Recent Foraminifera from the North Sea (Forties and Ekofisk areas) and the continental shelf west of Scotland

The regions studied are all of mid continental shelf depth (70–145 m) and have bottom waters of normal marine salinity. The North Sea has lower bottom water temperatures than those to the west of Scotland. However, the major difference between the two regions is one of tidal and/or wave energy: the northern North Sea is a low energy environment of muddy sand deposition whereas the sampled part of the continental shelf west and north of Scotland is a moderate to high energy environment of medium to coarse biogenic carbonate sedimentation. The physical differences between the two main areas are reflected in the living and dead foraminiferal assemblages. The northern North Sea is a region of free-living species whereas the continental shelf west of Scotland has immobile and mobile attached species living on firm substrates. The northern North Sea is very fertile and has high standing crop values. The dead assemblages are small in size and very abundant. To the west of Scotland the sea is less fertile, standing crop values are low, the dead assemblages are moderate to large in size and reasonably abundant due to the slow rate of dilution by sediment.


INTRODUCTION
The North Sea is an extensive epicontinental area opening northwards into the Norwegian Sea. The Forties sampling area (approx. Long. 57" 44"; Lat. 00" 54'E) extends over a depth range of 100-130 m. The Ekofisk sampling area (approx. Long. 56" 32"; Lat. 03" 13'E) has a depth of approximately 70m and is further removed from the influence of the open ocean (Fig. 1).
The two North Sea areas have similar bottom water characteristics. Ekofisk has a mean temperature in the winter of 5.5"C and in the summer of 7.0"C. For Forties, the values are 6.5" and 7.0"C respectively. Likewise, the values for bottom salinity are 35.0%, and 34.0%, for winter and summer at Ekofisk and 35.1%, throughout the year at Forties (Lee & Ramster, 1981). In both areas, the surface waters develop thermohaline stratification during the summer months. Ekofisk has a slightly muddy fine sand substrate with 3.6-6.596 fines < 63 pm and a median particle diameter of 130-144 pm (Addy er al., 1979). Forties is characterised by a higher proportion of fine fraction, generally in the range 10-39% (Hartley, 1979).
Forties is fished commercially for the pink shrimp (Pandalus borealis) and for scampi Vephrops norvegicus) which digs extensive dwelling burrows in the cohesive sediment (Lee & Ramster, 1981).
The continental shelf of the Fair Isle Channel and to the west of Scotland is a region of carbonate sedimentation. This reflects the meagre supply of temgenous material thereby permitting the gradual accumulation of biogenic debris (Wilson, 1979(Wilson, ,1982. The mean bottom temperature is 7" C in the winter and 11" C in the summer for the Fair Isle Channel sampling area and -9" €or the summer for the shelf to the west of Scotland (no details available for the winter; Lee & Ramster, 1981). The bottom water salinity values are 35.1%, throughout the year in the Fair Isle Channel sampling area and 35.0 to > 35.25%, to the west of Scotland (Lee & Ramster, 1981). The area develops thermohaline stratification in the summer months. Tidal current velocities are small to the west of Scotland (rippled sand), but are powerful in the Fair Isle Channel (gravel sheet; Wilson, 1982). All areas are subject to vigorous wave activity (35m, 30m, for West Scotland, Fair Isle Channel; and 28m for Forties and Ekofisk -50 year wave height, Lee & Ramster, 1981).
In summary, all four areas are of mid-shelf depth and normal salinity and they differ only in lower bottom temperatures and terrigenous sedimentary substrate (North Sea) and higher bottom temperatures and a bioclastic sediment substrate (Fair Isle Channel and West Scottish Shelf).

MATERIAL AND METHODS
Three samples from the Forties area of the North Sea were collected in June 1978 and a further 17 in June 1981. Five samples were collected from the Ekofisk area of the North Sea in August 1981. All were taken with a Day grab. Once on the ship, a plastic ring enclosing an area of 45 cm2 was pressed into the sediment surface and the contents scooped out to a depth of 1 cm. The samples from west and north of Scotland were collected in 1979 and 1981 using a Murray grab which takes a sample 100cm' in area and l c m thick (Table 1).
All samples were preserved in alcohol. In the laboratory, they were washed on a 240 mesh (63 pm aperture) sieve, stained with rose Bengal, washed again on a 240 mesh sieve, and dried in an oven at 60°C. When dry and cold, the foraminifera were foated off in trichloroethylene. The flotation was weighed and a portion of known weight picked for both living and dead assemblages. The standing crop was calculated to a standard area of 30cm2. Material finer than 63 pm was collected in a dish, decanted into a filter paper and dried. Small pieces of the dried material were mounted on stubs, coated with gold, and examined in a Philips S.E.M. 501B. Searches for nannoplankton were carried out at magnifications of x 5000 or x 2500.
The standing crop values are generally high (Table 2  and Figs. 3,4) The pattern of abundance at Forties shows no clear relationship with either depth or sediment grain size. Biomass values are also high and they show considerable variation ( Table 2). The biomass distribution pattern at Forties also shows no clear correlation with depth or sediment grain size (Fig. 5). However, the abundance of the tiny species Fursenkoina fusiformis and the generally low numbers of larger species mean that there is a reasonsable correlation between biomass and standing crop (Fig. 6).

DEAD ASSEMBLAGES North Sea
These are more diverse than the living ( Table 2) but some of the values are still low for a shelf sea. These assemblages fall in field 1 of the classification proposed by Murray (1984). The Rotaliina dominate the two areas but the proportion of Textulariina is between 12 and 19% at Ekofisk in comparison with < l o % at Forties (Fig. 2).
Fursenkoina fusiformis is generally the dominant species (Table 2)  The abundance of foraminifera, both benthic and planktonic, can be expressed by the weight of the flotation. This shows a fairly good correlation in the Forties area with the sediment grain size; muddy samples yield a greater weight of flotation. This suggests that the muddy areas are either more productive or that small tests are being deposited there from elsewhere. In view of the pattern of distribution of the standing crop (Fig. 4) and the small size of many of the dead tests, the latter interpretation is favoured.  At Ekofisk, individuals of H . balthica in particular, and to a lesser extent C. obtusa and B . marginata, have opaque, white tests which appear to have been etched through dissolution. In some cases, the tests are so fragile that they crumble to dust. In the Forties area, some tests of H. balthica, C . carinata and Melonis pompilioides have etch-pits made by boring organisms. These seriously weaken the tests and cause them to break.

Continental shelf west and north of Scotland
The assemblages fall into two groups: stations 3109 and 3110 from the Fair Isle Channel gravel sheet (in fields IIIlIV of Murray, 1984) and stations 3111-3116 from the western shelf rippled sands (fields I/III of Murray, 1984).
Although some specimens are damaged through abrasion, none shows any effects of dissolution.

PLANKTONIC: BENTHIC RATIO
Only one planktonic test (150 p m in diameter) was observed in the Ekofisk samples whereas in the Forties samples the planktonic: benthic ratio is between 19 : 81 and 46 : 54. In general, the muddier sediments have the higher planktonic values. The maximum size of planktonic tests varies from 300-500 pm but the average size is in the range 110-150 p m and this reflects the dominance of thin-walled juvenile tests.
Planktonic forms are absent in the Fair Isle Channel samples. On the shelf to the west of Scotland, the ratio is 10 : 90 to 45 : 55. The maximum size varies from 420-660 wm.

COCCOLITHS
Whole and fragmented coccoliths are present in all the samples examined but they are more abundant at Forties than they are at Ekofisk. The dominant species is Emiliana huxleyi. At Ekofisk, only one or two species are present in each sample and the second species is either Coccolithus pelagicus or Coronosphaera mediterranea. At Forties, three to five species are present, the additional forms being Calcidiscus leptoporus, Gephyrocapsa oceanica and Helicosphaera carteri (Table 3). Table 3. Occurrence of nannoplankton in the sediment fraction <63 pm.
This pattern of distribution resembles that of the English Channel , in showing a decrease in diversity with increase in distance from the ocean. It is also consistent with the known movement of oceanic water into the area (Fig. 1). The occurrence of Emilina huxleyi as the dominant species accords with the observation by Braarud et al. (1953) that this is the characteristic species of Atlantic water masses entering the North Sea through the Faeroe-Shetland Channel.
The shelf to the west of S.cotland comes under the influence of Atlantic water. Whole and fragmentary coccoliths are generally abundant and Coccospheres (of Emiliana huxleyi) are present at some stations. The coarse sediments from the Fair Isle Channel have a very small proportion of the < 6 3 p m component and this has only 2-3 species of coccoliths (Table 3).

DISCUSS I 0 N
In the early part of this century, Heron-Allen and Earland wrote a series of papers on individual species of foraminifera present in samples from the northern North Sea collected by the Scottish Fisheries Board cruiser "Goldseeker". The first assemblage study was that of Stephen (1923) who described the 'Foraminifera community' based on the abundance of foraminifera > 1.5mm in size on muddy bottoms from 100m and deeper. Astrorhiza arenarea, Saccammina sphaerica 'and Psammosphaera fusca were present together with the bivalve Thyasira flexuosa. Thorson (1957) considered this community to have a small standing crop and low productivity, which is at variance with the results reported here.
The Fladen Ground, studied by McIntyre (1961), yielded 983 Saccammina spherica per m2 sea floor ( > 1.3 mm sieve). However, examination of 1000 tests showed that only 267 contained the animal and 636 were dead. Other species recorded by McIntyre were Alveolophragmium crassimargo and Reophax scorpiurus, but he also looked at the < 1.3mm size fraction and commented on the great abundance of small foraminifera. In a further paper (1976) he stained the samples with rose Bengal and looked at the > 76 pm fraction. Bulimina (which may include Fursenkoina as used here) and Cancris were said to be present in abundances of 2000-105,000 per m2 (= 6-315 per 30cm2) in samples from the Fladen Ground and Loch Nevis on the west coast of Scotland. These values are significantly lower than those recorded here, perhaps partly because of the use of a 76 pm rather than a 63 pm sieve.
In the classification of marine communities proposed by Jones (1950), the northern North Sea is a boreal deep mud association.
Previous studies of assemblages of foraminifera from the whole North Sea include those of Jarke (1961) and Gabel (1971). These authors used a 63 pm sieve and concentrated the foraminifera by the carbon tetrachloride technique. However, they did not distinguish between living and dead faunas. The Ekofisk area lies within the Bulimina fusiformis, B . marginata, Anomalina balthica assemblage of Jarke (1961) and the Bulimina fusiformis, B . marginata assemblage of Gabel (1971). This is consistent with the present results.

(Bulimina fusiformis = Fursenkoina fusiformis; Anomalina balthica = Hyalina balthica).
The living assemblages of the mid-shelf off Northumberland (depth 85 m) were studied by Collison (1980). He took core samples having a surface area of 3.9cm2, and examined 1 cm thick slices down to a depth of 11 cm below the sediment/water interface. Fursenkoina fusiformis was the dominant species at every station. It formed an average of 57% of the living assemblages from all cores, but only 21% of the assemblage in the top 1 cm (average of 5 cores). In core 1, it formed 6% in the top 1 cm but reached a peak of 96% at 5-6 cm. (Absolute abundance 10 and 205 respectively). The other five species discussed, Bulimina marginata, Hippocrepina sp., Reophax subfusiformis, Ammonia batavus, and Quinqueloculina sp. were most abundant in the top 2 cm although they also occurred in small numbers below this depth.
The results from Forties and Ekofisk show some differences from shelf seas in general (see Murray, 1973, for a review). In particular, the living assemblages show high dominance by one species and low diversity. Both of these features are more characteristic of marginal marine environments. It is possible that the high abundance of Fursenkoina fusiformis might be a seasonal feature as most of the samples were collected in June (as also were those by Collisoq,, 1980). Three samples from the Celtic Sea had living assemblages with F. fusiformis forming 58, 67 and 92% and these were collected in October 1970 and July-August 1972 (Murray, 1979). A seasonal blooming would also help to account for the reduced abundances in the dead assemblages (Table 2). Alternative explanations are the selective post-mortem loss of this species or the introduction of tests of other species not normally resident in the area. The latter process probably takes place anyway as the dead assemblages are somewhat more diverse. These results are in marked contrast to those for the living macrofauna. In the 1978 (Hartley, 1979) and 1981 (Hartley & Ferbrache, 1983) sampling surveys, the Forties area was found to be rich in species and low in species dominance.
The standing crop and biomass values range from average to very high in comparison with other shelf seas and this indicates that the area is very fertile. The range previously recorded for shelf seas is 0-4.230 mms/ 30cm2 biomass (Murray, 1973(Murray, , 1979 thus the values for the North Sea are especially high. The living assemblages are composed entirely of free forms (sensu Sturrock & Murray, 1981). This is true also of the dead assemblages although these include rare attached immobile forms (Cibicides fobatufus). This is consistent with a low-energy area of fine grained sediment accumulation.
Both Forties and Ekofisk come under the influence of Atlantic water which flows into the North Sea at the surface. The evidence for this is recorded in the sediment by the planktonic: benthic ratio and by the coccoliths. Plantonic tests are present at Forties but most are small juveniles. They are virtually absent from Ekofisk. This accords with the model for determining the proximity of marginal seas to an ocean put forward by Murray (1976). Similarly, the coccoliths show a reduction in diversity from Forties to Ekofisk.
The samples studied were originally taken during surveys to monitor the effects of pollution from oilfield development. The macrobenthos of the Ekofisk oilfield underwent changes between 1975 and 1977 with a modified area extending to -3 km away from the central storage and production complex in 1977. Although some species were reduced in number in the central area, others (Chaerozone serosa and Arctica isfandica) were most abundant there. Unfortunately, no studies of living foraminifera were undertaken prior to the development of the oilfield. However, the results presented here suggest that there has been no detrimental effect on living assemblages.
Few studies have been made of foraminifera from the west coast of Scotland. Brady (1865) discussed the Shetland fauna and Heron-Allen and Earland (1916) the continental shelf. Recently, Edwards (1982) reported on the faunas of the North Minch Channel. None of these studies differentiated between living and dead, although a few of the samples studied by Heron-Allen and Earland were preserved in alcohol. McIntyre (1961) made a study of the fauna > 1.3 mm on the muddy bottom of Loch Nevis and found Crithrionina granum to be abundant. There appears to be no previous studies of the mid-shelf region.
In this study, it has been shown that the faunas of the North Sea are very different from those of the Scottish shelf. The latter have a modest proportion of free-living forms and a dominance of attached forms, both immobile and mobile.This is indicative of the higher-energy environment to the west of Scotland, where, although the tidal currents are weak (Wilson, 1982, p. 165), it is possible that wave energy disturbs the sea floor especially during storms. The highest energy area is that of the Fair Isle Channel where tidal currents are powerful (Wilson, 1982, p. 163). The living assemblages resemble those of the English Channel and Celtic Sea (Groups 1 and 3 of Sturrock &Murray, 1981). The dead assemblages differ markedly from the living due to transport. This causes mixing of assemblages from adjacent micro-environments and winnowing away of small mobile attached species which live on firm sub-strates. The abundance of the dead faunas is thought to be controlled mainly by the slow rate of dilution by sediment.

CONCLUSIONS
The mid continental shelf muddy bottom living foraminiferal assemblages from the northern North Sea have a high standing crop, low diversity, and are generally dominated by Fursenkoina fusiformis. This contrasts with the high diversity, low dominance macrofauna of the same area. The dead foraminiferal assemblages are more diverse, probably due to the introduction of small tests winnowed from elsewhere but are nevertheless very similar to the living assemblages. Both are made up of free-living species.
Living assemblages from the mid continental shelf west of Scotland are largely confined to attached forms on hard substrates such as bivalve shells, bryozoa or hydroids. The dead assemblages are made up of large tests and they differ significantly from the living due to post-mortem transport effects.