Journal of Plankton Research Vol.20 no 1 pp.85-103, 1998
Vertical distribution of zooplankton in the frontal zone of the Gulf
Stream and Labrador Current
Michael E.Vinogradov, Elvira A.Shushkina, Nikolay P.Nezlin and Genrikh
N.Arnautov
P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences,
36 Nakhimovskiy Avenue, Moscow, 117851, Russia
Abstract. Zooplankton data collected during September 1995 in the North West Atlantic at 41°39'N,
49°58'W (the location of the site of the 'Titanic' wreck) were analysed. The region investigated was
characterized by a very sharp frontal zone between the Gulf Stream and the main stream of the
Labrador Current. The total plankton biomass in the water column was very high. The macroplankton biomass values below the 600 m layer were significantly higher as compared with the similar values
measured before in other productive boreal regions of the Atlantic and Pacific oceans. A lot of dead
mesoplankton animals occurred in the deep layers. The reason was that the cold-water mesoplankton advected by the Labrador Current died off intensively within the deep layers of the frontal zone
and were used as a food resource by the macroplankton carnivores and scavengers that were very
abundant there.
Introduction
The frontal zone of the Gulf Stream is one of the most vigorous in the World
Ocean. It is classified with the climatic frontal zones of planetary scale (Fedorov,
1983). It can be traced from surface to near-bottom layers. The huge extension
of the frontal zone and the alternative nature of the central and the subarctic
water masses being in contact there led Stommel (1958) to suggest that all the
system of Gulf Stream currents is a phenomenon of secondary order resulting
from the zone of contiguity of these waters.
The frontal zone at the northern boundary of the Gulf Stream is very sharp and
narrow near the surface. According to Baranov (1988), its width at the surface
could be as narrow as 1 km and the surface temperature could change from 9 to
18°C. The frontal zone is inclined towards the direction of the Sargasso Sea with
an angle of 0.33-0.50', in some seasons up to 2°30' (Baranov, 1988). Below the
depth of 600 m, the frontal zone is traced better by oxygen concentration than by
temperature and salinity (Baranov, 1988; see also Figure 2).
During cruise 37 of R/V 'Akademik Mstislav Keldysh', a small area of size 6 x 6
nautical miles with the centre at 41°39'N, 49°58"W was investigated (from 8 to 22
September 1995). This is the location of the 'Titanic' wreck. The area investigated
was located directly in the sharp frontal zone between the deep stream of cold
enriched by plankton waters of the Labrador Current and the waters of the Gulf
Stream characterized by low productivity. The frontal zone is extremely sharp
there. As observed by Baranov (1988) at 50°W, the gradient of surface temperature was 2-3 times higher than both westward and eastward (at 60°W and 40°W),
especially in September-October. This region is the deep stream of the Labrador
Current, and exactly there icebergs that concentrate near the frontal zone
penetrate especially far southward. Evidently, the collision between the 'Titanic'
© Oxford University Press
85
M.E.Vinogradov el al.
and the iceberg there was by no means a rare event. After the 'knock' of the
Labrador Current, the Gulf Stream divides into the set of branches (the delta of
the Gulf Stream).
During the whole period of investigations, the underwater operations were
carried out by manned submersibles at the sunk hull of the 'Titanic'. That is why
the vessel maintained its position, while the frontal zone moved, the cold
(Labrador) water changing to the warm (Gulf Stream) one, and vice versa.
The most intensive southward flow of arctic waters by the Labrador Current
occurs during autumn months exactly along 50°W (Baranov, 1988). This phenomenon is evident from remote observations. Figure 1 represents the mean September distribution of surface phytoplankton pigment concentration averaged
over the period of 8 years (1978-1986) obtained by Coastal Zone Colour Scanner
(CZCS) radiometer at the 'Nimbus-7' satellite. The permanent tongue of waters
rich with chlorophyll is evident along 50°W. The frontal zone is not narrow at the
mapped averaged long-term data, it results from the unsteadiness of the Gulf
Stream, which changes its position with time; hence, the resulting image appears
to be smoothed. At the same time, during field observations a sharp change in
water colour was clearly seen by the unaided eye. From Figure 1, we notice that
58H
-70PW
Longitude
Fig. 1. CZCS surface plant pigment distribution in September, averaged over the period 1978-86. The
colour coding is given as mg rrr3.
86
Vertical distribution of zooplankton
the area investigated was located near the southward tongue of waters enriched
by chlorophyll; the latter constitute the main stream of the Labrador Current.
The contact of two water masses of such different genesis inevitably results in
cardinal differences between the communities of pelagic organisms in both directions from the frontal zone. In the present paper, we made an effort to evaluate
these changes through the whole water column, from the surface to the nearbottom layers.
Method
Hydrographic data were obtained with a 'Neil-Brown' CTD. Twenty-seven
hydrocasts were conducted from the surface to the bottom. The values of temperature, salinity, density, oxygen concentration and turbidity were obtained. The
overall productivity level of the investigated region was evaluated by the climatic
monthly images in surface chlorophyll concentrations obtained by CZCS
radiometer at the 'Nimbus-7' satellite (Feldman et ai, 1989), Level 3 files,
received from Goddard Space Flight Centre.
The vertical distribution and taxonomic composition of mesozooplankton
were investigated with the help of large water bottles and vertical closing plankton nets. The zooplankton were divided into three size groups: small 'bottle'
mesoplankton (0.2-3.0 mm length, chaetognaths up to 5 mm), net mesoplankton (3.0-30 mm, chaetognaths up to 50 mm) and net macroplankton (3.0-10 cm).
Bottle mesoplankton were collected by 150-180 1 water bottles. The total water
column was sampled with special emphasis on the near-bottom layers. A series
of 25 samples of large water bottles from the surface to the bottom (3700-3800
m deep) was made: 0, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, 500, 600, 750,
1000, 1300, 1500, 1800, 2000, 3200, 3240, 3540, 3640, 3690 and 3735 m. The last
five samples were collected from the depths of 500, 200,100, 50 and 5 m above
the bottom, respectively. Acoustical pinger 'Benthos' was used to achieve such
a depth accurately. The series within the 0-600 m layer was sampled twice
(during the day and at night-time) in order to estimate diel changes. The first
0-600 m series was obtained on 9 September during 14:00-20:00 h; the second
one on 12-13 September during 22:00-2:00 h. After the bottle was brought on
deck, samples were thoroughly wet sieved through a 64 um mesh and preserved
in 2% neutral seawater-formalin solution. Before preservation, the samples
were stained with neutral red dye with the aim of distinguishing live and dead
animals.
The vertical distribution of net mesoplankton was obtained from stepwise
vertical hauls by closing BR 113/140 nets with 1.0 m2 mouth opening area and 530
um mesh. Ascent speeds were 0.9-1.0 m s"1. The angles of wire resulting from
vessel drift ranged from 0 to 30°. The water column was sampled twice over 12
depth strata: 0-25, 25-100, 100-200, 200-400, 400-600, 600-1000, 1000-1300,
1300-1800, 1800-2300, 2300-2900, 2900-3500 and 3500-3670 m (3557-3774 m in
the second series). In order to evaluate the diel changes in plankton distribution,
the samples of the first series from the 0-600 m layer were collected during the
87
M.E.Vinogradov et al.
daytime (8 September, 14:00-18:00 h), and the samples of the second series
during the night-time (8 September, 21:00-23:00 h). The samples were also
preserved in a 2% neutral seawater-formalin solution.
In addition, the near-bottom macroplankton were collected by traps with bait
(fish tissue). The traps were equipped with an acoustic unit which disconnected
it from the weight and enabled it to float to the surface. They were exposed at the
bottom during 5 days.
In the samples collected by plankton nets, the macroplankton and large size
mesoplankton animals (chaetognaths, decapods, mysids, hyperiids, medusae,
etc.) were picked out first. They were counted, measured and weighed by assay
balance. Then the samples collected by both large water bottles and plankton
nets were analysed by a unified method. The animals in the samples were
counted using a binocular amplifier at 8 X 4 or 8 X 2 magnification. All large
or rarely occurring animals were counted and measured using an ocular micrometer. In the samples containing few plankton, all the animals were divided into
taxonomic and size groups, counted and measured. In rich samples, a 1/2, 1/10
or 1/20 aliquot of the total sample was taken with a Stempel pipette and analysed
by the same procedure. The weight of each measured specimen was calculated,
taking into account its body length and shape, with the help of Chislenko's
(1968) regression equations or specific empirical equations. The organic carbon
content in the tissues of plankton animals was determined by the method of
bichromate liquation. Then the obtained conversion coefficients were used to
recalculate the wet weight biomass values as Corgones. Then, the weights of individual animals were summed using the special computer program PLANKTY.
This program for IBM-compatible computers is used at the P.P.Shirshov Institute of Oceanology RAS for typing in the results, of treatment of plankton
samples (numbers of each species and measured body length values) and for
calculations of required structural characteristics of a planktonic community
(the wet and carbon biomasses of each species, each group of species, and the
total biomass).
In addition, the detritus particles were calculated and measured in the large
bottle samples and the biomass of detritus [particulate organic carbon (POC)]
was estimated in the same manner as zooplankton biomass.
The data obtained with the help of a combination of the above-listed sampling
equipment give a rather adequate pattern of the vertical distribution of mesoplankton and reliable absolute values of its biomass. This is evident from numerous comparisons of the results obtained by different types of sampling equipment
(Shushkina et al., 1980; Vinogradov et al., 1987; Tutubalin et al., 1987; Musaeva
and Nezlin, 1995), and especially from the results of direct counts of meso- and
macroplankton animals from submersibles (Vinogradov and Shushkina, 1994;
Vinogradov et al., 1995, 1996). At the same time, the accuracy of the biomasses
of macroplankton animals, especially shrimps of size >5 cm, obtained by nets with
1.0 m2 mouth opening area, seems to be underestimated. Hence, these values
should be analysed as a sort of rough approximation.
88
Vertical distribution of zooplankton
Temperature ("CI)
-50
5 10 15 20
Oxygen (Mml"1)
Salinity (ppt)
323334353637
4
5
6 7
8
9
500 -
1000-
1500 -
2000-
B
2500 -I
Fig. 2. Profiles of the vertical distribution of temperature (A), salinity (B) and oxygen concentration
(C) at Station 3606 (9 September 1995; solid line) and Station 3618 (13 September 1995; dashed line).
Results
The surface temperature in the investigated area ( 6 x 6 nautical miles) changed
from 15.2 to 22.7°C. Sometimes the surface temperature was >20°C, and sargasso
seaweeds with their typical tropical fauna were floating at the surface; at the same
time at the depth of 40-150 m (sometimes up to 35 m), the water temperature was
about zero or below it (down to -1.3°C). Cold water layers were interleaved with
the warmer ones within the depth range from the surface to 500-600 m (Figure 2).
Figure 2 illustrates the vertical profiles of the main hydrological properties at
two stations where the series of bottle samples within the 0-600 m layer were
collected. The first series was collected on 9 September during the daytime, and
the second series was collected on 13 September during the night-time. It is
obvious that on 9 September the region of investigations was occupied by cold
waters of the Labrador Current, and on 13 September these waters were changed
to the warm ones of the Gulf Stream. The salinity of the Gulf Stream waters
within the upper 0-600 m layer was much higher and the oxygen concentration
was much lower. It should be noted that the temperature at the surface changed
insignificantly as compared with the changes in temperature, salinity, and oxygen
concentration within the 0-600 m layer.
Plankton biomass distribution in the frontal zone
The vertical distribution of plankton biomass from the surface to the 200-600 m
layer depends from what side of the frontal zone (warm or cold) the samples were
collected. Thus, in the 0-200 m layer, the plankton biomass varied three times
(Table I), as measured by both net and bottle samples. In the 0-600 m layer, the
89
M.E.Vuiogradov et al.
Table L The stock of mesozooplankton, wet weight in g nr 2 and g C n r 2 (in parentheses) over
different depth ranges measured by both large water bottle and net samplers. Bz is the total biomass
of the sample, Bm that of animals 0.2-3 mm and flg that of animals 3-30 mm
Sampled layer (m)
0-200
600-3700 (bottom)
0-600
Measured by large water bottles
Labrador side of the
frontal zone (day)
Gulf Stream side of the
frontal zone (night)
Deep water
(600-3700 m)
Measured by BR net
Labrador side of the
frontal zone (night)
Gulf Stream side of the
fiz
Bm
Bz
Bm
27.9
(1.92)
9.1
(0.58)
23.3
(1.63)
7.2
(0-47)
38.6
(2.63)
20.8
(1.29)
30.9
(2.15)
15.1
(0.94)
B,
Bi
Bz
36.8
(251)
13.5
32.1
(222)
123
516
(3.43)
34.3
6
Bz
Bm
Two series of samples
averaged
47.2
(3.24)
12.6
(0.88)
B
B%
59.3
(3.33)
57.7
(3.22)
z
46.8
(3.03)
31.2
frontal zone (day)
(0.84)
(0.77)
(205)
(1.84)
Deep water
(600-3700 m)
Total biomass from the samples of BR net and large water bottles (Bm + Bg)
Labrador side of the
frontal zone
55.4(3.85)
77.7(5.18)
Gulf Stream side of the
frontal zone
19.5(1.24)
46.3(278)
Deep water
(600-3700 m)
70.3(4.1)
difference was 1.5-1.8 times (Table I, Figures 3A and 4A). However, the plankton
net samples were collected in the cold water during the night-time and in the
warm water during the daytime. Thus, the biomass increase in the upper layers
might be affected by the night ascent of plankton animals. On the contrary, the
series of large bottle samples were collected in the cold water during the daytime
and in the warm water during the night-time. Nevertheless, the ratio between the
biomass in cold and warm waters was in this case similar to that determined by
the BR net.
The ratio between the biomass of small (<3 mm) zooplankton measured by
large bottle samplers and the biomass of large (>3 mm) zooplankton measured
by BR net was nearly equal within the 0-200 and 0-600 m layers. Namely, 'bottle'
zooplankton were 40-42% of the total at the cold side and 33-37% at the warm
side of the frontal zone. A slight increase in the percentage of small zooplankton
in the cold water was caused by a high percentage of Oithona and juveniles of
Calanoida. Small plankton in warm waters were dominated by Appendicularia
and Oncaea.
In the deep waters below 600 m down to 3600 (3700) m (bottom), the biomass
of small mesozooplankton (Bm) was twice lower than the biomass in the 0-600
90
Vertical distribution of zooplankton
Biomass (mg m" )
100
200
300
0
100
200
B
3000
3500
Fig. 3. Vertical distribution of the mesozooplankton biomass measured by a 1501 water bottle sampler.
The fraction of large animals (>3 mm) is shown by hatching. (A) Cold water, 0-600 m; (B) warm
water, 0-600 m; (C) 600-3600 m.
m layer (cold water), and the biomass of large mesozooplankton (Bg) was slightly
higher (Table I). The percentage of small (<3 mm) mesozooplankton deeper than
600 m (Bm in the 600-3700 m layer; Table I) was lower than the similar value in
the 0-600 m layer (Bm in the 0-600 m layer; Table I); it was -18% of the total
biomass. In the region investigated, the ratio between the biomasses of the
surface (0-600 m) plankton and the deep (600-3700 m) plankton differed from
the values typical for other regions of the ocean. Usually, the total value of net
mesozooplankton biomass (Bz) in the 0-500 m layer is on average 2/3 of the total
biomass in the 0-4000 m layer (Vinogradov, 1968), although in the area investigated it was less than 1/2 (52.6 and 111.9 g m"2 from the cold side, and 34.3 and
93.6 g m~2 from the warm side; Table I). The reason seems to be that the deepwater plankton were advected by the Labrador Current from the cold regions of
the Atlantic Ocean, where the plankton biomass in the whole water column was
essentially higher than that in the frontal zone in the region investigated.
91
M.E.Vinogradov et al.
Biomass (mgnr 3 )
100
0
100
200
300 100
0
100 200
0
J
100
200
300
400
500
600
4 0 2 0 0 2 0 4 0 6 0 8 0 8
4
100 200
^
"
"
0
4
8
I
"
•
400 500600 -
10 8
6
4
2
0
2
4
6
8
10
0 -,
100 200 300 -
E
400 500600-
Fig. 4. Vertical distribution of the mesozooplankton biomass measured by BR net in the 0-600 m
layer. The biomass during night-time on the cold side of the frontal zone is depicted to the right, the
biomass during the daytime on the warm side of the frontal zone is depicted to the left. (A) Total
biomass; (B) Calanusfinmarchicus;(C) Metridia (M.longa + M.lucens); (D) Pareuchaeta sp.; (E)
Euphausiaceae.
The total stock of mesozooplankton (Bm + B^) in the investigated region in the
total water column from surface to bottom (-3700 m) was 148 g nr 2 if plankton
of the upper 600 m layer were measured from the cold side of the frontal zone,
and 117 g nr 2 if plankton were measured from the warm side of the frontal zone.
The stock of small plankton (<3 mm) in the total water column was 43.5 g m~2,
the stock of large (>3 mm) plankton was 104.5 g nr 2 . In the total water column
of cold waters, the biomass of mesozooplankton (measured as wet weight) was
1.3 times greater than the biomass in the warm waters. In terms of carbon units,
this value was 1.35.
Thus, the mesozooplankton of the Gulf Stream frontal zone were rather rich.
Their biomass was slightly lower than that in the highly productive north-western
part of the Pacific Ocean (Kuril-Kamchatka region). In the latter, the average
biomass measured by plankton net in the 0-4000 m layer was 157 g nr 2 (Vinogradov, 1968). In the Norwegian Sea in the 0-1700 m layer, the sum value of
92
Vertical distribution of zooplankton
plankton biomass measured by plankton nets and large bottles was 170 g nr 2
(Vinogradov et ai, 1995).
Vertical distribution ofmesozooplankton
In the upper 600 m layer, the contrast between the oceanographic characteristics
of the Gulf Stream and the Labrador Current was extremely sharp. Their interaction was reflected by rapidly fluctuating with time pattern of cold and warm
.ayers interleave. Thus, the composition, biomass and vertical distribution of
plankton also varied with time. Regular diel migrations of some common species
made a contribution to these variations. They were most pronounced in the
richest layer from the surface to the depth of 50-100 m. At the warm side of the
frontal zone, the upper 25 m layer was formed by the water of the Gulf Stream.
From time to time, the Gulf Stream waters covered all the investigated region
and the presence of subtropical fauna was particularly striking. The plankton
consisted of copepods (Corycaeidae, Nanocalanus minor, Paracalanus, Clausicalanus, Calocalanus pavo, Mecynocera, tropical Eucalanidae), Hyperiidae
(Lestrigonus and Hyperietta), colonial radiolaria Colozoum, etc. At the cold side
of the frontal zone, these species were almost absent.
To both sides of the frontal zone, the boreal species Calanus finmarchicus,
Metridia longa, Metridia lucens and Sagitta elegans dominated in the surface layer,
especially during night-time. Cold water with a temperature near or below zero
in the 35-150 m layer was densely inhabited by these species. In addition, typical
representatives of arctic-boreal plankton Aglantha digitate, Themisto compressa,
Clione limacina, Limacina retroversa, etc., appeared. In the 100-200 m layer, the
typical arctic species Calanus hyperboreus was found, its biomass increased with
depth and became maximum (31.2 mg nr 3 ) in the 1800-2300 m layer. The
biomass of this species was even higher than that measured in the open part of
the Norwegian Sea (Vinogradov etai, 1995), where this species is very important
as a source of food for planktivorous fish.
The two series of net plankton samples on 8-9 September in the 0-600 m layer
during day and night reflected both water change and diel migrations of plankton. At night, the mesozooplankton biomass in the upper 600 m layer was about
twice as high at the cold side of the frontal zone than the biomass at the warm
side of the frontal zone (52.9 and 35.6 g nr 2 , respectively). The highest plankton
concentration was at night-time in the 25-100 m layer (>200 mg nr 3 ) and during
the daytime in the 100-200 m layer (-80 mg nr 3 ; Figure 4A).
The highest concentrations of the most common species (C.finmarchicus)
during both day- and night-time were in the 25-100 m layer. However, its biomass
in this layer exceeded 175 mg nr 3 in the cold water and never reached 30 mg nr 3
in the warm water (Figure 4B). The second most common group consisted of two
arctic-boreal species (M.longa and M.lucens). Unlike C.finmarchicus, these
species performed well-defined diel migrations (Figure 4C). They concentrated
within the 100-200 m layer during the daytime and within the 25-200 m layer
during the night-time. The copepods of the Pareuchaeta group (Figure 4D) and
euphausiids (Figure 4E) demonstrated evident diel migrations.
93
0.9
2.2
0
0
0
0
Themislo compressa
Euphausiaceae
(Acanthephyra, etc.)
Sagitta maxima
Cyclothone
Total macroplankton
biomass
125.2 172.4
Total mesoplankton
biomass
0
0
0
0
19.3
3.2
4.1
0
4.1
0
11.2
1.7
93.2
50.1
12.1
36.0
4.4
0.2
13.6
2.1
82.4 100.5
0
0
4.8
1.3
2.0
3.9
Calanusfinm archicus
C.hyperboreus
Sagitta elegans
Pareuchaeta
Metridia longa +
M.lucens
2.5
0
2.5
0
3.4
3.1
43.8
5.6
2.9
0.4
10.1
3.0
0-25 25-100 100-200 200^*00
Layer (m)
34.7
31.1
3.2
0.4
3.6
3.7
50.5
2.4
6.5
1.5
10.3
2.0
69.9
67.8
0.3
1.8
0.9
0.8
19.2
0.4
1.6
2.4
3.7
1.2
400-600 600-1000
145.6
139.2
0
0.2
0
0.8
44.4
43.0
0.8
0.6
0
1.1
21.3
0.8
0.9
23.2
0
7.3
2.6
0.5
1300-1800
0.4
9.8
2.8
2.4
1000-1300
43.1
40.8
2.3
0
0
0.9
43.9
0.9
0
31.2
3.9
0.3
1800-2300
10.9
9.8
1.1
0
0
0.25
6.3
0.4
0
2.4
0
0.1
2300-2900
6.4
5.4
1.0
0
0
0.1
3.2
0.2
0
1.2
0
0.04
2900-3500
0
0
0
0
0
0
0.4
0.03
0
0.1
0
0.02
3500-3700
(bottom)
Table II. The biomass (mg nr 3 ) of the main species and groups of mesoplankton and macroplankton (according to BR net samples) at 41°39'N, 49°58'W. The
two series of samples were averaged
if
PI
Vertical distribution of zooplankton
Below 600 m, the abundance of small mesozooplankton (<3 mm) decreased
sharply. Below 1500 m, the biomass was as low as 0.2-6.7 mg m~3 (Figure 3B).
Meanwhile, the total mesozooplankton biomass in the large bottle samples from
the depths of 1300 and 1500 m was as high as 50-70 mg nr 3 , i.e. it was comparable
with the biomass in the subsurface layers. Such a high value of mesozooplankton
biomass consisted of copepodid stages V and VI of C.finmarchicus and M.longa,
and stage V of C.hyperboreus.
According to BR net samples, the mesozooplankton biomass varied slightly
within the depth range from 200 to 2300 m (Table II). The biomass value varied
from 20 to 50 mg nr 3 (Table II, Figure 5). In the upper 200-500 m layer, the arcticboreal species C.finmarchicus and Metridia dominated. Deeper, cold-water
C.hyperboreus appeared instead, its maximum biomass being in the 1800-2300 m
layer (Figure 6). Down to 2300 m depth, Sagitta aff. elegans continued to be one
of the most important parts of the total mesozooplankton biomass (Table II,
Figure 7). Below 600-1000 m, the deep-water species were common in the
samples: copepods Lucicutia grandis, Heterorchabdus, various Aetideidae,
Pareuchaeta; below 2000 m, copepods Augaptilidae and Euaugaptilus (2900-3500
m) were found, amphipods Lanceola loveni, Vibilia propinqua, Scina incerta,
Halice aculeata, H.secunda, Cleonardo, deep-water medusas Atolla, Halicreas,
etc., and ostracods. Below 2300 m, a lot of deep-water polychaetes, chaetognaths
Eukrohnia fowleri, Sagitta macrocephala and S.maxima occurred. Within the
1300—1800 m layer, the rare hyperiid Pegohyperia
50
Biomass (ITIQ m
100
150 200
250
princeps
was caught.
300
0 -,
500-
1000 -
1500 £
2000 -
Q.
8
2500-I
3000 -
3500-
4000
Bottom
Fig. S. Vertical distribution of the mesozooplankton biomass measured by BR net in the 0-3700 m
(30 m above the bottom) layer.
95
M.E.Vinogradov et at.
10
i
Biomass (mg m )
20
30
160
i
i
/
/
,
170
180
,
7/
500-
1000 -
1500-
£
2000 -
Q.
3
2500-I
300035004000 J
Bottom
Fig. 6. Vertical distribution of Calanusfinmarchicus(solid line) and Calanus hyperboreus (dashed
line) biomass measured by BR net.
Macroplankton animals dominated in terms of biomass within the depth range
from 400-600 to 3000-3500 m.
The relatively high mesozooplankton biomass confirms that the investigated
region could be characterized by a cold-water boreal type of plankton community. Such high biomass values are typical of arctic-boreal waters of high
productivity (e.g. the Norwegian Sea) rather than waters of the northern part of
the North Atlantic Current. From this point of view, it is interesting to compare
the biomass values in the frontal zone with those obtained earlier in similar ocean
regions (Table III). The data obtained before by the English National Oceanographic Institute (kindly provided to us by the late Dr Foxton) and during the
R/V 'Mikhail Lomonosov' cruises (Yashnov, 1961; Vinogradov, 1968) were
collected by methods that were different to some extent (other layers sampled,
thinner mesh size, higher tow speed, etc.). Thus, these data can be compared only
approximately.
The cited values indicate that plankton of the investigated region are significantly more rich than those in the neighbouring regions of the North Atlantic
Current. An important point is that their decrease with depth is less evident. This
phenomenon reflects the regularities observed in the arctic waters located northward.
The large amount of dead plankton (mainly partly decomposed copepods and
their exuviae) in the deep waters has engaged our attention. At various depths
and in different samples, their quantity varied significantly. Thus, in one of two
96
Vertical distribution of zooplankton
0 -
_l
1
I
U.
5001000-
1500£
2000-J
Q.
<D
Q
2500-
3000-
3500-
4000 J
Bottom
Fig. 7. Vertical distribution of Sagitla elegans biomass measured by BR net.
net samples from the horizon of 1000-1300 m, the ratio between the number of
live and dead partly decomposed copepods C.hyperboreus, C.finmarchicus,
Chiridius aff., etc., with their exuviae, was 2.4:1. In this sample, the biomass of the
live plankton was only 8 mg m~3, although it had been 36.2 mg m~3 in the sample
obtained from the same depth several days before and only a few dead copepods
have been found. It is interesting that the 1300-1800 m sample, which belongs to
Table III. The biomass of mesozooplankton measured by plankton nets (mg nr-1) in various regions
of the North Atlantic
Layer
(m)
Our data:
'Titanic'
wreck
region"
Continental slope
38°-41°N
(English National
Oceanographic
Institute)6
Gulf Stream
40 o -43 o N
(R/V 'Mikhail
Lomonosov')0
Gulf Stream
38"M1°N
(R/V 'Mikhail
Lomonosov')0
Norwegian
Sea 73°44'N,
13°16'E"
0-100
100-200
200-500
500-1000
1000-2000
2000-3000
160.5
93.2
43.8
34.8
23.2
21.3
146.5
35.1
30.8
19.0
7.8
-
58.4
26.2
25.4
15.7
5.0
-
44.9
23.3
11.1
6.0
2.8
2.1
345.5
99.0
145.2
63.0
37.7
-
"Our data;
T.Foxton, personal communication.
Yashnov, 1961.
c
97
M.E.Vinogradov et al.
the series where mass dying of plankton has been observed at the depth of
1000-1300 m, contained the chaetognaths, the majority of which had Calanus
specimens in their guts. The biomass values in the 1300-1800 m layer in both
series were almost equal: 19 and 23.6 mg nr 3 . The significant difference between
the biomass values in the two series of samples was also noted at the 2900-3500 m
horizon (0.82 and 5.56 mg m~3). Here, a large number of dead copepods and their
exuviae occurred in the poor sample as well. The ratio values between the
number of live and dead animals were 4.7:1 and 4.3:1 in these samples. In the
sample from 2300-2900 m depth, this ratio was 1.6:1, in the sample from
3000-3500 m depth it was 3.1:1 and in the near-bottom stratum at 3557-3774 m
(10 m above the bottom) it was 3.6:1.
These data illustrate the following: after contact of the arctic waters with the
those of the Gulf Stream, the plankton advecting from the north seem to die
within the whole water column down to the near-bottom layers. As noted above,
the plankton die more intensively within some layers than within others. That is
why as mesozooplankton biomass decreases with depth the layers with poor zooplankton biomass (600-1000 m) are interleaved with the layers of enlarged zooplankton biomass (1800-2300 m) (Table II, Figure 5). The intensity of plankton
dying off and its biomass at the same depths may change rapidly.
Generally, the 'net' mesozooplankton biomass decreases with depth by 2-3
orders of magnitude: from >100 mg nr 3 within the 0-100 m layer to 0.4 mg nr 3
within the near-bottom horizon at 3500-3700 m (Table II). The range of variation
of 'bottle' plankton biomass was about the same: from 50-250 mg nr 3 within the
surface 150 m layer to 0.2-6 mg nr 3 at 3000 m depth.
It should be emphasized that neither BR net nor large water bottle samples
indicated any regular variations of zooplankton biomass at the horizons 5,50 and
200 m above the bottom. It is unclear whether these data reflect the natural processes or the fact that the methods used are not suitable to detect minor differences in the near-bottom plankton biomass.
Macro plankton
The plankton advected by the stream of deep waters from the north and dying
off within the frontal zone are a food resource for the species of macroplankton
carnivores and scavengers, mostly shrimps, which occur in the region investigated
within the depth range from 400-600 to 3500 m. The deep-water macroplankton
were absolutely dominated by the shrimps (Table II), namely Acanthephyra
purpurea above 750 m, A.pelagica together with the more uncommon species
Hymenodora gracilis, and more rare H.glacialis and small Gennades within
750-2500 m. Below 2500 m, they were replaced by the other abyssal species of
the family Pasiphaeidae, although A.gracilipes was caught at the depth of
2275-2904 m.
Near the bottom (within the 3500-3800 m layer), the macroplankton shrimps
were rare. They were not found in the BR net samples, although the pilots of
'Mir' submersibles (these submersibles executed under-water operations during
98
Vertical distribution of zooplankton
the cruise) noticed 'large red shrimps' just near the bottom. The shrimps in the
deep layers were accompanied by other carnivores. Small fishes (Cyclothone)
occupied the 400-2000 m layer. Sagitta maxima, being 7-8 cm long, occurred
down to 3500 m depth, etc. Ten young (20-50 mm long) specimens of the typically benthopelagic scavenger amphipod Eurythenes gryllus were found in the
bottom trap with bait positioned 2 m above the bottom. The deep-water whiptail
Coryphaenoides armatus was caught by the trap positioned 5-7 m above the
bottom.
The biomass of macroplankton sampled by BR nets in the frontal zone was
extremely high (Table IV). It was an order of magnitude higher than the macroplankton biomasses in the productive Norwegian Sea and in the equatorial divergence of the Eastern Pacific; it was two orders of magnitude higher than the
macroplankton biomasses in the Kuril-Kamchatka region of the North-Western
Pacific. The total macroplankton biomass in the water column (0-3700 m) was
130 g rrr2, whereas it was 10.2 g rrr2 in the Norwegian Sea (0-1700 m), 11.6 g nrr2
in the productive equatorial region of the Eastern Pacific (0-4000 m) and only 4.5
g rrr2 in the North-Western part of the Pacific Ocean (0-4000 m). In the region
investigated, the macroplankton biomass of the total water column ranged up to
one-half (53%) of the total macroplankton + mesoplankton biomass measured
by BR net samples.
Similar to other regions of the ocean, the highest concentrations of
macroplankton were located under the layers of mesozooplankton concentration
(the upper and intermediate layers). In the BR net samples, the macroplankton
animals occurred below 500 m. Their biomass increased rapidly with depth and
peaked within the 1000-1300 m layer (-150 mg nr 3 ). Then it decreased down to
40-70 mg m"3 in the 1300-2300 m layer. Within the 600-1300 m layer, the percentage of macroplankton in the total meso- and macroplankton biomass was as
much as 75-87%. In the deeper layers (2300-3500 m), the macroplankton
biomass exceeded 60% (Table IV). Within the deeper layers, the species composition of macroplankton animals changed and their biomass decreased down
to 7-10 mg nr 3 .
The macroplankton dominated (>50% of the total biomass of mesoplankton
and macroplankton) over a huge depth range. This depth range covered all the
mesopelagial, bathypelagial and even the upper layers of the abyssopelagial. This
phenomenon seems to be determined by the rich mesozooplankton biomass in
this region and by its extensive dying off in the vigorous frontal zone between two
water masses characterized by alternative hydrophysical features.
Discussion
The investigated region of the frontal zone between the Gulf Stream and the
Labrador Current exemplifies the typical contact between the two water structures inhabited by the communities at different stages of succession. As indicated
earlier (Fedorov, 1983; etc.), the zone of contact between water masses is more
volume than surface, with turbulent diffusion within this volume. Not the continuous gradual changes in the environment, but the mosaics of parcels of more
99
8
4.6
5.4
38.4
69.7
86.2
67.5
48.2
63.2
68.0
0
68.2
145.6
44.3
40.8
10.8
6.4
0
Bottom
%
2.1
2.5
31.1
B
Frontal zone between the
Labrador Current and the
Gulf Stream, 41°39X
49°58'W, September
(2 series of hauls
averaged)'
•Our data.
b
Vinogradov, 1968.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
3400
3600
4000
(m)
Depth
6.9
19.5
5.6
9.9
48.1
0
0
13.6
Bottom
0
%
0
B
Norwegian Sea
73°44'N, 13°16'E, July
(5 series of hauls
averaged)*
0.11
1.3
1.3
3.4
0
0.3
B
6.6
11.2
5.4
3.8
0
0.05
%
NW Pacific
(Kuril-Kamchatka region),
July-September
(9 series of hauls
averaged) 6
0.02
5.9
2.7
4.2
2.2
B
8.0
74.0
34.2
6.0
22.3
%
Pacific and Indian oceans.
Equatorial region
(12°N-12°S)
(21 series of hauls
averaged)1"
0
0.3
1.6
0
0.8
B
0
14.2
25.0
0
10.5
%
Pacific and Indian
oceans. Oligotrophic
regions of the tropical
gyres (11 series of hauls
averaged) 6
Table IV. The macroplankton biomass (£, mg nr 3 ) and its role in the total biomass of mesoplankton and macroplankton (%) in various regions of the ocean
a
•<
0
a.
1
Vertical distribution of zooplankton
or less uniform water alternating with the parcels (layers) of the other water mass,
are observed there.
In terms of the conception of ecosystem succession, the processes in the frontal
zone may be considered as a contact between two communities characterized by
different levels of maturity. In this situation, the flux of energy and matter goes
from the system of lower 'level of organization' towards the system whose level
is higher. It results in conservation or even sharpening of boundary contrasts. The
more mature community exploits the younger one. Some part of the biomass of
the latter is utilized by the former and it grows up more intensively as compared
with the situation when the community is limited by its natural resources only.
The energy consumed by the mature system is lost for the younger one (Margalef,
1968).
In the frontal zone, the production and biomass of the higher trophic levels of
a more mature community increase rapidly (Margalef, 1968; Odum, 1969;
Vinogradov, 1977; Frontier, 1978; Peres, 1982). They undergo sharp changes in
their structure and functioning, and these changes do not coincide with the
general pattern of successive development. These changes may vary and depend
on the hydrophysical features within the frontal zone, its structure, stage of
succession, and other peculiarities of the communities in contact. The biomass of
carnivores increases sharply in the communities located to the warm side of a
frontal zone. The nekton animals feeding on these carnivores concentrate here
(Knauss, 1957; Murphy and Shomura, 1972; Laurs and Lynn, 1977; etc.).
The contacts between extremely different communities in the surface waters of
the frontal zone were studied in detail at the continental slope of the Peruvian
Upwelling region (Vinogradov and Shushkina, 1987). The communities of the
early stage of succession were characterized by mass development of phytoplankton; the communities of the later stage of succession were characterized by
the development of mesoplankton phytophags.
In the area described in the present paper, an unusual situation occurs. The
younger community of the cold side of the frontal zone is at the stage of mass
development of mesozooplankton; the more mature community of the warm side
of the frontal zone is at the stage of mass development of carnivores. The
macroplankton carnivores and scavengers dominating in the mature community
consume food sources from the younger one; in other words, the mature community is exploiting the other one.
The pattern given above clarifies the apparent contradiction that in the investigated region the biomass of macroplankton carnivores and scavengers in the
water column below 600 m depth is 2.3 times higher (within the 1000-1200 m
layer, it is 6.3 times higher) than the biomass of their prey. The mass of shrimpsscavengers forms a peculiar kind of sieve. Weak and dying off in the 'alien' water
zooplankton are filtered through this sieve. Usually, the macroplankton animals
form a similar sieve under the rich layers of the productive zone of the ocean
(Barham, 1966; Vinogradov, 1968). Additional examples of a similar nature
could be noted. The concentrations of crustaceans-phytophags were up to 27 g
m~3 and even greater at the shelf edge of the Peruvian Upwelling (Shushkina et
ai, 1978). Yet another example are the systems of coral reefs that develop on
101
M.E.Vinogradov et al.
the supply of nutrients advected by the flow of the oligotrophic ocean waters
(Sorokin, 1990).
Acknowledgements
We are grateful to A.L.Vereshchaka (IO RAS) for the identification of the
shrimps, and to V.Yu.Dyakonov (IO RAS) for developing the software for our
calculations. We thank V.A.Artemyev and all the crew of the R/V 'Akademik
Mstislav Keldysh' for help during the collection of samples. This work was funded
by the Russian Ministry of Science (World Ocean Program) and the Russian
Foundation for Basic Research, project no. 96-05-64248.
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Received on May 5, 1997; accepted on September 8, 1997
103