ICES Journal of Marine Science, 55: 1095–1111. 1998
Article No. jm980418
Plankton distributions in relation to physical oceanographic
features on the southern Malin Shelf, August 1996
R. J. Gowen, R. Raine, M. Dickey-Collas, and
M. White
Gowen, R. J., Raine, R., Dickey-Collas, M., and White, M. 1998. Plankton distributions in relation to physical oceanographic features on the southern Malin Shelf,
August 1996. – ICES Journal of Marine Science, 55: 1095–1111.
The distribution of plankton in relation to physical oceanographic features on the
southern Malin Shelf and in the North Channel of the Irish Sea was investigated
during August 1996. The near surface 13.5C isotherm and 35.20 isohaline marked the
location of the Islay and Irish coastal fronts, respectively. An additional salinity front
was observed to the south-east of Islay. A mergence of the Irish coastal front and the
salinity component of the Islay front suggested that, at certain times, a frontal
boundary separating coastal and oceanic water may extend from south-west Ireland
north along the western Scottish Shelf. The presence of highly saline (>35.35) water on
the outer shelf indicated the intrusion of Atlantic water.
The frontal boundaries influenced the distribution of plankton on the southern
Malin Shelf and in the North Channel. The outer shelf region was characterized by the
presence of Chaetoceros atlanticus, Gonyaulax polygramma, Oxytoxum scolopax,
Doliolum gegenbauri, Sagitta tasmanica, and S. maxima. Halosphaera minor and
Limacina retroversa were distributed between the shelf edge and the Irish coastal and
Islay fronts. Gyrodinium aureolum dominated the phytoplankton of the Islay front but
in contrast, Ceratium longipes was restricted to the east of the Islay Front. The distinct
spatial separation of species suggested that there was no exchange of southern Malin
Shelf and North Channel water at the time of the study.
Key words: Malin Shelf, plankton, physical oceanography.
Received 21 April 1997; accepted 29 July 1998.
R. J. Gowen, and M. Dickey-Collas: Agricultural and Environmental Science Division,
Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, BT9 5PX,
Northern Ireland. R. Raine, and M. White: Martin Ryan Institute, University College
Galway, Galway, Ireland. M. Dickey-Collas: University of Liverpool, School of Biological Sciences, PortErin Marine Laboratory, Isle of Man, IM9 6JA, UK. Correspondence
to R. J. Gowen: tel: +44 1232 255511; fax: +44 1232 38 244; email:
[email protected]
Introduction
The shelf seas of north-western Europe are characterized
by the presence of tidal mixing fronts in summer. These
fronts form the boundary between thermally stratified
and tidally-mixed waters, and their location can be
successfully predicted by the relationship between water
column depth and the amplitude of the tidal current
(Simpson and Hunter, 1974; Pingree et al., 1978;
Simpson and Pingree, 1978). In addition to these tidal
fronts, salinity fronts form boundaries between coastal
water and water of more oceanic characteristics
and have been observed off the coast of Norway (Orvik
and Mork, 1993) and the west coasts of Scotland (Ellett
1054–3139/98/061095+17 $30.00/0
and Edwards, 1983) and Ireland (Huang et al., 1991;
McMahon et al., 1995).
Distinct biogeographical patterns in the spatial and
temporal distributions of planktonic organisms have
often been associated with the seasonal development
of stratification. For example, regional differences in
phytoplankton production and standing crop (Pingree
et al., 1978; Joint and Pomroy, 1993; Gowen and
Bloomfield, 1996) and species composition (Holligan
et al., 1980; Jones and Gowen, 1990) have been
observed. Furthermore, intense blooms of dinoflagellates have frequently been associated with tidal mixing
fronts (Holligan, 1985). The distribution of certain
species of zooplankton has also been related to the
1096
R. J. Gowen et al.
Figure 1. A map of the study area showing the location of sampling stations and the positions of transects A, B, C, and D. The
dashed lines represent depth contours. The heavy solid lines are the approximate boundaries between the four regions (1–4) derived
from TWINSPAN analysis of the net flora collected at each station.
seasonal development of stratification. Williams et al.
(1994) suggested that the two calanoid copepods
Calanus finmarchicus (Gunnerus) and C. helgolandicus
(Claus) are more abundant in stratified waters.
Fronts may act as boundaries separating oceanic and
coastal species of phytoplankton (Raine et al., 1993).
Similarly, the distribution of chaetognaths has been
related to specific temperature and salinity regimes
(Fraser, 1952). Superimposed on these seasonal distributions are patterns which result from the circulation of
water on the northern European shelf. It has been
argued, for example, that the presence of C. finmarchicus
(Rees, 1949; Backhaus et al., 1994) and the Doliolid,
Doliolum nationalis (Lindley et al., 1990) in the
northern North Sea and C. finmarchicus in the Irish Sea
(Williamson, 1956) is the result of inflow of North
Atlantic and Malin Shelf water, respectively.
In the southern Malin Shelf two prominent oceanographic features have been observed. To the south-east
of the area the Islay front separates thermally stratified
water on the shelf from mixed water of the North
Channel (Simpson et al., 1979; Hill and Simpson, 1989).
The northern section of the Irish Shelf front is located to
the south-west of the Malin Shelf. With the exception of
earlier studies of the seasonal distribution of calanoid
copepods on the Malin Shelf (Rees, 1949; Mathews,
1969) and a limited study of phytoplankton in relation
to the Islay front (Pingree et al., 1978; Simpson et al.,
1979) little is known of the distribution and abundance
of plankton in this region. This paper describes the
results of a survey carried out in August 1996 to
investigate the distribution of plankton in relation to the
main physical oceanographic features of the southern
Malin Shelf.
Methods
A grid of stations was worked in the North Channel
and on the southern Malin Shelf (Fig. 1) between 5 and
10 August onboard the RV ‘‘Lough Foyle’’. At each
station, vertical profiles of temperature and conductivity
were recorded using a Hydro-Bios CTD. The CTD was
calibrated for temperature using mercury reversing
thermometers and for salinity by salinometer measurements on discrete water samples.
Near-surface (4 m) water samples were collected using
the ship’s clean seawater supply for the determination of
phytoplankton chlorophyll and dissolved inorganic
nitrate and nitrite. Chlorophyll concentration was estimated following the method of Tett (1987). Measurements of extracted chlorophyll fluorescence were made,
before and after acidification to distinguish pheopigments, using a Turner Designs Model 10 filter fluorometer. For the determination of dissolved inorganic
nitrate and nitrite, water samples were stored frozen for
Plankton distributions on the southern Malin Shelf
later analysis using a Bran and Luebbe segmented,
continuous-flow analyser (TRAACS) employing
automated colorimetric methods.
Phytoplankton samples were collected at each station
from both the ship’s clean seawater supply and using a
25 cm diameter plankton net fitted with a 20 ìm mesh
net. The net was lowered to a depth of 100 m, or 20 m
above the seabed whichever was shallower, and slowly
hauled to the surface. Phytoplankton samples were
preserved with acidic Lugol’s iodine or neutralized formaldehyde. For species identification and enumeration
of discrete water samples the sedimentation technique
described by Tett (1973) was used. Net-haul samples
were initially screened using an inverted microscope.
Subsequently, one drop was taken from each sample,
placed on a microscope slide and examined with a
conventional microscope. Species were recorded on a
‘‘presence or absence’’ basis. Multivariate statistical
analyses were carried out using Vespan III (Malloch,
1995), a PC based package which provides de-trended
correspondence analysis (DECORANA; Hill, 1979a)
which is used to plot ordination diagrams and two-way
species indicator analysis (TWINSPAN; Hill, 1979b)
which gives cluster diagrams. The data set used for the
analysis comprised 145 species from 49 stations.
Authorities for diatoms are given in Hasle and Syvertsen
(1996). For dinoflagellates authorities are given in
Dodge (1982) and Steidinger and Tangen (1996).
Zooplankton samples were collected from selected
stations using a modified, high-speed plankton sampler,
Gulf VII (Beverton and Tungate, 1967; Nash et al.,
1998), fitted with a 280 ìm mesh net. The sampler was
deployed in a double-oblique manner from the sea
surface to within 2 m of the sea-bed (or to a depth of
200 m where the sea-bed >200 m). Measurements of
water flow by internal and external flow meters were
recorded continuously during each tow. This allowed net
clogging to be monitored and the total volume filtered
by the net estimated. Samples were preserved in 4%
buffered formaldehyde. Species abundance for both
copepod adults and late copepodite stages, was determined by microscopic examination of subsamples
taken by volumetric subsampling using a 250 ml Schott
Stempel pipette flask. Copepod and chaetognath species
were identified following Farran (1951) and PierrotBults and Chidgey (1988), respectively. Abundance is
expressed as numbers of individuals m 2.
Results
Physical oceanography
Near-surface temperature ranged from 12.1C in the
North Channel to 14.8C at the western edge of the
study area (Fig. 2a). South of Islay there was a marked
horizontal gradient (ÄT=1.5C) with temperature
1097
increasing to the west. Lowest salinities (]34.3) were
recorded in the North Channel (Fig. 2b). Salinity
increased towards the north of Malin Head where there
was a strong gradient (34.8–35.2) between 710 and
740. West of Malin Head a band of 34.8–35.0 salinity
extended around the north-west coast of Ireland. This
was delineated seawards by a gradient across which
values rose to above 35.2. Maximum salinities of up to
35.39 were found at the shelf edge.
The water column was thermally stratified across the
entire southern Malin Shelf (Fig. 3a) with a thermocline
depth of 50 m. At the shelf edge a high-salinity core was
evident at a depth of 150–200 m. Within this core
salinity exceeded 35.43 (stations 24, 30, and 31; Figs 3b
and 3c). An intrusion of saline water (35.35) below the
thermocline and onto the shelf was clearly evident at
56N (Fig. 3c) but not at 5530N. Further on-shelf an
increase in the horizontal salinity gradient was observed
at Station 20 (Fig. 3b) which separated fresher coastal
water from shelf water. This salinity gradient was less
apparent in the section further north and away from the
coast (Fig. 3c).
Vertical sections of temperature across the southern
Malin Shelf show a clear delineation between thermally
stratified water to the west and mixed water to the east
(Figs 4 and 5). This frontal region, referred to as the
Islay front, was located close to Station 39 (Section C)
and Station 40 (Section D). The surface signature of the
Islay front, therefore, corresponded to the 13.5C isotherm (Fig. 2b). A sharp horizontal salinity gradient
from 34.75 to 35.25 was associated with the thermal
component of this front (Fig. 4b). Further east a second
salinity front (at Stations 3 and 4) separated waters of
the southern Malin Shelf and North Channel (Fig. 5b).
Chlorophyll and nitrate
Highest values of nitrate (up to 2.6 mmol m 3) were
measured in samples from the tidally-mixed waters of
the North Channel (Fig. 6a). A marked nutricline
(]1.2 mmol m 3) was associated with the salinity front
located to the south of Islay and elevated near-surface
nitrate concentrations (]1.3 mmol m 3) were
measured near the shelf edge. Maximum chlorophyll
concentrations (up to 12 mg m 3) were measured southwest of Islay in the region of the salinity front (Fig. 6b).
On the outer shelf and shelf edge chlorophyll values
were generally less than 0.5 mg m 3.
Plankton distributions
The maximum abundance of phytoplankton in nearsurface waters coincided with the location of the 13.5C
isotherm which has been previously identified as the
location of the Islay front (Fig. 7a). In this region, the
phytoplankton was dominated by dinoflagellates, in
1098
R. J. Gowen et al.
14
.0
13 5
.
13 80
.55
13
.30
12
.5
5
100 m
12
.30
12.55
55
14
.80
14
.55
150
0m
100
0m
500
m
100
Latitude (N)
m
56
5
14.0
0
14.3
14
.30
(a)
.5
.4
0
34
100 m
0
35
.1
35 0
.
34 00
.
34 90
.
34 80
.70
m
150
0m
100
0m
500
m
Latitude (N)
100
0
35.3
5
35.3
56
35
.20
(b)
10
34
55
9
8
7
Longitude (W)
6
5
Figure 2. The distribution of near-surface temperature (C) and salinity on the southern Malin Shelf and North Channel during
August 1996. (a) temperature, with a contour interval of 0.25C; (b) salinity, with a contour interval of 0.1. The dashed lines
represent depth contours.
particular Gyrodinium aureolum which exceeded
300 cells ml 1 (Fig. 7b). Total cell counts were markedly reduced in the North Channel and outer Malin
Shelf.
Cluster analysis of phytoplankton data
Species preferentials from the TWINSPAN analysis
showed five species to have a cosmopolitan distribution
across the study area, being present in >50% of the
samples examined. The species were the diatom Proboscia (Rhizosolenia) alata (79%) and the dinoflagellates
Ceratium furca (68%), C. fusus (75%), C. tripos (75%),
and Protoperidinium depressum (54%).
The first separation of the phytoplankton data set
(Fig. 8) divided the study area into an eastern and
western section with a boundary along the near-surface
35.1 isohaline. Diatoms with a benthic habit (for
example, Paralia sulcata and Navicula spp.) had high
scores at stations in the eastern section which probably
reflected re-suspension of bottom sediments in the shallower locations and in the vicinity of the North Channel
where tidal currents are strong. The distribution of the
diatom Ditylum brightwellii (Fig. 9a) and dinoflagellate
Ceratium longipes (Fig. 9b) are illustrative of those
diatoms and dinoflagellates restricted to the eastern
section. Other diatoms with high scores in the eastern
section included, Rhizosolenia setigera (70%), R.shrubsolei (53%), Stephanopyxis turris (56%), and Gyrosigma
spp. (70%). Of the dinoflagellates, Ceratium horridum
(70%), Dinophysis acuminata (78%), and Prorocentrum
micans (56%) were recorded frequently in the eastern
area. Species with high scores in the western area were
the diatom Leptocylindrus mediterraneus (58%) and the
chlorophyte Halosphaera minor (Fig. 9c) which was
recorded at all of the stations.
After two separations of the data four distinct groups
of stations could be identified (Fig. 8) with each group
Plankton distributions on the southern Malin Shelf
1099
Station
5 26 25
24
23
14.0
22
21
18
11.0
10.5
11.0
104
19
20
14.5
13.5
Depth (m)
10.5
203
10
.0
302
401
(a)
500
5
35.40
Depth (m)
104
35.0
0
35.30
35.30
35.35
35.10
35.20
35.43
203
302
35.43
401
500
5
0
10
27
28
21
29
31
52
62
Distance (km)
Station
32
33
83
72
93
34
35.40
103
10
35.30 35.25
35.3
5
35.43
203
41
30 31
35.43
104
Depth (m)
(b)
40
35.
302
40
35.
401
(c)
500
0
14
29
43
58
72
87
Distance (km)
101
116
130
145
Figure 3. The horizontal and vertical distribution of temperature (C) and salinity on the southern Malin Shelf during August 1996.
(a) temperature along transect A; (b) salinity along transect A; (c) salinity along transect B. The contour intervals are 0.5C for
temperature and 0.05 (with an additional contour at 35.43) for salinity. The dotted lines show the depth range over which data were
collected.
corresponding to a geographical region (1–4 in Fig. 1)
that could be characterized by a particular physical
regime (Table 1). Species preferentials for each group are
given in Table 2, although it should be stressed that the
score reflects differences between each pair after the
second separation. Thus, Leptocylindrus mediterraneus,
which had a high score in both groups 3 and 4, is listed
in group 2 as its presence reflects a difference between
these stations and those in group 1.
The highest species diversity was found in the stations
of group 1. For example, a total of 14 species of
Protoperidinium were recorded from all the stations in
the group. Group 1 could be divided geographically into
northern (Stations 2, 4, 5, 39–43) and southern (Stations
1100
R. J. Gowen et al.
5
35
37
12.8
12
.3
11.3
44
43
13.3
45
Depth (m)
41
12.3
13.8
85
Station
39
38
10.3
125
(a)
165
5
Depth (m)
34.60
34.50
34.70
34.80
.90
34
.20
.00
.10
35
35
35
35.30
45
85
35.40
125
(b)
165
5
Depth (m)
.3
26
26.5
26.7
26.9
45
26.1
27.1
85
125
(c)
165
0
16
33
49
65
82
98
Distance (km)
114
131
147
163
Figure 4. The distribution of temperature (C) salinity and density along transect C on the southern Malin Shelf during August
1996. (a) temperature, with a contour interval of 0.5C; (b) salinity, with a contour interval of 0.05; (c) density, with a contour
interval of 0.1. The dotted lines show the depth range over which data were collected.
1, 3, 44–51) sections. This was based on higher scores for
Cosinodiscus spp., Ceratium horridum, and Protoperidinium ovatum, P. curvipes, P. excentricum, and P. pyriforme in the southern section and higher scores for the
diatoms D. brightwellii, R. setigera, and T. nitzschiodes
in the northern section. Analysis of the flora from
stations associated with group 2 gave high scores for
Leptocylindrus mediterraneus, Pseudo-nitzschia delicatissima, and Halosphaera minor. High scores for the diatoms Rhizosolenia setigera and Paralia sulcata and the
dinoflagellates Amphidoma caudata, Gyrodinium aureolum together with two unidentified species of Gymnodinium were recorded from stations within group 3.
Stations associated with group 4 were notable for the
presence of species associated with warm temperate or
oceanic waters, including the diatom Thalassionema
frauenfeldii, the dinoflagellate Oxytoxum scolopax
(Fig. 9d) the coccolithophorid Daktylethra pirus, and a
filamentous species of the Cyanobacterium genus
Oscillatoria.
Plankton distributions on the southern Malin Shelf
13.4
.4
13
12.4
53
3
12.9
13.9
4
42
12.4
Station
40
12
13.4
5
13
1101
Depth (m)
10.9
101
148
196
Depth (m)
34.40
34.50
34.60
34.70
35
53
34
34..765
0
0
.2
0
.1
35
35
0
0
34.8
34.7
35.30
.0
0
34.9
0
(a)
244
5
34.80
101
148
196
(b)
244
5
26.0
26.4
26.5
53
26.6
26.2
26.1
26.9
Depth (m)
27.0
101
148
196
(c)
244
0
16
31
47
63
79
94
Distance (km)
110
126
142
157
Figure 5. The distribution of temperature (C) salinity and density along transect D on the southern Malin Shelf during August
1996. (a) temperature, with a contour interval of 0.5C; (b) salinity, with a contour interval of 0.05; (c) density with a contour
interval of 0.1. The dotted lines show the depth range over which data were collected.
Ordination analysis
Results of the DECORANA analysis are portrayed as
an ordination diagram of the first two axes (Fig. 10). In
this analysis the eigenvalues of the axes were 0.44 for
axis 1, 0.2 for axis 2, 0.16 for axis 3, and 0.11 for axis 4.
The first two axes represented the largest proportion of
the variation and so only these two have been plotted.
The stations show a marked spread along axis 1.
Stations clustered in group 1 had the lowest score rising
progressively to the highest score for stations in group 4.
Axis 2 showed much less variation. Three stations stand
out. Station 19 had the highest score on axis 2 which was
60 points higher than all other stations. Stations 8 and
R. J. Gowen et al.
1.0
0.5
(a)
1.0
1102
0.5
0.2
0m
100 m
1.
5
0.2
0.1
150
0m
100
0m
500
m
Latitude (N)
10
0.2
56
1.0
2.0
55
2.0
0.
10
2.
7.0
4.0
6.
0
2.
0
55
10
0
100 m
.0
7.0
m
150
0m
100
0m
500
m
Latitude (N)
100
0.5
56
1.0
1.0
2.0
4.0
0.5
(b)
2
9
8
7
Longitude (W)
6
5
Figure 6. The distribution of dissolved inorganic nitrate and nitrite and chlorophyll in near-surface waters of the southern Malin
Shelf and North Channel during August 1996. (a) nitrate (mmol m 3); (b) chlorophyll (mg m 3). The dashed lines represent depth
contours.
11 had scores close to zero. Axis 1 represents variation
based on near-surface salinity or the potential energy
anomaly (Phi, Simpson et al., 1979) which is a measure
of water column stability (Fig. 11) since correlation
between axis 1 scores and these two variables were high
with r2 values of 0.878 and 0.817, respectively. There was
little or no correlation between axis 1 score and nearsurface temperature, depth and distance from land
(Table 3).
Zooplankton
Analysis of the zooplankton data showed that certain
species had distinct geographical distributions within the
study area. The Thaliacean Doliolum gegenbauri (Ulj)
was confined to the oceanic and slope regions (Fig. 9e)
and the Pterapod Limacina retroversa (Fleming) was
distributed across the shelf but did not extend east of
Islay (Fig. 9f). Regions of high chaetognath abundance
(Fig. 12) were observed in the North Channel, dominated by Sagitta elegans Verrill, and near the shelf
edge. The latter was dominated by Sagitta tasmanica
(Thomson), whose distribution extended from the shelf
edge north-eastwards onto the shelf in a manner similar
to the incursion of warm saline water noted in Section B
(Fig. 3c). Low numbers of S. maxima (Conant) were
recorded at the shelf edge, and showed a distribution
similar to that of D. gegenbauri.
With the exception of the shelf region occupied by L.
retroversa planktonic copepods dominated the zooplankton of the southern Malin Shelf and North
Channel. Maximum abundances were found in the
mixed waters to the south and east of the Islay front and
in the North Channel (Fig. 13). Four species dominated
the copepod populations (Fig. 13). Acartia clausi
(Giesbrecht) was the most abundant throughout the
area of study with Pseudocalanus elongatus (Boeck) the
second most abundant. Maximum numbers of Temora
Plankton distributions on the southern Malin Shelf
1103
(a)
1.
5
15
10 0
0
25
5
500 m
100 m
100
10
50
150
0m
1000
m
10
Latitude (N)
0m
30
25 0
0
20
0
15
0
15
1
5
0
5
56
55
0
30
0
20
0
15
0
15
0
100 m
25
15
150
0m
100
0m
500 m
100
m
5
10
100
75
5
Latitude (N)
56
1
1
(b)
55
10
10
9
8
7
Longitude (W)
6
1
5
Figure 7. The spatial distribution of phytoplankton (cells ml 1) in near-surface waters of the southern Malin Shelf and North
Channel during August 1996. (a) total phytoplankton; (b) the dinoflagellate, Gyrodinium aureolum. The dashed lines represent
depth contours.
longicornis (Müller) (73.4103 individuals m 2) were
located to the east of the Islay front with <1000 individuals m 2 in waters of the shelf and shelf edge. Calanus
spp. was generally more abundant on the shelf with
fewer numbers in the North Channel and at the shelf
edge. The abundance of C. finmarchicus (Gunnerus) and
C. helgolandicus (Claus) were similar and there was no
clear spatial pattern to the distribution of either species.
Discussion
Previous studies of Malin Shelf oceanography have
generally concentrated on the coastal waters and fjords
of the south-west of Scotland (Pingree et al., 1978;
Simpson et al., 1979; Jones et al., 1995; Rippeth et al.,
1995) and the northern Malin Shelf (Ellett, 1979; Ellett
and Edwards, 1983; McKay et al., 1986). A principal
feature of the former is the Islay Front (Simpson et al.,
1979; Hill and Simpson, 1989). This front has a type I
salinity component (sensu Hill and Simpson, 1989) and a
type II tidal-mixing component separating thermally
stratified from vertically-mixed water. The location of
the front can be seen clearly in Figures 4 and 5 as the
boundary between stratified and mixed waters. During
this study the overall temperature and salinity structure
was similar to that measured in July 1983 but further
north at 5610N by Hill and Simpson (1989).
Observations have shown that the Irish Shelf front is
a consistent feature on the western Irish seaboard. This
is a type I salinity front which separates Irish coastal
water from water with more oceanic characteristics
(Huang et al., 1991; McMahon et al., 1995). The front
has been observed off the south-west of Ireland at 52N
(Raine and McMahon, 1998) and north-west Ireland
between 53–55N (Bowyer and Ward, 1996). In
addition, there is some evidence that the front is continuous between these two locations (Huang et al.,
1991). The front can be seen at the eastern end of
Section A shown in Figure 3b. Our results (Fig. 2b)
indicate that this salinity front merges into the salinity
component of the Islay Front north of Malin Head. This
confirms what has hitherto only been inferred from
satellite images (Bowyer and Ward, 1996). Furthermore,
a merger of the two fronts indicates that at certain times
a frontal boundary may extend from south-west Ireland
north along the western Irish and Scottish Shelves.
A northward-flowing slope current of the order 10 cm
s 1 exists along the western edge of the Malin and
Hebridean Shelves (Booth and Ellett, 1983; White and
1104
R. J. Gowen et al.
Figure 8. A cluster diagram derived from TWINSPAN analysis
of the phytoplankton data analysed by station.
Bowyer, 1997). A component of this current is a warm,
saline core situated between depths of 200 and 400 m
(Booth and Ellett, 1983; Hill and Mitchelson-Jacob,
1993) which is also evident in our data (Fig. 3b and c).
Previous observations of a tongue of high-salinity water
extending across the Malin Shelf (Ellett, 1979; McKay
et al., 1986) suggest that Atlantic water may come onto
the shelf in this region. Theoretical analysis of the
dynamics of a barotropic slope current (Hill, 1995) has
also raised the possibility of on-shelf leakage of a slope
current, due to an along-shelf increase in bottom slope.
Hill (1995) identified the Malin/Hebridean Shelf as an
area where this is likely to occur. The observations
presented here indicate an incursion of warm saline
water onto the shelf between Sections A and B (55.5N
and 56N) in accordance with Hill (1995) and observations of a drogued buoy which came on-shelf at 55.5N
(Booth and Meldrum, 1987) and a drifting mooring
array which came on-shelf near 56N (White and
Bowyer, 1997). Together these observations indicate
cross-shelf exchange of oceanic water in this region
which may also transport planktonic organisms.
The four groups of phytoplankton species derived
from multivariate analysis had a clear geographic delin-
eation and the significant correlations between axis 1
scores, Phi and salinity (Table 3) suggest that this
distribution was related to the temperature and salinity
structure, which in turn reflected the different physical
processes on the shelf. The observed decrease in the
diversity of diatom species with increased water column
stability is consistent with previous studies of the relationships between phytoplankton class dominance and
environmental conditions (Margalef, 1978; Bowman
et al., 1981; Jones and Gowen, 1990). The latter were,
however, based on quantitative measurements of algal
abundance and biomass rather than the qualitative
analysis of the net flora in this study. The coincidence of
the high score of Gyrodinium aureolum in group 2 (Table
2) with the abundance of this species measured quantitatively in samples from discrete water samples (Fig. 7b)
provides a measure of confidence in the qualitative data
set. The association between species more typical of
oceanic environments and the highest recorded salinities
together with the presence of species with a benthic habit
at low-salinity coastal stations is the most likely explanation for the high correlation between axis 1 scores and
salinity.
Species diversity was greatest at stations in the cool,
vertically-mixed waters to the east of the Malin shelf.
The diatoms characteristic of this region (group 1 in
Table 2) included large species such as Ditylum brightwellii, Guinardia striata (Rhizosolenia stolterfothii),
and Stephanopyrix turris, which in our experience are
common in nutrient-rich conditions such as may be
found during the spring bloom and, by inference, in
nutrient-rich, tidally-mixed waters. The dinoflagellates
Ceratium lineatum, C. longipes, and Dinophysis norwegica, present at these stations also tend to be more
abundant in cooler temperate zones (Dodge and
Marshall, 1994).
Gyrodinium aureolum was identified as an indicator
species for group 2 having a score higher than other
species in the group. Maximum abundance of this
species corresponded with the geographic location of
the Islay Front. This is not untypical for G. aureolum.
The tidal front off Ushant, for example, is known to
support large populations of G. aureolum in late summer
(Simpson and Pingree, 1978). Previous observations at
the Islay Front have only recorded small (2–5 mg m 3)
enhancements of chlorophyll (Pingree et al., 1978;
Simpson et al., 1979) and mixed diatom-dinoflagellate
populations (Simpson et al., 1979). The latter were based
on samples collected in June 1977, which is generally
earlier than the establishment of large populations of G.
aureolum in stratified shelf seas (Holligan, 1985; Raine
et al., 1993). There have been no descriptions of blooms
of this organism at the western Irish Sea front to date,
despite considerable research (Richardson et al., 1985;
Gowen et al., 1995). A bloom of G. aureolum was
recorded by Jones et al. (1982) in the Firth of Clyde but
Plankton distributions on the southern Malin Shelf
1105
56°
55°
(a)
Ditylum
brightwellii
(b)
Ceratium
longipes
(c)
Halosphaera
minor
(d)
Oxytoxum
scolopax
(e)
Doliolum
gegenbauri
(f)
Limacina
retroversa
Latitude (N)
56°
55°
56°
55°
10°
9°
8°
7°
6°
5° 10°
9°
Longitude (W)
8°
7°
6°
5°
Figure 9. Distributions of plankton species in southern Malin Shelf and North Channel waters during August 1996. (a) the Diatom
Ditylum brightwellii; (b) the Dinoflagellate Ceratium longipes; (c) the Chlorophyte Halosphaera minor; (d) the Dinoflagellate
Oxytoxum scolopax; (e) the Thalacian Doliolum gegenbauri; (f) the Pteropod Limacina retroversa. Presence at a station is denoted
by filled circles.
other than this there have been no observations of
blooms off the Scottish or northern Irish coasts prior to
the current study.
A notable feature of the outer shelf phytoplankton
community (group 4 in Table 2) was the presence of
three species reported to have an oceanic distribution.
These were the diatom Thalassiothrix frauenfeldii (Hasle
and Syversten, 1996), the dinoflagellate Oxytoxum scolopax (Doge, 1982; Steidinger and Tangen, 1996), and
the coccolithophorid Daktyethra pirus (Heimdal, 1993).
Warm temperate/sub-tropical species such as Ceratium
azoricum, C. arietinum, C. hexacanthum, and Podolampas palmipes are a regular feature of the summer,
plankton-net flora from waters immediately offshore of
the Irish shelf front off south-west Ireland (Raine and
Joyce, 1996). However, it is clear that the flora observed
in the net-hauls taken from the oceanic side of the Irish
Shelf front in this study was impoverished compared
with the warm temperate/sub-tropical species found to
the south-west of Ireland.
The reasons for this difference in the flora are unclear.
The physical environment on the oceanic side of the
Irish shelf front encountered during this study was
similar to the environment described by Raine and
McMahon (1998) for offshore waters south-west of
Ireland. Furthermore, samples taken off the south-west
of Ireland immediately before and after this study, all
contained a broad spectrum of the warm water species
(R. Raine, unpubl. obs.). The stations worked on the
outer shelf region of this study were ]250 km north of
those studied by Raine and McMahon (1998) and north
of the geographic feature of the Porcupine Bank. There
is a complex oceanography associated with the Bank
but whether this influences the northerly transport of
1106
R. J. Gowen et al.
Table I. The four groups of stations identified by the first two divisions of two-way indicator species
analysis (TWINSPAN) and some physical characteristics of the geographic region within which each
group of stations was located. The potential energy anomaly [Phi (ö) J m 3] is a measure of
water-column stability. Values <10 and >20 J m 3 denote mixed and stratified water, respectively
(Simpson et al., 1979).
Group
Stations
Physical characteristics
1
1–5, 39–51
2
6–9, 14, 16–19, 38
3
10–13, 15, 20, 21, 37
4
23–35
All stations located east of Malin Head
Near-surface temperature <13.5C
Stations mixed or weakly stratified: phi 0.5–27 J m 3 (mean 7)
Bounded to the east by the near-surface 13.5C isotherm
Bounded to the west by the near-surface 35.1 isohaline
Depth <100 m
Stations mixed or weakly stratified: phi 0.8–40 J m 3 (mean 27)
Near-surface salinity within the range 35.10–35.30
All stations thermally stratified: phi 31–87 J m 3 (mean 60)
Temperature gradient across the thermocline >3.0C
All stations located in the outer-shelf region
Near-surface salinity >35.30 (except Station 23; 35.25)
Depth >100 m
All stations thermally stratified: phi 69–109 J m 3 (mean 86)
Table II. Species preferentials for the four groups of stations generated by the first two divisions of two-way indicator species
analysis (TWINSPAN). Only species present at d50% of stations within a group are listed. Indicator species (denoted by an
asterisk) were those species with the highest score.
Group 1
Chaetoceros decipiens
Chaetoceros densus
Chaetoceros didymus
Chaetoceros difficile
Coscinodiscus radiatus
Ditylum brightwellii*
Guinardia flaccida
Guinardia striata
Stephanopyxis turris
Thalassionema nitzschiodes
Thalassiosira spp.
Ceratium lineatum
Ceratium longipes
Dinophysis acuta
Dinophysis norwegica
Dinophysis rotundatum
Protoperidinium curtipes
Protoperidinium excentricum
Protoperidinium ovatum
Protoperidinium pyriforme
Group 2
Group 3
Group 4
Chaetoceros breve
Chaetoceros danicus
Chaetoceros laciniosus
Cylindrotheca closterium
Leptocylindrus danicus
Leptocylindrus mediterraneus
Pseudo-nitzschia delicatissima
Paralia sulcata
Rhizosolenia setigera
Chaetoceros atlanticus*
Proboscia (Rhizosolenia) indica
Thalassionema frauenfeldii
Amphidoma caudata
Gyrodinium aureolum*
Scrippsiella spp.
Amphidoma caudata
Dinophysis rotundatum
Gymnodinium spp.
Gonyaulax polygramma
Halosphaera minor
plankton in oceanic waters to the west of the Irish Shelf
front has yet to be determined.
The identification of distinct geographical regions on
the Malin Shelf based on the distribution of phytoplankton is supported by the correspondence between the
distribution of certain zooplankton species (Fig. 9e and
f) and the physical structures. A number of species were
confined to the high-salinity waters of the shelf edge of
which the chaetognaths, S. maxima and S. tasmanica
Daktylethra pirus
Ocsillatoria spp.
have previously been recorded in waters to the west of
Scotland (Fraser, 1952). The occurrence of S. tasmanica
and Doliolum gegenbauri on the outer shelf corresponded to the distribution of warm, saline water on the
outer shelf, suggesting that these species were transported onto the shelf by the incursion of Atlantic water.
The Pteropod Limacina retroversa can be regarded as
a shelf species and has been observed to reach maximum
seasonal abundance in this location during August
Plankton distributions on the southern Malin Shelf
1107
250
Axis 2 score
200
150
100
50
0
50
100
150
200
Axis 1 score
250
300
350
Figure 10. An ordination diagram of the first two axes of the DECORANA analysis of the net phytoplankton from 49 stations.
Group 1, ; Group 2, ; Group 3, ; Group 4, .
350
Axis 1 score
300
250
200
150
100
50
0
34
34.5
35
35.5
Salinity
350
Axis 1 score
300
250
200
150
100
50
0
20
40
60
Phi (J m–3)
80
100
120
Figure 11. Plots of axis 1 DECORANA scores for the net phytoplankton samples against salinity and the potential energy anomaly
(Phi, J m 3). Values of Phi<10 and >20 J m 3 denote mixed and stratified-waters, respectively (Simpson et al., 1979). Group 1,
; Group 2, ; Group 3, ; Group 4, .
(Vane and Colebrook, 1962). To the east of Malin Head
and in the North Channel the dominant chaetognath
was Sagitta elegans which is common in more saline
coastal waters around the British Isles (Russel, 1932;
O’Brien, 1976) including the Irish Sea (Khan and
Williamson, 1970; Alvarez-Cadena, 1993), but rare in
the outer shelf region (Frazer, 1952). The absence of
chaetognaths from the frontal region is consistent with
observations made by Dickey-Collas et al. (1996) in the
Irish Sea, but is difficult to explain by physical processes
alone and requires further investigation to determine if
this is a characteristic of chaetognath ecology.
The distribution of planktonic copepods was generally
as expected. Maximum copepod abundance was associated with the region of elevated phytoplankton chlorophyll, to the south-east of Islay. Acartia clausi the most
abundant, is a late-summer/autumn dominant in coastal
and shelf seas around the British Isles (Fransz et al.,
1991) and higher abundances of T. longisima, a typical
neritic species (Herdman, 1918; Rees, 1949) were
restricted to the shallower coastal waters and North
Channel. The similarity in abundance and spatial distribution of the two species of Calanus is consistent with
continuous plankton recorder studies of the southern
1108
R. J. Gowen et al.
Table III. Values of the correlation coefficient (r2) between axis
1 scores resulting from DECORANA analysis of samples
against physical variables. Details are given in the text.
Correlation
coefficient
(r2)
Variable
Distance from shore (km)
Depth (m)
Temperature (C)
Salinity
Stratification parameter (Phi, J m 3)
0·163
0·228
0·139
0·878
0·817
Malin Shelf (Rees, 1949; Matthews, 1969) which has
been identified as a region of co-occurrence of the two
species (Planque and Fromentin, 1996).
It is notable that none of the shelf edge and shelf
species were observed to the south-east of the Islay front
and species characteristic of the North Channel and
south-eastern shelf region did not extend west of the
front. One inference which could be drawn from these
spatial distributions is that during the summer the
apparent continuity between the two frontal systems,
together with the salinity front to the southwest of Islay,
form a physical boundary which restricts the advection
(a)
Latitude (N)
56
50
200
100
400
600
55
800
10
9
8
7
6
5
7
6
5
6
5
(b)
400
10
0
50
0
20
50
10
50
0
Latitude (N)
20
0
30
100
0
56
55
10
9
8
2
8
5
Latitude (N)
2
56
5
(c)
2
55
10
9
8
7
Longitude (W)
Figure 12. Chaetognath abundance (ind. m 2) on the southern Malin Shelf and in the North Channel during August 1996.
(a) Sagitta elegans; (b) Sagitta tasmanica; (c) Sagitta maxima.
Latitude (N)
50.6
59.8
56
54.6
29.4
30.9
42.0
61.7
83.6
15.2
27.5
13.0
30.2
12.1
234.0
45.5
31.9
115.1
45.5
71.9
19.4
72.5
55
113.9
Total abundance
137.6 629.3
Latitude (N)
1.6
4.3
56
9.1
8.7
1.1
6.4
2.3
11.6
1.3
9.2
30.3
4.9
7.1
4.3
7.3
2.7
15.6
8.3
13.9
13.9
15.0
55
33.6
Pseudocalanus elongatus
64.7 139.4
Latitude (N)
0.3
0.0
56
1.3
2.7
0.0
24.7
0.0
0.2
6.2
0.1
4.1
0.3
73.4
18.5
9.3
39.1
0.3
21.0
25.8
0.2
22.6
55
24.5
Temora longicornis
2.5 88.5
Latitude (N)
43.4
0.5
7.7
56
1.0
2.9
24.7
17.3
3.3
40.6
6.1
1.2
87.3
2.1
2.2
0.3
5.4
63.0
50.7
23.9
27.7
0.2
30.3
55
48.0
Acartia clausi
58.9 347.8
Latitude (N)
0.5
26.5
56
32.0
10.6
0.8
23.1
2.3
14.6
12.7
0.6
1.6
16.4
25.6
4.7
7.0
1.9
3.5
1.5
2.2
3.0
1.0
55
2.2
Calanus spp.
10
9
8
7
3.5 17.3
6
5
Longitude (W)
Figure 13. Total copepod abundance (103 ind. m 2) and the abundance of the four dominant species Pseudocalanus elongatus,
Temora longicornis, Acartia clausi, and Calanus (finmarchicus and helgolandicus) in southern Malin Shelf and North Channel
waters during August 1996. Estimates of abundance were based on sampling the upper 200 m of the water column at those stations
where the depth >200 m.
1110
R. J. Gowen et al.
of species. This hypothesis has implications for the
potential inflow of Atlantic water and oceanic/shelf
species of plankton into the Irish sea. There is a general
net outflow of Irish Sea water through the North
Channel (Mckay et al., 1986; Brown and Gmitrowicz,
1996) although Ramster and Hill (1969) report a southward flow of water into the Irish Sea adjacent to the
Northern Ireland coast. However, tracks of sub-surface
drogues indicate that some of this southward flow may
be northward-flowing Irish Sea water which mixes
across the North Channel and flows south (Hill et al.,
1996). Our data suggest that inflow of Atlantic water
and the introduction of oceanic species of plankton into
the Irish Sea via the North Channel is unlikely during
the summer.
Acknowledgements
The support of colleagues during the cruise and the
assistance of the Captain, Officers and crew of the RV
‘‘Lough Foyle’’ is gratefully acknowledged. The authors
also thank G. McCullough for identification and
enumeration of copepod species and D. Wallace for
preparation of the figures. Crown copyright.
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