Hiatella Arctica Research Articles

Bioclastic carbonate sediments on the southwestern Svalbard shelf

The poleward continuation of one of the largest known Arctic carbonate factories, the western Spitsbergenbank of the Barents Sea, has been surveyed along the narrow SW Svalbard shelf. On this shelf, formation of cold-water carbonate sediments occurs under glaciomarine environmental conditions. Mya truncata and Hiatella arctica colonized a soft glacio marine diamicton 3000–6000 years after the final retreat of the icedome from the SW Svalbard shelf. Since 2600 yr B.P., an increase of along-shelf bottom currents washed out the fine terrigenous deposits leaving behind a shelly lag-deposit and winnowed boulders on a morainic shelf topography. The most favourable depth interval promoting the establishment of an amalgamated bioclastic blanket is located in 50–80 m water depth. In this interval, strong bottom currents prevent sedimentation, thus, facilitating colonisation of filter-feeding benthic communities dominated by Balanus balanus and Chlamys islandica. Additional skeletal components are supplied by echinoderms, bryozoans, serpulids, brachiopods and gastropods. In shallower water depths, carbonate production is hampered by intense iceberg-scouring and release of ice-rafted debris from the still glaciated hinterland of Svalbard. In deeper water (>80 m), olive grey muds with dispersed pebbles rich in benthic foraminiferans, Clinocardium ciliatum and taxodont bivalves are present on all surveyed shelf banks. The deepest unit is a blue mud in water depths > 120 m with benthic foraminiferans, taxodont bivalves and sipunculids. Compared to Spitsbergenbank, carbonate production is much more restricted and impoverished on the SW Svalbard shelf, but is still an instructive modern example for the formation of skeletal carbonates resting on tillites in the geological record.

Experimental collection of great scallop postlarvae and other benthic species in the Bay of Brest: settlement patterns in relation to spatio-temporal variability of environmental factors

Settlement patterns of great scallop (Pecten maximus) postlarvae and other benthic species in the Bay of Brest were studied from 2-D and 3-D collectors to clarify biotic interactions at the postlarval stage. Plane tiles became colonized mainly by epifaunal suspension feeders, while the 3-D substrata collected a more diverse fauna. Both species diversity and total abundance were higher in Netlon bags, compared with sandstone tiles. However, hydroids, bryozoans, tubicolous polychaetes, a number of epifaunal molluscs and macroalgae were more abundant on tiles. Between-sites differences in taxonomic group abundances mostly reflected the benthic fauna and flora beneath the collectors and would be explained by differential settlement and survival, according to local environmental conditions. Sediment type, depth, salinity, water turbulence, food and biotic interactions would be the factors regulating settlement. Bryozoans, hydrozoans, Anomia ephippium, Crepidula fornicata, mussels, tubicolous polychaetes, Pisidia longicornis, Hiatella arctica, Aequipecten opercularis and Chlamys varia were the main potential competitors of Pecten maximus collected. Crabs and Nassaridae would account for most of the potential scallop predators. Possible ways of improving scallop spat collection and cultivation are discussed with reference to the efficiency and placement of collectors, to environmental characteristics and to the intensity of biotic interactions at the postlarval stage.

Optimizing the design of giant scallop (Placopecten magellanicus) spat collectors: field experiments

Field experiments were carried out in 1991 and 1992 in the Gulf of St. Lawrence (Quebec, Canada) to examine the influence of various structural components of spat collectors on recruitment of the giant scallop (Placopecten magellanicus) and associated species. Components considered were monofilament diameter, heterogeneity (presence of knots), mesh size of external bags combined with monofilament density, and spatial arrangement of monofilament inside collectors. P. magellanicus spat were more abundant in collectors made with bags of intermediate mesh size (0.4x0.5 cm) containing low densities (2030 cm2) of monofilament than in other types of collectors. significant variations in spat numbers were, however, observed with other taxa such as Mytilidae, Hiatella arctica, Cerastoderma pinnulatum, Mya arenaria, Anomia simplex, Tellina sp., Veneridae, Dacrydium vitreum, Mactridae, Buccinidae, Littorina sp., Ostracoda and Foraminifera. Abundance of scallop spat did not vary significantly with spatial arrangement, diameter and heterogeneity of monofilament.

Directional selection on directional asymmetry: testes size and secondary sexual characters in birds

A standard example of directional asymmetry with one side of the body having a larger character value than the other is testes size in many vertebrates. The relation between directional asymmetry in testes size and the expression of secondary sexual characters (a measure of phenotypic quality) in two bird species, the barn swallow, Hirundo rustica, and the house sparrow, Passer domesticus, was used to test whether directional asymmetry was subject to directional selection. Testes size, which was positively related to the size of the secondary sexual character, demonstrated directional asymmetry, with the left testis generally being larger than the right testis. The amount of directional asymmetry was positively related to the expression of the secondary sexual character, suggesting that males with the largest degree of directional asymmetry were at a selective advantage, since males with the most extravagant secondary sexual characters have the highest mating success. These relations were not confounded by variables such as body mass, body size and age. Males in poor condition (with small secondary sexual characters) may be unable to develop large degrees of directional asymmetry, and deviations from directional asymmetry may therefore be viewed as a measure of developmental homeostasis.

Oxygen and carbon isotope composition of Quaternary bivalve shells as a water mass indicator: Last interglacial and Holocene, East Greenland

Oxygen and carbon isotope composition of arctic bivalve shells are used in an attempt to reconstruct surface water temperature and salinities in Scoresby Sund, East Greenland. The oxygen isotope compositions of Mya truncata, Hiatella arctica and Tridonta borealis have been compared with present day hydrological parameters. Modern shells yield oxygen isotope values that, on the whole, reproduce the environmental temperature and sea water isotopic composition. Furthermore, it is possible to estimate the living depth of the analysed specimens. Analyses of growth increments from single shells show that there are large variations from year to year in temperature and oxygen isotope composition of the surface waters of Scoresby Sund and that these variations decrease with depth. Analyses of Holocene shells indicate that the Polar Current water, which flows from north to south along the East Greenland coast was also present during the Holocene climatic optimum 8000-7000 yr B.P. Analyses of bivalve shells from the last interglacial show that Scoresby Sund during that time was well circulated and that meltwater from the Greenland ice sheet and sea ice meltwater was important for the temperature, salinity and isotopic composition of the surface waters.

Early Holocene deglaciation of Expedition and Strand fiords, Canadian High Arctic

Terrestrial and marine records provide the first reconstruction of past glacial events on west-central Axel Heiberg Island. Lateral moraines along Expedition Fiord define an early Holocene ice margin when a trunk glacier occupied the inner half of the fiord. Limited sediment accumulation in the outer fiord (< 10–40 m) questions whether this ice margin records the limit of the last glaciation. Ice retreat began > 8.4 ka BP, principally by calving of an unstable ice front to a position well upvalley of the present ice limit. Rapid deglaciation is evidenced by (i) an absence of ice marginal landforms and related marine biofacies; (ii) a scarcity of raised marine sediments; and (iii) exceptionally thin sediments (< 5–20 m) in the inner fiord. A transition from a Hiatella arctica – Mya truncata macrofossil assemblage to an Astarte borealis – Mya truncata assemblage reflects a change from moderate to low energy depositional environments at the fiord head by 7.9 ka BP.The extensive ice advance into Expedition Fiord is contrasted by a more restricted advance in adjacent Strand Fiord, which contains both glacial and marine features that appear to predate the last glaciation. The difference in ice extent between the two fiords is explained by drainage basin morphometry and fiord bathymetry, and reflects modern analogues. Initial retreat of trunk glaciers may have occurred in response to rising sea level, with regionally synchronous ice retreat occurring at 8.4 ka BP when climatic factors were dominant. Glacier dynamics controlled the rate of ice retreat, with tidewater glaciers retreating much more rapidly than those with terrestrial margins.

Fouling development and its effect on the growth of juvenile giant scallops ( Placopecten magellanicus) in suspended culture

We examined the development of the fouling community on pearl nets and its effect on the growth of juvenile Placopecten magellanicus at Grande-Rivière and Gascons in the Baie des Chaleurs. The total biomass of fouling organisms was 2–3 times greater at Gascons than at Grande-Rivière and at both sites decreased with increasing depth. Throughout the study, at Grande-Rivière, at the mouth of the bay, the fouling community was dominated by two bivalves, Mytilus edulis and Hiatella arctica, and the hydroid Obelia gelatinosa was third in abundance. In contrast, at Gascons, 40 km inside the bay, the hydroid Tubularia larynx predominated in the summer while M. edulis and H. arctica predominated in the autumn. Factors potentially explaining the success of M. edulis and H. arctica in the fouling community are the loss of hydroids due to selective grazing by nudibranchs, the rapid growth rate of the bivalves and their ability to inhibit settlement of competitors. Changing the pearl nets regularly results in a sharp increase in the yield of muscle and other soft tissues, but only a slight increase in shell height of the cultured scallops. For example, for a 30-mm scallop suspended at 9 m in depth for 4 months, the increase in muscle mass is 68% greater in cleaned than in fouled nets whereas the difference is only 4.8% for shell height. Because fouling decreases with depth and also varies markedly between localities, judicious choosing of culture sites in term of fouling development could substantially improve meat production.

Feeding habits of newly hatched juveniles of an intertidal predatory gastropod, Nucella emarginata (Deshayes)

The feeding habits of juvenile Nucella emarginata ( = Thais emarginata), a marine prosobranch gastropod with direct development, were examined to determine when predatory feeding begins, and if ontogenetic shifts in mode of feeding occur. Most hatchlings did not attack live prey during the first 3 days after emerging from their egg capsule. By day 10, however, over 80% of the hatchlings had attacked prey. The late onset of predatory feeding appears to be ontogenetic, rather than a consequence of handling or experimental conditions. Hatchlings did not use alternate food types (mussel feces, algae, biofilm, or barnacle moults), as determined indirectly by growth, survival and organic content measurements of hatchlings. Once N. emarginata started attacking prey, six species of invertebrates which co-occur with the hatchlings in the field were consumed: three bivalves ( Lasaea spp., Musculus taylori and Mytilus spp.) and three barnacles ( Balanus glandula, Chthamalus dalli and Pollicipes polymerus). Hatchlings did not attack limpets ( Lottia pelta and Lottia digitalis), a littorine ( Littorina scutulata), the bivalve Hiatella arctica, or other hatchlings. In addition, hatchlings were able to survive for up to 120 days without food. Although no ontogenetic shift in mode of feeding was observed in young N. emarginata, at least two prey species consumed by hatchlings cease to be used during ontogeny. Possible reasons for the late onset of feeding are discussed.

The pre‐Holocene marine limit on Hopen, Svalbard

A pre‐Holocene marine level is found at 109 m a.s.1. on Hopen. Fragments of Mya truncata and Hiatella arctica occurring on a raised coastal spit at that altitude have been radiocarbon dated to >45,000 BP. The amino acid epimerization of these shells, a clearly mixed sample, correlates with samples from Kongsøya that are of Eemian or Early Weichselian age (alloisoleucine/isoleucine ratios, hydrolysed fraction, between 0.084 and 0.213). No direct evidence, such as striations or roche moutonées, of overriding glacier ice has been found, and no erratics were found above the pre‐Holocene marine limit. The existing Holocene shoreline displacement curve, with its upper limit at 60 m a.s.1., is supported by four new radiocarbon dates.

Identification of in situ Canadian Arctic bivalves using underwater photographs and diver observation

In situ bivalves in the Barrow Strait region of the Canadian Arctic can be identified accurately to genus from siphon characteristics or general appearance. Identification characteristics are described for seven taxa found in the area including Mya truncata, Serripes groenlandicus, Hiatella arctica, Musculus spp., Astarte spp., Macoma spp. and Delectopecten greenlandicus.

Macrofauna of Canadian arctic fjords

Deposition of fine glacial sediments in Canadian arctic fjords decreases downfjord from the source. Soupground substrates near the head give way to softgrounds and then to firmgrounds in the outer regions. Intertidal areas are strongly affected by sea ice. Faunal communities reflect substrate characteristics. The muddy, soupground substrate supports a community of benthic infauna, in which errant polychaetes such as Nereis pelagica coexist with mobile deposit-feeding bivalves, including Portlandia arctica, Nucula belloti and Nuculana pernula. Forms are typically inflated and possess attached bristle-like hydrozoans. The firmground communities are dominated by epifaunal species, especially pectens, Chlamys islandica or Delectopecten groenlandicus, old shells of which litter the bottomcreating shell pavements. Hardground communities occur on sidewalls and in microcosm on isolated ice-rafted cobbles to form faunal islands. Shell and gravel surfaces support epifauna including anthozoans (octocorals), bryozoans, tunicates, porifera and serpulid polychaetes. Echinoderms, especially ophiuroids (brittlestars) dominate the epifaunal communities, increasing in species diversity, density and size toward the mouths of the fjords. Intertidal areas support a biota dominated by bivalves, polychaetes and fucoid algae. In the subarctic marine zone, a distinct subarctic macrofauna consisting of Macoma balthica, Mytilus edulis, Arenicola marina, Littorina saxatilis and Semibalanus balanoides cohabit with panarctic shallow-water species such as Hiatella arctica, Mya truncata and Musculus discors. In the arctic marine zone, the intertidal macrofauna consists solely of panarctic shallow-water species, including Macoma calcarea and Balanus balanus, in addition to H. arctica, M. truncata and M. discors.

Biotic interactions revealed by macroborings in arctic bivalve molluscs

The presence of organisms whose bodies have low preservation potential may be deduced by searching for the traces produced by them. The addition of predatory gastropods and soft-bodied epizoans to Quaternary marine faunas dominated by bivalves was facilitated by an examination of borings in bivalve shells. Borings attributed to predatory gastropods (ichnogenus Oichnus) were observed in shells of Astarte spp., Hiatella arctica and Macoma calcarea. Astarte, Hiatella and Macoma were preyed upon in preference to other members of a diverse suspension-feeding bivalve community. Borings attributed to epizoans (ichnogenus Cautostrepsis) were observed in bivalve shells (Astarte spp. Hiatella arctica), calcareous algae and limestone clasts. Biotic interactions revealed by trace fossils are employed, for the first time, to reconstruct the trophic structure of arctic Quaternary marine benthic faunas. ▭ Arctic molluscs, palaeoecology, Oichnus, Caulostrepsis.

Bioerosion of intertidal limestone, Co. Clare, Eire — 2: Hiatella arctica

The boring bivalve Hiatella arctica (L.) is common in the lower intertidal and upper sublittoral zones of the Carboniferous Limestone coast of NW Co. Clare, Eire. Age determinations using growth rings are discussed and boring rates are calculated from the relationships between age and depth of boring to range between 0.125 and 1.0 cm a −1. The significance of Hiatella borings in weakening rock surfaces is discussed, suggesting that death and colonisation cycles are more frequent than rock fracture cycles.

Refuges, Biological Disturbance, and Rocky Subtidal Community Structure in New England

The effects of two sources of biological disturbance–predation and sea urchin grazing–on the structure of benthic communities inside and outside beds of the horse mussel, Modiolus modiolus, were examined in the rocky subtidal zone off the Isles of Shoals, New Hampshire, USA. Multivariate analysis revealed three major communities between 8 and 30 m in depth: (1) a Modiolus community; (2) a 30 m community; and (3) an 8—18 m community. At all depths, mussel beds contained significantly higher densities of infauna than did other subtidal habitats. The hypothesis that Modiolus beds provide a refuge from predation for the associated community was tested in five manipulative field experiments. Three members of the mussel bed community, the bivalve Hiatella arctica, the sea urchin Strongylocentrotus droebachiensis, and the ophiuroid Ophiopholis aculeata, were used as experimental prey. Consumption of these prey by a guild of generalist predators was observed outside but not inside mussel beds, which indicates that subtidal Modiolus beds provide a spatial refuge from predation. Deaths from predation were significantly higher at night than during the day for Hiatella but not for Strongylocentrotus. At night, crab (Cancer borealis, Cancer irroratus) and lobster (Homarus americanus) predation accounted for all attacks that were directly witnessed, while fish (Tautoglabrus adspersus, Pseudopleuronectes americanus) predation accounted for 71% of the total prey consumed during the day. Such diel differences in predation corresponded with predator abundance patterns. The sea urchin Strongylocentrotus droebachiensis was the most significant agent of biological disturbance during the 1979—1984 study period. Strongylocentrotus intensively grazed the benthos at 8 m in 1982, causing a 79% reduction (from predisturbance levels) in the mean population density of invertebrates outside the mussel beds. This event served as a natural experiment in which to test the hypothesis that Modiolus beds function as a refuge from severe grazing disturbance. Re—sampling of communities inside and outside mussel beds immediately after the grazing disturbance indicated that the Modiolus community exhibited less change in species composition, dominance, and diversity than communities outside the mussel beds. Such comparisons indicate that Modiolus beds afford protection from severe grazing disturbance for infaunal invertebrates. Long—term photographic monitoring of marked mussel beds at 8, 18, and 30 m depth showed that Modiolus beds at all depths persisted for more than 5 yr. Mortality rates of adult Modiolus were low; however, mortality was highest at the shallow site (8 m). Mussel beds successfully resisted the grazing disturbance which eliminated all other biogenic habitats except those created by crustose coralline algae. Modiolus beds are effective refuges because they persist for many years and resist biotic disturbance. In the subtidal communities examined here, lower levels of disturbance inside mussel beds can account for the abundance and spatial distribution of Modiolus—community species. These results demonstrate the functional significance of mussel beds in cold—temperate subtidal regions where predation and sea urchin grazing are major determinants of community organization.

Marine bivalve bores above present high tide level in a limestone cliff near Ord, southern Skye

Bores of marine molluscs above high water mark have been used as evidence for former higher than present sea levels, for instance in the Mediterranean ([Zeuner 1945][1], p. 229). In Scotland, empty bores attributed to the marine bivalve Hiatella arctica (Linne) have been reported up to 2 m above high tide level in limestone at Seafield Point, Loch Kishorn (Fig. 1) by Smith ([1978][2]). She also recorded the presence of the living animal near low water level at this locality. The bores described below were discovered during the field meeting of the Edinburgh Geological Society held in the western Highlands and Skye in May 1984. The foreshore and cliff immediately north of the mouth of the Ord River at Ord (Fig. 1) are formed of cherty dolomitic limestones of the Sailmhor and Eilean Dubh formations of the Lower Palaeozoic Durness Group ([Peach et al. 1910][3]). There is an intertidal rock platform up to 200 m wide which bears traces of moutonee-like forms. These suggest that the platform was glaciated and was therefore cut prior to the last ice movement across the area. The intertidal platform is separated from the base of the cliff by a gently sloping ramp and a poorly developed higher platform. The angle between the latter and the base of the cliff is at a height of c 1 m above high water as indicated by the distribution of black and yellow lichens, seaweeds and land plants. In the intertidal zone the bedrock and limestone boulders lying on . . . [1]: #ref-10 [2]: #ref-6 [3]: #ref-4

Intrashell variations in amino acid concentrations and isoleucine epimerization ratios in fossil Hiatella arctica

Twenty-four valves of fossil Hiatella arctica were analyzed to determine if amino acid ratios varied from one region of a shell to another. The ratio of D-alloisoleucine/L-isoleucine, routinely used as a stratigraphic correlation tool and an indicator of relative age, did not vary significantly between five anatomically different shell parts in Hiatella arctica . Sampling only the hinge or central part of all valves, however, resulted in less variation about the average value. Analyses of only this part of the shell should improve the resolution of stratigraphic units by amino acid geochronology. The absolute concentrations of aspartic acid, threonine, serine, glutamic acid, glycine, alanine, valine, alloisoleucine, isoleucine, and leucine (in picomoles/milligram of shell) are significantly higher in the hinge and central part of the shell, whereas the outer growth edge appears to have lower levels of amino acids. This is true in both the FREE and TOTAL hydrolysate fractions. The reasons are not clear; however, the high value may be caused by a thin, protein-rich inner layer lining the valve out to the pallial line and/or differences in the proportion of inorganic carbonate to protein produced in different areas during shell growth. Alternatively, it may suggest leaching of the thinner, more vulnerable part of the shell growth edge.

Pozzolanic aggregates of fly ash in the sea

Pulverized fuel ash (pfa) readily forms pozzolanic aggregates when wetted, and these are common at sites of ash dumping at sea, where they can be up to 2 m across. Field observations on the structure and fauna of aggregates from an ash dumping ground off the Northumberland (UK) coast were undertaken in 1975, together with experimental studies of aggregation, to determine the influence of ash aggregates on the benthic substrate and the faunistic colonisation of aggregates in situ. The aggregates are of a soft sandstone consistency, but are persistent at the sea bed. Particle analysis of aggregates showed negligible contribution by inorganic particles other than the ash, and an overrepresentation of larger diameter particles (3.5 to 4 Φ) compared to pure ash. There was an exponential relationship between aggregation and ash content of experimental sediments. It is suggested that only 15 to 17% of the ash reaching the sea bed will be involved in aggregation. The naturally occurring aggregate fauna consisted of a sparse but diverse sessile epifauna, which depended on encrusting bryozoans for initial colonisation, and a few boring species (Zirfaea crispata, Hiatella arctica, Polydora flava). The presence of 7 yr-old Z. crispata in aggregates confirms their persistence at the sea bed, and indicates that the ash is not toxic.

Quaternary Depositional Episodes, Western Spitsbergen, Norway: Aminostratigraphy and Glacial History

Quaternary events interpreted from deposits along the central west coast of Spitsbergen are subdivided and correlated by amino acid enantiomeric ratios in enclosed molluscan fossils. Deposition is characterized by relatively short episodes of rapid sedimentation associated with glacial activity and high relative sea level. Depositional episodes are separated by relatively longer periods of subaerial exposure and pedogenesis. At least five major depositional episodes, each terminated by an emergence cycle, are identified by characteristic isoleucine epimerization ratios (aIle/Ile) in Mya truncata and Hiatella arctica. Absolute age limitations are derived by calibrating the aIle/Ile ratios against radiometrically dated samples and diagnostic faunal communities. Glacial activity and sea-level change are inferred from the facies relationships within each depositional episode. The oldest recognized unit, episode H, is characterized by aIle/Ile Total ratios of 0.15 ? 0.02. At least three emergence cycles and two distinct glacial events are recorded within deposits of this episode. The absolute age is estimated to be between 300 and 1000 ka. The next younger episode, D, is represented at only one site, although reworked shells with the same ratios were also found. Total aIle/Ile ratios are ca. 0.08 and the age is estimated to be between 170 and 480 ka. Deposits of the three youngest depositional episodes, C, B, and A, are widespread throughout the study area, and are characterized by Total aIle/Ile ratios of 0.051 ?0.006, 0.033 ?+0.0022, and 0.019 ?0.001, respectively. Episode C deposits represent the last major regional glaciation of the area, an event estimated to have occurred between 90 and 260 ka ago. Episode B deposits are associated with a restricted regional glaciation estimated to have occurred between 40 and 130 ka ago. The associated fauna indicates marine conditions as warm or warmer than present. Episode A deposits are related to a latest Weichselian/earliest Holocene marine transgression/regression cycle during a period of restricted local glacier expansion, ca. 9 to 12 ka ago. These findings place constraints on the timing and extent of Spitsbergen/Barents Sea glacial events. The 18 ka BP open-ocean 6180 minimum is not associated with extensive glacierization of the Forlandsund region of western Spitsbergen, nor did outlet glaciers of the postulated Barents Sea ice sheet reach the study area in the last 40 ka or more.

Aminostratigraphy of Quaternary marine and glaciomarine sediments, Qivitu Peninsula, Baffin Island

A complex sequence of cyclical marine and glaciomarine lithofacies exposed along the coastal lowlands of the Qivitu Peninsula was deposited during the marine transgressions and regressions that affected this area during repeated glaciation of the region. Multivariate analysis of amino acid ratios measured on shell fragments of the pelecypods Hiatella arctica and Myatruncata in the lithostratigraphic units provides a basis for their chronocorrelation through the use of informal regional aminostratigraphic units termed aminozones. All but the oldest aminozones correspond with particular lithostratigraphic units deposited during transgressive–regressive periods when relative sea level was higher than present sea level. Discriminant analysis shows total amino acid fraction allo/iso ratios from either species are the best overall criteria for assigning a shell sample to a particular aminozone, but free fraction allo/iso ratios are more useful in the younger zones. The distribution of ratios in lithostratigraphic units indicates a large proportion of shell fragments is reworked.Interpretation of all 14C, uranium-series, paleomagnetic, and amino acid data from eastern Baffin Island suggests aminozone 1 is [Formula: see text] years BP, aminozone 2 is [Formula: see text] years BP, aminozone 3 is [Formula: see text] years BP, and aminozone 4 is [Formula: see text] years BP. The extent of isoleucine epimerization in shell samples indicates some aminozones may be of early or pre-Quaternary age.

Groupement et ordination appliqués à l'étude de la répartition de l'épifaune benthique de l'estuaire maritime et du Golfe du Saint-Laurent

Forty-seven navigation buoys were used to collect quantitative samples of epibenthic fauna in the Lower Estuary and Gulf of St. Lawrence. Stations, as described by their sessile species and biomass composition, were grouped using multivariate analysis. A Gower similarity index, modified to take into account the importance of each species in the ecosystem, was used to compare stations. Results obtained by cluster and principal coordinate analyses showed comparable relations between stations. North Shore and Gaspé Coast stations were separated from those of the Lower North Shore, the Mingan Island sector and the Lower Estuary. Stations from the Lower North Shore were also separated from those of the Mingan Island region and of the Lower Estuary, but at a higher level of similarity. All animal assemblages consisted of the following sessile species: Obelia longissima (Pallas), Hiatella arctica (L.), Mytilus edulis L., Balanus balanoides (L.) and B. crenatus (Bruguière). These species formed over 95% of biomass of all sessile species. Differences between the various station assemblages were mainly due to changes in the relative abundance of these species. The geographical gradient of summer surface water temperature best explains the observed clusters. In the Lower Estuary, the spatio-temporal variations of physico-chemical factors and the surface circulation may influence the number of species and their abundance. The circulation systems of the Lower Estuary and near the Mingan Islands might account for the similarity between their assemblages. The same dominant species, Obelia longissima, characterizes the fauna of these two regions.