SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

NUMBER 6

Daniel J. Stanley
and Noel P. James

Distribution of Echinarachnius parma (Lamarck) and Associated Fauna on Sable Island Bank, Southeast Canada

SMITHSONIAN INSTITUTION PRESS
CITY OF WASHINGTON
1971

ABSTRACT

Stanley, Daniel J., and Noel P. James. Distribution of Echinarachnius parma (Lamarck) and Associated Fauna on Sable Island Bank, Southeast Canada. Smithsonian Contributions to the Earth Sciences, number 6, 24 pages, 1971.—A combined bottom photographic and sampling survey of Sable Island Bank southeast of Nova Scotia, Canada, reveals locally high densities (to 180 individual/m²) of the northern sand dollar Echinarachnius parma. Populations of this form are closely related to texture of the sea floor and generally concentrated on moderately sorted fine to medium sand surfaces. Topography and current regime are also correlatable factors; depth, time, salinity, and temperature apparently are not. Sand dollars are second in importance, after current activity, in reworking surficial sediments, and these organisms modify at least a third of the total Bank surface in the study area. Bioturbation is particularly intense in the sector north of Sable Island. Associated epifauna and infauna populations occur in two east-west trending areas on the Bank north and south of Sable Island. Absence of conspicuous fauna, save E. parma, in an east-west zone along the crest of the Bank and near Sable Island results from extremely strong current activity concentrated in this region.

Official publication date is handstamped in a limited number of initial copies and is recorded in the Institution’s annual report, Smithsonian Year.

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Contents

Copyright

Introduction

Geographic and Hydrologic Setting of Study Area

Methods

Sediment Texture and Evidence of Current Activity

Echinarachnius parma on Sable Island Bank

Other Epifauna

Infauna

Discussion

Summary of Observations

Acknowledgments

Literature Cited

Distribution of Echinarachnius parma (Lamarck) and Associated Fauna on Sable Island Bank, Southeast Canada

Daniel J. Stanley and Noel P. James

Daniel J. Stanley, Division of Sedimentology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560
Noel P. James, Department of Geological Sciences, McGill University, Montreal, Quebec, Canada

Introduction

The present study was conducted as an outgrowth of a sedimentological survey (James and Stanley 1968) of the sand-covered, current-dominated shallow outer banks of the Nova Scotian Shelf off southeastern Canada. It is an attempt to evaluate more precisely than heretofore possible the fauna likely to affect surficial sediments on a shallow platform, Sable Island Bank, located far from the present mainland. It is possible as a result of this investigation to define the regional distribution pattern and density of the northern sand dollar, Echinarachnius parma (Lamarck), and associated fauna commonly encountered on Sable Island Bank. Previous investigations (James 1966; James and Stanley 1968) have shown that this circumboreal echinoderm is particularly abundant on Sable Island Bank, the largest of the outer banks on the shelf. Availability of detailed geographic and hydrologic data, together with information on the sediment regimen, suggested that a study of the benthic fauna of this area would be particularly profitable.

Data evaluated are derived mainly from bottom photographs, supplemented by surficial samples collected at numerous stations distributed over most of Sable Island Bank and immediately adjacent areas. The present analyses serve, first of all, to supplement the surprisingly small amount of quantitative data and field information on this common sand dollar (Moore 1966). This echinoid occurs on both margins of northern North America, in the western North Pacific, and in the western North Atlantic the known range of E. parma extends from Cape Hatteras to Labrador and Greenland (Mortensen 1948). We relate the distribution of E. parma to such parameters as time of observation, depth, topography, sediment type, current regime, and associated fauna. The problem of correlating fauna visually observed on the bottom with that collected in relatively small bottom grabs is also considered.

Geographic and Hydrologic Setting of Study Area

MORPHOLOGY.—Sable Island Bank is a sandcovered platform approximately 250 km long with a maximum width of 115 km. The 90 m (50 fms) isobath defines the bank margin (Figure 1). Sable Island, at approximately 60°W longitude and 43°55’N latitude, is located on the eastern margin of the Bank, 334 km southeast of Halifax on the Nova Scotian mainland. Topography and sediments of the island, a low (relief of 1 to 20 m) east-west trending arcuate bar of sand flats and dunes about 39 km long and 1.5 km wide, have recently been described by James and Stanley (1967).

Extremely gentle gradients characterize the bank top east and south of Sable Island (approximately 1:880 and 1:330 respectively) as well as north (1:280) of the island. Along its steep northern edge, the Bank is bounded by the Gully Trough, a depression (90 to 210 m deep) characterized by hummocky and dissected topography. This depression curves southward around the northeast margin of the Bank and deepens to form the southeast trending submarine canyon known as The Gully Canyon (Marlowe 1967). The southeast margin of the Bank is the boundary between the outer shelf and the upper continental slope. The outer margin topography and sediments have been detailed elsewhere (Stanley and Silverberg 1969).

GENERAL WATER MASS PROPERTIES.—Water mass likely to affect sediments and organisms in the study area is composed of Atlantic water diluted by 20% coastal water (Hachey 1961) and covers the outer banks of the Nova Scotian Shelf, including Sable Island Bank. Between this “shelf water” and the Gulf Stream to the south lies a band of “slope water” having intermediate surface salinities and relatively low temperatures at middepth. Shelf water is generally well stratified into three layers during the summer months and less so during the winter. The upper layer may be as much as 40 fms (73 m) thick, with temperatures ranging between 5°C and 20°C and salinities less than 32‰. The intermediate layer ranges between 17 and 80 fms (31 to 146 m) in thickness, with temperatures from 0°C to 4°C and salinities between 32‰ and 33.5‰. Bottom water, occupying depths of 50 to 110 fms (91 to 201 m), has a temperature range from 5°C to 8°C and salinity gradient from 33.5‰ to 35‰.

The intermediate layer is most important in determining the water characteristics of the outer banks including Sable Island Bank (Hachey 1961). The presence of this cold water on the shelf is due to the large volume of water transported from the east and northeast by the Labrador Current, and chilling in situ is a continuous feature, at least during part of the year. Incursions of slope water (a mixture of surface, coastal, Gulf Stream, Labrador Current, and upwelling deep Atlantic waters, as described by McLellan et al. 1953) onto the Nova Scotian Shelf results is salinity variations on the outer margin of Sable Island Bank ranging from 33‰ to 33.5‰ and temperature variations from 5°C to 20°C.

CURRENTS, WAVES, AND TIDES.—The regional distribution of benthic organisms whose feeding and burial habits make them dependent on the texture of surficial sediments would almost certainly be controlled to a considerable degree by water-mass movement affecting Sable Island Bank. There is ample evidence that bottom currents do, in fact, modify the sedimentary cover of the bank top. A circulatory pattern centered around Sable Island displays an average rate of drift ranging from 0.11 to 0.15 knots (5.6 to 7.7 cm/sec) (Trites 1958). This surface circulation pattern varies seasonally (Bumpus and Lauzier 1965). The northeasterly and easterly drift along the outer Bank margin constitutes a general off-the-shelf movement and is consistent except in spring when there is a westerly drift east of Sable Island. Bottom-circulation patterns in the study area are modified by marked topographic features including The Gully Canyon and Sable Island.

In fall and winter, when the Bank top is most likely to be affected by storms, the number of waves over 20 feet or more are reported 5% of the time (Climatological-Oceanographic Atlas for Mariners 1959). In spring the number and size of waves decrease (waves less than 4 feet in height account for over 45% of all waves), and in summer waves over 5 feet in height are rare. Fall and winter are times of long period, high waves from the west and northwest. In spring and summer the direction changes 90 degrees and the waves set from the open ocean to the south.

Tidal currents are particularly important in determining the current circulation around Sable Island (Cameron 1965) and modifying bed forms on the shallow Bank surface. Cotidal lines, trending in an ENE-WSW direction across the shelf (Sverdrup et al. 1942), suggest that the predominant tidal flow may be NW-SE. Currents do, in fact, tend to set westward (Canadian Hydrographic Service 1960). The mean range of tides on the bank is 3.7 feet. The ebb stream sets southward at 1.5 to 2 knots (77 to 103 cm/sec) on the submerged terminal bars of Sable Island, and the flood stream runs 0.5 knots (26 cm/sec) less northward (Canadian Hydrographic Service 1960). Terminal bars of Sable Island directly affect the flow pattern of tidal currents in the study area; the morphology of the terminal bars is, in turn, directly molded by these currents (James and Stanley 1967).

FIGURE 1.—Chart of Sable Island Bank and adjacent area on the outer Nova Scotian continental shelf showing major features discussed in text. Sable Island is a narrow, lunate feature (shown in black) on the Bank crest. Dots represent grab sample stations; circled dots represent stations with camera coverage (Table 1). Unnumbered dots in The Gully Canyon are piston cores collected by Marlowe (1967).

TABLE 1.—Sample stations on Sable Island Bank, Nova Scotian Shelf

SEDIMENT DISPERSAL.—Bottom currents mold the surface of Sable Island Bank in response to water-mass movement of the type described in the previous section. Large sedimentary features, including sand waves and subaqueous dunes in water less than 40 fms (73 m) deep, show that net current movement north and south of Sable Island is in opposite directions: toward the east in the region north of the island, and toward the west, south of the island (James and Stanley 1968). Intensity of current activity increases, as would be expected, in the shallower water close to the island. This circulatory pattern is confirmed by an independent examination of the texture and mineralogy of the sand on the Bank.

Most of the sediment on the Bank surface is now (or in the recent past) being moved by bottom currents. Winnowing and bypassing of finer sediment would explain the absence of fine silt and clay on the Bank surface (Stanley and Cok 1968). Although much of the Bank is undergoing active erosion, several zones of probable sediment accumulation are on the Bank margins (shelf-break) south, east, and northeast of Sable Island (James and Stanley 1968, their Figure 10).

Methods

A total of 114 bottom grab sample stations were taken on the Bank and surrounding area during the period from 5 to 9 May 1965 (Table 1 and Figure 1). The sample grid was planned so that the density of stations increases toward Sable Island. A Van Veen type grab sampler, which covers approximately 0.15 m² of surface area and when full recovers about 20 kg of sediment, was used on 110 of these stations. Small Phleger cores were collected at 4 of the deeper stations. Camera lowerings were made at 32 of these stations with an EG&G bottom camera rig, below which was suspended at 7.6-cm-diameter compass for orientation of physical and biogenic features. The camera was automatically triggered (not bottom-activated) and photographs were taken every 12 to 15 seconds while the camera rig was about 1 to 4 meters above the sea floor. The camera rig remained near the bottom for approximately 5 to 10 minutes as the ship drifted on station. The direction of drift on station is indicated by a 25-cm-long vane attached to the compass.

A total of 307 (1 to 30 per station) usable black and white 35-mm photographs were obtained at 28 of the 32 camera stations. A small clock showing the exact time when photographs were taken and a number recording the frame sequence, noted in the data chamber, were also photographed. Depth of stations ranged from 10 to 250 meters. Position, water depth, and textural composition of bottom sediment of each sample station are listed in Table 1. Complete cruise data and results of mineralogical analyses of the bottom samples collected at the 114 stations are detailed in James (1966). Negatives of all photographs are maintained in the bottom-photograph collection in the Division of Sedimentology, National Museum of Natural History. Data obtained at each of the 28 camera stations are listed in an extensive table available from the National Oceanographic Data Center, Washington, D.C. (NODC Accession number 70–1010). From each usable photograph the area photographed was calculated and the following information recorded: quality of photographs, presence of compass in photo frame, presence of ripple marks, sand waves and other current indicators, mottling, bioturbate structures (tracks and burrows), and fauna. Where one particular species was abundant (e.g., E. parma or anemones), the total number of individuals was counted. In the case of E. parma, apparently stationary, partially buried and moving individuals were counted separately. The occasional white, often overturned, sand dollar was interpreted to be dead. Finally, the number of individuals per square meter was calculated.

Sediment Texture and Evidence of Current Activity

Bottom photographs supplement information obtained from the mineralogical and textural analyses (Figure 5). Distinctly different types of sea-floor microtopography and texture were noted (Plate 1): clean sand, generally rippled; shelly sand with granular texture; silt and silty sand bottom; and sea floor completely modified by burrowing organisms (bioturbation). A light and dark mottled bottom was observed at a number of stations. The dark material is almost certainly organic; the patchy distribution is produced by burrowing organisms and bottom-current activity that disrupt this bottom veneer.

FIGURE 2.—Bottom current information from camera stations on Sable Island Bank. Current direction ascertained from bed forms (sand ripples and waves) and orientation of organisms. Note circulatory clockwise pattern of bottom currents immediately adjacent to Sable Island. The presence of mottling is also depicted. Current evidence and mottling are generally mutually exclusive.

Bottom photographs also provide evidence of short-term bottom-current activity (Plate 2). Most useful in detailing current direction are sand ripples noted at 14 camera stations (Figure 2). In one instance, the predominant orientation of anemones, presumably bending in the current direction (Station 13), was also noted. The NW-SE trending crests at Station 82 are those of larger sand waves. The ripple-crest orientation was recorded at all 14 stations, and the sense of bottom-current movement determined at 8 of these (Figure 2).

In shallow water surrounding Sable Island, the orientation of ripple marks confirms the circulatory pattern described earlier. Northeast of the island, at Station 59, a NW trend off the Bank is suggested. Current directions recorded in the head of The Gully Canyon (Stations 9 and 13) indicate a movement seaward, generally parallel to the canyon axis. A predominant downslope current movement is also noted at the shelf-break near the head of Sable Island Canyon (Station 18).

It is noteworthy that areas presenting evidence of strong current activity and those illustrating mottled sediment are generally mutually exclusive (Station 55 is an exception). At Station 9, what appears to be eroded algal matter has been concentrated in ripple troughs by bottom-current activity.

Echinarachnius parma on Sable Island Bank

POPULATION DENSITY.—The northern sand dollar Echinarachnius parma (Lamarck) was observed in 13 of 28 camera stations. Where present, the density ranged to as many as 185 individuals per square meter (at Station 21). The number of photographs showing this species varied from 5 to 22 at any one station. The photo-to-photo variation in density at each station is depicted on the histograms shown in Figure 3.

Immediately apparent is the irregularity of concentration (Plate 3) caused by clustering of individuals. From comparison of the number of animals counted within the area covered by the photograph (shown in black on histograms in Figure 3), it appears that if the area covered exceeds one square meter, the number counted is relatively close to the mean for that particular station (see Stations 8 and 21). Only a few photographs suffice then to give a reasonably close approximation to the overall distribution at any one position on the bank.

It is noteworthy that correlation between individuals photographed at the surface and individuals retrieved in a small grab at the same station is poor (Figure 3). Those areas that contain more than 50 sand dollars per m² yielded two or more individuals in the accompanying small Van Veen grab sample. This low retrieval reflects the small area covered by the grab but also suggests that not many sand dollars were buried at the time of the survey.

REGIONAL DISTRIBUTION.—A chart showing the distribution and density of E. parma on Sable Island Bank is given in Figure 4. This chart is compiled from both bottom photographs and grab samples (the presence of at least 2 individuals or more retrieved in a grab sample at a station where no photos are available indicates the probable abundance of at least 50 individuals/m²). The northern sand dollar is present over one third of the total Bank surface. Highest densities are found in an arcuate trending belt north of Sable Island as earlier postulated in a sedimentological study of the Bank (James and Stanley 1968). Concentrations of up to 180 individuals per m² are found just north of the two terminal submerged bars extending from Sable Island. High densities (>20/m²) are rarely found in water deeper than 30 fms (55 m).

While a large area of relatively few sand dollars is found on the Bank southeast of Sable Island, the crests of the island’s two submerged terminal bars, nearshore stations, and the area southeast of the island appear devoid of sand dollars as well. Sand dollars do occur, however, in two restricted areas southeast of the island: (a) a linear NE-SW trend in shallow water just south of Sable Island’s eastern submerged bar, and (b) at depths of 100 meters on the eastern margin of the Bank adjacent to The Gully Canyon.

The deepest stations where individuals are encountered are near the heads of Sable Island Canyon (Station 78, 175 m) and The Gully Canyon (Station 9, 256 m).

A number of trends were observed in the detailed photographic analysis. Comparison of sand dollars with the known compass diameter indicates that the

FIGURE 3.—Histograms showing photo-to-photo variation in concentration of the northern sand dollar Echinarachnius parma (Lamarck) (density in m²) as observed at camera and grab stations on Sable Island Bank. The area covered in each photograph and time of sampling is also shown.

FIGURE 4.—Regional distribution and concentration in numbers per m³ of Echinarachnius parma on Sable Island Bank.

FICURE 5.—Distribution of mean grain size (in φ units) on Sable Island Bank (after James and Stanley 1968). Note close relation between the presence of E. parma (see Figure 4) and that of fine to medium sand grade.

predominant size of E. parma observed in photos is between 5 and 6 cm. This is in close agreement with dimensions of this form found off New England (Lohavanijaya and Swan 1965). It is difficult to estimate the amount of movement of individuals in some photographs due to the absence of reworked sand markings. In many cases, however, partially buried forms and those having left a trail are noted (Plate 4).

There appears to be no relation between numbers of individuals and time of day or in the tidal cycle (Figure 3). Only in a few cases were individuals found in concentrations such as those observed by Zenkevitch (1963) in the northwest Pacific. With the possible exception of Station 56 (Plate 4) individuals appear to be moving in a random pattern, confirming the observations of Weihe and Gray (1968) and Boehmer (1970) on southern United States species of sand dollars. In areas of high density this movement is responsible for modifying physically produced bottom structures, and the effect of this bioturbation can be seen on Plate 4. The movement, by rotation and progression, as described by Parker (1927) in response to feeding and burying, modifies both rippled and nonrippled surfaces. The broad, flat, generally arcuate tracks made by the sand dollars are characteristic and may be recognized in the fossil record.

At Station 9 (256 m) located in the head of The Gully Canyon, sand dollars were observed on a rippled sand bottom in which organic matter, probably algal detritus, was noted in ripple troughs (Plate 2f).

Other Epifauna

In addition to E. parma the presence of other organisms living on the sea floor as noted in bottom camera stations and grab recoveries include starfish, brittle stars, anemones, gastropods, and fish (hake and cod). Their distribution is shown in Figure 6 and examples are illustrated in Plate 5. Starfish [Henricia sanguinolenta (O. F. Müller) and others] are found at 8 camera stations from depths of 20 to over 200 m and appear more numerous north of Sable Island. Brittle stars (Ophiomusium

FIGURE 6.—Regional distribution of epifauna on Sable Island Bank. Note area barren of organisms (except E. parma) north and south of Sable Island.

FIGURE 7.—Regional distribution of infauna on Sable Island Bank. The east-west area barren of infauna is considerably narrower than the area devoid of epifauna shown in Figure 6.

FIGURE 8.—Regional distribution of sorting of sand on Sable Island Bank. The zones generally devoid of fauna north and south of Sable Island are very well sorted sands coinciding with areas of high current activity (Figure 2).

lymani Wyville Thomson at 2 camera stations and in 3 grab samples), on the other hand, were not recorded above 90 m. These ophiuroids appear not to be restricted geographically but are rather limited to deeper water (for example, Gully Trough, The Gully Canyon, and outer shelf near the shelfbreak).

Numerous attached anemones were observed at 6 deep water camera stations (>90 m); north of Sable Island, at Station 33 (50 m), several anemones were attached to shells. At Station 51 in the Gully Trough (depth of 146 m) as many as 98 (26/m²) were counted in a single photograph.

Gastropods, including the species Aporrhais occidentalis, were recognized at 8 camera stations and recovered in 7 grab samples. These mollusks are more common north of Sable Island. Gastropods occur in water deeper than 55 m on the southern part of the Bank, while north of Sable Island they are found in depths over 35 m and in even shallower water in the lee of the submerged terminal bars of the island.

The organisms cited above generally occur together. The regional distribution of this epifaunal assemblage occurs in east-west trending zones as depicted in Figure 4. A belt 34 to 40 km wide that parallels the shallow crestal axis of the Bank is generally devoid of conspicuous epifauna.

Infauna

The regional distribution of organisms living within the surficial sediments cover of Sable Island Bank, evaluated from both photographs and grab recoveries, is shown in Figure 7. Included in this assemblage are tubes and polychaetes from grab recoveries and burrows and tubes observed in photos. Actual examples are shown in Plate 6. The regional distribution pattern of this assemblage is similar to that of the epifauna (Figure 6). The zone devoid of infauna, however, is narrower (12 to 22 km) than that devoid of epifauna and is more symmetrically oriented along the crest of the Bank.

Infauna is generally associated with a mottled sediment surface. Small mounds of light-colored sand surrounding burrow openings (Plate 6) are separated by darker areas, suggesting that this patchy coloring results, in some cases, from bioturbation (i.e., disruption of an originally continuous dark veneer).

Recognizable pelecypod shells, including Cyrtodaria siliqua, Pecten magellanicus, and Arctica islandica, are observed in bottom photographs. Fragments of shell, or shell hash, are ubiquitous. Bottom photographs are generally unreliable in estimating the density of pelecypods because of the large number of buried individuals. Bottom grab samples reveal a concentration of shell fragments in southeast, northeast, and northwest belts trending away from Sable Island toward deeper water on the Bank (in James and Stanley 1968, their Figure 6B). Shells are also abundant near the south shore of the island, at ends of both bars, and in shallow water surrounding submerged offshore terminal bars.

Discussion

INTERPRETATION OFE. parmaDISTRIBUTION.—There does not appear to be any recognizable correlation between density and regional distribution of E. parma and time of observation or depth of water on the Bank. The tolerance of this species to significantly large seasonal variations of temperature and salinities cited earlier is apparent. The salinity and temperature of bottom water north and south of Sable Island are not significantly different and cannot explain the remarkable faunal difference in these two regions (i.e., extremely large number of sand dollars north of the island as opposed to low densities south of the island). Predators that may affect regional distribution are unknown. The distribution pattern of the northern sand dollar (Figure 4) can, on the other hand, be closely related to that of the mean grain size of sand (Figure 5) which in turn reflects local conditions of bottom current agitation and topography.

The most abundant faunas appear in the fine (2.0 to 3.0 φ) and medium (1.0 to 2.0 φ) sand grades. An exception is Station 8 on the northeastern margin of the Bank where high densities occur on a coarse sand base. Sand dollars are absent in a large area southeast of Sable Island, an area covered by very fine (3.0 φ) sand. Sand dollars also tend to be absent from regions of very well sorted sand (Figure 8). Areas of well sorted sand on the Bank crest and shallow region adjacent to Sable Island coincide with zones of most intense current activity as shown in Figure 2. Although sand dollars do occur in rippled areas (such as Stations 54, 55, 111) reflecting moderate to intense water movement above the bottom, they are more commonly encountered on mottled covered bottoms indicative of somewhat less intense current activity.

The preference for clean sand by E. parma (Parker 1927) and other species of sand dollars (Kier and Grant 1965; Bell and Frey 1969) is well known. The preference for a distinct sand size related to its burrowing behavior, by Melitta quinquiesperforata (Salsman and Tolbert 1965; Bell and Frey 1969) is also clearly exhibited by E. parma. This selection of sand that is frequently mottled and not very well sorted is undoubtedly related to the detrital feeding habit of E. parma, a detritus feeder. The apparent relationship of population density with intense current agitation should not be overlooked. As has been suggested by others (Sokolova and Kuznetsov 1960; Zenkevitch 1963; Reese 1966), E. parma may obtain some of its food from suspension. Strong current activity like that near Sable Island would not be conducive to either substrate stability or successful suspension feeding.

Furthermore, there is some relation between density and topography. Like the species Mellita quinquiesperforata (Weihe and Gray 1968), E. parma aggregates in the lee of sand bars, in this case the large terminal bars of Sable Island. Similarly, the “clumping” response of this species is similar to that reported for M. quinquiesperforata (Weihe and Gray 1968).

Population densities calculated in the study area are somewhat less variable than those of M. quinquiesperforata whose concentrations range from 44 to 821 per m² (Salsman and Tolbert 1965). Concentrations of the northern sand dollar on the Bank are also less dense than those depicted by Sokolova and Kuznetsov (1960) and Zenkevitch (1963), whose photographs show extremely high densities of E. parma, with individuals actually overlapping.

The observation by Emery et al. (1965, their Figure 9) that more sand dollars were recovered in grab samples than were counted in bottom photographs on the continental shelf southwest of the study area (suggesting that most forms are buried and not detectable) is not confirmed by the present study. Our investigation shows that by far more individuals occur on the surface (as revealed in bottom photographs) than in grab samples, indicating that most individual sand dollars were living on or just at the surface at the time of the survey (May 1965), and that photographs in this instance are a more valid means of estimating population (Figure 3).

The distribution of medium and fine moderately sorted sand bottom selected by E. parma is closely related to the present near-bottom current regime on Sable Island Bank. This texture probably traps an optimum amount of particulate matter and grains covered by organic films that tend to sustain high populations. Coarser grain sizes, representing lag deposits in stronger eroded regimens, are probably depleted of sufficient organic matter by abrasion. Finer grain sizes, on the other hand, trap perhaps too much organic matter thus producing conditions unsuitable to E. parma. The high number of sand dollars on the surface may, in fact, be due to higher concentrations of organic matter in the sand just below the sediment-water interface. It has been suggested that the dislike by E. parma of “foul” (lacking oxygen) sand due to decomposition of organic matter causes it to remain on the surface (Parker 1927). At Station 21, an area containing as many as 180 individuals per m² on the surface, a dark malodorous layer of organic matter was recovered just below the thin light-colored sand veneer.

The importance of sand dollars as modifiers of the surficial sediment of Sable Island Bank is clearly evident. Mellita quinquiesperforata has been known to level completely a rippled field 6 to 10 cm in height in a single night (Salsman and Tolbert 1965). As the activity of E. parma is the same as this southern species, bioturbation must be regarded as almost as important as current activity in molding the microtopography and reworking the sediment of at least one third of the Bank surface.

DISTRIBUTION OF ASSOCIATED FAUNA.—Other organisms living on the sea floor show a dominant distribution pattern that, like that of E. parma, displays a predominant east-west trend. When abundant, the epifaunal and infaunal associations resemble faunas on other shallow outer banks of the continental shelves of northeastern North America (Wigley 1961; Emery et al. 1965). The area barren of epifauna (except E. parma) appears independent of mean grain size (Figure 5) but coincides with the well to very well sorted sand (Figure 8) and areas of abundant sand ripples and waves. This would suggest that currents are too strong in this area for typical northern shelf bottom dwellers. The general absence of Foraminifera in this barren east-west area adjacent to Sable Island (James and Stanley 1968) tends to confirm the above conclusion.

Organisms living within the sediment also require a reasonably stable bottom and consequently are not found on the crest of the bank or close to Sable Island, areas of strongest current activity and moving sediment. The close association of conspicuous infauna and mottling (the latter suggesting a more tranquil current regime) is significant in this respect.

The northern limit of the infauna zone south of Sable Island is located closer to the island than that of the epifauna zone. The narrower barren zone of infauna would suggest that buried or partially buried organisms are, as a group, somewhat more tolerant to current activity than those living on the Bank surface.

Summary of Observations

This investigation of faunal associations on Sable Island Bank on the outer margin of the Nova Scotian Shelf shows the following:

1. The distribution of Echinarachnius parma is more closely associated with mean grain size and sorting of the surficial sand cover (in response to current activity) and with local topography than with any other measured factors.

2. Densities of E. parma reach to a maximum of 180 individuals per m² and are highest in moderately sorted fine- to medium-grained sand.

3. After current activity, sorting and bioturbation as a result of feeding and movement by E. parma are the most important factors modifying the surficial sediment cover of the Bank (more than one third of the total Bank surface area including the entire sector north of Sable Island).

4. Absence of E. parma from regions of strongest current activity and also from areas of well sorted and very fine sand grades is probably related to feeding and burial habits.

5. Underwater photography applied to northern sand dollar population studies in environments such as broad, shallow outer banks proves to be considerably more reliable than bottom sampling surveys.

6. The shallow, east-west trending axis of Sable Island Bank, subjected to intense current actiyity, is devoid of the typical neritic northwest Atlantic epifauna and infauna. Population of typical shelf benthic organisms (other than E. parma) appear in the deeper, somewhat less agitated waters beyond this shallow axis and, consequently, their regional distribution shows a good correlation with sorting independent of mean grain size.

7. The regional population distribution shows that conspicuous infaunal assemblages on Sable Island Bank are somewhat more tolerant of strong near-bottom current agitation and related sediment movement than are epifauna.

Acknowledgments

Funds to conduct a survey on Sable Island Bank were provided by the Institute of Oceanography, Dalhousie University, Halifax, Nova Scotia. We would like to thank the Captain, officers, and men of the CNAV Sackville, Dr. F. Medioli, and Dalhousie University graduate students in the Oceanography Program for their help in collecting bottom samples and photographing the study area. Our appreciation is also expressed to Drs. D. L. Pawson and P. M. Kier, Smithsonian Institution, for critically reading the manuscript. Financial support to complete this investigation was provided by Smithsonian Institution Research Foundation Grant 436330.

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PLATE 1.—Variation of sediment texture and microtopography on Sable Island Bank: a. Station 59, on northwest margin of Bank, depth 25 m. Coarse, sinuously rippled sand with granules in trough. Some granular material in ripple troughs may be organic in origin. Wave length of ripple approximately 6 to 12 cm. b. Station 9, in head of The Gully Canyon, 256 m. Dark patchy nature of sand bottom referred to as mottling in text. Unlike the sea floor at this station, mottling and current activity are generally mutually exclusive (see Figure 2). c. Station 8, on northeast margin of Bank, 52 m. Echinarachnius parma (Lamarck) on a shelly granular sand bottom. Individual and articulated valves and current concentrated shell hash are noted. d. Station 13, on eastern part of Gully Trough, 223 m. Silty sandy sea floor completely modified (bioturbation) by benthic organisms. Note worm tube in upper-right corner of photo, e. Station 78, on southern margin of Bank near head of Sable Island Canyon, 175 m. Silty sandy (note cloud produced by compass striking bottom) sediment covered by old rounded ripples, indicating past current activity. Ripple crests are oriented NNE-SSW. Compass is 7.6 cm in diameter; compass vane is 25 cm in length. f. Station 51, in Gully Trough between Middle and Sable Island Banks, 146 m. Boulders (arrow) on a silty shelly sand bottom. Note profusion of anemones and other benthic organisms at this locality. Boulders are glacially transported (Stanley and Cok 1968).

PLATE 2.—Evidence of current activity on Sable Island Bank: a. Station 8, on northeast margin of Bank, 52 m. Sand waves (scale provided by Echinarachnius parma) with shell hash granules and organic detritus concentrated in linear troughs. b. Station 82, on Bank just south of the western tip of Sable Island, 18 m. Two sets of current ripples on an essentially sand bottom: predominant sand wave crest orientation of NW–SE; secondary ripple crests oriented NE–SW. c. Station 59, on northwest margin of Bank, 25 m. Straight asymmetric sand ripples showing predominant current direction toward the southeast. Compass and E. parma provide scale, d. Station 9, in head of The Gully Canyon, 256 m. Sinuous and interference sand ripples showing somewhat variable current directions. e. Station 78, on southern margin of Bank near head of Sable Island Canyon, 175 m. Somewhat vague “relict” ripples veneered with silt indicative of current activity sometime in recent past. f. Station 9, in head of The Gully Canyon, 256 m. Dark material, possibly of algal origin, concentrated in ripple troughs.

PLATE 3.—Distribution pattern of the northern sand dollar Echinarachnius parma (Lamarck) on Sable Island Bank: a. Station 21, on Bank north of eastern margin of Sable Island, 50 m. High density of northern sand dollar covering nearly entire sandy sea floor surface. Note starfish below compass. See also Station 21 in Figure 3. b. Station 21, same as Figure 1. Much lower concentration of E. parma. c. Station 8, on northeast margin of the Bank, 52 m. Tendency of sand dollars to cluster on sand wave crests is shown. Shelly and organic matter and granules are concentrated in troughs. d. Station 8, same as in Figure 3. Clustering of E. parma in linear belts trending parallel with sand wave crests. e. Station 9, in the head of The Gully Canyon, 256 m. Relatively low sand dollar population on an asymmetrically rippled sand bottom. Sand is moving parallel with the major down-slope trend of canyon. f. Station 33, on margin of Bank north of Sable Island, 50 m. Population of E. parma on a mottled and burrowed sand bottom, suggesting tranquil bottom conditions. Note articulated pelecypod valves concave down and gastropod shell in upper center.

PLATE 4.—Bottom markings produced by Echinarachnius parma on the sandy surface of Sable Island Bank: a. Station 21, on margin of Bank north of the eastern sector of Sable Island, 50 m. Evidence of bioturbation produced by the northern sand dollars on a mottled sand surface. Evidence of markings produced by rotation and by progression and combinations of both are noted. Partially buried forms are visible. b. Station 56, on the Bank adjacent to the submerged western terminal bar of Sable Island, 22 m. Sand dollars completely modifying sand surface of Bank. A possible preferential orientation of individuals moving toward the lower part of photograph is suggested. c. Station 88, on Bank south of Sable Island, 54 m. Close view of shallow parallel track made by moving E. parma on sand surface. d. Station 48, near margin of Bank north of western sector of Sable Island, 44 m. Close view of partially buried individuals indicating movement by rotation. Note starfish Henricia sanguinolenta (O. F. Müller) in right of photograph, and also in a. e. Station 54, on Bank northwest of Sable Island, 37 m. Bioturbation by E. parma on rippled sand bottom. Note curved parallel track of individual in lower center part of photograph. f. Station 22, on Bank north of east terminal bar of Sable Island, 52 m. Sand dollars on silty bottom covered by dark organic film. Evidence of burrowing also present.

PLATE 5.—Association of organisms living on the surface of Sable Island Bank: a. Station 8, on northeast margin of the Bank, 52 m. Benthic fish resting on sand wave trough. Note clustering pattern of E. parma. b. Station 48, near margin of Bank north of western part of Sable Island, 44 m. Large starfish in upper part of photograph. Note abundant individual valves of pelecypods concave up and partially buried. c. Station 86, on margin of Bank south of Sable Island, 92 m. Abundant brittle star (probably Ophiomusium lymani Wyville Thomson) on mottled sand surface. Sand dollars give scale. d. Station 16, in Gully Trough north of Sable Island, 128 m. Gastropod (see arrow) on a bioturbate silty sand bottom. Granular texture is probably organic in origin. e. Station 51, in Gully Trough, 146 m. Large population of small anemones on gravely silty sand bottom. f. Station 78, on southern margin of Bank near head of Sable Island Canyon, 175 m. Large anemones on gently rippled sand. g. Station 13, in Gully Trough near the head of The Gully Canyon, 223 m. Close-up of small anemone on sandy silt bottom.

PLATE 6.—Association of organisms living within and on the sediment on Sable Island Bank: a. Station 13, in Gully Trough near the head of The Gully Canyon, 223 m. High concentration of worm tubes on a sandy silt bottom. b. Station 13, same as Figure 1. Large number of worm tubes and anemones on a burrowed sandy silt bottom. c. Station 13, same as Figures 1 and 2. Trails of benthic organisms, probably worms, on burrowed sandy silt bottom. d. Station 33, on margin of Bank north of Sable Island, 50 m. Small raised burrows on a mottled shelly sand bottom covered with E. parma. Reworking by wormlike organisms probably resulted in patchy dark and light-colored bottom. Sand dollars provide scale. The associated features suggest relatively tranquil condition on the sea floor. e. Station 8, on northeast margin of the Bank, 52 m. Partially buried shell, for the most part individual valves concave down, concentrated in troughs of sand waves. f. Station 77, on southern margin of Bank, 105 m. Large individual valves concave up and partially buried. Mottled surface produced by burrowing organisms.