPM and sediments of all but two selected samples gave an analytical reproducibility of the order of + 0*12%0 (95% confidence level). However, the presence of plant macrodebris in the sediments of stations F2-G2 increased the standard deviation up to * 1~5!%0.

Results

Figures 2 and 3 show the suspended particulate matter loads (in mg 1-l) of the surface and intermediate watermasses of the sampling station for July and September, respectively. In addition, the bottom SPM concentrations are given for the deep stations of the Laurentian Channel [Figures 2(b) and 3(b)]. The stations are positioned on the horizontal axis according to their decreasing longitude, downstream from the Al-A2 transect. The two stations in the Saguenay Fjord, S3 and S4, appear on the north shore profile, between stations C3 and D2 [Figures 2(c) and 3(c)].

The isolated spring (May) turbidity measurements show a sharp enrichment in SPM concentrations at the surface as compared with the deeper watermasses (Table 1). In comparison, fall turbidity is generally lower and more uniform from surface to bottom (Figure 3). It tends to decrease downstream away from the Upper Estuary, and reaches record low SPM concentrations ( < 0.5 mg 1-l) at the head of the Laurentian Channel, around stations F2 and G2 [Figure 3(b)]. The July particulate load is contrastingly higher


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Figure 3. Concentrations of suspended particulate matter (mg 1 ‘) in September 1988. (a) South shore, (b) Laurentian Channel, (c) north shore. n , Surface; q , intermediate; 0, bottom.

TABLE 1. Suspended particulate matter (SPM) concentrations and 6°C values for the particles in suspension in the surface(S), intermediate (I) and bottom (B) watermasses of the St Lawrence Lower Estuary in May 1988

Stations SPM

Cm3 1-I) 6°C (560)

Watermass S I B S I s3 2.3 1.2 -28.3 -28.4 Gl 2.2 - 24.7 G2 1.2 05 0.3 G3 1.1 -26.3 H2 2.1 0.5 0.4 -25.4 -24.1 H3 3.1 0.6 0.3 -26.8 -25.4

than that of September. While summer turbidity also decreases from the surface of the Upper Estuary (2 10 mg 1-l) down to the bottom of the Lower Estuary ( < 0.5 mg I-‘), a turbidity peak is noticeable in the vicinity of the head of the Laurentian Channel (stations Gl-F2-G3). High turbidity is then observed in the whole water column of the shallow stations of the south and north shores (m2.5 mg 1-t at stations Gl-HI and F3-H3). On


Stotions Al El Cl 0, El FI GI HI II JI KI LI c2 E2 F2 G2 HZ 12 J2 K2 L2

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J Figure 4. PC (?&) of the particulate organic carbon in July 1988. (a) South shore, (b) Laurentian Channel, (c) north shore. l , Surface; q , intermediate; 0, bottom.

the other hand, the high SPM concentrations at the head of the Laurentian Channel (central stations E2-H2) are restricted to the surface waters (up to 35 mg l-l), while the corresponding deep waters are much less turbid ( GO.9 mg l- ‘).

The r3C/‘2C ratios of the SPM samples (Table 1, Figures 4 and 5) exhibit a strong seasonality in POC origin. In both spring and fall, the uniformly 13C-depleted mainly terrigenous POC (613C < - 24O&; Table 1, Figure 5) dominates the water column, down to the bottom watermass of the Laurentian Channel [Figure 5(b)]. In July, these same low S13C values [Figure 4(a) and (c)] which characterize the turbid waters of the Upper Estuary mainly extend down to stations Fl-D2. The transition with the adjacent surface waters of the Lower Estuary is marked by a sharp increase of 2 to 4% in SPM 613C values (downstream stations Gl-E2-E3; Figure 4) indicative of an increased contribution of marine POC. The previously recorded surface turbidity peak, centred around stations Gl-F2-G3, exactly corresponds to the maximum 613C values (up to - 19+3%). The t3C/i2C ratios of the suspended particulate organic matter in the intermediate and deep watermasses ( - 22 to - 24%0) fall halfway between the low ratios upstream and the high surface ratios downstream.

The carbon isotopic composition of the surface sediments (Figure 6) testifies to the two ‘3C-naturally labelled POC sources in the estuarine waters. The terrigenous signal (613C < - 24%) dominates the POC of the sparse Upper Estuary sediment samples and partially influences that of the shallow south platform deposits of the Lower Estuary down to station J 1 (613C % - 23%). A similar continental influence ( - 22.6% < 613C < - 22-9O&)


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Figure 5. PC (%o) of the particulate organic carbon in September 1988. (a) South shore, (b) Laurentian Channel, (c) north shore. H, Surface; q , intermediate; 0, bottom.

is noticeable in the recent sediments of the north platform under the plume of the Manicouagan-Outardes Rivers (stations 13-H3; Figure 1). The sediments at the head of the Laurentian Channel are characterized by 613C values (- 22.5%0 < 8’C < 22.0%) intermediate between the high values of the summer surface SPM and the low values of the SPM typical of the rest of the year. The marine component of recent sediments then becomes dominant towards the entrance of the gulf (6i3C 2 - 22.0%).

The rapidly accumulating sediments at the head of the Laurentian Channel (station G2) are a particular case and show large heterogeneities in 613C vertical distribution (Figure 7) because of the presence of organic macrodebris (size of a few millimetres and up). The average C-isotope composition is nevertheless in the order of -22.5%0, which is quite comparable with the mean annual ratio of nearby stations of the Laurentian Channel with slow sedimentation rates.

Discussion

Origin of the suspended particulate matter The mean flushing time for the spring freshet of the St Lawrence River from the Upper Estuary to the head of the Lower Estuary has been estimated at about 1 month (El Sabh, 1979). As the maximum spring discharge at Quebec City occurs in early May (Pocklington & Tan, 1987), its effect should not be felt on the Lower Estuary before early June. The


I 0" 69"

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Figure 6. Average PC (%o) of the particulate organic carbon in the top centimetre of the sediments.

smaller terrigenous particles (mean diameter < 1.5 pm, Coulter counter observations) first arrive in the Lower Estuary, being advected with the surface currents. These particles, characterized by 13C-depleted POC, appear to make up most of the POM in suspension in May in the Lower Estuary (613C < - 24.7’33). In early July, the SPM load at the downstream end of the Upper Estuary ( 2 2.5 mg 1-l) and at the bottom of the Laurentian Channel (6 1 mg 1-l) correspond to the larger particles (mean diameter of 4 to 6 urn, Coulter counter observations) of the spring runoff which arrive after their long transit in the brackish turbid zone. This assumption is again corroborated by the strong terrigenous character of the suspended particulate matter POC ( - 26% < 6i3C < - 245%0). These C- isotope compositions are consistent with those observed for the particulate matter carried by the St Lawrence River at the head of the Upper Estuary in mid-May and June (- 26.4% and -26.0%0; Lucotte, 19896). The slight increase in 613C values during the transit of terrigenous POC through the Upper Estuary testifies to the weak addition of authigenic POC. Indeed, microscopic observations of the SPM showed very few planktonic cells or faecal pellets. On the other hand, it is doubtful that attached auto-


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-50m ’ ’ ’ ’ ’ ’ -27 -26 -25 -24 -23 -22 -21 -20

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Figure 7. PC (540) of the particulate organic carbon in a core at station G2. x , Macrodebris.

trophic bacterial colonies also seen under the microscope could contribute significant carbon to alter the 613C values (Coffin et aE., 1989).

The low SPM concentrations found in the water column of the Lower Estuary in late September correspond to the low riverine discharge and the reduced particulate load which are found in the Upper Estuary (Pocklington & Tan, 1987) during the summer. Again, the ‘3C/‘2C ratios defining the seasonal upstream pole of POC in the Upper Estuary (613C = - 25%0 in August; Lucotte, 19896) are quite similar fo those of the particles found later in the waters of the Laurentian Channel ( -25%<6L3C d -24%,,). The POC thus appears almost completely free of any authigenic particles at that time.

All year round, the St Lawrence River is the dominant carrier of dissolved and particulate terrigenous matter to the estuary and the gulf (Pocklington, 1988). In fact, the influence of terrigenous POC discharge from the Saguenay River (8°C =Z - 28 to - 27%) does not extend further than its confluence with the Lower Estuary (stations D2-Dl; Figure 1) at any given time of the year. Strong currents might explain the rapid dilution of the fjord minor contribution to the estuary.

A major annual diatom and flagellate bloom in the Lower Estuary repeatedly occurs in early summer (Levasseur et al., 1984). The corresponding highly 13C-enriched POC (613C up to - 19.8%) of the surface turbid zone (centred around stations Gl-F2-G3; Figure 1) could be directly attributed to the phytoplanktonic populations, or indirectly to the related copepod or salp populations. The isotopic signature is indeed transferred from primary to secondary productions with minor fractionation (Gearing et al., 1984). In that context, most of the terrigenous POC has disappeared or is overshadowed in the photic zone. The 13C-depleted values of the POC observed during this period in the intermediate and deep watermasses at the head of the Laurentian Channel suggest that the biogenic particles have not reached these depths yet. In the upwelling zone, the vertical downward flux of diatoms and flagellates could be effectively limited by the ascending current (35 m h-‘) and by the strong density gradient separating the surface watermass from the intermediate glacial watermass (Gratton et al., 1988). Nevertheless, the marked marine POC component found down to the bottom of the shallow north and south platforms


[Figure 4(a) and (c)] can be attributed to the sweeping down of these biogenic particles towards the bottom by higher turbulence, or to the presence of copepods below the photic zone.

Residence time of the particles With an average size of 2 to 6 pm, the sinking rate of terrigenous suspended particles of the Laurentian Channel (Coulter counter observations; Syvitski et al., 1983) can be estimated at about 10 m day-’ (Krone, 1978). Once the phytoplankton bloom has begun, their aggregation to biogenic particles, such as Oihopleura houses, increases their settling velocity to 30-100 m day-’ (Syvitski et al., 1985). The transfer of spring runoff continental inputs to the bottom is therefore of the order of a few days from the time the particles enter the slow-moving intermediate and deep watermasses of the Laurentian Channel (currents < 15 cm s-l; Syvitski et al., 1983). This suggests that most terrigenous particles brought by the St Lawrence River are deposited within the limits of the Lower Estuary. This assumption will be corroborated later in this paper with the establishment of a first-order carbon budget.

Biogenic autochthonous particles sink even more rapidly towards the bottom of the Lower Estuary. For example, aggregated diatom cells may settle at the rate of 60 to 160 m day-’ (Alldredge & Gotschalk, 1989). Similarly, faecal pellets produced by deep- living copepods (Syvitski et al., 1983) reach the bottom of the sea-floor within a few days (Fowler & Knauer, 1986). The swiftness of biogenic SPM settling explains the disappear- ance of most marine 13C-enriched POC in the SPM of the water column in late September (Figure 5), about 2 weeks after the bulk of planktonic production has ceased (Levasseur et al., 1984). Once deposited, both autochthonous and allochthonous particulate matter can be resuspended mainly by benthic biological activity (Syvitski et al., 1983). Subsequent horizontal transports nevertheless seem quite limited due to weak bottom currents (< 15 cm s-l; Syvitski et al., 1983).

Surface sediments The mean sedimentation rate in the Laurentian Channel has been evaluated at around 2 mm year-’ both from radionuclide measurements (Silverberg et al., 1986) and seismostratigraphic records (Syvitski & Praeg, 1989). In a first approximation, we applied this rate to the entire Lower Estuary, even though it overestimates sedimentation in the shallow south platform and underestimates it at the head of the Laurentian Channel. A dry particle density of 2.6 g cmm3 and a mean water content of 50% were systematically observed in selected surface sediment samples, in agreement with Silverberg et al’s measurements (1985,1986). With the average 2% POC content (dry weight at 1 cm depth) reported by the same previous authors, the annual POC accumulation rate can be estimated at approximately 50 gC me2 year- ‘. This value is in the same order as the carbon fluxes determined by Silverberg et al. (1987) at a site close to station H2.

A proportional estimate of the allochthonous vs. autochthonous sources of POC found in recent sediments can theoretically be calculated using a simple two-components 13C- mixing equation (Tan & Strain, 1979a). This is based on the fact that the 6i3C of the POM incorporated in the sediments is only very weakly altered by bacterial degradation or geochemical transformation at a depth greater than the top centimetre (Figure 7; Silverberg et al., 1987; CofBn et al., 1989). In fact, the sedimented POC is made up mostly of refractory carbon: the terrigenous fraction is in the form of cellulose or lignin fragments


and pollens which remain undegraded after a transit of several weeks in the estuary (Pocklington & Leonard, 1979; Lucotte, 19896), whereas the bulk of the marine fraction is made of faecal pellets containing very high percentages of inorganic particles (Syvitski et al., 1983; Silverberg et al., 1985; Mayer et al., 1985). In order to apply the 13C-mixing equation, precise 613C values for each end-member, namely the terrigenous and the marine poles, must be determined.

The St Lawrence Upper Estuary POC is governed by a linear dilution of a seasonally fluctuating upstream 613C pole with increasing salinities (Lucotte, 19896). As little biodegradation or isotopic alteration occurs during the mixing of this upstream SPM pole along the Upper Estuary, one can take the corresponding 613C values as terrigenous end- members for the 13C-mixing equation. These values are tightly distributed around two poles, - 26.2’?? and - 25.0’%0 in spring and in summer-fall respectively, each representing about equal amounts of SPM on an annual basis (Lucotte, 1989b). An intermediate value of - 25.6OmO f 0.6% can therefore be taken as the characteristic mean 613C terrigenous pole for the surface sediments of the Lower Estuary.

The 613C marine pole is somewhat more ticklish to assess as biogenic production is not constant over time, and fractionation occurs among different taxa and between trophic levels. The early summer and later summer phytoplankton blooms represent close to two- thirds and one-third, respectively, of the Lower Estuary primary production on an annual basis (Levasseur et al., 1984). Most of the photosynthetically fixed carbon is then transferred to the secondary level within a few days (Rainville & Marcotte, 1985). As this transfer is accompanied by a slight 613C increase of the order of 0.6%0 (Gearing et al., 1984), we chose the highest 613C values repeatedly found during the periods of bloom in the surface waters at the head of the Laurentian Channel as representative of the seasonal 6i3C value of the pellet snow POC. These values come up to - 19.8% and - 20.4% for early July (this study) and late August (Tan & Strain, 1983), respectively. An intermediate value of - 20.0 f 0.3X characterizes then the authigenic POC isotopic composition.

The average 613C value of the surface sediment POC of the Lower Estuary (stations Fl-F2-F3 down to Jl-J2-J3) is -22.4% (n = 15, cr= 0.5). This ratio is similar to the one found by Tan and Strain (19796) for the sediments of the Laurentian Channel in the gulf. According to our previously selected end-members, it consists of approximately 43 f 8O,, and 57 f 8% of terrigenous and marine POC, respectively. With the mean POC accumulation rate given above, this means that on average * 22 g of allochthonous organic carbon and w 28 g of autochthonous organic carbon are incorporated annually per square metre of the Lower Estuary floor.

Carbon budget A first-order POC budget for the St Lawrence Estuary (Table 2) was established using all available data. The partition of the POC participating in the various exchanges within the limits of the Upper Estuary (turbid zone, temporary flat deposition, macrophyte standing crop) was calculated applying the respective POC concentrations (Lucotte, unpubl. data) to each sedimentary reservoir (Lucotte, 1989~). The total primary production in the Lower Estuary (M lo6 tonnes year-‘) was obtained by integrating Therriault and Levasseur’s (1985) regional annual estimates to the corresponding surface areas. The approximate amounts of terrigenous (-220 x lo3 tonnes year-‘) and marine POC (=280 x lo3 tonnes year-‘) deposited at the bottom of the Lower Estuary were determined by extrapolating the respective POC sedimentary rates (see above) to the entire surface of the estuary ( % 1 x lo4 km’).


TABLE 2. First-order annual particulate organic carbon budget ( lo3 tonnes year ‘) for the St Lawrence Estuary

POC budget (10’ tonnes year-‘)

Upper Estuary Terrigenous inputs” Turbid zone loadb Temporary sedimentation on tidal flats6 Macrophyte standing crop6

Lower Estuary Primary production’ Sedimentation of allochthonous particle& Sedimentation of autochthonous particlesd

300 50 70 11

1000 220 280

“At Quebec City (Tan, 1987). 4Deduced from Lucotte (1989a). ‘Extrapolated from Therriault and Levasseur (1985). This study.

This carbon budget could be improved by a more detailed differential integration of the specific surface sediment characteristics. In particular, accurate sedimentation rates taking into account the bioturbation are needed. Moreover, it would be interesting to go further into the determination of the 13C end-members by combining the seasonal variations of the isotopic ratios with direct microscopic identification of the particles.

Conclusions

The detailed seasonal study of the isotopic carbon distribution of the SPM and recent sediments of the St Lawrence Lower Estuary allows one to determine the intensity and nature of particulate fluxes to the estuarine floor. The St Lawrence River is the main source of continental particles to the Upper Estuary. The terrigenous particles stay in suspension in the turbid zone for a period of 1 to a few months before being flushed downstream, the residence time of the particles being inversely related to their size. This freshwater load along with the solid inputs originating from the marshes adjacent to the head of the Upper Estuary dominate the upstream POC pole with characteristic 6% values of = - 26.2%0 in the spring and = - 25.0% in the summer-fall. This pole keeps its 6i3C signature all the way downstream to the Lower Estuary as its carbon is quite refractory to biodegradation, consistent with Hedges et at.‘s (1988) conclusions for Puget Sound. These terrigenous particles must then reach the Lower Estuary since no major zone of permanent sedimentation is known in the Upper Estuary (Lucotte, 1989a). As soon as these particles enter the Lower Estuary, they sink rapidly (within a few days) towards the bottom. Their settling rate can also be greatly enhanced by aggregation onto faecal pellets. According to the carbon budget presented in Table 2, about 75% of the terrigenous particulate organic matter introduced in the Upper Estuary is deposited within the limits of the Lower Estuary. This figure is comparable with those estimated for the estuaries of the Gironde (80%) and the Amazon (94%) (Fontugne & Jouanneau, 1987; Showers &Angle, 1986). It strongly emphasizes the role of the Lower Estuary as the major sink of continental inputs to the St Lawrence system. Most of the terrigenous POC is


deposited in the form of lignocellulosic compounds, but, according to preliminary counts yielding = 25 000 grains crne3 (d e V ernal, pers. comm.), pollens could make up to loo,, of this POC.

Copepod faecal pellets are the main carriers of biogenic POM to the estuarine floor. They are most abundant during the early summer phytoplanktonic bloom, and are characterized by %-enriched POC (6i3C = - 19.8”/,). With settling rates in the order of 100 m day-‘, these biogenic particles remain in the water column for a few days at most. This indicates that very few particles remain in the area of the surface currents long enough to escape the Lower Estuary, and agrees with Pocklington’s (1988) considerations on reduced POC mass transport from the estuary to the gulf. Since the equivalent of less than 30% of the annual phytoplanktonic carbon production settles on the Lower Estuary floor (Table 2), we are led to conclude that over 709b of the carbon fixed in the photic zone is likely to be biodegraded and recycled within the water column. Such a high regeneration rate would be close to that found in British Columbian fjords or in Puget Sound (Sancetta, 1989; Hedges et al., 1988), but is far below the 99.59, rate observed in the open ocean water column (Honjo, 1978).

The various sources of sedimentary organic particles are not uniformly distributed throughout the estuary. The south platform down to Matane and the north platform around the plume of the Manicouagan-Outardes Rivers preferentially accumulate coarser terrigenous particles because of the local along-shore currents and turbulence. The marine component of the surface sediments becomes dominant only towards the gulf-end of the Lower Estuary.

Most of the deep Laurentian Channel is covered by about equivalent fractions of fine organic allochthonous and autochthonous particles. In most areas, the recent sediments are very homogeneous in depth as bioturbation extends through the whole oxic layer (a 2 cm) which is about 10 times as thick as the annual sedimentary accumulation (Silverberg et al., 1985,1986). A good fraction of the terrigenous particulate matter begins to settle as soon as it leaves the turbulent Upper Estuary for the less dynamic conditions of the Lower Estuary. In this same region, an intense biological production is triggered by deep nutrient-rich upwelled waters. In addition, the heavy load of suspended particles of the region is trapped by a local gyre (El Sabh, 1979). These conditions are responsible for the record high sedimentation rates of both terrigenous and biogenic POC at the head of the Laurentian Channel. Nevertheless, the accumulated particles cannot be directly explained by a simple vertical downward flux since strong upward water currents prevail. In fact, the deep watermasses of this region are virtually particle-free all year round (< 0.5 mg I-‘); rather, sedimentary accretion is due to advective fluxes along the steep slopes of the channel. The resulting sedimentary layers are more heterogeneous than those of the downstream regions of the Laurentian Channel.

Acknowledgements

This research was supported by a grant from the Fonds pour la Formation de Chercheurs et 1’Aide a la Recherche (FCAR-Quebec) to the first author, and by research grants from Universite du Quebec and the Fonds FCAR to the second author and to the GEOTOP. The shiptime granted by fisheries and Oceans Canada and the assistance of the crew members of the R/Vs L.M. Lather and Navimar Z are gratefully acknowledged. We also thank Danielle Giroux for her help in the laboratory.


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