Dense aggregations of a hexactinellid sponge, Pheronema carpenteri, in the Porcupine Seabight (northeast Atlantic Ocean), and possible causes

cn79- 661l/90$0.00+ .50 01990 Pergamon Rss plc

Prog. oceanog.Vol. 24,pp. 179-l%, 1990. Printedin GreatBritain,All lightsreserved

Dense aggregations of a hexactinellid sponge, Pheronema carpenter-i, in the Porcupine Seabight (northeast Atlantic Ocean), and possible causes A.L. RICE, M.H.

THURSTON

and A.L.

NEW

InstituteofOceanographicSciences Deacon Laboratory, Wormley, Surrey GU8 SUB, UK Abstract-Dense aggregations of the hexactinellid sponge, Pheronema carpenteri, were encountered in the Porcupine Seabight at depths between about 1000 and 13Khn. In restricted areas within this bathymetric range the sponges attain numerical abundances of more than 1.5m-’ and an estimated biomass of up to 372g mAzwet weight or about log m-* ash-i&e dry weight. These recently acquired samples, together with historical data, suggest that Pheronema occurs close to, but not within, regions of the upper continental slope where the bottom topography is expected to result in significant enhancement of the near-bottom tidal current velocities. It is suggested that the sponges may not be able to withstand the enhanced currents, but may nevertheless be dependent upon the resuspended or undeposited organic matter carried to them from these regions of increased tidal energy.

1. INTRODUCIION

The amphidiscophoran hexactinellid sponge genus Pheronema was originally erected by LEIDY(1868) for a specimen in the Museum of the Academy of Sciences in Philadelphia, collected near Santa Cruz in the West Indies. The following year, THOMSON(1869) described a similar species from material collected at a depth of 530 fathoms (970m) to the southwest of what later became known as the Wyville Thomson Ridge, during the cruise of HMS Lightning in 1868. By the time THOMSON’S account was in press, he had obtained additional material collected during the cruises of HMS Porcupine in 1869, both from close to the original locality and from southwest of Ireland on the southern flank of the Goban Spur at a depth of 725 fathoms (1326m). In ignorance of LEIDY’Spaper, THOMSONnamed his new species Holteniu carpenteri, the generic name “to compliment His Excellency M. Holten, the accomplished Governor of the Faroe Islands”, and the specific epithet in honour of THOMSON’S friend and colleague W.B. Carpenter. The sponge ultimately gave its name to the “Holtenia ground”, a region on the edge of the Hebridean Terrace from which the Lightning and Porcupine cruises obtained particularly rich benthic samples and which figures prominently in THOMSON’S general account of the cruises, The Depths of the Sea, published in 1873. In the meantime, KENT(1870a and b) had described a third species, very similar to carpenter?, obtained from a depth of 400-600 fathoms (730-l 1OOm)off Setubal, Portugal, during the cruise of theNorm earlier that same year (see RICE,1986). KENTrecognised that both his species, grayi, and THOMSON’scarpenteri, were congeneric with LEIDY’Sand placed them all in the genus Pheronema, where they remain.

179

18o

A.L. Rac~et al.

Pheronema carpenteri has been recorded subsequently outside the northeast Atlantic from off the coast of Brazil (Sca-oLa'ZE, 1887) and from off Zanzibar (Scmn~TZE, 1904), though the difficulties and uncertainties of hexactinellid taxonomy make the validity of these records uncertain. It is equally uncertain whether or not carpenteri and grayi are distinct species; several authors have treated them as synonyms, usually wrongly attributing seniority to the name grayi (see, for instance, Sa-EPrmNs, 1909; LE DANOIS,1948). In the northeast Atlantic, Pheronema species under one or other of these names have been recorded from the northern Rockall Trough at about 59°37'N in the north, through the Porcupine Seabight, Bay of Biscay, Portuguese coast and Moroccan coast to the Azores in the south, at depths ranging from 650m to 1557m (KENT, 1870a,b; TOPSENT, 1892, 1896, 1904; Sa'EPrmNs, 1915; LE D~a,~ois, 1948; LUTZEandTi-m~i~,in press). Although several of these records refer to the sponges as being locally abundant, there are no quantitative data available. This paper provides such data for dense aggregations ofPheronema carpenteri and offers a possible explanation for their strictly defined bathymetric distribution. The IOSDL benthic group has been conducting an intensive study of the benthic megafauna of the Porcupine Seabight, to the southwest of Ireland, since 1978. As part of this study, a series of 34 samples, taken with the lOS epibenthic sledge (Rac~.,ALDRED,DhRLn~GrONand WIND,1982) on a roughly north-south transect through the Seabight at depths ranging from 525 to 4080m, were used in a study of the relationship between megafaunal biomass and depth in this region (LA~n,rrr, BmhEa-rand RACE,1986). At each of two localities along this transect, one at about 1300m and the other at 4000m, several sledge samples were taken from restricted areas to investigate the small scale variability of the megabenthos. Among the ten samples from the shallower of these repeat stations, at mean depths ranging from 1283m to 1327m, a twenty-fold range in the total megafaunal ash-free dry-weight was encountered, largely dependent upon the presence or absence from these samples ofPheronema carpenteri. Available phototransect data suggested that the shallow repeat station had been located close to the lower bathymetric limit of dense aggregations of the sponge, and that somewhat shallower than the repeat samples even higher Pheronema abundances were to be found than the 475 individuals 1000m-2recorded in one of the sledge samples. This paper reports the results of our efforts to investigate this curious distribution further. 2. MATERIALSAND METHODS The benthic megafauna, including Pheronema, has been sampled during the IOS study with the lOS epibenthic sledge (Rtcr~ et al, 1982) and a semi-balloon otter trawl (MERe,~a'a"and MARSHALL, 1981). In addition, during joint cruises between IOS and the Scottish Marine Biological Association, further samples have been obtained with a single-warp trawl, Granton trawl and Agassiz trawl (GoRDoN and DUNCAN, 1985). Between November 1977 and December 1986, some 250 megabenthic samples were obtained with these gears in the Porcupine Seabight and adjacent abyssal plain at depths between about 400 and 4500m (see Fig. 1). Most of these samples were obtained on a transect extending from the Porcupine Bank roughly along 13°W to soundings of about 3000m, and then in a southwesterly direction through the mouth of the Seabight and onto the Porcupine Abyssal Plain. A second group of samples was obtained from the Goban Spur, to the southeast of the Seabight, while scattered additional stations were worked on the Porcupine Bank and in the northeastern part of the Seabight. However, the eastern part of the Seabight in general, and the Gollum Canyon

Aggregations of the sponge Pheronema carpenteri

181

system in particular, are poorly sampled because the irregular topography in this area makes the use o f towed benthic gear difficult and risky 1.

15°W

14 °

!

13 °

52 ° ' PORCUPINE N BANK

12 °

\

••

10 °

11 ° I

• °s~ 52 °

o~

O

%,

•

oo

00

51°1~ ~

.c~ / f - ' / e '

51

o

GOLLUM SYSTEM

50° I

50 °

•\

0

•

•

149 °

49' 15°W

14 °

13 °

12°

11 °

lO ~

FIo. 1. Bathymetric chart of the Porcupine Seabight showing the positions of about 250 stations from which megabenthos samples were obtained between 1977 and 1986.

1The rough topography in this area did not, apparently, dissuade the scientists involved in the work of the Fisheries Branch of the Department of Agriculture and Technical Instruction for Ireland in the early years of this century. Between 1901 and 1911, numerous trawl samples were obtained at depths around 1000m in the eastern part of the Porcupine Seabight from the vessels Helga and Helga 1I (see RIc~, 1986, and station lists in LE DANO~S,1948).

182

A.L. RtcBet al.

The I t S epibenthic sledge has been used routinely in conjunction with I t S automatic underwater cameras taking photographs of about 2m 2 of the seafloor in the path of the net (RacE, ALDRED, BILLETTand TrroRsroN, 1983; Pace et al, 1982; PaCE and COLLINS,1985). With a sampling time on the seafloor of between 30 minutes and one hour, and with an interframe interval of 15 or 30 seconds, the maximum requirement was for about 250 frames. The I t S Mark IV camera (see COLLINS, 1984), with a capacity of 15m of standard 35mm film giving up to 400 flames in a conventional 35mm format, satisfied this need. However, in order to investigate the detailed distribution of megabenthic organisms, including Pheronema, much longer photographic transects were required. Accordingly, a modification of the Mark IV camera was developed (designated Mark IVa) in which a movie format was used, producing a smaller frame size of approximately 24 x 18ram, but with the same field of view. This increased the capacity of the camera to almost 800 frames on standard film and about 1400 frames on thin-based surveillance film. The new camera has been used routinely since 1983, mounted on the I t S epibenthic sledge frame as before, but with no benthic nets attached. Used in this way as a photosledge, runs of up to 11 hours bottom time, and covering up to 15nm, have been achieved. In the Pheronema study, 11 such photosledge runs have been obtained, roughly at right angles to the depth contours between about 750 and 1600m, and in a restricted area of the Seabight (see Figs.2 and 3). The resulting photographs were grouped into 10m depth bands and the number of Pheronema "rooted" in each frame was counted (Fig.4). From these data, mean numerical densities were calculated for each 10m depth horizon on each phototransect. Sponge biomasses were estimated from the photographs in several stages. First, the equatorial diameters of the photographed sponges were estimated by projecting each frame on to a 5xScm grid which had been photographed in a water-filled tank (see Rice et al, 1979). Second, these measurements were checked against mean diameters of simultaneously collected specimens assumed to be representative of the photographed population. Unfortunately, the sponges were usually extensively damaged during collection, so that such a comparison was possible in only one case (station 51403) in which a sample of 28 undamaged sponges were obtained at the same time as a series of good photographs. Finally, the relationship between the mean equatorial diameter and wet weight was based on a series of preserved specimens which were weighed after being allowed to drain onto absorbent paper for about 5 minutes. These steps are subject to considerable inaccuracies. Consequently, the final biomass estimates may be "wrong" by a factor of two, but are likely to be considerably more accurate than the best estimates based on trawl or sledge catches despite recent improvements in monitoring such towed gears (see pace et al, 1982). 2.1 Bathymetric and horizontal distribution Pheronema carpenteri was taken in 22 hauls fished at depths ranging from 980m to 1370m in the IOSDL study (Table 1), but the species was absent from a further 29 hauls taken within the same depth range (Fig.2). Figure 2 also includes the positions of the fifteen stations in the eastern part of the Porcupine Seabight at which Pheronoma was recorded during the scientific investigations of the Irish Fisheries Branch between 1906 and 1911 (from STEPnENS, 1915), together with the stations at similar depths, and during the same period, from which Pheronema was apparently absent. The depths attributed to these Helga and Helga H samples, obtained with a beam trawl, are between 500 and 800 fathoms (about 900 and 1550m). These depths are based on soundings taken before

Aggregations of the sponge Pheronema carpenteri

,\

183

, , °O

::

52 ~

i 200

51 °

50 °

•

0

o 49 ° 14 °

13 °

12 °

11 °

IZio.2. The known distribution ofPheronema carpenteri in the Porcupine Seabight. Closed symbols represent the presence, and open symbols the absence of the sponge. Samples taken during the current lOS programme are marked by circles, while Irish Fishery Investigations stations are marked by squares. The shaded areas represent those regions where the "ray slope" is exceeded by the slope of the seafloor and near the upper boundaries of which enhanced near-bottom current speeds are to be expected. See discussion section for further explanation. The boxed area in the north-western part of the Seabight is enlarged in Fig.3.

184

A.L. Rtc~ et al.

52°00

51°30

13°30

13°00

1~o.3. Localities of photosledge transects and benthic samples between 950 and 1500m in the northwestern part of the Porcupine Seabight. Closed circles represent the presence and open circles the absence of Pheronema. Some of the symbols represent more than one sample (see also Table 1). The thickened parts of the photosledge transect lines similarly mark the regions where Pheronema was present. The asterisks mark the positions of two Bathymap deployments (see discussion).

Fro.4. (right) In situ photographs of Pheronema carpenteri taken during photosledge haul 52018. The upper photograph shows a single sponge at a depth of about 1210m. The lower photograph, at a depth of about 1250m, was recorded as having six sponges rooted within the frame area. Five of these axe obvious, while a further specimen, or possibly two, are less clearly seen in the upper left part of the photograph.

Aggregations of the sponge Pheronema carpenteri

185

A.L. Race et al.

186

TABLE 1. Station data for hauls in which Pheronema carpenteri was collected in the Porcupine Seabight. Station positions up to 51023 are ship positions based on satellite fixes. Thereafter they are best estimates of gear positions based on triangulation. Station

50503 50505 50508 50713 50815 51008 51023 51206 51208#1 51208#3 51306 51403#1 51403#5 51403#6 51403#7 51420#1 51420#2 51420#3 51420#4 51707 52009 52204

Gear*

OTSB GR OTSB GR OTSB OTSB OTSB OTSB BN BN OTSB BN BN BN OTSB BN BN BN BN BN BN BN

Date

01 Jtm 79 01 Jun 79 03 Jun 79 20 Oct 79 05 Aug 80 02 May 81 09 May 81 18 Sep 81 19 Sep 81 20 Sep 81 19 Feb 82 25 Mar 82 26 Mar 82 26 Mar 82 26 Mar 82 02 Apl 82 02 ApI 82 02 Apl 82 02 ApI 82 12 ApI 83 19 Aug 84 16 Jul 85

Depth Range (m) 992-1042 1270-1300 980-985 1245-1275 1280-1344 1290-1335 1270-1275 1200-1210 1170-1200 1170-1185 1205-1230 1292-1314 1289-1297 1278-1295 1255-1300 1326-1328 1304-1309 1293-1298 1279-1287 1205-1230 1206-1236 1295-1310

Position N

W

51037 ' 51044 ' 51034' 51022 ' 51036 ' 51036' 49030' 51040' 51041 ' 51041 ' 51044' 51037' 51037 ' 51037 ' 51037 ' 51037 ' 51037 ' 51038 ' 51038 ' 51040' 51°49 ' 51037 '

13015 ' 12046 ' 13018 ' 13018 ' 13004' 13002' 12°11' 13000' 13001 ' 13001 ' 12053 ' 13000' 12059' 12059' 12059' 12059 ' 12059 ' 12059' 13001 ' 13000' 12059' 13000'

*Gear Key: OTSB - semi-balloon otter trawl; GR - Granton trawl; BN - I t S epibenthic sledge. and after each haul (see KEMP, 1910) and cannot, therefore, be considered accurate. Nevertheless, the Irish Fishery Branch results, along with the I O S D L data, suggest that P h e r o n e m a is widely dislributed in the Porcupine Seabight at depths o f around 1000m to 1300-1400m, being present on the Go b an Spur and the eastern and northern flanks o f the Seabight, but perhaps being absent from the southerly parts o f the Porcupine Bank. This suggestion o f a westerly limit o f the species within the Seabight at about 5 1 ° 2 0 ' N ; 1 3 ° 2 0 ' W is supported by the photosledge results (Fig.3). Apart from 3 specimens photographed in haul 52022 (and one specimen taken in a nearby trawl sample, 50713), P h e r o n e m a was absent from the four southwesterly photosledge hauls which entered the depth range 1000-1300m, but was abundant some 10nm to the north and east (51734).

2.2 N u m e r i c a l abundance LAMPITT et al (1986) reported a m a x i m u m numerical abundance o f P. carpenteri of 0.475m -2 and an ash-free dry weight biomass o f about 2g m -z, based on an epibenthic sledge sample. This sponge biomass exceeded that o f the remainder o f the benthic m e g a f a u n a at the same locality by more than an order o f magnitude. Nevertheless, it was recognized as probably representing a conservative estimate o f the true m a x i m u m abundance o f the species, both because the sample was almost certainly not from the centre o f the sponge patch, and because o f the

Aggregationsof the spongePheronema

187

carpenteri

clogging effect of such large organisms in reducing the fishing efficiency of the net. Consequently, numerical abundances in this study are based on analysis of the four photosledge hauls which impinged on the sponge populations. One of these (52018) crossed both the upper and lower boundaries of the sponge community, while the other three each crossed only one boundary (Fig.3). Hauls 51709 and 51742, however, were so nearly continuous that they can be treated together as a single transect, but with a gap in the 1220-1250m depth zone. The results of these analyses are given in Fig.5. Densities for the 10m depth zones were based on 10-105 photographs (mean 28.55 + 13.91). The mean density encountered for all of the 10m depth zones sampled between the upper and lowerbathymetric limits of thesponge was 0.34m 2. However, this figure is relatively meaningless since the actual densities encountered in the depth zones varied between zero and a maximum of more than 1.5m -2, about three times the maximum density estimated from epibenthic sledge catches.

Number 0 i

0"5 i

1-0

0

0-5

I

I

m -2 1-0

1.5 ]

0 I

0-5

1.0

1.5 ,

0.5

0 I

=

1000

1100

E" m m m

Depth

m m m m

(m) 1200

m m

1300

51734

52018

51724

F/o.5. Abundanceof Pheronema (numbersm-2)in each 10m depth zone based on the photosledg¢ results.

tO9

188

A.L. Raceet al.

In the most complete transect, 52018, the bathymetric limits were respectively at about 1100m and 1310m. Over most of this range the P h e r o n e m a density was generally less than 0.1 m 2, while densities greater than about 0.5m -2were restricted to a narrow depth zone between about 1210 and 1260m. In the nearby transect 51724, the upper bathymetric limit was slightly shallower at about 1070m, while the maximum density was encountered in the deepest zone sampled, 1210-1220m, that is at a similar depth to the maximum density recorded on 52018. However, the P h e r o n e m a seem to be rather more evenly distributed on this transect, with a relatively gradual increase in density with increasing depth from about 1110m and much higher densities than on 52018 shallower than about 1200m. On the deeper extension of this transect (51709) no sponges were encountered at 1250 and 1260m, whereas they were still quite abundant (c. 0.5m -z) at this depth on 52018, only about 4kin to the southwest. The photographs from the upper part of 51709 show large quantities of what we have assumed to be sponge debris, perhaps indicating severe, but local, disturbance of the Pheronema community and thus explaining the low densities of living sponges recorded. The more southwesterly transect, 51734, some 20km from 52018, shows a quite different distribution pattern, Here, the upper bathymetric limit of P h e r o n e m a was some 60m shallower at about 1030m. With increasing depth along this transect, the sponge density increased fairly consistently to a maximum of 0.7-0.8m -2 at depths of 1120-1140m, thereafter decreasing abruptly. Since this transect was terminated at a depth of 1180m, it is not possible to say whether higher densities would have been found at deeper levels. However, the indications are that the whole P h e r o n e m a community, including both the total range and the zone of maximum abundance, was shallower in this region than in the more easterly area. 2.3 B i o m a s s estimates

Figure 6 shows the frequency distribution of the equatorial diameters of 118 sponges estimated from photographs taken during epibenthic sledge haul 51403 taken in March 1983 and of the mean equatorial diameters of 28 sponge specimens collected during the same haul. Although the range of diameters estimated from the photographs (5-21cm) is much greater than that based on specimens (9-15cm), the average diameters obtained from the two techniques (12.3 and 12. lcm) are not significantly different (Mann-Whitney U-test, d=0.80), suggesting that the photographs provide reasonable estimates of mean sponge size. Figure 6 also shows the sponge size frequency distributions based on three phototransects; 52018 ran very close to the position of 51403, while 51724 was obtained some 5nm to the north and 51734 about 10nm to the southwest of this position. The mean equatorial diameter estimated from haul 52018 (13.3cm) differs significantly from that based on 51403 (Mann-Whitney U-test, d=2.31, P<0.05). These two samples were separated in time by 29 months, the latter having been collected in March 1982 and the former in August 1984. Unfortunately no estimate of growth is possible as nothing is known about timing and frequency of recruitment to the population. Mean diameters estimated from hauls 51724 (17.2cm) and 51734 (16.4cm), obtained in May 1983 also differed significantly one from the other (d=1.99, P-0.05) and both differed from the figure obtained from haul 52018 (d=10.65, P<<0.001 and d=7.97, P<<0.001 respectively). The relationship between mean equatorial diameter and wet weight (Fig.7) is given by wet weight = 0.0501 (diameter) 3 + 27.9205

(1"2--0.50)

Aggregations of the sponge Pheronema carpenteri

189

51734 170

20

~n = 1 6 " 4 c m

0 40 24

co 63 "13 > o

-

.

I

-

"0 c"

156

20

:~n = 1 7 - 2 c m

0 20

52018 N=130

0

M e a n = 13-3 cm O) ..Q

0 E 20

51403

z

N =28 Mean=12"l

cm

0 51403

20

18 In = 12-3 cm

0 1'0

1'5

2,0

2'5

Diameter (cm) FIG.6. Size frequency distributions of sponges based on data from one epibenthic sledge haul (51403) and three photosledge hauls. The distributions based on photographs are shown i n solid black, while that based on specimens collected during 51403 is hatched.

and is based on the 28 specimens from haul 51403 together with 12 relatively intact specimens from other hauls providing a total range in equatorial diameters from about 8.5 to 14.25cm. Although the main bodies of the sponges used in this exercise were undamaged, the correlation between wet weight and diameter is not good. This results partly, at least, from the very variable size of the spicular root system which anchors the sponge to the sediment. Consequently, the regression probably tends to underestimate the sponge wet weight for a given equatorial diameter. Nevertheless, in the absence of better data, this relationship has been used in conjunction with the numerical biomass figures and mean diameters to derive maximum wet-weight sponge biomasses for three of the transects (Table 2).

190

A.L. P~cEet al.

200

•

i

E ~3 03 v

10£ o ~

•

•

:'.

I

10'00

.

'

(Diameter

2

i

(3'00

I

3 000

(cm))3

FIO.7. Relationship between wet weight and mean diameter in 40 intact specimens of Pheronema carpenteri (see text for details).

TABLE2. Estimates of maximum wet-weight biomass of Pheronema carpenteri based on phototransects. Transect Mean diameter of Pheronema (cm)

Depth (m)

10m depth horizon Maximum Wet-weight abundance biomass (m "2) (g m "z)

Individual photograph (2mz) Depth Maximum Wet-weight (m) abundance biomass (m -z) (g m -2)

51724

17.2

1210-1220

1.6

453

(1170-1180) (1210-1220)

4.0

1131

51734

16.4

1130-1140

0.8

199

1130-1140

2.5

498

52018

13.3

1230-1240

1.4

204

1220-1230

5.0

729

Aggregations of the sponge Pheronema carpenteri

191

The relationship between Pheronema wet weight, dry weight and ash-free dry weight (see LAMvrrr et al, 1986) was based on an analysis of some 35kg of sponge material representing about 220 individual sponges. As a percentage of the wet-weight, the dry weight of these samples represented an average of 23.6% (range 14.4-27.7%), while the ash-free dry weight represented an average of 2.75% (range 1.96-3.19%). Thus, the maximum sponge biomasses estimated by the photographic technique represent ash-free dry weights of up to about 12g m 2 through a 10m horizon and a spot value of about 31 gm 2 in the area covered by a single photograph. 3. DISCUSSION The highest benthic megafaunal biomass recorded in the Porcupine Seabight by LAMPITTet al (1986), was at station 51403#5, at a depth of 1293m. Here, the total ash-free dry weight biomass based on sledge hauls was 2.16g m "2,the bulk of which, some 2g m -2, was made up of Pheronema carpenteri. The techniques used to estimate the sponge abundance in the present study are subject to a variety of errors, but the results nevertheless suggest that Pheronema may reach biomass values at least five times as high as these previous estimates, though over a very restricted bathymetric range. LAMPITTet al (1986) derived a regression for the relationship between megabenthic biomass and depth in the Porcupine Seabight based on 34 hauls taken at depths between 525 and 4080m, and compared the results with available data on megabenthic biomasses in other areas. The Porcupine Seabight figures were somewhat higher than those recorded off southern New England by HAEDPaCH,ROWEand POLLOm(1980), but were similar to those obtained in the Bay of Biscay, on the Demerara Abyssal Plain and on the upper slope off Japan by KHRn'OUNOFF,DESBRUYERES and CHARDY(1980), SIaUETet al, (1984) and OHTA(l 983) respectively. On the other hand, SMITI~ and HAMILTON(1983) reported a megafaunal wet weight biomass of68g m -2 at a depth of 1300m in the Santa Catalina Basin, based almost entirely on a single ophiuroid species. This figure is some 18 times higher than the 3.85g m -2wet weight biomass indicated for this same depth by the Porcupine Seabight regression, and some 25 times higher when converted to ash-free dry weight. The data reported here completely reverse this situation, for the estimated Pheronema biomasses are more than ten times as high as the Santa Catalina Basin figures on the basis of wet weights and four times as high as the ash-free dry weights (based on conversion factors in LAMPITTe t al, 1986). Clearly, a simple regression does not describe adequately the distribution of benthic biomass in the Porcupine Seabight. Although the sponges are generally less abundant towards the upper and lower bathymetric limits, these limits, and the depth of maximum abundance, vary considerably so that order of magnitude differences may be encountered at the same depth over distances of no more than a few kilometres (see Fig.5). Why should the sponges occur in such unusually high abundances in this restricted bathymetric zone in the Seabight? The environmental factors must be peculiarly favourable for them, but it is not immediately obvious why this should be. The coccolith-foram marl sediments within the Seabight change character gradually with increasing depth, particularly in becoming finer grained (see LAMPITTet al 1986), but there are no clear discontinuities associated with the upper and lower bathymetric limits of the sponges which would explain their strict vertical distribution. It seems much more likely that the sponge distribution is controlled by the local hydrography resulting in an ample supply of suitable food. In the absence of productivity data, biomass estimates alone are not good indicators of the energetic requirements of an ecosystem or of a component faunal group. Such productivity data are not available for hexactinellid sponges, but intuitively one would expect their metabolic JPO 24:1-4-H

192

A.L. PdC~et al.

requirements to be relatively low, as is typical of sedentary suspension feeders. Consequently, the high sponge biomass at between 1000 and 1300m depth does not necessarily imply an enhanced organic input to the sea-floor in this region, but simply, perhaps, that the organic particles remain available to the sponges, that is in suspension close to the bottom, longer in this depth zone than either above or below it. The most obvious agency by which this might occur is the resuspension of sedimented organic particles by near-bed water currents. The relatively few direct measurements of near-bed currents in the Porcupine Seabight area suggest that such currents have a strong tidal component and rarely exceed a speed of about 15cm s-1 at a height of lm above the sea-floor (LAMPrrr, 1985 and personal communication). Two bathysnap deployments (see LAMPrrT and BURNHAM, 1983) in the vicinity of the P h e r o n e m a populations (see Fig.3) recorded current speeds over periods of 75 hours and 134 days respectively mainly between about 3 and lcm s -~ and mostly in a westerly or southwesterly direction, that is more or less along the depth contours (R.S. LA~prrr, personal communication). Such currents seem to be part of the northward boundary (or slope) current which, apparently, flows along the European continental margin over a wide bathymetric range (DICKSON, GOULD, GRIFFrrHS,MEDLERand GMrrROWlCZ, 1986; HUTHNANCE,1986). LAMPITT(1985) found no evidence of significant resuspension of what might be called the "normal" benthic sediments in the Seabight even at the highest current speeds recorded. However, he found that near-bed currents in excess of about 7cm s-~ resulted in resuspension of the flocculent phytodetrital material deposited seasonally at all depths in the region and first reported by BILLETT,LAMPITT,RICE and IVIANTOURA(1983). Thick carpets of phytodetritus have been encountered at depths below about 1300m, but not at shallower levels. LAMPITT(1985) suggested, but without any direct evidence, that generally higher near-bottom current speeds at these shallower depths kept the phytodetritus in suspension and prevented it from settling. Theoretical considerations, however, suggest that near-bottom across-slope tidal currents may be enhanced at certain depths. This component of the flow would be expected to interact with the bathymetry and generate "internal tides" (internal waves of tidal frequency) travelling as beams of energy along certain characteristic "ray" paths (NEw, 1988) which have a slope given by 1

S = + ~

(52

(1)

w here o is the semidiumal frequency ( 1.405 x 10-4s-1),f is the Coriolis frequency (1.133 x 104s -1 at 51°N) and N is a quantity related to the density profile 9(z) by N 2 = -g/p- dp/dz

(2)

where g is the acceleration due to gravity and z is the vertical coordinate. In general, significant generation of internal tides would be expected only if the slope of the bottom topography (ct, say) somewhere exceeds the ray slope, s, at the same depth (SANDSTROM,1976). In this case, the beams of internal tidal energy are predicted to emanate from the region of the "critical" slope, that is, where the bottom slope equals the ray slope (ct = s), and this is the region in which the bottom currents would be expected to become intensified. To explain this from a simple physical viewpoint, we imagine that water particles near the sea floor are being forced by the surface tide to oscillate tidally up and down the bottom topography, at the same slope as the topography (tx). When this slope is equal to the slope s, given by equation (1) above, with which the water particles would naturally oscillate if all boundaries were removed,

Aggregations of the sponge Pheronema carpenteri

193

a resonance occurs and large amplitudes of oscillation, together with enhanced bottom currents, may result. The situation is analogous to that occurring in a pendulum, which has its own natural frequency of oscillation, and can be excited by a small force (e.g. as might be provided by gentle blowing) applied periodically at exactly this natural "resonant" frequency. A numerical model for internal tidal generation in a continuously stratified ocean has been developed by PR~SENB~-Ra and RATTRAY(1975), and compared with observations in the Bay of Biscay by NEW (1988), who found encouraging agreement. The results indicate that the nearbottom currents in the critical slope region of the Bay of Biscay may be intensified by a factor of 2 or more as compared with the normal surface tide, and we may expect a similar phenomenon to occur in the Porcupine Seabight wherever the topographic slope becomes suitably steep. The areas in the Seabight in which the bottom slope exceeds the ray slope (t~>s) have been estimated by using the hydrographic data of Pm6Rm~ and MORRISON(1973) and measuring the topographic slopes over 200m intervals along transects crossing the topography along lines of steepest descent. These transects were spaced approximately every 10km along the slope and extended to the 2500rn contour. The resulting areas, shown shaded in Fig.2, occur in two main regions, on the eastern and western flanks of the Seabight respectively. The greatest enhancement of the near-bottom currents is to be expected near the upper boundaries of these areas, where tx N s. On the eastern flank of the Seabight this upper boundary largely follows the 500m contour, but is somewhat deeper, nearer to 1000m, at both the northern and southern ends. On the western flank the upper boundary is close to, or below, the 1000m contour. The across-slope surface tidal velocities at these upper boundaries were estimated from a numerical model (A.M. DAvms, personal communication) to be approximately 10cm s -1 on the eastern flank, but only about 5cm s-~ on the western flank. Allowing for some decrease due to turbulence near the sea-bed, we would consequently expect the near-bottom currents to be enhanced at these boundaries to approximately 15-20cm s -1 and 5-10cm s-~ respectively. Some enhancement may also occur near the lower boundaries of the shaded patches in Fig.2, but, because of the greater depth, any tidal currents, enhanced or otherwise, are likely to be correspondingly weaker. In summary, the strongest across-slope near-bottom tidal currents in the Porcupine Seabight (excluding those on the shelf in shallow water) are probably of the order of 15-20cm s -i, and occur around the 500-1000m depth contours on the eastern flank. Such currents are certainly sufficiently powerful to resuspend flocculent phytodetrital material (see LAMPrrT, 1985) and might even resuspend less flocculent sedimented material. They would also delay the deposition of any sinking material entering the region compared with areas where the near-bed currents are less rapid. Therefore these circumstances might be expected to favour the existence of suspension feeders like Pheronema, but the known distribution of the sponge in the Seabight does not support this suggestion. The high densities of Pheronema encountered during the IOSDL sampling programme are all well outside the regions of expected current enhancement, either on the Goban Spur or on the north-western flank of the Seabight where the sea-floor topography is relatively gentle (see Figs.2 and 3). Unfortunately, no samples have been taken within the area of steep topography on the western flank, but the negative records from either end of this region suggest that the sponge does not extend into this part of the Seabight. Most of the Irish Fishery Investigations records of Pheronema from the eastern flank of the Seabight are also apparently outside the enhanced current regions, generally rather deeper. Conversely, most of the Helga and Helga II samples from within the enhanced current regions did not contain Pheronema (see Fig.2). The accuracy of the station positions given for these early

194

A.L. Raceet al.

investigations probably cannot be relied upon to be better than several kilometres. Nevertheless, the results as a whole suggest an inverse relationship between the occurrence of Pheronema and the areas of predicted current speed enhancement. On the other hand, these regions on the eastern flank of the Seabight are associated with other suspension feeders, for the depth zone between about 500m and 1000-1500m is known to be populated by colonies of scleractinian corals, particularly Lopheliapertusa (L.) and Madrepora oculata L. (ZmRowros, 1980; LE DANOIS,1948). LE DANOIS summarized all of the benthic work on the continental slope off north-western Europe up to that time and recognised a number of "massifs coralliens", dominated by these and other coral species, including two on the eastern flank of the Porcupine Seabight. The most northerly of these, his "massif de la baie de Dingle", which he located between 51 °15' and 52°00'N and 11 °30' and 12°20'W is centred on the northern edge of the enhanced current area on the eastern flank of the Seabight marked in Fig.2, while the second, the "massif de Hurd Bank" at 50035 ' - 50°45'N and 11°15 ' - 11°35'W, is located further to the south, near the centre of this main area of enhanced currents. LE DANOIS considered Pheronema to be typical of an "infra-corallien" community which was to be found at the base of these coral patches and which he thought was particularly attracted to "coral mud", sediment containing a high proportion of debris carried down-slope and derived from the corals and their associated fauna. In this way, LE DANOIS maintained that Pheronema and other suspension feeding sponges and echinoderms occur not only beneath the coral patches in the Porcupine Seabight, but also beneath similar patches off La Chapelle Bank, off the Armorican Shelf and smaller patches along the northern coast of Spain. According to BAINES (1982), the La Chapelle region between 47°N and 49°N, probably produces the highest internal tidal energy fluxes anywhere in the world. Within this region and, more generally, within the whole of the northwestern European margin, the corals seem to be associated particularly with those areas in which the bottom topography produces further enhanced near-bottom currents. While Pheronema is not, in general, found within these areas of enhanced current, its distribution seems nevertheless to be associated with them, the sponge being particularly abundant along their lower boundaries and "downstream" of them. Perhaps Pheronema is unable to withstand exposure to the high current speeds directly, but is dependent upon the resulting increased organic particulate load being deposited downslope or carried along the slope in the generally northward drift of the slope current.

4. ACKNOWLEDGEMENTS

Our thanks are due to the officers and crews of NERC research vessels, and to our IOS colleagues for help both at sea and in the laboratory. We are grateful to Miss T.M.H. Araujo and Mrs P.A.B. Jackson for technical assistance. Comments by two anonymousreferees led to improvementsin this paper. Part of this work was carried out with the support of the Procurement Executive, Ministry of Defence.

5. REFERENCES

BAn~S, P.G. (1982) On internal tide generation models. Deep-Sea Research, 29, 307-338. Bmt.grr, D.S.M., R.S. ~ r r r , A.L. RacEand R.F.C. MArcrotw,A (1983) Seasonal sedimentation of phytoplankton to the deep-sea benthos. Nature, London, 302, 520-522.

Aggregations of the sponge Pheronema carpenteri

195

COLLINS,E.P. (1984) Recent developments in deep-sea photography at the Institute of Oceanographic Sciences. In: Underwaterphotography: scientific and engineering applications. P.F. SMITH,editor, New York, Van Nostrand Reinhold, 247-256. DICKSON,R.R., W.J. GOLq.,D,C. G~tevnas, K.J. I-B~L~ and E.M. GMrmovncz (1986) Seasonality in currents of the Rockall Channel. Proceedingsof the Royal Society of Edinburgh, B, 88, 103-125. GORDON,J.D.M. and J.A.R. DtrNcAN(1985) The ecology of the deep-sea benthic and benthopelagic fish on the slopes of the Rockall Trough, northeastern Atlantic. Progress in Oceanography, 15, 37-69. HAEDPaCn,R.L., G.T. ROWEand P.T. POLLOm(1980) The megabenthic fauna in the deep sea south of New England, U.S.A. Marine Biology, 57, 165-179. Htrrr~aNcE, J.M. (1986) The Rockall slope current and shelf-edge processes. Proceedingsof the Royal Society of Edinburgh, B, 88, 83-101. KEMP,S. (1910) The Decapoda Natantia of the coasts of Ireland. ScientificInvestigations of the FisheriesBranch, Departmentfor Agriculturefor Ireland, 1:1-190 + Pls I-XXIII. IO~¢r, W.S. (1870a) Notice of a new vitreous sponge, Pheronema (Holtenia) Grayi. Annals and Magazine of Natural History, Set.4, 6, 182-185. KENT, W.S. (1870b) On the Hexactinellida taken in the Norna expedition off the coast of Spain and Portugal. Monthly Microscopic Journal, 1870, 241-252. Krmu,otmo~, A., D. D E S B R ~ and P. Ct-tAm~X(1980) Les peuplements benthiques de la faille Vema: donn6es quantitatives et bilan d'6nergie en milieu abyssal. OceanologicaActa, 3, 187-198. ~ , R.S. (1985) Evidence for the seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension. Deep-Sea Research, 32, 885-897. ~ r r r , R.S., D.S.M. BmLETrand A.L. RICE(1986) Biomass of the invertebrate megabenthos from 500 to 4100m in the northeast Atlantic Ocean. Marine Biology, 93, 69-81. l..xMprrr, R.S. and M.P. BtrRrmaM(1983) A free-fall time lapse camera and current meter system "Bathysnap", with notes on the foraging behaviour of a bathyal decapod shrimp. Deep-Sea Research, 30, 1009-1017. I~ DANOXS,E. (1948) Les Profondeurs de la Mer, Paris, Payot, 303pp. LZtDY, J. (1868) Meeting of Biological Department of the Academy, October 5th 1868. Proceedings of the Academy of Natural Sciences, Philadelphia, 1867-1868, p.9. LtrrzE, G.F. and H. Ti-n~ (in press) Epibenthic foraminifera from elevated microhabitats: Cibicoides wuellerstorfi

and Planulina ariminensis. Journal of Foraminiferal Research ~ ,

N.R. and N.B. MARSHALL(1981) Observations on the ecology of deep-sea bottom-living fishes collected off northwest Africa (08°-27°N). Progress in Oceanography, 9, 185-244. N~w, A.L. (1987) Internal tidal currents in the Bay of Biscay. In: Advances in UnderwaterTechnology, Ocean Science and Offshore Engineering, 12, London: Graham and Trotman, 279-294. Nzw, A.L. (1988) Internal tidal mixing in the Bay of Biscay. Deep-SeaResearch, 35, 691-709. OrrrA, S. (1983) Photographic census of large-sized benthic organisms in the bathyal zone of Surnga Bay, central Japan. Bulletin of the Ocean Research Institute of the University of Tokyo, 15, 1-244. P~omm, R.D. and G.K. MomusoN (1973) The relationship between stability and source waters for a section in the Northeast Atlantic. Journal of Physical Oceanography, 3, 280-285. PRes,'cameo, S.J. and M. RATrRAY(1975) Effects of continental slope and variable Brunt-Valsala frequency on the coastal generation of internal tides. Deep-SeaResearch, 22, 251-263. RacE, A.L. (1986) British oceanographic vessels 1800-1950. London, Ray Society, 193pp. RacE, A.L., R.G. ALDRED,D.S.M. Bna.m'r and M.H. TmmSTON(1979) The combined use of an epibenthic sledge and a deep-sea camera to give quantitative relevance to macro-benthos samples. Ambio Special Reports, 6, 59-72. RACE, A.L., R.G. ALD~EO,E. DAm-~GTONand R.A. WInD (1982) The quantitative estimation of the deep-sea megabenthos; a new approach to an old problem. OceanologicaActa, 5, 63-72. RAcr~, A.L. and E.P. COLLINS(1985) The use of photography in deep-sea benthic biology at the Institute of Oceanographic Sciences. In: Underwaterphotography and televisionfor scientists, J.D. GEOROE,G.I. Lv'ruoor~ and J.N. Lrraoon, editors, Oxford, Clarendon Press, 153-164. SAr~S"rRO~I,H. (1976) On topographic generation and coupling of internal waves. GeophysicalFluid Dynamics, 7, 231-270. SCmrL~, F.E. (1887) Hexactinellida. Report of the Scientific Results of the Voyage of H_M.S. Challenger, 18731876. 21,513pp 104pls. Scrlta.zE, F.E. (1904) Hexactinellida. WissenschaftlicheErgebnisse der Deutschen Tiefsee-Expedition auf dem Dampfer "Valdivia" 1898-1899, 8, 1-266.

196

A.L. RICEet al.

Stutter, M., C. Mom,uOT, D. DESBRUYER~,A. Dn~ET, A. KHRn~OUNOF~,G.T. ROWE and M. SEOO~,rZAC(1984) Peuplements benthiques et charact6ristiques trophiques du milieu dans la plaine abyssale de Demerara clans l'Oeean Atlantique. Oceanologica Acta, 7, 345-358. SMrrH, C.R. and S.C. HAMmTON(1983) Epibenthic megafanna of a bathyal basin off Southern California: patterns of abundance, biomass and dispersion. Deep-Sea Research, 30, 907-928. STEVnn~s,J. (1915) Sponges of the coasts of Ireland. I. The Triaxonida and pan of the Tetraxonida. Scientific Investigations. Fisheries Branch, Department of Agriculture for Ireland, 4, 1-43 + pls.I-V. THOMSOn,C.W. (1869) On Holtenia, a genus of vitreous sponges. Philosophical Transactions of the Royal Society, 159, 701-720. TopsE~rr, E. (1892) Spongiaires de l'atlantique Nord. R~sultats des Campagnes Scientifiques accomplies par le Prince Albert 1, Fasc 2, 1-165. TovsEsrr, E. (1896) Eponges. In: Rdsultats scientifiques de la campagne du Caudan dans le golfe de Gascogne, Fasc II, KOP.tmER,editor. Annales de l'Universit3 de Lyon, 273-297. Tovs~srr, E. (1904) Spongiaires des A~ores. R3sultats des Campagnes Scientifiques accomplies par le Prince Albert 1, Fast.25, 1-280. Zmsowr~s, H. (1980) Les scl6ractiniaires de la M6diterran6e et de l'Atlantique nord-oriental. M~moires de l'lnstitut oc~anographique, Monaco, 11, 1-284 + 107pls.