Marine Biology 95, 93-101 (1987) Marine o , - . - . . . . . . . . . . BiOlOgy

© Sprmger-Vedag 1987

Distribution and community structure of the benthic fauna of the north shore of the Gulf of St. Lawrence described by numerical methods of classification and ordination

B. Long * and J. B. Lewis

Institute of Oceanography, McGill University; Montreal, Quebec, Canada

Abstract

The invertebrate benthic macrofauna of the northern part of the Gulf of St. Lawrence was sampled with a van Veen grab along a series of transects from 37 to 285 m m May, 1981. The pattern of spatial distribution was determined by classificauon and ordination techniques and by di- versity and evenness statistics. Classification and ordina- tion resulted in eleven station groups which could be divided into three broad faunal zones related to tempera- ture, depth and sediment composition. The proportions of suspension feeders increased with sand content and non- selective deposit feeders increased with mud content. Fatmal diversity decreased with depth

Introduction

The objective of this study was to attempt to describe, by means of numerical techniques of classification and ordi- nation, the spatial distribution and community structure of the benthic macrofauna of the northern part of the Gulf of St. Lawrence, m relation to environmental parameters and feeding strategies. An additional purpose was to determine to what extent this community description, based on family taxi levels, was similar to the results obtained from previous species analyses m the Gulf and hence might be used In the future to reduce time consuming and often uncertain species identifications.

In spite of its importance for commercial fishenes, the fauna of the Gulf as a whole is still poorly known (Robert, 1979). This lack is especially evident along the north shore (Dunbar et al., 1980). Subsequent to the earlier investiga- txons of Prefontalne and Brunel (1962), Peer (1963) and Brunel (1970), the most recent studies of the distribution

* Present address Zoology Department, Unlvers]ty of Queens- land; St. Lucia 4067, Austraha

of the benthic fauna have been those In the lower estuary by Robert (1979) on the molluscs, by Massad and Brunel (1979) on the polychaetes and a general account of faunal distribution by Lewis et al. (1980).

A chart of the area investigated in the Gulf of St. Lawrence is shown in Fig. 1. It is marked by an extensive flat-topped offshore bank (Banc Beauge) with minimum water depth of less than 60m. This bank forms the junction of the north shore shelf and the Quebec/Labra- dor shelf and separates the Anticosti and Esqulman channels.

The topography, geomorphology and surface sediments have been described in detail by Lorang and Nota (1973). The sediments of the shelf and slopes are fair to poorly sorted. A general progradxng offme sands from the shelves down the slopes is the result of winnowing effects by bottom currents. ]'he sediments in the channels range from petite to sandy pelite.

The pattern of surface circulation in the Gulf has been described by E1-Sabh (1976). The water masses are strati- fied for most of the year in a two layered system but three layers are established during summer months with the formation of a shallow thermoclme around 20 m. A cold, intermediate layer, derived predominantly from winter cooling of surface waters, with ItS core between 75 and 100 m and temperatures lower than 1 °C, overlies a warmer, more saline bottom layer which has its origins outside the Gulf m Atlantic and Labrador current waters (Lauzier and Bailey, 1957; Lauzier and Trltes, 1958). An oxygen mini- mum layer exists between 175 and 300 m in the central region of the Gulf(Dunbar et al., 1980).

Material and methods

From the 13th until the 28th of May, 1981 the study area was sampled for macrobenthos and sediments. Sampling was done systematically along depth gradients A weighted 0.1 m 2 van Veen grab was used to obtain all samples. A

94 B Long and J.B Lewis Benthic fauna described by numerical methods

Table 1. Station group composmon

Station Statmns (no replicates in brackets) groups

SG 1 A2 (3), B3 (4), B4 (1), C4 (1). C5 (1), E6 (1), F3 (1) SG 2 G1 (l), G2 (l) SG 3 A 1 (4), A3 (4), 63 (4), D4 (4) SG 4 C2 (4), D2 (4), E2 (4) SG 5 E5 (3) SG 6 E4(4) sG 7 E1 (2), r 1 (3) SG 8 H3 (1) SG 9 B2 (4), F2 (6), E3 (4) SO 10 B 1 (4), C 1 (4), H 1 (3) S G l l H2 (1),I 1 (1)

Statmns A4 (1), D 1 (1) and D5 (1) were unclassffiable and have been excluded

total of 85 grab samples from 32 stations was obtained (Fig. 1). Attempts were made to obtain four replicate grabs per station, but at 13 stations only single samples were obtained (see Table 1).

From each sample a small ahquot of sediment was retained for particle size analysis and the remainder was sieved through 0.5-ram square wire mesh. This mesh size was chosen because all the metazoans retained by a 0 5- m m sieve are regarded as macrobenthos (McIntyre, 1971). The residue containing the benthm organisms was fixed m 5% neutral formalin. After sorting, the animals were rinsed m distilled water and preserved in 70% alcohol. They were identified to family level whenever possible and the number of individuals counted. Colonial animals were recorded as present or absent. Porifera, Nemert inea, Ne- matoda, Bryozoa, Cnxdaria, Brachiopoda and other mis- cellaneous taxa were not identified beyond the phy lum

Fig. 1. Map of the northern part of the Gulf of St Lawrence showing station posmons and the 100- and 200-m 1so- baths

level either because of taxonomic difficulties or because so few specimens were collected. To avoid confusion all taxa will be referred to as 'families'. A list of all taxa identified m the collections is given in Table 2.

Granulometry of the sediments was determined by the rapid partial analysis of sediments (Buchanan and Kam, 1971). The Wentworth grade scale classification scheme was used to establish grain size intervals This resulted m three grades; pehte ( < 6 2 / 0 , sand ( 6 2 / ~ - l . 0 m m ) and gravel (1 .0 -64mm) . Cobbles were not included in the particle size analysis but were recorded as present or absent.

Diversity and evenness indices

The Shannon-Weaver mformat lon content index was used to describe the family diversity and evenness. Although this index is normally applied to species, Ptelou (1974) has shown that the overall diversity of a commumty IS com- prised of hierarchlal components, family, genus and spe- cies, and thus the concept can be applied to families. Examples of the calculation of hierarchlal diversity are provided by Lloyd et al. (1968) and Valentine (1973). We have considered family diversity and evenness as useful for describing trends within our communit ies and do no imply a direct comparison with species radices.

Classification and ordlnahon

The data base used in the classification and ordination of stations consisted of taxon abundances. In order to elimi- nate skew and normalize the data a square root trans-

B Long and J B Lewis' Benthic fauna described by numerical methods 95

formation was applied prior to cluster analysis (Field et aL, 1982). Ordination operated on Ln transformed data. The Canberra metric dissimilarity measure was used for clus- tering (Lance and Williams, 1967). Lance and Wllhams flexible, sortmg algorithm (beta level of 0.25), contained in a computer package by Wlshart (1978) called CLUSTAN, was chosen and expressed in graphical form as a dendro- gram.

Classification and ordination o f the benthic survey data consisted of alternating cluster analysis and ordma- t]on with removal of stable station groups at mtermedmte stages Clusters of stations which were distmgmshed as

Table 2. Last of taxa collected m grab samples along the north shore of the Gulf of St Lawrence

Class Polychaetea Order Cumacea 1 Splomdae 49 Leucomdae 2 Cirratuhdae 50 Dlastyhdae 3 Maldamdae 51 Nannastacldae 4 Sabelhdae 5 Capltellldae Order Amphipoda 6 Lumbnnendae 52 Phoxocephahdae 70rblnddae 53 Ampehscidae 8 Ampharetldae 54 Photldae 90nuphldae 55 Oedlcerotldae

!0 Terebelhdae 56 Atyhdae 11 Glycendae 57 Lys]anassldae 12 Paraomdae 58 Stenotholdae 13 Sylhdae 59 Gammandae 14 Aphrodmdae 60 Syrrholdae 15 Nephtyldae 16 Opheludae Phylum Echmodermata 17 Cossundae 61 Ophmroldae 18 Scahbregmidae 62 Asteroldea 19 Phyllodocldae 63 Regulana 20 Flabelhgendae 64 Irregulana 21 Nere]dae 22 Pectlnanidae Phylum Cmdarta

Phylum Mollusca 65 Hydrolda 23 Thyasmdae 66 Alcyonacea

67 Zoatharla 24 Nuculamdae 25 Nucuhdae 26 Mytllidae Other Phyla 27 Astart]dae 68 Nematoda 28 Aplacophora 69 Nemert]nea 29 Pectmldae 70 Slpuncuhda 30 Turridae 71 Brachmpoda 31 Turntelhdae 72 Ponfera 32 Trochidae 73 Bryozoa 33 Rassoldae 34 Cephalaspadea 35 Telhnldae 36 Scaphopoda 37 Arcldae 38 Natlcldae 39 Cuspadaridae 40 Polyplacophora 4J Flssurelhdae 42 Llmldae 43 Acmaeldae 44 Trophonamdae 45 Carthtadae 46 Tnchotropldae 47 Hiatellidae 48 Velutlmdae

common m both the f rs t classification and first ordination procedures were regarded as stable station groups. Omit- tlng stable configurations from further analysis serves to remove redundant information so that subsequent cluster- mg and ordination will be more sensmve m detecting relationships within the reduced data matrix (Anderberg, 1973; Field et al., 1982).

R e s u l t s

From the recurrence o f stations in the cluster analysis and ordination o f the abundance data (Figs. 2 to 5), eleven station groups were obtained (Table 1). Three stations, D1, D5 and A4, were unclass]fiable and were omitted. A summary of station environmental data, abundances and diversities is shown m Table 3 There were clear rela- tionships between the station factor loadings and depth, temperature, salinity and granulometric data. Sigmficant correlations between the second eigenvector and the en- vironmental data are shown in Table 4. From the first ordination of stations, station factor loadings for the first three components together accounted for 71% of the variance and from the second ordination, 79% of the variance is explamed

d~ss~m~lar~ty 0.1 -0"3 -I'0 -2-0

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Fig. 2. Dendrogram showing relative similarities of groups of stations for the first clustering of stations by Lance and Wllhams (1967) flexable agglomerative strategy. The dissimilarity index was the Canberra metric

96 B. Long and J. B. Lewis: Benthic fauna described by numenca l methods

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Fig. 3. (a) Station positions on pnnclpal components one and two from the first ordmatlon of staUons. (b) Station posmons on pnncipal components two and three from the first ordination of stations. Station groups are enclosed by sohd hnes. Horizontal and vertical scales are the same

Table 3. Summary ofstaUon environmental and biological parameters

Station % Pehte % Sand % Gravel Temp. Sahnlty Slgma-t Depth Abundance H' J number (0 062 mm) (1-0.062 ram) (1-64 m m (°C) (%o) (m) (0 1 m 2)

A 1 84 5 15.5 0.0 A2 99.0 1.0 0.0 A3 85 0 11.0 4.0 A4 86 0 8.0 6.O B 1 20 0 68.0 12.0 B 2 14 0 60.0 26.0 B 3 97.0 3.0 0 0 B4 99 0 1.0 0.0 C 1 26.0 70.0 4 0 C 2 44.0 48.0 8 0 C 3 24.0 64.0 12.0 C4 98.0 2 0 0.0 C 5 99.0 1.0 0.0 D 1 17.0 57.0 26.0 D2 37.0 31.0 32.0 D 4 53.0 46.0 1.0 D5 56 0 18 0 26.0 E1 16.0 71 0 13.0 E2 21.0 68 0 11.0 E3 13.0 63 0 24 0 E4 32.0 35.0 33 0 E 5 32 0 64.0 4.0 E6 94.0 1.0 0.0 F 1 29.0 57 0 14.0 F2 28.0 58.0 14 0 F3 97.0 3.0 0.0 G 1 96 0 4.0 0.0 G2 98.0 2.0 0 0 H 1 30.0 50.0 20 0 H2 6.0 65.0 24 0 H3 7.0 89 0 4.0 I 1 5.0 72.0 23 0

4 733 34.207 27.099 190 325.2 1 073 0.186 5.806 34.716 27.374 285 102 0 0.835 0 182 4.794 34.254 27 130 180 342 5 0.817 0 141

-0.628 32.079 27.798 75 73 0 0 746 0.174 -0.433 32 314 25.901 70 136.5 1 029 0.299 -0.447 31.997 25.926 65 168.5 1 205 0 278

5.279 34740 27.192 215 92.7 0.882 0.196 5.790 34 703 27.366 280 135.0 0.861 0.176

-0.548 31.985 25.721 40 287.5 1.092 0.193 - 0 026 32.642 26 216 92 254 0 1.071 0.239

2.444 33 609 26 803 125 377 5 0.993 0 171 5 531 34.489 27.229 265 113.0 0.943 0.199 5 814 34.769 27 415 284 157.0 0.755 0.149

- 0 041 31.982 25,698 70 75 0 1.234 0.286 -0.075 32.588 26 187 96 293.2 1 176 0.243

2.646 33 513 26 752 130 412.0 1.129 0.188 4.530 34.147 26.180 188 91 0 1.108 0.246

- 0 229 32.025 25.740 55 64.0 1.212 0.292 -0.047 32.421 26.069 95 159.6 1 205 0.252 - 0 566 32 535 26 164 75 231.0 1 114 0.208

2.863 33.587 26.793 135 67 2 0 984 0 269 5 669 34 561 27.269 220 174 3 1.007 0.197 5.856 34.856 27.440 270 119 0 0.989 0.207 0.033 31.904 25.631 37 100.3 1 089 0.236

-0.734 32.281 26.207 95 189 6 1 078 0.247 5.805 34 703 27 364 260 108.0 1.102 0.235

- 1.469 32 513 26.174 161 54.0 0.995 0.249 0 086 32.871 26.407 250 33 0 1.038 0.297 2.494 31.870 24 994 78 305.6 1.132 0.211 1.248 31.644 25.360 55 375.0 1.299 0.219

-0.689 31 870 25.573 66 216.0 1.132 0.211 0 197 31.938 25.651 95 525.0 1 226 0,196

B Long and J B Lewis Benthic fauna described by numerical methods 97

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Fig. 5. (a) Station positions on principal components one and two from the second ordination of stations (b) Station positions on princi- pal components two and three from the second ordination of stations Station groups are enclosed by sohd lines. Horizontal and vertical scales are the same. Deep water stations (group 1C3 of Fig 2) r e m o v e d - see text for explanation

Table 4. Spearman Rank correlation matrix for environmental variables and station loadlngs on the principal component axes ** ( P < 0 01), * (P<0.05)

Depth Temp. Salinity % Gravel % Sand % Pehte

First Ordmat. PC 1 - 0 054 0 849 0.051 0 108 -0.239 0 180 P C l l -0892** -0.716"* -0.896** 0275 0713"* -0690** PC 111 - 0 196 0 326 0 160 0 397 * 0.045 - 0 273

Second Ordmat PC 1 0.092 0 081 0.027 0.200 -0.141 0.162 P C l l - 0 5 2 9 * -0.477* -0.712"* 0470* 0.493* -0.718 P C l l l - 0 5 4 3 * -0.359 -0377 -0356 0434* -0.111

98 B. Long and J B. Lewis: Benthic fauna described by numencal methods

Table 5. Percent abundance of feeding categories for the Gulf station groups ** Dominant (> 1070) and * sub-dominant (> 570 < 10%) taxa ofstatlongroups. P polychaetea, M mollusca, A amphlpoda, E echinodermata; C cumacea

Station Deposit (%) Suspension (70) Herbivores (70) Carnivores (70) Mtxed (%) group

** Capxtellid P 35 6 Thyasxnd M 11 1 Lumbnnereld P 12.9 SG 1

* Nereid P 8.0

** Splomd P 13.7 Lumbnnereld P 19 5 SG 2

* Nuculanld M 8.0 Thyasxnd M 8.0 Nephytld P 6 9 Lyslanassxd A 6 9

** Capltelhd P 27.9 Sabelhd P 18 0 SG 3

* Maldanld P 8.4 Thyasind M 5 2 Nematoda 5.3

** Sabelhd P 22 0 SG 4

* Clrretulxd P 9.7 Nephytxd P 69 Lumbnnereld P 54 Maldanxd P 5 4 Nematoda 5.2 Ampharetld P 5.2

** Capltelhd P 17 8 Nematoda 39.1 SG 5

* Thyasxnd M 5 9

** Ophmroldea E 24 4 SG 6

* Nannastlcld C 7 4 Regulana E 8.6 Aphrodmt P 7 4 Capltelhd P 7.1 Stenothoid A 6.4

** Sabellld P 13 4 Nematoda 19.2 SG 7

* Maldamd P 7.9 Capltellid P 6 5 Splonid P 6 1 Ampharetld P 5 6 Paraomd P 5.4

** Sabelhd P 21 6 Nematoda 17 9 SG 8

* Ampharetld P 7.3 Astartid M 8 7 Sylhd P 5.0 Paraomd P 5.5

** Ophxuroxdea E 13.7 Sabellid P 11 0 Nematoda 10.5 SG 9

* Cap~telhd P 6 0 Sylhd P 5.4

** Splomd P 12.8 Nematoda 29 1 SG 10

* Sabelhd P 7.2 Nephytld P 6 4

** Nematoda 21 0 SG 11

* Ophmroldea E 7 1 Astamd M 7 1 Sylhd P 96 Mytlhd M 6 0 Sabelhd P 4.9

A summary of percent abundances of feeding catego- ries of station groups and their dornlnant and subdomi- nant taxa is presented in Table 5. Station group d ive rmy and evenness values arranged by depth are given in Table 6. In general, diversities were high m shallow water and decreased with depth. Max imum diversity was found between 75 and 94 m. Peaks m molluscan species diversity between 75 and 120m on the southern shore of the St. Lawrence were repor ted by Rober t (1979) who con- sidered that this pat tern was typical of the transects sampled m the estuary.

Diversity d ropped progresswely down the slope to the station group in the Esquiman and Antxcosti channel (SG1). q-his group showed the lowest diversity and even- ness was modera te ly low. When the diversmes for all station groups were compared it appeared that family diversity increased on mixed or coarse sediments in-

dependent of the general d ive r s i ty -dep th relationship.

Similarly. Massad and Brunel (1979) and Rober t (1979) noted that species nchness increased with sand content, independent of bathymetr lc position.

Trends relatmg feeding strategy to sediment type emerged when the relative proport ion of deposit feeders and suspension feeders at stations was correlated with sedi- ment grain size. Sediment fraction data were arcsme transformed prior to correlation analysis as r ecommended by Sokal and Rohlf (1982). Deposi t feeding was signifi- cantly correlated with pelite content ( r = 0.897; P < 0.001) and suspension feeding was sigmficantly correlated with sand content ( r = 0.78; P < 0.001).

Thus there was an increase m deposit feeders with a concomitant decrease in suspension feeders as the ratio of sand to pelite increased. This relat ionship was evident when station group proport ions (percentage) of deposit

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Table 6. Station group dwersmes (H') and evenness (J) values ar- ranged by depth

Station group Depth in m H' J

SG 1 266 0934 0 204 SG 5 220 1 000 0 194 SG 2 206 1 017 0 269 SG 6 135 1 000 0.238 SG 3 128 1.048 0.185 SG 4 94 1 215 0.242 SG 9 78 1 187 0.216 SG 11 75 1.228 0 201 SG 8 66 1 080 0201 SG I0 63 1.102 0.199 SG 7 46 1 170 0248

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Fig. 7. Relataonshlps between proporaons (percentages) of (a) deposit feeders, (b) herbwores and (c) suspension feeders plotted according to station group cen- trolds. Station group centro]ds are located relative to a composite of axes two and three from the first and second ordmaUons of stations

100 B Long and J B. Lewis- Benthic fauna described by numencal methods

and suspension feeders were plotted according to station group centroids along factor two from the second ordina- tion of stations (Fig. 7). The deep water station groups were assigned positions on this axis based on their station group centroid position from the first ordination of sta- tions. Oblique lines passing through the origin indicate trend direction Thus in waters less than 100 m (positive values on elgenvector two) the proportion of herbivores increased with increasing values of eigenvector three (Fig. 7 c).

Trophic mode segregation by suspension and deposit feeders has been well documented (Sanders, 1958; Parker, 1963; Rhoads and Young 1970; Rhoads 1974) The sedi- ment preferences of deposit and suspension feeders has been reported from many sub-tidal benthos studies (Sanders, 1958, 1960; McNulty et al., 1962; McKoy, 1970; Young and Rhoads, 1971)

Discussion

Cluster analysis combined with ordination has allowed the resolution of station taxon abundances into a deep water station group located in the Anticostl and Esquiman channels, a station cluster in the Mecatina trough, a group of stations situated in a layer of intermediate cold water, a solitary station in the deep warm layer and a shallow water group located near the shore. Results based on family taxa indicated three broad faunal zones: a shelf region of depths to 100 m, a slope region in depths of 100 to 200 m and a bottom zone at depths greater than 200 m. Robert (1979) reported a marked faunal division at 75 m in the lower estuary.

Low diversity values were obtained from samples in the Antlcosti and Esquiman channels. This was largely due to the dominance of capitellid polychaetes and thyasirid bivalves and the low mean abundance of the pooled taxi. Based on sigma-t values, all stations sampled in the deep water were in the oxygen minimum layer. Mirza and Gray (1981) attributed low diversity and the presence of Ca- pttella sp. and Polydora sp. m the oxygen poor muds in Oslofjord, Norway to the life histories of these and other opportunists. Thus the low diversity, evenness, abundance and oxygen concentration in the St. Lawrence Gulf bottom zone samples suggest the presence of a fauna dominated by opportunists and limited by oxygen

In the Mecatina trough, where oxygen concentrations are high (d'Anglejan and Dunbar, 1968), diversity was moderately high. Sample abundance in the trough was the lowest of all station groups. Steven (1975) recorded low levels of primary production m the surface waters of the same area and thus Massad and Brnnel (1979) argued that food scarcity in overlying waters may explain the low abundances in deep waters of the Laurentian channel.

On the slope, dwersities were moderately low and abundances high. Capitellids dominated the fauna while thyasirid bivalves were sub-dominant. Robert (1979) sug- gested that factors associated with upwelling were re- sponsible for low diversities on the slopes in this area.

In general, diversities were moderately high on the shelf but showed, in some instances, a maximum at the shelf/slope break. Nearshore stations were sampled close to major northshore river outlets and the low pehte content in the samples indicated a non-deposltional en- vironment.

Diversities were high at the shelf/slope break and near the southern entrance to the Strait of Belle Isle. Sediments of these stations were heterogeneous with sizeable propor- tions of gravel. The high evenness suggested that these diversity levels were a result of a micro-habitat complexity associated with that of the substratum (Robert, 1979).

The results of this study have largely corroborated the findings of Robert (1979) and Massad and Brunel (1979) on diversity patterns and depth zonation in the Gulf. The use of family abundance data for broad characterization of distribution and community structure in the Gulf thus appears justified.

Acknowledgements. This work was supported by a Grant in Aid of Research from the Natural Sciences and En- gineering Research Council of Canada (J. B. Lewis) and a McGill University Fellowship (B. Long). We are grateful to Professors M. Curtis, B M. Marcotte and H. R. Reiswlg for advice and criticism. S. Salley, B. Stacey, V. Conde, S. Peck and M. Wysykowski provided valuable assistance.

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B Long and J. B. Lewis. Benthic fauna described by numerical methods 101

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Date of fmal manuscript acceptance. May 23, 1986. Commumcated by R W. Doyle, Halifax