Distribution of benthic foraminiferal populations in surface sediments of the Saguenay Fjord, before and after the 1996 £ood Julie Leduc a; , Guy Bilodeau a , Anne de Vernal a , Alfonso Mucci b a Centre de recherche en ge ¤ochimie isotopique et en ge ¤ochronologie (GEOTOP), Universite ¤ du Que ¤bec a ' Montre ¤al, P.O. Box 8888, Succursale Centre-Ville, Montreal, QC, Canada H3C 3P8 b Earth and Planetary Sciences, McGill University, 3450 University street, Montreal, QC, Canada H3A 2A7 Received 6 March 2001; accepted 19 September 2001 Abstract In the summer of 1996, a ‘flash’ flood occurred in the Saguenay-Lac-St-Jean region (Quebec) leading to the deposition, in less than 2 days, of an estimated 6^15U10 6 m 3 of sediments at the head of the Saguenay Fjord. In order to evaluate the impact of such a sedimentary event on the benthic meiofauna, foraminiferal analyses were performed in surface sediment at eight stations in 1994, 1997, 1998 and 1999 along the main axis of the fjord. A 10-cm sedimentary sequence taken from a box core collected in 1997 from the deepest part of the fjord and spanning the last ca. 35 years provides a reference state for pre-flood foraminiferal assemblages. A significant change in foraminiferal assemblages is observed downcore, with the decrease of Spiroplectammina biformis relative to Adercotryma glomerata. 210 Pb measurements suggest that this change occurred during the late 1970s. Surface samples (0^1 cm) collected between 1997 and 1999 in the inner basin also reveal a decrease of S. biformis percentages. Given the relative stability of this environment, such a change in faunal assemblages could be related to reduced industrial waste inputs or to a differential preservation of taxa. The 1996 flood had a major negative impact on the concentration and species diversity of benthic populations near the head of the fjord (Baie des Ha! Ha!), where the flood material is up to 50 cm thick. However, two years after the flood, benthic foraminiferal populations recolonized the sediments in the Saguenay Fjord. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: foraminifera; benthic; distribution; Saguenay Fjord; £ood 1. Introduction The Saguenay Fjord is a strati¢ed marine envi- ronment characterized by variable sedimentation rates, from 0.2 cm/yr to a few centimeters per year (e.g. Smith and Walton, 1980). It has been the site of many dramatic geological events over the past centuries (Smith, 1962). These include several strong earthquakes (Smith, 1962; Basham et al., 1985) and landslides, such as the Kenogami and St-Jean-Vianney landslides which occurred in 1924 and 1971, respectively (Schafer and Smith, 1988). Moreover, the Saguenay Fjord has been a¡ected by industrial activities upstream and 0031-0182 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0031-0182(01)00429-1 * Corresponding author. Fax: +1-514-987-3635. E-mail address: [email protected] (J. Leduc). PALAEO 2788 21-5-02 Palaeogeography, Palaeoclimatology, Palaeoecology 180 (2002) 207^223 www.elsevier.com/locate/palaeo
Transcript
Distribution of benthic foraminiferal populations in surfacesediments of the Saguenay Fjord, before and after
the 1996 £ood
Julie Leduc a;�, Guy Bilodeau a, Anne de Vernal a, Alfonso Mucci b
a Centre de recherche en ge¤ochimie isotopique et en ge¤ochronologie (GEOTOP), Universite¤ du Que¤bec a' Montre¤al, P.O. Box 8888,Succursale Centre-Ville, Montreal, QC, Canada H3C 3P8
b Earth and Planetary Sciences, McGill University, 3450 University street, Montreal, QC, Canada H3A 2A7
Received 6 March 2001; accepted 19 September 2001
Abstract
In the summer of 1996, a ‘flash’ flood occurred in the Saguenay-Lac-St-Jean region (Quebec) leading to thedeposition, in less than 2 days, of an estimated 6^15U106 m3 of sediments at the head of the Saguenay Fjord. Inorder to evaluate the impact of such a sedimentary event on the benthic meiofauna, foraminiferal analyses wereperformed in surface sediment at eight stations in 1994, 1997, 1998 and 1999 along the main axis of the fjord. A 10-cmsedimentary sequence taken from a box core collected in 1997 from the deepest part of the fjord and spanning the lastca. 35 years provides a reference state for pre-flood foraminiferal assemblages. A significant change in foraminiferalassemblages is observed downcore, with the decrease of Spiroplectammina biformis relative to Adercotryma glomerata.210Pb measurements suggest that this change occurred during the late 1970s. Surface samples (0^1 cm) collectedbetween 1997 and 1999 in the inner basin also reveal a decrease of S. biformis percentages. Given the relative stabilityof this environment, such a change in faunal assemblages could be related to reduced industrial waste inputs or to adifferential preservation of taxa. The 1996 flood had a major negative impact on the concentration and speciesdiversity of benthic populations near the head of the fjord (Baie des Ha! Ha!), where the flood material is up to 50 cmthick. However, two years after the flood, benthic foraminiferal populations recolonized the sediments in theSaguenay Fjord. @ 2002 Elsevier Science B.V. All rights reserved.
The Saguenay Fjord is a strati¢ed marine envi-ronment characterized by variable sedimentationrates, from 0.2 cm/yr to a few centimeters per year
(e.g. Smith and Walton, 1980). It has been the siteof many dramatic geological events over the pastcenturies (Smith, 1962). These include severalstrong earthquakes (Smith, 1962; Basham et al.,1985) and landslides, such as the Kenogami andSt-Jean-Vianney landslides which occurred in1924 and 1971, respectively (Schafer and Smith,1988). Moreover, the Saguenay Fjord has beena¡ected by industrial activities upstream and
0031-0182 / 02 / $ ^ see front matter @ 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 2 9 - 1
along its shore for more than sixty years. Papermills and other industries, including a chlor-alkaliplant, were responsible for the discharge of largevolumes of chemical and organic waste (Loring,1976; Pocklington, 1976). These materials, en-riched in trace metals such as Hg and Pb (Loring,1976; Pocklington, 1976), caused environmentalstress to the benthic habitat (Schafer et al.,1991). Foraminiferal populations in the fjord aredominated by Spiroplectammina biformis, a spe-cies which has been associated with polluted en-vironments in eastern Canadian estuaries and em-bayments (Schafer et al., 1991) and known to bethe most frequent species with a wide range ofenvironmental tolerance (Alve, 1991; Alve,1995a,b).A spectacular £ood occurred in the Saguenay
region in July 1996. During this event, an esti-mated 6^15U106 m3 of sediment were transportedand deposited in the fjord within two days (Pelle-tier, 1997). Following this event, monitoring ofthe sediments was undertaken within the contextof a collaborative project whose main objective isto assess the e⁄ciency of the £ood layer to isolatethe underlying contaminated sediments from theoverlying water (see web site : www.saguenay.ggl.ulaval.ca). Our speci¢c contribution to this proj-ect is a study of the benthic foraminiferal popu-lations, which was undertaken to meet the follow-ing three objectives :(1) to evaluate the immediate impact of the
£ood on benthic populations;(2) to estimate the rate of recovery of the
benthic environments and the recolonization bymeiofaunal populations after the £ood;(3) to verify the e⁄ciency of the £ood deposit
on environmental recovery using benthic fora-minifers as tracers of environmental stress. Pre-vious studies have indeed shown that foraminiferaconstitute useful bioindicators of pollution (e.g.Schafer et al., 1991) and physico-chemical charac-teristics of the sediment^water interface in marineenvironments (e.g. Culver, 1993).In this study, we document the recent evolution
of the benthic foraminiferal populations in theSaguenay Fjord since the 1996 £ood, from theanalysis of surface sediment samples collected in1994, 1997, 1998 and 1999 at eight stations lo-
cated along the main axis of the fjord. The tem-poral variability of benthic foraminiferal popula-tions is further completed by the analysis of ashort sedimentary sequence cored in the deepestpart of the fjord.
2. Environmental context
2.1. Physiography
The Saguenay Fjord is located about 200 kmnortheast of Quebec City and joins the St. Law-rence estuary at Tadoussac (Fig. 1.1). It is a typ-ical fjord from a geomorphological point of view:it is characterized by a U-shaped cross-sectionand it occupies a long (80 km) and narrow (1^6km) glacial valley. The sheer walls reach eleva-tions of 350 m. From west to east, the fjord iscomposed of three main basins, 275, 180 and250 m deep, which are separated by sills at 130m, 65 m, and 20 m below sea level, respectively(Fig. 1.2). In the deepest and westernmost basin,three stations were sampled and studied for theirforaminiferal content. At the head of the fjord,the fjord valley is subdivided into two branchescalled La Baie des Ha! Ha! and the North Arm.Samples were collected at four stations in the Baiedes Ha! Ha! and one station in the North Arm(St-Fulgence).
2.2. Hydrography and oceanography
The drainage basin of the Saguenay Fjord in-cludes more than 15 rivers and occupies a water-shed of about 85 500 km2 (Fortin and Pelletier,1995). About 90% of the freshwater input to thefjord is derived from the Saguenay River whichhas a mean annual discharge of about 1500 m3/s.The freshwater discharge and lack of turbulentmixing are responsible for important vertical sa-linity gradients and a sharp strati¢cation of watermasses. A shallow surface layer (0^10 m) is char-acterized by a salinity of about 0 near the head to29 at the mouth of the fjord. Because of the buoy-ancy of the thin mixed layer, the surface waterfreezes in winter and reaches up to 16‡C insummer. The thermohalocline is located between
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Fig. 1. The Saguenay Fjord (Quebec, Canada). (1) Location of sampling sites. (2) Longitudinal pro¢le along the Saguenay Fjordshowing the bottom topography and salinity gradients.
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8 and 25 m, its position varying seasonally andhorizontally according to freshwater discharge.Below the thermohalocline, the deep layer consistsof marine water originating from the St. Law-rence estuary: it is characterized by a salinity of30.5 (Syvitski and Schafer, 1996) and a temper-ature varying from 0.4‡C in winter to 1.7‡C insummer (Fortin and Pelletier, 1995).The 1996 £ash £ood is also likely to have af-
fected the physical and chemical properties of thewater column. However, temperature and salinity,which control the strati¢cation in the fjord, weremeasured at the end of September 1996, twomonths after the £ood, and data show that themassive and sudden freshwater discharge fromthe £ood left no noticeable evidence of its occur-rence (Pelletier et al., 1999).
2.3. Sedimentology
The valley of the Saguenay Fjord is carved intoPrecambrian rocks of the Canadian Shield(Drainville, 1968). After deglaciation of the Lau-rentian dome from the Laurentide Ice Sheet, thepostglacial La£amme Sea invaded the basin. Dur-ing this marine episode, which spanned from 8000to 11 000 yr BP, slightly carbonate-enriched clayswere deposited (Lasalle and Tremblay, 1978).Above these marine clays, the Holocene sedimentsconsist of sand at the head of the fjord, and ofsilty clay in the inner basin (Perret et al., 1995).The thick accumulations of Holocene sedimentsre£ect the high sedimentation rates. Accordingto seismic pro¢les, the fjord is ¢lled with post-glacial sequences which are, on average, 800 mthick and reach up to 1300 m in the intermediatebasin (Syvitski and Praeg, 1989). Sediment accu-mulation processes include hypopycnal sedimen-tation (Schafer et al., 1990) in addition to fastdepositional processes such as turbidite or debris£ows triggered by landslides or earthquakes (Sy-vitski and Praeg, 1989; Praeg and Syvitski,1991).Regarding the recent sedimentation, 210Pb mea-
surements reveal an exponential decrease in sedi-mentation rates with distance from the mouth ofthe Saguenay River (Smith and Walton, 1980).Near St-Fulgence, in the North Arm, the sedi-
mentation rate is up to 7.0 cm/yr, whereas inthe inner basin it is only about 0.2 cm /yr (Smithand Walton, 1980; Perret, 1994; Zhang, 2000).The recently accumulated sediments contain al-most no carbonate, but high percentages of or-ganic matter (up to 7% at the head of the fjordand between 2 and 4% in the inner basin) (Schaferet al., 1990). The organic matter is mainly com-posed of wood ¢bers originating from pulp andpaper mills (Perret et al., 1995; Louchouarn et al.,1997, 1999).In the Baie des Ha! Ha! and the North Arm,
the 1996 £ood layer shows a very di¡erent sedi-mentological composition from the backgroundsediments (St-Onge and Hillaire-Marcel, 2001).This layer contains an average of 1.7% organiccarbon (OC) and 1.4^2.5% CaCO3. It is also char-acterized by high Corg/Ntot ratios (v 20) and lowN13C values (mean value 326.5x). The thicknessof the 1996 layer is highly variable along the Sa-guenay Fjord, ranging from 50 cm in the Baie desHa! Ha! (Locat et al., 1998; Savard, 2000) to8 cm in the North Arm (Savard, 2000; St-Ongeand Hillaire-Marcel, 2001), whereas it is barelydetectable in the inner basin (St-Onge and Hil-laire-Marcel, 2001).
3. Methods
3.1. Coring sites and on-board sampling
Sediments analyzed for this work were collectedusing a box corer (Ocean Instrument Mark II)with a surface area of 0.12 m2 and a height of60 cm. This box corer permits the recovery ofsurface sediments with minimal disturbance.Eight sites were sampled in the Baie des Ha!
Ha! (SAG-02, -07, -09 and -13), the North Arm(St-Fulgence) and the inner basin (SAG-15, -30and -36) (Fig. 1.1; Table 1). In order to evaluatethe impact of the 1996 £ood on the distribution ofbenthic foraminiferal populations, box cores werecollected in 1997, 1998 and 1999. Samples col-lected in 1994 provide information on the popu-lations that developed prior to the £ood. A boxcore collected at SAG-30 was also analyzed at1-cm intervals over the upper 10 cm to examine
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the recent variability of the benthic foraminiferalpopulations.Subsampling was done immediately after the
recovery of the box cores on the ship’s deck.Push cores, 10-cm-diameter PVC tubes, were in-serted in the box core. One push core was cappedand stored vertically on deck and a second wassubsampled in the vertical position using a hy-draulic extruding piston system. Sediment distur-bance due to handling appeared to be negligible.The cores collected prior to 1999 were subsampledat 1-cm intervals. The cores collected in 1999 weresubsampled at 0.5-cm intervals. The subsampleswere stored in a cold room except those collectedin 1999 which were immediately frozen.
3.2. Micropaleontological analyses
In the laboratory, subsamples of about 5 cm3
were dried at room temperature, weighed, andthen washed through a 63-Wm screen. The frac-tion s 63 Wm was examined under a binocularmicroscope (20U). All the foraminifers and the-camoebians were hand picked, identi¢ed andcounted (Appendix 1: Systematic taxonomy).Concentrations are expressed in the number oftests per gram of dry sediment. The percentageof taxa was calculated when the sum of specimenscounted was greater than 25. In subsamples con-taining fewer than 25 specimens, only the presenceof taxa was reported in the diagrams. Detailed
Table 1Location and water depth of the surface samples and cores in the Saguenay Fjord
a=Alcide C. Horth; b=Martha L. Black; c =Louis St-Laurent; n.a. = not available; *in the text = SAG-9730.
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results of the counts can be found in Tables 2^4.These results are likely a¡ected by the di¡erentialpreservation of tests resulting from the dissolutionof calcium carbonate in the sediments (Alve,1995a,b; Leduc, 2001).
3.3. Geochemical and isotopic analyses
An aliquot of sediment was dried at room tem-perature, ground and analyzed for its total carbon(TC) and total nitrogen (TN) content with a Car-lo Erba1 elemental analyzer. Inorganic carbon
(IC) was analyzed independently using a UICCoulometrics coulometer equipped with an acid-i¢cation unit. The OC content was obtained bysubtraction of IC from TC.The isotopic composition of the OC was mea-
sured with a Carlo Erba1 elemental analyzer on-line with a Micromass IsoPrime1 instrument.Data are reported in N units (x) with referenceto V-PDB (Coplen-Tyler, 1995) and the analyticaluncertainty is V 0.15x.Radioisotopic analyses were also performed on
a reference core collected at SAG-30 in 1997.
Table 2Total benthic foraminifer and thecamoebian percentages and concentrations (in specimens per gram of dry weight) for the 1994and 1997 surface samples collected in the Saguenay Fjord
210Pb was measured using Q spectrometry of thedaughter 210Po (Canberra1). The chemical treat-ment, puri¢cation and deposition on a disk weredone following routine procedures at GEOTOP(e.g. Courcelles, 1998). In this particular case,210Pb measurements were carried out only onthe upper 10 cm. The data are compared withanother 210Pb record from a box core collectedat the same site (SAG-30) in 1996, 1 month afterthe £ood (Zhang, 2000). Sedimentation rates are
thus calculated on the basis of the composite210Pb record (Fig. 2).
4. Results
4.1. Decadal variations of micropaleontologicalassemblages in core SAG-9730 (inner basin)
One of the sites from the inner basin, SAG-30,
Table 3Total benthic foraminifer and thecamoebian percentages and concentrations (in specimens per gram of dry weight) for the 1998surface samples collected in the Saguenay Fjord
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was used as a reference to document recent sedi-mentary and micropaleontological variations.The sediment recovered from box core SAG-
9730 is composed of dark gray, bioturbatedclayey silts to silty clays. The 1996 £ood layermaterial is not visible nor is its geochemical sig-nature detectable at this site. In fact, it was onlyvisible as a thin discontinuous ¢lm upon recovery
of a box core at this site three weeks after the£ood.Based on the 210Pb data, a uniform sedimenta-
tion rate of about 0.35 cm/yr can be calculated atthe coring site (Fig. 2; Zhang, 2000). This esti-mate is in agreement with those reported byothers (Smith and Walton, 1980; Barbeau et al.,1981) for the inner basin, on the basis of 210Pb
Table 4Total benthic foraminifer and thecamoebian percentages and concentrations (in specimens per gram of dry weight) for the 1999surface samples collected in the Saguenay Fjord
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and 137Cs measurements. The sediment sequencerecovered in core SAG-9730 represents about 35years of deposition. The subsamples, which cover1-cm intervals, integrate about 4 years of sedi-mentation, and probably more when taking intoaccount the biological mixing which results in asmoothing window of up to 8 cm (Zhang, 2000).The geochemical signature of the core is char-
acterized by a high OC content (2.4% average;Fig. 2). Most of the OC is believed to be of ter-restrial source originating from the pulp and pa-per industry (Schafer et al., 1980; Smith andSchafer, 1987; Louchouarn et al., 1997, 1999).The CaCO3 content is low, about 0.7%, and theCorg/Ntot ratios are about 20 (Fig. 2; Table 5).Similar geochemical characteristics have been re-ported for this site by Zhang (2000).The foraminiferal content of core SAG-9730 is
moderately low (Fig. 3). Concentrations varyfrom 12 to 30 tests/g. The assemblages are dom-inated by the agglutinated epibenthic species Spi-roplectammina biformis (35^75%) and Adercotry-ma glomerata (5^45%) accompanied by the
agglutinated species Cribrostomoides crassimargo(2^15%) and the calcareous species Quinqueloculi-na stalkeri (2^10%).The faunal assemblages does not vary much
with depth, but there is a signi¢cant change inthe relative proportion of Spiroplectammina bifor-mis and Adercotryma glomerata at 6 cm. Below
Fig. 2. Geochemical and isotopic properties of the core SAG-9730 from the inner basin. The OC concentrations represent 5 yearsof collected data (from 1996 to 2000) from di¡erent box cores. The OC data are available on request.
Table 5Geochemical and isotopic properties of box core SAG-9730subsamples from site SAG-30 in the inner basin
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6 cm, S. biformis largely dominates (70^75%),whereas A. glomerata represents less than 15%of the assemblages. Above 6 cm, the percentageof A. glomerata increases (20^45%), whereas theproportion of S. biformis decreases (6 60%).
4.2. Post-£ood variations in micropaleontologicalassemblages of surface sediments
4.2.1. Inner basinIn addition to box core SAG-9730 (Fig. 3), two
other box cores were collected at the same site in1998 and 1999 respectively (Site SAG-30; Fig. 4).In the surface sediments (0^1 cm in 1998 and 0^0.5 in 1999), the concentrations are less than 27tests/g. Due to the small number of foraminifersrecovered in the 1999 subsamples (n=13), the rel-
ative composition (i.e. percentages) of the assem-blages are reported only for subsamples collectedin 1997 and 1998. The foraminiferal analyses re-veal abundant agglutinated relative to calcareoustaxa. The agglutinated species Spiroplectamminabiformis and Adercotryma glomerata dominatethe assemblages (30^45%) and are accompaniedby some agglutinated and calcareous species,such as Cribrostomoides crassimargo (6 15%)and Quinqueloculina seminulum (6 8%). There isan increase of A. glomerata relative to S. biformisfrom 1997 to 1998, as observed in core SAG-9730.Box cores were collected at station SAG-36 on
the eastern slope of the inner basin (Fig. 1) in1997, 1998 and 1999 (Fig. 4). As with SAG-30,the £ood material is not visible in the surface sedi-ments. According to the foraminiferal content,
Fig. 3. Concentration and percentages of foraminifers in box core SAG-9730 (SAG-30). This sedimentary sequence correspondsto approximately 35 years of sedimentation, according to a sedimentation rate of 0.35 cm/yr.
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the concentrations are relatively uniform, rangingfrom 29.4 to 44.2 tests/g. However, the assem-blages show signi¢cant variations in diversity. Inthe surface sample collected in 1997, the diversityis low and Spiroplectammina biformis largely dom-inates the assemblage, accounting for nearly 90%of the tests. In the 1998 sample, S. biformis stilldominates but only accounts for about 50% of thetests. In the 1998 sample, the diversity increasessigni¢cantly with the presence of calcareous spe-cies, such as Cassidulina reniforme, Elphidium ex-cavatum and Islandiella norcrossi. In the 1999sample, the assemblages are dominated by Cri-brostomoides crassimargo (65%), whereas S. bifor-mis is almost absent (2%). In the 1999 sample, we
also observed new accompanying taxa such asQuinqueloculina seminulum, Quinqueloculina agglu-tinata and Reophax fusiformis. As reported at siteSAG-30, S. biformis recorded a decrease in per-centages from 1997 to 1999.At station SAG-15 on the western slope of the
inner basin (Fig. 1), box cores were collected, in1994, 1997, 1998 and 1999 (Fig. 4). According tovisual observations, radioisotopic and geochemi-cal data (Savard, 2000), the 1996 £ood layer isabout 5 cm thick. Due to the small number offoraminifers recovered in the 1994 (n=19) and1997 (n=4) subsamples, we calculated relativecompositions only in the subsamples collected in1998 and 1999. In the 1998 sample, the concen-
Fig. 4. Concentration and percentages of foraminifers in surface samples at stations SAG-15, SAG-30 and SAG-36, in the innerbasin. The stars stand for the samples in which there were fewer than 25 specimens, not enough to report meaningful percentages(see Table 2).
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trations are highest, and reach 119 tests/g. Theassemblage is dominated by Cassidulina reniforme(70%) and Spiroplectammina biformis (30%). Inthe 1999 sample, the concentration decreases sig-ni¢cantly (6 20 tests/g) and the diversity increases
notably with the occurrence of calcareous taxaPyrgo williamsoni and Robertinoides charlottensis.C. reniforme still dominates the 1999 sample(80%) whereas S. biformis displays a signi¢cantdecrease (6 5%).
Fig. 5. Concentration and percentages of foraminifer species in surface sediments from the Baie des Ha! Ha! in 1998 and 1999.
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4.2.2. Baie des Ha! Ha!Based on visual observations and 210Pb data,
the £ood layer is up to 50 cm in the Baie desHa! Ha! (Savard, 2000; Locat et al., 1998). Thesedimentation rate is quite uniform at thestudied stations, ranging from 0.1 to 0.15 cm/yrbased on 137Cs measurements (Barbeau et al.,1981).Four stations were sampled in the Baie des Ha!
Ha!: SAG-02, SAG-07, SAG-09 and SAG-13(Fig. 1.1). Surface sediments in box cores recov-ered in 1998 and 1999 were analyzed for theirforaminiferal content (Fig. 5). Between 1998 and1999, the concentration increased signi¢cantly,doubling at all stations, except at SAG-02, whereit decreased. The diversity also increased from1998 to 1999 with the occurrence of calcareousspecies such as Bulimina exilis, Quinqueloculinaseminulum, Robertinoides charlottensis and Pyrgowilliamsoni. Finally, we observed a general de-crease in Cassidulina reniforme at all stations inthe Baie des Ha! Ha! except at SAG-02 where thepercentages of this species remained constant inthe 1998 and 1999 samples.
4.2.3. North ArmAt St-Fulgence, the £ood layer is about 20^30
cm thick (Maurice and Locat, 2000; De£andre etal., 2000). Based on 210Pb and 137Cs measure-ments the sedimentation rate was calculated tobe about 2^7 cm/yr (Smith and Walton, 1980;Barbeau et al., 1981).Box cores were collected at station St-Fulgence,
in the North Arm in 1997, 1998 and 1999 (Fig. 1).Because few foraminifers tests were found in the1997 (n=1) and 1999 (n=27) samples, no inter-pretation can be drawn from foraminiferal assem-blages. Among the meiofaunal assemblages, fresh-water Rhizopoda occur in low numbers withconcentrations less than 4.6 tests/g. The observedspecimens belong to the thecamoebian speciesCentropyxis aculeata, Centropyxis constricta, Dif-£ugia oblonga and Di¥ugia protaeiformis. In the1998 sample, D. oblonga is the dominant species.It is associated with a fresh water habitat. In the1999 sample, C. aculeata dominates, a species re-lated to fresh or slightly brackish water habitats(Scott et al., in press)
5. Discussion
5.1. Temporal changes in the inner basin
The geochemical and isotopic records of coreSAG-9730 show very little variation and suggestrelatively uniform sedimentary processes over thepast ca. 35 years. The relative uniformity of therecord is also explained as smoothing by biotur-bation. Nevertheless, core SAG-9730 seems toshow a change in foraminiferal populations. Inparticular, a relative decrease in Spiroplectamminabiformis at 6 cm appears signi¢cant (Fig. 3). Theprecise date of the change in foraminiferal assem-blage is di⁄cult to assess because of the biologicalmixing in sediments. Nevertheless, assuming uni-form mixing through time, the 210Pb measure-ments suggest that the main change occurred inthe late 1970s. A previous study by Schafer et al.(1991) reports a decrease in S. biformis in theNorth Arm of the Saguenay Fjord. These authorsassociated the decline of S. biformis with increas-ing sedimentary organic matter (OM) concentra-tions, they interpreted this change as a responseto reduced industrial discharges following the im-plementation of governmental regulations. Ac-cording to Corg data from that site (Fig. 2), wecould link the decrease of S. biformis with an in-crease of OM in the sediments. However, the con-tinuous decrease of Adercotryma glomerata withtime, probably due to early diagenetic destructionof foraminiferal tests, cannot be excluded (Nagyand Alve, 1987).At sites SAG-30, SAG-36 and SAG-15, varia-
tions in the assemblages in surface sediments be-tween 1997 and 1999 are consistent and show adecrease in Spiroplectammina biformis (Fig. 4).Furthermore, as Hess and Kuhnt (1996) observedfor the Mt. Pinatubo, we observe di¡erent stagesof recolonization between 1997 and 1999: in the1997 samples (initial stage), the concentrationsand the diversity are low and the assemblagesare dominated by agglutinated species. In the1998 samples (recolonization by opportunisticspecies), the concentrations increased signi¢cantlyand the assemblages are dominated by S. biformiswhich is particularly opportunistic. Finally, in the1999 samples (later stage), we see a return to a
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more diverse assemblage with the presence of cal-careous species.
5.2. Variations in the upstream branches of theSaguenay Fjord
In spite of the thick £ood layer at stations inthe Baie des Ha! Ha!, the benthic foraminiferalpopulations recolonized the sediments within twoyears of the £ood, but we cannot observe a cleartrend in the assemblages. Analyses of pre-£oodsamples would be required to document the evo-lution of the populations.The low numbers of foraminifers found in the
samples from the North Arm (Fig. 5) do not al-low an unequivocal interpretation, such as varia-tions in the assemblages through time. The bot-tom salinity of the waters in the North Arm,which varies from 28 to 31 (Schafer et al.,1991), does not allow an in situ productivity ofthe thecamoebian species observed. The presenceof these thecamoebians, as reported in the samearea by Schafer et al. (1991), re£ects a sourcefrom freshwater and brackish environments nearthe mouth of the Saguenay River and transportinto the North Arm basin.
6. Conclusions
The 1996 £ood material had only a short-termimpact on the benthic foraminiferal populationsof the fjord. The sedimentary input caused bythe £ood probably a¡ected the benthic meiofau-na. However, only two years after this event, rel-atively abundant and diverse foraminiferal assem-blages developed in surface sediments, even in theBaie des Ha! Ha! where the £ood layer is partic-ularly thick. A rapid rate of recolonization of thebenthic environment by the meiofauna can there-fore be inferred. After the 1996 £ood, changes inthe assemblages of benthic foraminifers occurredat most sampling sites of the fjord, notably de-crease in relative abundance of Spiroplectamminabiformis. Inasmuch as this species is indicative ofpolluted benthic environments (Schafer et al.,1991), the signi¢cant decrease of its relative abun-dance in the inner basin sediments could be inter-
preted as evidence that the £ood deposit helpedisolate contaminated sediments. However, manyfactors can be considered when interpreting fau-nal assemblages because they are susceptible tothe di¡erential preservation of the species (postmortem e¡ects) as well as seasonal variations inpopulation (Alve, 1995a,b).Monitoring the benthic foraminiferal popula-
tions in parallel with measuring trace metals, al-kalinity, salinity, water temperature and dissolvedoxygen concentration of the bottom water wouldbe necessary to better de¢ne the causes of therecent variations of the meiofaunal assemblagesin the Saguenay Fjord. Unfortunately, the mosaiccharacter of the benthic environment of the Sa-guenay Fjord does not allow ¢rm conclusionsconcerning its evolution.
Acknowledgements
The authors express their gratitude to LouiseCournoyer, Bassam Ghaleb and ConstanceGuignard (GEOTOP UQAM-McGill) for theirtechnical assistance in the laboratory and tothe crew of the Alcide C. Horth. We are gratefulto EŁmilien Pelletier (Universite¤ du Que¤bec a'Rimouski) and David Scott (Dalhousie Univer-sity) for their critical review of the manuscript.Special thanks are also due to Marit-SolveigSeidenkrantz (University of Aarhus) and MichaelKaminski (University College, London) fortheir critical comments which helped to revisethe manuscript. This study was supported bya strategic grant from the Natural Sciencesand Engineering Research Council of Canada(NSERC).
Appendix. Systematic taxonomy
A.1. Foraminifera
Adercotryma glomerata (Brady) Barker, 1960, pl. 34, ¢gs. 15^18 ( =Lituola glomerata Brady, 1878, p. 433, pl. 20, ¢g. 1a^c).Bulimina exilis (Brady) Cushman and Parker, 1940, p. 11, pl.2, ¢gs. 18^21 ( =B. elegans var. exilis Brady, 1884, p. 399,pl. 50, ¢gs. 5^6).
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Cassidulina reniforme (NZrvang) Scott, 1987, p. 327, pl. 2,¢gs. 11^12 ( =C. crassa d’Orbigny var. reniforme NZrvang,1945, p. 41, ¢g. 6c^h).Cribrostomoides crassimargo (Norman) Loeblich and Tappan,1953, p. 29, pl. 3, ¢gs. 1^3 ( =Haplophragmium crassimargoNorman, 1892).Elphidium excavatum (Terquem) formae Miller et al., 1982,(all) ( =Polystomella excavata Terquem, 1876, p. 429, pl. 2,¢g. 2a^d).Fursenkoina fusiformis (Williamson) Loeblich and Tappan,1964, pp. C732^C733 (=Bulimina pupoides d’Orbigny var. fu-siformis Williamson, 1858, p. 63, pl. 5, ¢gs. 129^130.Hyperammina sp. of Brady, 1878.Islandiella norcrossi (Cushman) Rodrigues et al., 1980, p. 49,pl. 4, ¢gs. 1, 4, 10; pl. 6, ¢gs. 8, 9 ( =Cassidulina norcrossiCushman, 1933, p. 7, pl. 2, ¢g. 7).Lagena mollis (Cushman) Loeblich and Tappan, 1953, p. 63,pl. 11, ¢gs. 25^27 ( =Lagena gracillima (Seguenza) var. mollisCushman, 1944, p. 21, pl. 3, ¢g. 3).Lagena striata (d’Orbigny) of Barker, 1960 ( =Oolina striatad’Orbigny, 1839, p. 21, pl. 5, ¢g. 12).Miliolinella circularis (Bornemann) Wiesner, 1931, p. 63( =Triloculina circularis Bornemann, 1855, p. 349, pl. 19,¢g. 4).Paratrochammina challengeri of Bro«nnimann and Whittaker,1988.Pyrgo williamsoni (Silvestri) Loeblich and Tappan, 1953,p. 48, pl. 6, ¢gs. 1^4 ( =Biloculina Williamsoni Silvestri, 1923,p. 73).Quinqueloculina seminulum (Linne¤) d’Orbigny, 1826, p. 301( =Serpula seminulum Linne¤, 1758, p. 786, pl. 2, ¢g. 1).Quinqueloculina stalkeri (Cushman) Loeblich and Tappan,1953, p. 40, pl. 5, ¢gs. 5^9 ( =Quinqueloculina fusca Cush-man, 1948, p.33, pl. 3, ¢gs. 16^17).Reophax scottii of Chaster, 1892, p. 57, pl. 1, ¢g. 1.Robertinoides charlottensis (Cushman) Loeblich and Tappan,1953, p. 108, pl. 20, ¢gs. 6^7 ( =Cassidulina charlottensisCushman, 1925, p. 41, pl. 6, ¢gs. 6^7).Spiroplectammina biformis (Parker and Jones) Cushman,1927, p. 23, pl. 5, ¢g. 1 ( =Textularia agglutinans d’Orbignyvar. biformis Parker and Jones, 1865, p. 370, pl. 15, ¢gs. 23^24).
A.2. Thecamoebians
Centropyxis aculeata (Ehrenberg) Medioli and Scott, 1983,p. 39, pl. 7, ¢gs. 10^19 ( =Arcella aculeata Ehrenberg, 1832,p. 91).Centropyxis constricta (Ehrenberg) Medioli and Scott, 1983,p. 41, pl. 7, ¢gs. 1^9 ( =Arcella constricta Ehrenberg, 1843,p. 410, pl. 4, ¢g. 35; pl. 5, ¢g. 1).Di¥ugia oblonga Ehrenberg 1832, p. 90.Di¥ugia protaeiformis Lamarck 1816, p. 95.
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