ELSEVIER
Marine Micropaleontology 25 ( 1995) 269-288
Benthic foraminiferal distribution and abundance changes in
Skagerrak surface sediments: 1937 (Hiiglund) and 1992/ 1993
data compared
E. Alve ‘, J.W. Murray b
“ Department c$Geology, University of Oslo. P.O. Box 1047 Blindern, N-0316 Oslo, Norway
h Department ofGeology,Southampton Oceanography Centre. European Way, Southampton SO14 3ZH, UK
Received 20 June 1994; accepted 16 March 1995
Abstract
Both living (stained) and dead (unstained) foraminiferal assemblages from surface sediments (O-2 cm) in the northwestern
part of the Skagerrak have been studied in order to (1) define and characterize the distribution of various modern benthic
environments and (2) by comparing these findings with surface samples collected 40-60 years ago, to document possible fauna1
changes that might have occurred. The investigated area is physiographically divided into the Norwegian slope, the Skagerrak
Basin, and the Danish slope. The latter is under the influence of the Jutland Current, while the basin and the investigated parts
of the Norwegian slope are bathed in Atlantic water. All areas have bottom waters with a high oxygen concentration. Three
living (stained) and three dead (unstained) assemblages occupy the three physiographic areas. Only one assemblage (on the
Norwegian slope) is common to both the living and dead assemblages but the boundaries between them lie at comparable depths.
The higher standing crops are found on the fertile Danish slope while the lower ones are in the deep basin where the diversity is
at a maximum. In the dead assemblages, the relative abundance of agglutinated tests increases with depth. Comparison with data
collected 40 to 60 years ago shows increases in absolute numbers of tests, especially in the deep basin. There are changes in
assemblage compositions in all areas. The dominant species found in 1937 are different from those of 1992/ 1993. There is a
major change in the basin where one agglutinated species has changed its depth distribution downslope and two present day
abundant species are new arrivals. These fauna1 events are probably linked to environmental changes.
1. Introduction
Since 1991 the Norwegian Geological Survey has
been carrying out a marine geological investigation of
the Norwegian sector of the Skagerrak. As part of this
a study of surface sediments (O-2 cm) is being undertaken. We report here the lirst detailed analysis of recent
foraminiferal distributions and dynamics in the northwestern part of the Skagerrak.
Although there are several publications which consider the recent foraminifera of the Skagerrak and adja0377-8398/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDIO377-8398(95)00026-7
cent areas (Van Weering and Qvale, 1983; Corliss and
Van Weering, 1993; Moodley et al., 1993; Conradsen
et al., 1994), the classic work is that of Hoglund
(1947). In 1937 he collected core samples from the
Skagerrak and 9 of his stations were within the present
study area. Except for one station, his quantitative data
have never been published but they have been made
available to us, by courtesy of T. Cedhagen. This has
enabled us to make a study of long term changes in
surface sediment foraminiferal
faunas of this shelf
basin. During the five decades between Hoglund’s
270
E. Ah,
J. W. Murray/Marine
Micrr,paleontoloXy
1937 sampling and ours from 199211993, there have
been progressive environmental
changes due to the
input of nutrients and other pollutants from man-made
sources quite apart from any natural alterations in
hydrography which may have occurred. Assessment of
such long term fauna1 changes is important both from
geological and from environmental management viewpoints. These data also represent a baseline for monitoring effects of future oil exploration
or other
anthropogenic alterations to the environment.
2. Description of the environment
The Skagerrak forms the eastern continuation of the
North Sea and has an open connection with the Baltic
through the Kattegat (Fig. 1). The water depth
increases gradually from the shoreline towards the
Skagerrak Basin (maximum
depth 700 m, Fig. 2)
which is separated from the rest of the North Sea by a
sill at about 270 m water depth. This is the deepest of
those forming the Norwegian Channel and represents
a continuation of the Oslo Rift Zone. The surface water
circulation is anticlockwise. Coastal surface water from
the southern North Sea, which has received the outflows from the major European rivers (North Sea Task
Force, 1993)) flows, as the Jutland Current, northeastwards into the Skagerrak, along the western and northern coast of Denmark. As it turns northwards in the
eastern Skagerrak, it mixes with surface water flowing
out from the Baltic Sea and flows westwards along the
Norwegian coast as the Norwegian Coastal Current
(Svansson,
1975; Larsson and Rodhe, 1979). The
deeper part of the basin is occupied by a stable water
mass derived from the Atlantic. This water enters the
North Sea between the Orkney and Shetland Islands
and some of it flows episodically
into the deeper
( > 200 m) Skagerrak Basin via the Norwegian Channel (Ljoen, 198 1) . The Atlantic Water in the Skagerrak
has salinities of > 35%~ the temperature is generally
between 6.0 and 65°C with occasional values below
5.O”C (Mork et al., 1976; Larsson and Rodhe, 1979)
and it is well oxygenated ( > 87% saturation in our
investigation area, Svansson, 1975; Bakker and Helder,
1993). Recent investigations
have shown that deep
water renewal in the Skagerrak Basin below sill depth
(350 m) occurs every 1-3 years with an average residence time of 25 months, a mean oxygen depletion rate
25 (1995) 269-288
of 0.04 ml 0,/l per month at 600 m water depth but
with no significant long term variation since 1947
( Aure and Dahl, 1994). However, a minimum concentration of 4.8 ml 0,/l was recorded in September 1990
(the lowest since 1950) due to a prolonged residence
time (40 months) between 1987 and 1991 caused by
unusually high temperature regimes in the North Sea
in the late 1980s (Aure and Dahl, 1994).
The Skagerrak is the major depository of hnegrained sediments in the North Sea (e.g. Van Weering
et al., 1987) with about 70% finer than 63 pm (Eisma
and Kalf, 1987). These sediments are derived from the
southern North Sea and transported, in suspension,
along the Danish coast by the Jutland Current (Eisma,
198 1; Qvale and Van Weering, 1985). A major depositional area is found north of Skagen (northern tip of
Denmark) and between Skagen and the Swedish west
coast (Qvale and Van Weering, 1985). Additionally,
substantial amounts of sediments are deposited in the
basin below the low-velocity currents in the central
Skagerrak vortex (Eisma and Kalf, 1987). The organic
carbon, most of which is refractory, is associated with
the clay and silt fractions; the lower values are in the
shallower, well-sorted sands ( < 0.5%) and higher values ( > 3%) in the moderately-sorted silts in the deeper
parts of the area where the sedimentation rate is lowest,
about 0.1 cm/year (Van Weering and Qvale, 1983;
Van Weering et al., 1987). Bottom current velocities
in the deeper parts of the Skagerrak are generally low
( < 10 cm/s) in contrast to the variable, but signihcantly higher (sometimes > 30 cm/s), values at depths
< 100 m (Larsson and Rodhe, 1979).
The Skagerrak receives about 2.5 million tonnes of
total nitrogen per year from the central North Sea and
the Atlantic current. It is not clear whether part of this
is of anthropogenic origin. However, it has been estimated that > 800,000 tonnes of anthropogenic total
nitrogen is introduced to the Skagerrak from the Jutland
Current and the Baltic, through atmospheric precipitation, and through riverine inputs and direct discharges
from Denmark, Sweden, and Norway. Anthropogenic
inputs of phosphorus from the same sources is estimated to be about 67,000 tonnes per year (North Sea
Task Force, 1993).
3. Previous foraminiferal
studies
The classic taxonomic work on Skagerrak foraminifera is that of Hoglund ( 1947). Additional useful
E. Abe, J. W. Murruy /Marine Micropaleontology 25 (I 995) 269-288
8’00
8’60
271
1 o;oo
1 odoo
12”OO
Fig. 1. General water circulation in the Skagenak (after Svansson, 1975). Inset map (after Eisma and Kalf, 1987) shows the the Skagerrak
(Sk) in relation to the North Sea and Norwegian Channel (N.Ch.). Continuous arrows = surface water circulation; broken arrows = deep water
circulation.
taxonomic
studies are those of Feyling-Hanssen
(1964) and Feyling-Hanssen
et al. (1971). Also,
Gabel ( 197 1) gave line drawings of many of the taxa.
The commonly occurring bolivinid identified as Bolivim cf. robusta in the older literature was named B.
skugerrukensis by Qvale and Nigam ( 1985). According to modern generic usage (Loeblich and Tappan,
1987)) this species should be referred to Brizalina.
Previous distribution
studies have generally been
based on widely spaced samples collected by various
methods (grabs, cores, etc.), some stained to distinguish living from dead (e.g. Corliss and Van Weering,
1993) others not stained so only total data were
recorded (Lange, 1956; Jarke, 1961; Gabel, 197 1; Van
Weering and Qvale, 1983)) and processed on sieves of
varying mesh size (63-150 pm). The earliest study is
that of Hoglund ( 1947) but quantitative distributional
information was provided for only one station. Some
of his data were used by QvaIe et al. (1984) in a
comparative study of fjords.
Lange ( 1956) studied the total assemblages ( > 63
pm) of 12 surface samples and 4 gravity cores from
the Skagerrak and Kattegat. Only 3 of the samples
(collected in 1949 and 1951) were from within the
212
E. Alve, J. W. Murray /Marine
8’bO
Micropaleontology
9*bo
25 (I 995) 269-288
lo:00
Fig. 2. Bathymetric map (contour interval 200 m) of investigation area showing the location of samples collected in 1992/ 1993 (this study)
and samplescollected by Hiiglund in 1937 and Lange in 1946/ 1951
discussed here (Fig. 2, Danish slope). Station 24
(196 m) was dominated by Cassidulina laevigata,
Anomalina baltica ( = Hyalinea balthica) and Elphidium incertum which made up 24, 14, and lo%, respectively. Station 26 (422 m) was dominated by Boliuina
cf. robusta ( =Brizalina skagerrakensis)
making up
44% with subsidiary C. laeuigata and Uuigerina peregrina, each 11%. The upper 3.3 m of core M5a was
also dominated by B. skagerrakensis.
Two surface sediment samples from the general area
of the Skagerrak were included by both Jarke (1961)
and Gabel ( 1971) in their studies of the North Sea.
One sample had the dominant species Stainforthiafusiformis, (as Bulimina), Bulimina marginata, and Hyalinea balthica (as Anomalina)
and the other had
Brizalina skagerrakensis
(as Bolivina spathulata) in
the > 63 pm fraction. Van Weering and Qvale ( 1983)
area
using the > 125 pm fraction found a Bolivina robusta
( = B. skagerrakensis) total assemblage in the deeper
basin and a Cassidulina laevigata total assemblage on
its southern slope. The absence of S. fusiformis from
their results is no doubt due to the relatively coarse
sieve size used. Conradsen et al. ( 1994) have reviewed
some of the data on distributions in the Skagerrak and
Kattegat areas. The depth distribution of stained foraminifera ( > 1.50 pm) in the sediment has been investigated in 2 cores from within our study area (Corliss
and Van Weering, 1993). Core 13 (621 m) yielded
few stained forms. Core 4 (530 m) had 56% of the
stained forms in the top 2 cm of sediment with a
B.skagerrakensis assemblage in both the O-2 cm and
2-20 cm intervals. Variations in abundance patterns
based on core data have been discussed by Moodley et
al. (1993). A comparison
between original dead
E. Alve, J. W. Murray /Marine
assemblages and experimentally produced agglutinated
assemblages (derived by acid treatment) has been
made by Alve and Murray ( 1995).
4. Material and methods
The present study is based on surface sediment samples (top 2 cm of short gravity cores). Thirteen were
collected by the Norwegian Geological Survey (NGU)
in July 1992 [University of Bergen (UiB) cruise no.
92051 and eighteen by E.A. in July 1993 (UiB cruise
no. 9307) (Fig. 2; Table 1) . At eight of the 1993 stations, 250 ml of water from just above the sedimentTable 1
Details of samples collected in 1992/ 1993
Station no.
Latitude (“N)
Longitude
Yeur 1992
48
10
21
18
49
28
SO
32
40
44
51
52
53
(“E)
Depth (m)
58O24.41’
58O52.45’
58’40.52’
58O44.40’
58O20.36’
58”36.62’
58O16.94’
Y~S”28.84’
SS”21.31’
58O17.66’
58O13.33’
58OO9.87’
58OO6.44’
09”01.57’
09O50.52’
09O43.76’
09O50.95 ’
09”09.63’
09”36.58’
09O16.67’
09O37.60’
09O37.52’
09”30.68’
09O23.99’
09”3 1.08 ’
09”39.64’
283
349
377
393
394
556
558
594
647
662
666
598
431
58O16.30’
58”19.95’
58”12.01’
58OO8.70’
58O12.61’
58”16.56’
58”08.65’
58W.69’
58”01.39’
58OO8.98’
.58”12.64’
58OO9.28’
58OO5.66’
58OO1.59’
57O57.76’
58OO2.06’
57”58.21’
5lO59.3 1’
57Y5.86’
08’48.86’
08O55.37’
08’42.35’
Og”34.75’
08Y6.73’
09”03.04’
08O49.99’
08O43.19’
08O49.99’
09W.65’
09”10.84
09O17.39’
09”10.81’
09v4.15
08O57.42’
OY17.95
09”11.13’
OY23.87’
09”16.11
285
298
304
310
399
404
427
473
595
640
651
652
626
581
534
514
483
359
266
Year 1993
60
59
71
12
61
58
70
73
14
62
51
S6
63
68
75
64
67
65
66
Micropaleontology
25 (1995) 269-288
213
water interface (O-15 cm) in the core liner was transferred to glass bottles for determination of the dissolved
oxygen concentrations by the standard Winkler titration method (performed by the Institute of Marine
Research, Bergen, Norway).
The 1992 cores were stored in a cold room (7”
10°C) until they were opened and subsampled in September 1992. The 13 subsamples were transferred to
cylindrical containers and gently mixed with 70% ethanol. After about 24 hours, the sample volumes were
determined by measuring the height and diameter of
the sediment in the containers. The samples were processed by washing them on a 63 pm sieve and staining
the residues with rose Bengal for about one hour before
they were washed again on the same sieve and dried at
50°C. At least 250 dead and all stained individuals were
picked from these samples. However, the volumes of
most of the 1992 samples were too small (only subsamples of 7.5-15.1 cm were available) to give statistically significant numbers of stained individuals.
Therefore, the stained data from the 1992 samples will,
in the following, only be used with caution to supplement the stained data from the 1993 samples. All samples were dry-picked with a moistened brush and there
was no difficulty in recognising stained agglutinated
individuals. However, when, in some cases, there was
reason to question the presence of stain, particularly in
Haplophragmoides
bradyi, the specimens were completely soaked in water.
The subsampling of the 1993 samples were performed as follows: Most of the water overlying the
sediment in the core liner was carefully siphoned off
slowly. The sediment was gently pushed up through
the core liner and the last few millimetres of water was
carefully removed with a pipette and transferred to the
sample container. The surface 2 cm was then sectioned
off, put into the same container, and gently mixed with
70% ethanol. As two different core liners were used,
15 samples had a volume of 39.25 cm3, whereas the
remaining 3 had volumes of 51.0 cm”. After return to
the laboratory, the samples were processed and analysed in the same way as described for the 1992 samples. At least 250 stained and 250 unstained individuals
were picked from each of the 1993 samples.
Fragments of tubular and branching forms were
treated as a separate category and are not included in
the calculations (for discussion, see Murray and Alve,
E. Ah,
274
J. W. Murray /Marine
Micropaleontology
25 (1995) 269-288
5. Results
1994). All species mentioned in the text are given in
the fauna1 reference list.
Two measures ofdiversity have been used: the alphaindex of Fisher et al. (1943), which is an indicator of
species richness, relating the number of species to the
number of individuals, and, the information function,
H(S), which is a measure of heterogeneity taking into
account the evenness of the species abundances (Murray, 1991).
Varimax factor analysis was carried out on species
making up >5% of the assemblages in at least two
samples, using the Q-mode factor analysis program
CABFAC (Imbrie and Kipp, 197 1; Klovan and Imbrie,
1971).
A visual evaluation of the relative content of organic
detritus of the > 63 pm fraction was performed on the
wet 1993 samples during processing and recorded on a
scale from 1 to 4 (from little to abundant).
5.1. Organic matter and oxygen
Analyses of the total organic carbon (TOC) content
of the surface sediments (O-2 cm) were performed on
replicate cores from the 1993 stations by the Norwegian
Geological Survey and their data have been placed to
our disposal. The TOC content is relatively constant
(2.1-2.3 wt.%) except in the sandy sample (67) from
the Danish slope ( 1.1%) (Fig. 3). The distribution of
the TOC values is in accordance with earlier findings
(Van Weering and Qvale, 1983; Anton et al., 1993;
Bakker and Helder, 1993). However, the visual examination of the wet samples revealed that there are major
differences in the amount of fragile, delicate, fluffy,
easily oxidized, organic detritus in the > 63 hum fraction. The sediments from the Skagerrak Basin con-
NW
SE
120
n
-
loo-
4
Stained / cm3
-3,s
TOC (%)
-3
80;
.-?
s
*
-2,5
‘;;
5
*
8
l-
60-
-1,5
40-
-1
20-
‘)A”
60
59
71
72
61
58
70
73
74
62
57
63.68
75’
64’
67’
65’
Skagerrak Basin
1
Danish slope
66’
E 300
z
h 400
$2 500
2
600
700
Norwegian slope
I
Fig. 3. Upper diagram: numerical density of stained foraminifera and TOC (both 1993 data). Figures on abscisa refer to sample numbers. Lower
diagram: projection of stations onto a NW-SE profile through the investigation area. Figures in circles reflect the relative content in organic
detritus ( > 63 pm) on a scale from 1 to 4 (from little to abundant).
E. Alve, J. W. Murray/Marine
Micropaleontology
25 (1995)
269-288
27s
Table 2
Percent abundance
of important
Sample no.
Depth (m)
species (see text) and calculated
fauna1 parameters
of living (stained)
assemblages.
1993 data
60
285
59
298
71
304
72
310
61
399
58
404
70
427
73
473
74
594
62
640
57
651
63
626
68
581
75
534
64
514
67
483
65
66
35’3 266
2
2
4
3
3
10
2
5
0
4
I
2
2
2
I
2
0
1993, stained (%)
Cassidulina
iueuijiafu
Eggerelloides
me&us
Glohohulimina
Epistominella
auriculutu
clitreu
2
125310360311635006
12
40
II
13
4
4
I1
3
I
0
I
0
0
I
I
0
70
12
0
I
1
0
9
6
2
5
II
12
8
16
16
13
12
1
0
0
0
0
5
2
0
1
3
3
IO
2
2
4
4
0
6
0
0
0
I
0
4
0
21
7
10
II
14
9
7
16
16
19
19
0
0
0
Lrehusellu goesi
5
0
5
22
4
5
I
0
I
0
8
0
6
2
I1
5
I
I
I
5
2
0
0
0
Noniorzellu irideu
3
I
1
2
7
0
3
I1
16
21
0
4
3
0
3
0
17
9
0
5
10
4
2
Melonis barleeunum
6
11
9
I
0
1
Pullenia bullodes
0040113
10
3
7
21
5
2
4
4
0
0
0
Haplophragmoides
bra&i
Huplophragnroides
memhrunuceum
I
P ulleniu od0ensi.s
17
4
30
8
15
16
12
I
2
0
4
5
3
10
8
2
2
I
Reophax micuceu
17
I
6
25
6
9
8
9
2
4
6
0
I
I
1
I
2
17
Stainforthiafusiformis
13
32
3
3
4
17
5
10
3
13
2
4
3
7
5
86
6
19
Te.xtulariu tenuissimu
15
5
4
10
4
11
15
2
0
0
0
0
0
0
I
2
5
26
251
No. counted
No. species
% agglutinated
Alpha index
H(S)
Stained/cm’
Stained agglutinated/cm’
272
275
235
261
269
247
259
282
235
268
322
244
257
259
339
324
281
25
22
28
20
34
24
28
35
42
39
40
42
46
40
46
16
19
42
6.5
2.4
21
9
11
5.5
1.9
7
1
35
8
2.6
33
11
64
5
2.2
34
22
39
10.5
2.8
9
4
33
6.5
2.6
5
2
52
8
2.8
16
8
42
11
3.0
22
9
38
14
2.9
6
2
29
13
3.0
I1
3
30
12
2.9
16
5
40
14
3.1
8
3
51
17
3.1
15
8
32
13.5
2.9
13
4
42
14.5
3.0
35
15
6
17
3.5 4.5
0.8
1.3
120 48
7
8
tained little of this organic detritus compared with the
samples from the shallower areas on the Norwegian
and Danish slopes. Sample 67 (Danish slope), contained so much >63 pm organic detritus that it was
difficult to pick out the foraminifera.
The oxygen
results from the 1993 cruise revealed well-oxygenated
bottom water conditions for all 8 investigated stations
(6.3-6.6 ml 02/1).
5.2. Standing crop and number of tests
The standing crop is variable and ranges between 5
and 120 per cm3, indicating patchiness in the distribution of the living individuals (Table 2). However, there
is a pronounced pattern with intermediate values (average 18 per cm3, n = 8) on the Norwegian slope, generally low values in the Skagerrak Basin (average 11
per cm3, n = 5)) and maximum values on the Danish
slope (average 63 per cm3, n = 5) (Fig. 3). These high
values are found in the area of maximum abundance of
delicate organic detritus in the > 63 pm fraction (encircled values in lower diagram, Fig. 3).
35
56
11
2.4
98
55
The number of empty tests in the 1993 samples varies
from 18-1007 per cm3 and generally follows the abundance patterns of the stained (linear correlation;
r = 0.72, n = 18). The average values are higher; 94,
132, and 355 for the Norwegian slope, the Skagerrak
Basin and the Danish slope, respectively (Table 3).
The 1992 samples generally follow the same pattern
with average empty test values of about lOO/cm” for
the Norwegian slope and basin and a maximum value
of 236/cm” on the Danish slope (Table 4).
Only scattered juvenile individuals of planktic foraminifera were present (O-6 per 250 dead benthic individuals).
5.3. Distribution of species > 10%
In order to emphasise the distribution patterns of the
most abundant species, this section highlights the distribution of those species which comprise > 10% of
the assemblage in at least one sample. The only species
with frequent stained individuals at all water depths is
E. Alve, J. W. Murray /Marine Micropaleontology 25 (I 995) 269-288
276
Table 3
Percent abundance
of key species and calculated
fauna1 parameters
of dead assemblages,
1993 data
70
421
73
473
74
594
62
640
Sample no.
Depth (m)
60
285
59
298
7b
304
72
310
61
399
58
404
1993, dead (%)
Brizalina skagerrakensis
4
I
3
3
8
5
8
22
21
3
Cassidulina laevigata
Eggerelloides medius
Elphidium spp.
Epistominella oitrea
Haplophragmoides bradyi
10
3
5
4
3
15
2
2
3
0
8
3
3
3
1
12
5
4
2
0
11
10
2
1
6
5
4
1
2
2
6
9
0
4
4
10
13
0
0
7
6
10
2
2
17
7
11
2
3
11
Nonionella iridea
Pullenia bulloides
Pullenia osloensis
Reophar micacea
Saccammina spp.
Stainforthia fusiformis
Textularia tenuissima
Trochamminopsispusillus
1363326408
003003117691111000
19
26
25
19
538334421022122001
0
0
0
0
12
4
8
15
20
18
15
21
0
0
0
0
21
12
15
15
0
0
1
4
8
6
0
2
19
11
1
5
3
11
2
4
3
0
2
8
2
0
4
7
9
0
8
No. counted
No. species
% agglutinated
Alpha-index
H(S)
Dead/cm’
Dead agglutinated/cm3
212
35
36
10.5
2.7
88
31
262
33
35
10
2.8
45
16
256
48
30
18
3.1
18
5
261
41
42
16
3.1
44
19
253
42
35
13
2.8
297
104
257
39
51
13
2.9
45
23
260
47
47
16
3.3
50
23.35
Table 4
Percent abundance
273
38
26
12
2.6
46
12
262
38
32
12
2.1
90
29
of key species and calculated
261
36
31
11
2.7
126
39
51
651
63
626
68
581
75
534
64
514
67
483
5
1
9
13
1
1
12
1
5
0
0
20
65
359
66
266
5
1
0
1
6
0
5
12
4
9
2
0
12
2
2
2
3
6
0
1
0
1
0
2
3
0
4
2
9
3
0
3
1
14
4
0
3
7
8
8
3
2
3
10
1
0
8
19
3
0
15
4
10
0
10
9
25
0
3
2
43
1
6
0
65
3
0
0
45
6
0
0
39
4
0
288
43
54
13
3.0
102
55
258
34
79
10.5
2.5
41
32
275
45
52
15
3.2
423
222
258
41
47
13.5
2.8
55
26
294
46
29
15
2.5
128
37
283
38
8
12
1.8
362
28
255
44
18
15
2.5
222
41
288
47
18
16
2.8
1007
181
50342000
faunal parameters
of dead assemblages,
1992 data
Sample no.
Depth (m)
48
283
10
349
21
377
18
393
49
394
28
556
50
558
32
594
40
647
44
662
51
666
52
598
53
431
1992, dead (%)
Brizalina skagerrakensis
Cassidtdina laeoigata
Eggerelloides medius
Epistominella oitrea
Haplophragmoides bradvi
Nonionella iridea
Pullenia osloensis
Saccammina spp.
Sfaiflforthiafusiformis
Textularia tenuissima
Trochamminopsis pusillus
5
13
7
1
1
3
25
0
5
15
0
1
13
3
2
0
2
14
0
7
26
0
I
6
8
5
3
8
27
0
3
4
0
6
I
4
0
2
2
38
0
4
17
0
25
10
6
5
3
6
22
2
3
1
2
7
4
6
2
5
12
19
5
8
2
5
14
10
14
0
8
4
12
4
1
0
3
6
8
14
1
9
9
10
4
2
0
5
4
2
5
0
12
3
6
11
1
0
14
1
4
9
0
13
4
12
11
3
0
7
3
4
I
4
9
13
8
8
3
0
7
2
2
7
1
26
2
7
11
7
0
6
2
3
3
3
3
8
20
1
34
3
0
No. counted
No. species
% agglutinated
Alpha-index
Dead/cm3
Agglutinated/cm3
525
49
30
13
92
27
494
50
34
14
77
26
273
32
23
9.5
134
31
258
29
29
8.5
43
12
504
43
19
11
161
30
277
34
38
10.5
145
54
468
45
39
12.5
127
49
250
46
44
16
113
49
251
41
66
13.5
77
51
267
41
51
13,5
62
35
583
59
44
17
103
46
512
53
69
16
92
64
558
51
19
I3,5
236
44
E. Alve, J. W. Murray/Marine
Micropaleontology
25 (1995) 269-288
271
SE
NW
I
Stained, 1993
*
Norwegian slope
Factor assemblage
-+-
Pullenia osloensis
--c
Haplophragmoides
. 0 -
Globobulimina
I
Skagerrak Basin
(stained)
membranaceum
Factor assemblage
--a-
Pullenia osloensis
-c
Haplophragmoides
+-
auriculata
Fig. 4. Varimax factor values for stained ( 1993) and dead ( 1992/ 1993) assemblages
area.
Stainforthia fusiformis.
Globobulimina
auriculata,
Pullenia osloensis, Reophax micacea and Textularia
tenuissima are abundant on the northern and southern
slopes whereas Epistominella uitrea, Haplophragmoides membranaceum
and Nonionella iridea are con-
1 Danish slope
Stainforthia
(dead)
bradyI
fusiformis
projected onto a NW-SE transect through the investigation
fined to the part of the basin deeper than 450 m (Table
2).
For the distribution of empty tests, S. fusiformis is
abundant on both slopes (down to 404 m on the north
and 534 m on the south) ; Cassidulina laevigata (down
to 473 m) , T. tenuisssima (down to 427 m) and Bri-
E. Ah,
278
J. W. Murray/Marine
Table 5
Varimax factor score matrix for living (stained)
data. Significant values in bold
assemblages,
1993
Varimax factor score matrix
Variable
1
2
3
C. hwigufu
G. auriculuta
P. bulloides
P. osloensis
S. fusiformis
N. iridea
E. medius
E. uitrea
H. brudvi
L. goesi
H. membrunaceum
T. tenuissima
R. micacea
M. barleeanum
0.070
0.067
0.288
0.057
0.206
0.454
0.113
0.503
0.142
0.051
0.587
0.072
0.010
0.132
0.034
0.691
0.152
0.200
0.495
0.178
0.066
0.080
0.031
0.0.50
0.171
0.252
0.248
0.115
0.107
0.1 II
0.166
0.724
0.011
0.205
0.030
0.021
0.047
0.286
0.175
0.336
0.309
0.238
Table 6
Varimax factor score matrix for dead assemblages,
data. Significant values in bold
Micropaleontology 25 (I 995) 269-288
because of the exceptionally high dominance of S.fusiformis. Three varimax factors account for 82% of the
variance and have a clear distribution pattern (Fig. 4).
Factor 1, characterised by H. membranaceum, E. vitrea
and N. iridea, is restricted to the basin at depths greater
than 473 m (Table 5). Factor 2, comprising G. auriculata and S.fusiformis, is present on the shallower part
of the north and south slopes. Factor 3, P. osloensis
with subsidiary T. tenuissima and R. micacea, is found
only on the north slope between 285 and 427 m.
The dead assemblages (both 1992 and 1993 data)
also fall into three factor assemblages (Fig. 4; Table
6). These account for a higher proportion of the variance (91%) and show an even clearer pattern than for
the living. Factor 1, P. osloensis and T. tenuissima,
with subsidiary C. laevigata, is confined to the north
slope down to 473 m (Fig. 5). Factor 2, comprising H.
bradyi, Saccammina spp., E. medius, and T. pusillus,
is confined to the basin > 473 m. Factor 3, S.fusiformis,
occurs on the south slope down to 534 m.
1992 and 1993
Varimax factor score matrix
Variable
1
2
3
B. skagerrakensis
B. marginutu
C. lueoigutu
H. balthica
P. osloensis
S. fusiformis
E. medius
H. b&vi
T. pusillus
T. tenuissimu
Succammina spp.
0.225
0.036
0.301
0.068
0.765
0.026
0.118
0.115
0.102
0.472
0.084
0.253
0.006
0.146
0.014
0.100
0.027
0.403
0.620
0.385
0.172
0.423
0.125
0.000
0.016
0.016
0.018
0.988
0.030
0.036
0.025
0.069
0.002
59’00
zalina skagerrakensis (473-594 m) are abundant only
on the north slope. Eggerelloides medius and P. osloensis extend from the north slope into the basin, whereas
Haplophragmoides
bradyi, Trochamminopsis pusillus
and Saccammina spp. are abundant in the basin only at
depths > 534 m (Table 3 and Table 4).
58’30
58’oa
5.4. Assemblages
For the stained assemblages (only 1993 data), sample 67 was excluded from the varimax factor analysis
Fig. 5. Distribution of factor assemblages (empty tests) from 1992
and 1993 data. Diagonal lined area = factor 1, dotted area = factor 2,
squared area= factor 3.
E. Abe, J. W. Murray /Marine
80
Micropaleontology
25 (1995) 269-288
279
NW
SE
60-
01,
60
,,,(,,,,,,,,,,,,
59
71
72
61
58
Norwegian slope
Fig. 6. Relative abundance
5.5. Proportion
70
73
1
of stained (stippled line) and unstained
of agglutinated
74
62
57
63
68
Skagerrak Basin
75
1
64
67
65
66
Danish slope
(solid line) agglutinated tests ( 1993 data) projected onto a NW-SE profile.
tests
Whereas the stained assemblages show no clear pattern of distribution, and the values are highly variable
(Fig. 6; Table 2)) the proportions of dead agglutinated
tests show a significant increase with depth (r = 0.71,
II = 3 1, Table 3 and Table 4). Generally, the relative
abundance of agglutinated
forms is higher for the
stained than for the dead assemblages on both slopes,
whereas the contrary seems to be true for the basin.
5.6. Tubular and branching tests
Tubular or branching agglutinated forms are present
at all depths. The number of fragments depends to a
great extent on the fragility of different species. On the
slope the tubes are more fragmented so even though
the maximum number of tube fragments is approximately the same in both shallower and deeper water
samples (20-30 per 250 specimens of other taxa), the
absolute abundance of tubular or branching forms is
considerably higher in the deeper parts of the Skagerrak
Basin. Marsipella spiralis and Rhabdammina scabra
occur between 266 and 404 m water depth, whereas
Bathysiphon hirudinea occurs at all depths below 300
m. Larger, more robust, coarse grained tubes of Rhabdammina spp. are most common at water depths greater
than 500 m. These forms were excluded from diversity
calculations.
5.7. Diversity patterns
The alpha-values for the living assemblages increase
from around 6 on the north slope to 17 in the basin.
Anomalously low values are found on the south slope
(stations 65,67) due to high dominance of two species
(Fig. 7). For the dead assemblages the alpha-values
are high (all > 10) and variable with no clear trend
except that relatively lower values occur on the upper
part of the north slope compared to the basin and the
south slope.
The general pattern of H(S) for the living and the
dead assemblages basically follows that of the alphaindex with an increase from around I .9 and 2.5 on the
north slope to 3.1 and 3.2 in the basin, respectively.
E. Alve. J. W. Murray /Marine
280
Micropaleontology
25 (I 995) 269-288
6-
-1
4-
-c
Alpha-index
-0,5
2.-o.
0
I
60
H(S)
I
59
Dead
II
71
I
72
61
II,
58 70
Norwegian slope
Fig. 7. Distribution of Fisher-alpha (solid lines) and H(S)
onto a NW-SE profile ( 1993 data).
They also show anomalously
slope.
I
73
1
74
I
62
T
57
I
63
I
68
Skagerrak Basin
I
75
1
I
64
I
67
o
I
65
66
Danish slope
(stippled lines) diversity indices for live (stained)
low values on the south
5.8. Hiiglund’s ahta from 1937
In 1937, Hiiglund collected a series of cores from
the Skagerrak. Nine of these are from within the area
discussed here. The inner core diameter was 4.8 cm
and he studied the top 2 cm (sample volume 36 cm”).
He did not sieve the samples but removed the clay
fraction through careful decantation. This was repeated
until the samples were clean and the decanted water
was checked to ensure that there was no loss of foram-
and dead assemblages
projected
iniferal tests. He picked his samples wet and did not
differentiate
between living and dead individuals.
Because he should not have lost any fragile species
through drying and he probably included forms smaller
than 63 pm, his values should represent maximum
abundances compared to our data.
Hijglund did not publish his quantitative data, but,
by courtesy of Dr Tomas Cedhagen, we had access to
his counting sheets and fauna1 collection. Hijglundconcentrated on the taxonomy of the agglutinated forms.
He did not identify Epistominella vitrea (grouped as
rotaliids) which is relatively common in our material.
Other groupings of abundant species included Pullenia
E. Alve, .I. W. Murray /Marine Micropaleontology 2.5 (1995) 269-288
Table 7
Mean numerical density (no./cm3)
of important species (total =
live + dead populations) from the Norwegian slope, the Skagerrak
Basin, and the Danish slope. Comparison between 1937 (HGglund)
and 1992/1993 (present study) data
Area
Depth interval
(m)
No. of stations
B. skqerrakensis
C. luevigutu
E. me&s
Elphidium spp.
H. hrff&i
M. hdeeanum
N. idea
Pullenia spp.
R. micacrcr
Saccommina spp.
S.fusiffwmis
T. tenuissimcr
T. pwillus
Total/cm’
Agglutinated/cm’
Calcafeous/cm-’
Agglutinated
(%)
199211993
H-1937
1992/1993
H-1937
199211993
H-1937
1992/1993
H-1937
199211993
H-1937
1992/1993
H-1937
1992/ 1993
H-1937
1992/ 1993
H-1937
199211993
H-1937
199211993
H-1937
199211993
H-1937
199211993
H-1937
1992/1993
H-1937
1992/1993
H-1937
199211993
H-1937
1992/1993
H-1937
199211993
H-1937
1992/1993
H-1937
1992/1993
H-1937
Norwegian
slope
Skagerrak
Basin
Danish
slope
283473
242-500
13
5
11.1
3.4
9.9
25.0
1.7
13.0
1.8
3.2
3.3
2.2
2.4
7.0
4.5
2.0
22.7
7.3
4.2
7.6
1.5
0.0
6.9
0.4
II.2
4.6
0.8
0.0
55&666
520-700
12
3
6.9
2.2
5.2
7.0
10.0
2.7
0.9
0.3
14.0
3.7
2.6
2.0
6.8
3.1
13.6
5.4
1.2
0.3
7.8
0.0
6.6
0.3
0.5
0.1
9.2
0.0
266
201
1
3.5
0.0
29.5
19.0
19.7
18.0
136.7
190.0
3.5
0.0
0.0
0.0
4.3
1.1
28.8
1.8
26.9
9.0
0.0
0.0
410.0
83.0
67.4
56.7
0.0
0.0
111
116
36
33
75
83
33
31
122
46
61
16
61
30
52
36
1104
795
236
160
868
635
21
20
I
spp. and Elphidium spp. Because of this lack of differentiation it has not been possible to study the diversity
patterns or compare them with our data.
The following list includes names of important species which have changed since 1947 (genera according
to Loeblich and Tappan, 1987) :
281
Hoglund
This work
Bolivina robusta
Brizalina skagerrakensis
Bulimina fusiformis
Stainforthia fusiformis
Nonion labradoricum?
Nonionella iridea
Nonion umbilicatulum
Melonis barleeanum
Proteonina fusiformis
Reophax micacea
Trochammina pusilla
Trochamminopsis pusillus
Hoglund studied total (i.e. living plus dead) assemblages and, because the sample volume is known, it is
possible to express his results as number/cm”. We have
combined our living and dead data to make our results
comparable with his. The comparison is based on species making up 2 5% in two or more samples in either
data set. The absolute abundances are presented as
mean values for samples within three depth ranges corresponding with the Norwegian slope, the Skagerrak
Basin and the Danish slope (Table 7). Although the
depth ranges of Hoglund’s samples and ours are not
identical, they are similar.
The total abundance of tests per cm3 is remarkably
similar on the Norwegian slope with 111 in 1992/‘1993
and 116 in 1937 (Fig. 9). In the basin, the modem
abundance shows almost a threefold increase since
1937. An increase, although based on only one sample
from each data set, is also found on the Danish slope
(from 795 to 1104 per cm”).
6. Discussion
6.1. Modern stained abundance patterns
In general, the abundance of foraminifera is related
to the fertility of the area. In the southern North Sea the
average standing crop values are 2-10 with occasional
values > 10 per cm3 (Murray, 1992). These values are
slightly lower than those of the Norwegian slope and
Skagerrak Basin (average 18 and 11, respectively) and
significantly
lower than those of the Danish slope
(average 63). On the Norwegian slope at a depth of
304-3 10 m there is maximum abundance of 33-34 per
cm3 (Fig. 3). Similar high abundance of macrobenthos
was recorded by Aure et al. ( 1993) for annual sampling
in the same area (340-360 m water depth) during
1990-1992. The macrofaunal density at this depth
interval was about 2-5 times higher and the diversity
(information function) was lower than that of shallower and deeper areas. The assemblages were domi-
E. Alve, J. W. Murray /Marine
282
-y
Micropaleontology
25 (1995) 269-288
c
= 21.269 + 5.4165x R= 0.71646
II
,I
-800
-600
t
;
it+
n
-400
60
59
,
I
71
I
72
,
I
61
58
Norweglan slope
,
70
,
73
1
,
74
,
62
,
57
,
63
,
68
Skagerrak Sasln
,
75
1
,
64
,
87
,
65
86
Danish slope
Fig. 8. 1993 standing crop (stippled line) and absolute number of empty tests (solid line) projected onto a NW-SE profile. Inset graph shows
the linear correlation between the two.
nated by the polychaete Heteromastusfiliformis
which
often is abundant in organic rich environments.
All
these features were attributed to higher organic sedimentation.
The high standing crop values on the Danish slope
are probably related to the availability of organic material supplied by the Jutland Current and to the associated bacteria which provide an additional food source
for foraminifera. The area is also one of disturbance
from the activities of trawlers scouring the seabed for
prawns. This includes both physical disturbance of the
sediment surface and the consequent resuspension of
fine fractions, including organic matter. Some of this
may be transported downslope into deeper areas not
otherwise directly affected by the trawling. All these
factors may contribute to the greater numbers of living
foraminifera on the Danish slope.
The combination
of physical disturbance and frequent supply of organic matter may be particularly
favourable for opportunistic species as they can easily
recolonize and flourish in recently disturbed areas. One
such species is S. fusiformis (Alve, 1994) which is
dominant on the Danish slope and which probably
reproduces throughout the year in more southwestern
parts of the North Sea (Murray, 1992).
6.2. Comparison between modern stained and dead
assemblages
For almost all species, there are major differences in
living and dead abundances and distributions.
The
exceptions are P. osloensis and T. tenuissima which
both show similar high abundance and distribution only
on the north slope. Consequently, the single varimax
factor assemblage which is common to the living and
the dead is that of P. osloensis. However, overall the
boundaries between the stained factor assemblages and
those of the dead lie at very similar depths (Fig. 4).
Only one bloom of S. jiuiformis was sampled (station
67, Danish slope) but because it is the dominant species
in the dead assemblages, it is clear that it must flourish
here.
In the living assemblages, the fragile agglutinated
taxa R. micacea and H. membranaceum are common
on the slopes and in the basin, respectively, whereas
they are much reduced in abundance in the dead assem-
E. Abe.
*E
i
E
‘c
=r
B
*
J. W. Murray
/Marine
Microl,aleontolo~y
2.5 (1995)
60
60
30
30
6(
0
0
(
269-288
2x3
16l
Skagerrak Basin
Fig. 9. Comparison of average relative and absolute abundance
in I992/ I993 (this study) and I937 ( HGglund data).
of agglutinated
blages. Either these two taxa were unusually abundant
at the time of sampling or there has been postmortem
destruction of some of their tests. Likewise, the three
calcareous taxa G. auriculatu (a slope species), N.
iridea, and E. vitrea (both basin species) show reduced
dead abundances. Dissolution damage (hyaline walls
becoming white and etched and/or chamber breakage)
is evident in some specimens of these species and also
in other calcareous taxa such as B. skagerrakensis, C.
laeuigata, H. balthica, and Melonis barleeanum. However, other small taxa, such as S. fusiformis, do not
show these features. We conclude that some selective
tests and of total abundance
of tests between samples collected
dissolution may take place. This may explain the difference between the percent abundance of agglutinated
tests in the living and dead assemblages and why there
is an increase with depth only in the dead assemblages
(Fig. 6). On the other hand, there is a reasonably good
correlation (r = 0.72, n = 18) between the number of
living and dead tests per cm3 of sediment with the peaks
and troughs in phase (Fig. 8).
The factor analysis of both stained and dead assemblages show clear distributional
patterns (Fig. 4).
However, the marker species are different from those
284
E. Ah,
J. W. Murray / Murine Micropaleontology
of previous studies (e.g. Conradsen et al., 1994) due
to dissimilar size fractions.
Both alpha and H(S) show clear distribution patterns for the living and dead assemblages but they are
invariably lower in the living than in the dead assemblages on the slopes. From the hydrographic data
already discussed, it is known that the basin waters are
more tranquil than those of the slopes, which are under
the influence of various currents. This is also reflected
in the sandier nature of the slope sediments compared
with the muddy ones of the basin. Thus the slopes are
more disturbed environments and the dominant species
are mainly infaunal. For instance, the extremely low
diversity values on the Danish slope are due to the high
dominance of a single infaunal taxon in each of the two
samples (S.fisiformis
and G. auricuhta) . By contrast,
the tranquil basin supports a more diverse assemblage
with lower dominance including epifaunal suspension
feeders (e.g. Saccammina spp. and tubular agglutinated forms, although the latter were excluded from
the diversity calculations) and inferred vagile epifaunal
and shallow infaunal taxa (e.g. T. pusillus, H. bradyi).
Where the diversity values of the stained assemblages
are higher than those of the dead, this may indicate
some postmortem destruction of tests.
6.3. Comparison
with Hiiglund’s data from 1937
Abundance
From macrofaunal studies it has been suggested that
the Skagerrak area is undergoing eutrophication;
an
increase in the supply of organic matter has been
matched by an increase in the biomass and standing
crop of the macrofauna (Rosenberg et al., 1987; Josefson, 1990). Based on stained and dead foraminiferal
assemblages in box cores, two from our investigation
area but mainly from the eastern Skagerrak and northern Kattegat, Moodley et al. ( 1993) concluded that the
top 2 cm of sediment has fewer dead tests per unit
volume than the level representing 47 years ago. They
attributed this to increased predation by the macrofauna.
Moodley et al. ( 1993) argued that “Exaggeration
of foraminiferal densities in the 24-25 cm interval, as
a result of sediment compaction or the later contribution
of deep living Foraminifera to this fossil layer is repudiated because 1) the water content (calculated as per
cent of wet sediment)
exhibited limited variation
25 (1995) 269-288
between the different layers in individual cores ( ..) ,2)
higher densities were also encountered in the surface
layers in the Skagerrak in 1937” (referring to the only
station where quantitative data were given by Hbglund
in his 1947 work). Both arguments are hard to accept
because ( 1) the general trend in most marine sediments
(in particular muddy ones) is that the water content
decreases with increasing sediment depth due to compaction, and there is no reason to believe that Skagerrak
sediments
should deviate from this pattern. For
instance, a sediment core collected from 645 m depth
in the Skagerrak Basin had a water content of 74% at
2 cm depth and 57% at 25 cm (Paetzel et al., 1994).
This decrease of 23% from 2-25 cm represents a minimum for the total reduction from the surface to 25 cm,
as the water content of the surface O-2 cm undoubtedly
was higher than 74% (Paetzel, pers. commun., 1994).
(2) The single 1937 sample to which Moodley et al.
refer was from the south western part of the Skagerrak
(i.e. the Danish slope), whereas the majority of their
own samples were collected in the eastern parts. Our
results from the Skagerrak demonstrate strong variabilities in abundance from one regional area to another,
particularly between the main basin and the Danish
slope. Therefore, it is unlikely that these variations are
due to either patchiness or seasonal/annual
fluctuations. Consequently, we conclude that in the Skagerrak,
comparisons of temporal abundance changes should be
made only on samples from the same area.
The average total abundance on the Norwegian slope
does not seem to have altered since 1937 (Fig. 9). On
the other hand, the almost threefold increase in the
Skagerrak Basin indicates that the environmental conditions have changed here. To the authors’ knowledge,
no biological data are available from the middle and
western parts of the Skagerrak Basin to infer whether
or not the benthic macrofaunal biomass has increased
over the last 50 or 100 years. Consequently, it is not
possible to draw any conclusions about possible interactions between macrofaunal and foraminiferal abundances. However, it has been demonstrated through
several investigations that the Skagerrak Basin is one
of the major depositional areas in the whole North Sea
and acts as a sink for fine-grained sediments derived
partly from the southern North Sea (e.g. Van Weering
et al., 1987). Even though much of the organic material
is refractory it probably supports an abundant bacterial
flora, beneficial for some foraminifera. Additionally,
E. Abe, J. W. Murray /Marine Micropaleontology
the area has received an increasing amount of anthropogenically
induced nutrient salts which enter the
Skagerrak via the Jutland Current, the Baltic and from
adjacent countries (North Sea Task Force, 1993). A
reasonable conclusion is, therefore, that the abundance
of benthic foraminifera has risen in the deeper parts
( > 550 m) of the Skagerrak Basin over the last 50 to
60 years due to enhanced nutrient conditions. The
increased modern abundance compared to the 1937
values on the Danish side is noteworthy but no firm
conclusions can be drawn, as the comparison is based
on only one sample from each data set.
Assemblage
composition
Each of the total assemblages discussed here is made
up of the time-averaged successive dead assemblages
from a period of several years plus the modern living
assemblage at the time of sampling. The time-averaging process effectively eliminates any patchiness which
may have occurred either spatially or temporally.
Even though the average foraminiferal abundance
on the Norwegian slope does not seem to have changed
since 1937, the data sets indicate changes in the assemblage composition. Hoglund’s Norwegian slope samples were dominated by C. laeuigata, with subsidiary
E. medius, Pullenia spp., R. micacea and Hyalinea
balthica. By 199211993 Pullenia spp. had increased
and become dominant with subsidiary T. tenuissima
and S. fusiformis. The absolute abundance of C. laevigata has been more than halved whereas those of Pullenia spp. and T. tenuissima have been tripled and
almost doubled, respectively.
For the Danish slope, there is only one Hoglund
station but 3 stations from 1949 and 195 1 (Lange,
1956) with which we can compare our relative abundance data. At Lange’s station 24, close to Hoglund’s
station 10 and our station 66 (Fig. 2)) S. fusiformis was
only an accessory species, The absolute abundances of
Elphidium spp., T. tenuissima, and C. laevigata were
essentially the same in 1992/ 1993 as in 1937, but S.
fusiformis shows a fivefold increase (Table 7). We
believe this to be a real increase and not attributable to
the slightly different water depths as S. fusiformis is
known to dominate at shallower water depths as long
as the salinity is higher than about 30%0 (Alve, 1990).
Lange’s stations 26 and M5a, close to our station 53,
were dominated by B. skagerrakensis in 1949-1951
compared with S. fusiformis in 1992. Other changes
25 (1995) 269-288
include a significant reduction in relative abundance
285
of
H. balthica and C. laeuigata.
In 1937, the Skagerrak Basin was dominated by C.
Pullenia spp., N. labradorica, B. skagerrakensis, and H. bradyi (all calcareous, except the latter). By 1992/ 1993 the picture had
changed dramatically as the agglutinated H. brudyi had
become the dominant form and of the important taxa
from 1937 only Pullenia spp. was still frequent. On the
other hand, the agglutinated forms E. medius, Succammina spp. and T. pusillus had increased in both relative
and absolute abundance.
According to Hoglund ( 1947, p. 184), E. medius
had its maximum abundance between 150 and 250 m
water depth, and this is also reflected in the distribution
patterns represented by those of his stations that are
used in the present comparison (Table 7). The 19921
1993 data show, however, that its absolute frequency
in the Skagerrak Basin has tripled since 1937, indicating that it has moved its peak abundance towards water
depths greater than about 550 m.
A similar increase in the absolute frequency is evident for H. bradyi and T. pusillus but also for
B.skagerrakensis,
Pullenia spp., and S. fusiformis. Of
special interest is the fact that Hoglund found only
scattered individuals of T.pusillus (p. 202) in the Skagerrak in 1937 and it did not occur in any of his samples
used in this comparison. In contrast, it was one of the
most characteristic and common species in the Skagerrak Basin in 1992/1993. Other characteristic and
common species in the deep basin today are Succammina spp., which were not recorded at ail by Hoglund
in his Skagerrak material. It is highly unlikely that he
overlooked them as he focused particularly on the taxonomy of the agglutinated forms and was meticulous
with his work. Consequently, it seems that these forms
have only recently established themselves in the Skagerrak Basin.
Both the 1937 and the 199211993 data show a reasonably good correlation between the relative abundance of total (stained + dead) foraminiferal taxa and
water depth (r=0.66
for both; n = 9 for 1937 and
n = 3 1 for 199211993). Neither the absolute nor the
relative abundances of agglutinated taxa seem to have
changed significantly on the Norwegian and the Danish
slopes since 1937. For the Skagerrak Basin, however,
the situation is different, as the average relative abundance of agglutinated taxa shows an increase from 36%
laeuigata but with subsidiary
286
E. Abe, J. W. Murray/Marine
in 1937 to 52% in 199211993 and the absolute abundances show an increase from 16 to 61 individuals
(total) per cm3. A similar increase in the relative abundance of agglutinated taxa over the same time period
(based on sediment core data) was recorded by Moodley et al. (1993) in their 2 cores which were collected
in our investigation area (at 627 and 424 m of water).
However, they attributed these changes to represent
“local unfavourable conditions affecting primarily the
densities of calcareous Foraminifera”.
All the fauna1 changes enumerated above are possible indicators of modified environmental conditions in
the area over the last 50-60 years. These are particularly
obvious on the Danish slope and in the deep basin.
7. Summary
and conclusions
This study is based on 3 1 surface sediment (O-2 cm)
samples collected in 1992/ 1993 from the Norwegian
slope, Skagerrak Basin and Danish slope of the northwestern Skagerrak.
The TOC values are not very variable, whereas the
relative abundance of organic detritus > 63 pm shows
a pattern with low values in the basin and maximum
values on the Danish slope. Nevertheless, the oxygen
concentration of the bottom water is invariably high.
Stained (living) and unstained (dead) foraminiferal
assemblages ( > 63 pm) have been studied separately
in order to interpret ecology and taphonomy. The standing crop shows a patchy distribution of abundance with
the highest values on the Danish slope, in the area of
highest nutrient supply and probably maximum disturbance of the sediment surface due to trawling. There
are three living and three dead assemblages corresponding with the Norwegian slope, the Skagetrak Basin and
the Danish slope. However, the dominant species of
the living and dead assemblages are not the same in the
basin and on the Danish slope, possibly partly due to
dissolution of calcareous forms. Although there is no
clear distribution
pattern of the living agglutinated
abundance, there is a progressive depth related increase
in dead agglutinated tests which also may be explained
by dissolution effects. However, the data indicate that
the total amount of dissolution is modest. The highest
values of diversity are found in the stable environment
of the basin.
Micropaleontology 25 (1995) 269-288
Fauna1 comparisons between Hoglund’s data from
1937, Lange’s from 1949/1951 and ours from 1992/
1993 (all as total assemblages)
have shown major
changes to have occurred:
-Norwegian slope. There is no obvious change in
the absolute abundance of tests, but Pullenia osloensis
has replaced Cassidulina laeuigata as the dominant
species.
-Danish slope. An increase in absolute abundance
of tests represented principally by tests of Stuinforthia
fusiformis, probably related to a combination of an
opportunistic response to physical disturbance and an
elevated nutrient supply. Additionally, there has been
a significant reduction in both relative and absolute
abundance of C. laevigata and Hyalirzea balthica.
-Skagerrak Basin. This area has shown a threefold
increase in the number of tests and a marked increase
in the abundance of agglutinated tests especially of
Haplophragmoides bradyi, Trochamminopsispusillus,
Saccammina spp., and Eggerelloides medius. The latter
has changed its depth peak from 150-250 m in 1937 to
> 550 m. Neither Saccammina spp. nor T.pusillus were
found by Hoglund in 1937 so these taxa seem to be
new arrivals to the area.
These fauna1 changes, which are particularly obvious
on the Danish slope and in the deep basin, represent the
biological response to probable modifications of the
environment over the last 40-60 years. At present, we
can not identify the precise causes but, possibilities
which should be considered, are natural hydrographic
changes (such as varying intensity of water exchange
with adjacent areas) and anthropogenic
influences
(such as nutrient enrichment, due to sewage and fertiliser inputs, and physical disturbance through trawling).
Further studies targeting these aspects will be necessary. As a first step towards testing long term (last
couple of centuries) changes in the deep basin, the first
author is undertaking downcore analyses of the microfaunas. These analyses are linked with an absolute
chronology derived from isotopic datings.
Acknowledgements
First of all we sincerely thank Tomas Cedhagen for
kindly placing Hbglund’s raw data and sample material
at our disposal. We thank the Norwegian Geological
Survey (NGU) and the University of Bergen for pro-
E. Alve, J. W. Murray/Marine
Micropaleontology 2.5 (1995) 269-288
viding the 1992 samples and for the opportunity for
E.A. to participate in the 1993 cruise of the Htikon
Mosby. We are also grateful to Oddvar Longva at NGU
for the invitation to join the investigations in the Skagerrak; to NGU for 3 months funding for E.A.; to the
Department of Geology, University of Oslo for the use
of its facilities; to Per Ivar Steinsund for kindly providing the factor analysis program, to Lennart Sandberg
at Riksmuseet in Stockholm for the loan of type material from the Hoglund collection, to Joan Bernhard for
encouraging assistance during subsampling of the 1992
cores, to Brage Rygg for useful discussions, to Eigil
Whist for preparing some of the drawings, and to the
Institute of Marine Research, Bergen, for carrying out
the Winkler titrations. The referees, Anne Jennings and
Tjeerd van Weering, are acknowledged
for helpful
comments on the manuscript.
Appendix
A. Fauna1 reference list
The identification of most species has been checked with Hoglund’s collection. Generic names are in accordance with Loeblich
and Tappan ( 1987 1.
Bathysiphon hirudinea (Heron-Allen and Earland) = Hippocrepinella hirudinea Heron-Allen and Earland, 1932
Brizalina skagerrakensis (Qvale and Nigam) = Boliuina skagerrakensis Qvale and Nigam, 1985.
Cassidulina laevigata d’orbigny,
1826.
Eggerelloides medius (Hoglund) = Verneuilina media Hoglund,
1947.
Epistominella LaitreaParker, 1953.
Globobuliminu auriculata (Bailey) = Bulimina auriculata Bailey, 1851.
Haplophragmoides bradyi (Robertson) = Trochammina bradyi
Robertson, 189 I.
Huplophragmoides membranaceum Hoglund, 1947
Hyulinea balthica (Schroter) = Nautilusbalthicus Schroter, 1783.
Marsipella spiralis Heron-Allen and Earland, 1912
Melonis barleeanum (Williamson) = Nonionina barleeana Williamson, 1858
Nonionella iridea Heron-Allen and Earland, 1932
Pullenia osloensis Feyling-Hanssen,
1954
Reophar micacea (Cushman) = Proteonina micacea Cushman,
1918
Rhubdummina scabra Hoglund, 1947
Stainforthiafusiformis (Williamson) = Bulimina pupoides d’Orbigny var.fisiformis Williamson, 1858
Textularia tenuissima Earland, 1933
Trochamminopsis pusillus (Hoglund) = Trochammina pusilla
Hoglund, 1947
287
References
Alve, E., 1990. Variations in estuarine foraminiferal biofacies with
diminishing oxygen conditions in Drammensfjord, SE Norway.
In: C. Hemleben, M.A. Kaminski, W. Kuhnt and D.B. Scott
(Editors),
Paleoecology,
Biostratigraphy
and Taxonomy
of
Agglutinated Foraminifera. Kluwer, Dordrecht, pp. 66 l-694.
Alve, E., 1994. Opportunistic features of the foraminifer Stainforthia
jiisiformis (Williamson) : evidence from Frierfjord, Norway. J.
Micropalaeontol.,
13: 24.
Alve, E. and Murray, J.W., 1995. Experiments to determine the origin
and palaeoenvironmental
significance of agglutinated foraminifera1 assemblages. In: M.A. Kaminski, S. Geroch and M.A. Gasinski (Editors),
Proc. 4th hit. Workshop on Agglutinated
Foraminifera. (Krakow, Poland, September 12-19.1993.)
Grzybowski Found. Spec. Publ., 3: l-l I.
Anton, K.K., Liebezeit, G., Rudolph, C. and Wirth H., 1993. Origin,
distribution and accumulation of organic carbon in the Skagerrak.
Mar. Geol., 111: 287-297.
Aure, J. and Dahl, E., 1994. Oxygen, nutrients, carbon and water
exchange in the Skagetrak Basin. Cont. Shelf Res., 14: 965-977.
Aure, J., Dahl, E., Green, N., Magnusson, J., Moy, F., Pedersen, A.,
Rygg, B. and Walday, M., 1993. Langtidsovervaking
av trofiutviklingen i kystvannet langs Sor-Norge. .&rapport
1992. Rapp.,
528/93. (NIVA 2924) 99 pp.
Bakker, J.F. and Helder, W., 1993. Skagerrak (northeastern North
Sea) oxygen microprofiles and porewater chemistry in sediments. Mar. Geol., Ill: 299-321.
Conradsen, K., Bergsten, H., Knudsen, K.L., Nordberg, K. and Seidenkrantz, M.-S., 1994. Recent benthic foraminiferal distribution
in the Kattegat and Skagerrak, Scandinavia. Cushman Found.
Spec. Publ., 32: 53-68.
Corliss, B.H. and Van Weering, T.C.E., 1993. Living (stainecl) benthic foraminifera within surficial sediments of the Skagerrak.
Mar. Geol., I1 I: 323-335.
Eisma, D., 198 1. The mass balance of suspended matter and associated pollutants in the North Sea. Rapp. P. V. R&n. Cons. hit.
Explor. Mer. 181: 7-14.
Eisma, D. and Kalf, J., 1987. Dispersal, concentration and deposition
of suspended matter in the North Sea. J. Geol. Sot. London.,
144: 161-178.
Feyling-Hanssen,
R.W., 1964. Foraminifera
in Late Quaternary
deposits from the Oslofjord area. Norg. Geol. Unders., 225: 383
PP.
Feyling-Hanssen,
R.W., Jorgensen, J.A., Knudsen, K.L. and Lykke
Andersen, A.-L., 1971. Late Quatemary
Foraminifera
from
Vendsyssel, Denmark and Sandnes, Norway. Bull. Geol. Sot.
Denmark, 2 1: 67-3 17.
Fisher, R.A., Comet, AS. and Williams, C.B., 1943. The relationship
between the number of species and the number of individuals in
a random sample of an animal population. J. Anim. Ecol.. 12:
42-58.
Gabel, B., 1971. Die Foraminiferen
der Nordsee. Helgol. Wiss.
Meeresunters., 22: 165.
Hoglund, H., 1947. Foraminifera in the Gullmarfjord and the Skagerrak. Zool. Bidrag Uppsala, 26, 328 pp.
288
E. Alve, J. W. Murray /Marine Micropaleontolo,qy 25 (1995) 269-288
Imbrie, J. and Kipp, N.G., 197 I. A new micropaleontological
method
for quantitative paleoclimatology:
application to a late Pleistocene Caribbean core. In: K.K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale Univ. Press, New Haven, 3: 71-181.
Jarke, J., 1961. Die Beziehungen zwischen hydrographischen
Verhiiltnissen, Faziesentwicklung
und Foraminiferen-verbreitung
in
der heutigen Nordsee als vorbild filr die Verhiiltnisse wiihrend
der Miociin-Zeit. Meyniana, 10: 21-36.
Josefson. A.B., 1990. Increase in benthic biomass in the SkagerralKattegat during the 1970s and 1980s+ffects
of organic enrichment? Mar. Ecol. Prog. Ser., 66: 117-130.
Klovan, J.E. and lmbrie, J., I97 1. An algorithm and fortran IV program for large-scale Q-Mode factor analysis and calculation of
factor scores. J. Int. Assoc. Math. Geol., 3: 61-77.
Lange, W., 1956. Grundproben aus Skagerrak und Kattegat, mikrofaunistisch und sedimentpetrographisch
untersucht. Meyniana,
5: 51-86.
Larsson, A.M. and Rodhe, J., 1979. Hydrographical
and chemical
observations in the Skagerrak 1975-1977. Oceanogr. Inst., Gateborgs Univ., Rep., 29: I-40.
Ljoen, R., I98 I. On the exchange of deep waters in the Skagerrak
Basin. In: R. Sztre and M. Mork (Editors), The Nonvegian
Coastal Current. Univ. Bergen, pp. 340-356.
Loeblich, A.R., Jr. and Tappan, H., 1987. Foraminiferal Genera and
their Classification. Van Nostrand Reinhold, New York.
Moodley, L., Troelstra, S.R. and Van Weering, Tj.C.E., 1993. Benthic foraminiferal response to environmental change in the Skagerrak, northeastern North Sea. Sarsia, 78: 129- 139.
Mork, M., Saetre, R., Fumes, G., Wilborg, K.F. and Rey, F., 1976.
Some preliminary results from a synoptic experiment in the Norwegian coastal current (SEX 75). Rep., l/76, Geophys. Inst.,
Univ. Bergen, I1 I pp.
Murray, J.W., 1991. Ecology and Palaeoecology of Benthic Foraminifera. Longman, London, 397 pp.
Murray, J.W., 1992. Distributionand populationdynamicsofbenthic
foraminifera from the southern North Sea. J. Foraminiferal Res.,
22: 114-128.
Murray, J.W. and Alve, E., 1994. High diversity agglutinated foraminiferal assemblages from the NE Atlantic: dissolution experiments. Cushman Found. Spec. Publ., 32: 33-51,
North Sea Task Force, 1993. North Sea Subregion 8 Assessment
Rep. State Pollut. Control Authority (SIT), Oslo, pp. 1-79.
Paetzel, M., Schrader. H. and Bjerkli, K., 1994. Do decreased trace
metal concentrations in surficial Skagerrak sediments over the
last 15-30 years indicate decreased pollution? Environ. Pollut.,
84: 2 13-226.
Qvale, G. and Nigam, R., 1985. Boliuinaskagerrakensis, anew name
for Bolillina cf. B. robusta, with notes on its ecology and distribution. J. Foraminiferal Res., 15: 6-12.
Qvale, G. and Van Weering, T.C.E., 1985. Relationship of surface
sediments and benthic foraminiferal distribution patterns in the
Norwegian Channel (Northern North Sea). Mar. Micropaleontol., 9: 469-488.
Qvale, G., Markussen, B. and Thiede, J., 1984. Benthic foraminifers
in fjords: response to water masses. Nor. Geol. Tidsskr., 66: 32S332.
Rosenberg, R., Gray, J.S., Josefson, A.B. and Pearson, T.H., 1987.
Petersen’s benthic stations revisited. II Is the Oslofjord and eastern Skagerrak enriched? J. Exper. Mar. Biol. Ecol., 105: 219251.
Svansson, A., 1975. Physical and chemical oceanography
of the
Skagerrak and Kattegat 1. Open sea conditions. Rep., 1, Fishery
Board Sweden, Inst. Mar. Res.
Van Weering, T.C.E. and Qvale, G., 1983. Recent sediments and
foraminiferal distributions in the Skagetmk, northeastern North
Sea. Mar. Geol., 52: 75-99.
Van Weering, T.C.E., Berger, G.W. and Kalf, J., 1987. Recent sediment accumulation in the Skagetmk, northeastern North Sea.
Neth. J. Sea Res., 21: 177-189.