Zooplankton distribution across the Lomonosov Ridge, Arctic Ocean: species inventory, biomass and vertical structure

Deep-Sea Research I 47 (2000) 2029}2060

Zooplankton distribution across the Lomonosov Ridge, Arctic Ocean: species inventory, biomass and vertical structure K. Kosobokova, H.-J. Hirche* P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, 36 Nakhimov ave., Moscow 117853, Russia Received 17 November 1998; received in revised form 8 October 1999; accepted 10 December 1999

Abstract On a transect across the Lomonosov Ridge strati"ed zooplankton tows were made to the bottom at seven stations. A species inventory was established and compared with earlier observations in the Arctic Ocean. Di!erences between the Amundsen and Makarov basins are relatively small and correspond well with the general circulation patterns for Atlantic, Paci"c, and neritic waters, suggesting slow mixing rates for the di!erent basins. There were no remarkable di!erences in the species composition or their vertical distribution between the two sides of the Lomonosov Ridge. This indicates e!ective faunistic exchange across the ridge, although several bathy-pelagic species were almost or completely absent on top of the Ridge. Biomass showed a strong gradient along the transect, with a pronounced peak (9.5 g dry weight m\) in the core of Atlantic water over the ridge, and minima over the deep basins. These di!erences were related to the e!ect of bottom topography for deep-living species, and the dynamics of the Atlantic layer for the meso- and epipelagic species. The maximum was formed mainly by the copepods Calanus hyperboreus and Metridia longa together with chaetognaths and ostracods. The presence of young developmental stages in some of the abundant species (C. hyperboreus, M. longa) suggests successful reproduction at all stations but C. xnmarchicus was almost exclusively represented as old stages and adults. Comparison with earlier data on abundance and biomass from the Canada Basin (Russian Drift station `North Pole-22a) shows a pronounced di!erence with respect to both absolute quantities and relative composition. The copepod C. xnmarchicus is completely absent in the central Canada Basin, and the portion of non-copepod zooplankton is dramatically decreased. This points to a reduced advection of Atlantic water or more severe food conditions in this basin.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Zooplankton; Lomonosov Ridge; Arctic Ocean

* Correspondence address: Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse 1, D-27656 Bremerhaven, Germany. E-mail address: [email protected] (H.-J. Hirche). 0967-0637/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 0 0 ) 0 0 0 1 5 - 7

2030

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

1. Introduction In recent years, the Arctic Ocean has received increased scienti"c interest. Microwave remote sensing data have revealed a decrease in the extent of Arctic sea-ice (Parkinson and Cavalieri, 1989), which in recent years has accelerated (Johannessen et al., 1995). In both the Eurasian and Canadian Basins the Atlantic layer shows higher temperatures than in available climatological data (Rudels et al., 1994; Carmack et al., 1995, 1997; Macdonald, 1996). Primary production values higher by a factor of 10 as compared to earlier estimates have been reported from the Arctic Ocean Section by two ice breakers (Wheeler et al., 1996; Gosselin et al., 1997), changing our view of the Arctic Ocean as a biological desert. However, lack of basic biological data makes it as yet impossible to estimate the e!ect of recent environmental changes on the biological system. This holds also for the Arctic zooplankton. Most of the earlier zooplankton studies in the Arctic Ocean were performed from drifting Russian and American platforms, a few from submarines and ice-breakers. Spatial coverage of this work was strongly biased towards the Amerasian Basin. Only a few samples were obtained over the complete vertical range. One of their aims was to establish a relationship between zooplankton species distribution and water masses. Thus, Brodsky (1957) described the assemblages of copepod species for di!erent water layers and assessed the relative importance of particular species for each layer from the collections of `Frama, ice breakers `Sadkoa and `Sedova, aircraft `USSR P-169a, and Russian drift stations `North Polea (NP) and NP-2. Johnson (1963) presented station protocols of the drift station `Alphaa with the vertical distribution of more than 35 copepod species and other taxa down to 1000}2000 m depths. Dunbar and Harding (1968) reported on the vertical distribution of plankton from bottom to surface from the collections of `T-3a. Deep-water collections of later Russian and American drift stations have been used to describe seasonal variations in the vertical distribution of the large copepod species Calanus hyperboreus, C.glacialis and Metridia longa (Brodsky and Pavshtiks, 1976; Pavshtiks, 1976, 1977; Geynrikh et al., 1983; Dawson, 1978; Rudyakov, 1983). From collections of the drift stations NP-22 and NP-23 (1975}77) seasonal migrations of small copepods (Microcalanus pygmaeus, Oithona similis, Oncaea borealis and O. notopus) were described (Kosobokova, 1980), together with seasonal variations in the vertical distribution of the total zooplankton biomass in the entire water column (Kosobokova, 1982), and vertical ranges of the particular plankton species (Kosobokova, 1989). In contrast to the Amerasian Basin, in the Eurasian Basin sampling was restricted to shallow depths (Minoda, 1967; Hirche and Mumm, 1992; Mumm, 1993; Mumm et al., 1998), making comparison between zooplankton communities in the Canadian and Eurasian Basins impossible. However, regional di!erences in the distribution of biomass of the upper water layer within the Arctic Ocean have been studied in more detail in the Eurasian Basin. Thus, recent sampling along ships' transects in the upper layers (0}500 m) of the Nansen Basin revealed strong variability of species composition and biomass within this basin (Hirche and Mumm, 1992; Mumm, 1993; Mumm et al., 1998), which was closely related to hydrography. Biomass was generally higher near the shelf margin than in the deep basins, with the maximum related to the core

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2031

of the Atlantic in#ow along the margin of the Barents Shelf. Elevated biomass and an increased portion of Atlantic species were also encountered on top of the NansenGakkel-Ridge, which was attributed to recirculating Atlantic water. These observations suggested close coupling between hydrography, topography and regional zooplankton distribution also for the other basins of the Arctic Ocean. The central Arctic Ocean is subdivided by three nearly parallel submarine ridges which signi"cantly in#uence the exchange of water and the direction and magnitude of #ow (Schlosser et al., 1995). Over#ows of Atlantic water in the depth range 150} 1000 m occur only at the intersections of the ridges with the continental slope. A subsurface front has been observed recently above the Lomonosov Ridge, and exchanges between the basins are limited to the surface #ow of the Trans-polar Drift and to the spill-over of Eurasian and Canadian Basin deep waters through gaps of the Lomonosov Ridge. Thus, the Lomonosov Ridge may have a depth-dependent e!ect on lateral exchange of zooplankton communities: unlimited across-basin exchange in the surface layer, limited exchange in the Atlantic layer, and a faunistic boundary in the deep water. Furthermore, in analogy to observations by Hirche and Mumm (1992) mentioned above, a biomass maximum is expected in the recirculating Atlantic water on top of the ridge. During the present study, deep water zooplankton sampling was carried out in two basins of the Arctic Ocean, the Amundsen and Makarov Basins, on a transect across the Lomonosov Ridge. The aims were (1) to establish a species inventory for the major basins and to analyse the relative importance of species abundance and biomass in these basins; (2) to describe the vertical distribution patterns of mesozooplankton biomass and species in the Arctic Ocean; and (3) to understand the e!ect of bottom topography and circulation pattern on the horizontal and vertical distribution of zooplankton in the case of the Lomonosov Ridge. In order to extend the comparison of the data to all major basins of the Arctic Ocean, the zooplankton collections obtained earlier in the Canada Basin from the Russian ice drifting station NP-22 were also used.

2. Materials and methods Zooplankton was collected during RV Polarstern cruise ARK XI/1 between 20 August and 4 September, 1995 at seven stations (Fig. 1). Nine depth layers were sampled with a multinet (Hydrobios, Kiel, 0.25 m mouth opening, 150 lm mesh size) in two successive vertical hauls, one from the bottom and one from 300 m to the surface (Table 1). Samples were preserved in 4% borax-bu!ered formalin. All mesozooplankton organisms ('1 mm) from the samples were counted and measured under a stereo microscope. For the small plankton ((1 mm), an aliquot (1 : 8, 1 : 10) of the sample was counted after fractionation with a stempel-pipette. Most taxonomic groups, including Copepoda Calanoida and Cyclopoida, Decapoda,

2032

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Fig. 1. Station locations during Polarstern cruise ARK XI/I and Russian drift station NP-22. Position of the ice edge derived from SSM/I passive microwave images.

Pteropoda, Chaetognatha, Appendicularia, and Hydromedusae, were identi"ed to the species level. Copepodite stages of calanoid copepods were counted separately. Prosome length was used to distinguish adult females (AF) and copepodite stage V (CV) of the two closely related copepods Calanus xnmarchicus (AF(3.1 mm, CV(2.9 mm) and C.glacialis (AF'3.1 mm, CV'2.9 mm). Prosome length was measured from the tip of the cephalosome to the distal lateral end of the last thoracic segment. Earlier copepodite stages CI-CIV of Calanus belonged almost exclusively to C. glacialis and C. hyperboreus. They were separated by morphology and body size according to Hirche et al. (1994). Biomass was calculated from published (Richter, 1994) and unpublished taxonspeci"c length}dry weight relationships and individual dry weights (Kosobokova et al., 1998). For rare copepod species and juvenile stages of Clione limacina, "rst wet weights were calculated according to length}weight regressions established by Chislenko (1968). They were then converted to dry weight using a factor of 0.16 established

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2033

Table 1 Station list and sampling protocol. MN- Multinet, JN- Juday net Station

Date

Latitude

Longitude

Depth (m)

Sampling depths (m)

ARK XI/1 47

20.08.1995

80355,1N

132300,0E

3830

49

22.08.1995

81303,4N

136300,0E

2700

51a

23.08.1995

81307,3N

136300,0E

1750

52

24.08.1995

81310,5N

136300,0E

1290

55

25.08.1995

81310,6N

136300,0E

1690

57

27.08.1995

81312,5N

136300,0E

2640

75

04.09.1995

80355,6N

136300,0E

3580

MN 3500-1800-1000-500-300-200100-50-25-0 MN 2600-1000-750-500-300-200100-50-25-0 MN 1700-1000-750-500-300-200100-50-25-0 MN 1200-750-500-300-200-100-5025-0 MN 1600-1000-750}500-300-200100-50-25-0 MN 2500-1500-1000-500-300-200100-50-25-0 MN 3566-2000-1000-500-300-200100-50-25-0

NP-22 9

22.07.1975

83345N

162312W

2655

11

06.08.1975

83302N

160357W

3160

14

23.09.1975

83308N

161325W

3250

JN 1600-1000-500-300-200-100-5025-10-0 JN 2000-1000-500-300-200-100-5025-10-0 JN 3000-2000-1000-500-300-200150-100-50-25-10-0

for arctic zooplankton by Kosobokova (unpubl.). For chaetognaths a length}weight relationship established for Eukrohnia hamata by Hirche (unpubl.) was applied. The Euphausiacea, Decapoda and Cnidaria were not included in the calculations of the total biomass. Zooplankton from the Russian ice drifting station NP-22 was sampled with a vertically hauled closing Juday net (mouth opening 0.1 m, mesh size 176 lm) between 83308 and 83345N (Fig. 1) in the summer, 1975. From eight to ten depth strata from the bottom to the surface were sampled (Table 1) and preserved in 4% borax-bu!ered formalin. All samples were sorted (Kosobokova, 1981) and processed according to the procedure described above. In order to group copepod species according to their distribution patterns along the transect, their relative frequency at each station was calculated and submitted to agglomerative hierarchical cluster analysis (Gray et al., 1988; Field et al., 1982). Biomass was chosen as a measure of species frequency to minimize the e!ect of di!erences in stage composition between stations. Data were subjected to di!erent sets of combinations of approximation (Bray-Curtis index, Canberra metric) and agglomeration (single linkage, average linkage, complete linkage), but the basic structure of the dendrograms remained una!ected.

2034

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

3. Results 3.1. Ice cover and hydrography All stations were situated in the region covered with "rst-year ice with thickness between 1.2 and 2.5 m (Eicken et al., 1997) (Fig. 1). A transect of temperature and salinity together with the positions of the multinet stations is shown in Fig. 2a and b. Hydrographic features during this cruise were described in detail by Rudels et al. (2000). Two branches of Atlantic water from the Norwegian Sea are identi"ed by salinity maxima, representing the Barents Sea branch and the Fram Strait branch. The salinity minimum at the western side of the Lomonosov Ridge indicates that this part of the boundary current turned northward and #ows towards Fram Strait within the Amundsen Basin. The deep waters are warmer and more saline in the Makarov Basin. From parallel transects it is apparent that the deepest gaps in the Lomonosov Ridge are located at approximately 1600 m depth (the sill depth, Rudels et al., 2000), while at our transect the tip of the ridge was at about 900 m depth (Fig. 2, station 53). 3.2. Zooplankton inventory In Table 2 the speci"c composition of the zooplankton fauna and mean relative abundance and biomass in the Amundsen (stations 75, 47 and 49) and Makarov Basins (stations 55 and 57) are presented for each species. A total of 78 species including 52 species of copepods, 1 euphausiid, 6 amphipods, 1 decapod, 13 cnidarians, 1 pteropod, 2 larvaceans and 2 chaetognaths together with 3 other taxa not determined to the species level were identi"ed. Copepods were the most important group in terms of abundance and biomass in both basins. They contributed up to 95 and 97% of the total zooplankton abundance in the Amundsen and Makarov Basins (Table 2). The next important groups were chaetognaths (1.3 and 0.8%), appendicularians (1.4 and 0.4%) and ostracods (0.9 and 0.95%). The contribution of ostracods did not di!er between the two sides of the Lomonosov Ridge, but chaetognaths and appendicularians were considerably less important in the Makarov Basin (Table 2). In terms of biomass, copepods contributed 74 and 82% of the zooplankton in the Amundsen and Makarov Basins. Chaetognaths (18.1 and 12.5%,), appendicularians (4.1 and 0.8%) and ostracods (1.9 and 3.4%) were the major elements of non-copepod zooplankton. The importance of calanoid and cyclopoid copepods was di!erent for abundance and biomass. More than 54% of the total number in both regions were cyclopoids, and more than 37% were calanoids. However, cyclopoids made up only 4% of the total biomass, whereas calanoids contributed more than 70%. Harpacticoid copepods were of minor importance, both in terms of abundance and biomass (Table 2). Single copepod species also di!ered in importance between abundance and biomass. The small cyclopoids Oithona similis and Oncaea spp. and the calanoid Microcalanus pygmaeus made up together more than 69% of abundance (Table 2), while the large calanoids Calanus hyperboreus, C. glacialis and Metridia longa together made up

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2035

Fig. 2. Temperature (a) and salinity (b) along the transect across the Lomonosov Ridge.

49% of the biomass. C. hyperboreus was by far the most important component of biomass (28.5 and 34.8% in the Amundsen and Makarov Basin). Due to their high abundance, M. pygmaeus and O. similis followed in the ranking after these large species. A few large copepods (Pareuchaeta glacialis, Scaphocalanus acrocephalus)

Cnidaria Hydromedusae Aglantha digitale (O.F. Muller, 1766) Aeginopsis laurentii Brandt, 1835 Botrynema brucei Browne, 1908 B. ellinorae (Hartlaub, 1909) Homoenema platygonon Maas, 1893 Margelopsis hartlaubi Browne, 1903 Paragoetea elegans Margulis, 1989 Plotocnide borealis Wagner, 1885 Sminthea arctica (Hartlaub, 1909) Solmundella bitentaculata (Quoy & Gaimard, 1832) >akovia polinae Margulis 1989 Scyphomedusae Atolla tenella Hartlaub, 1909 Syphonophora Dimophyes arctica (Chun, 1897)

0.01 0.03 0 0.06 0.04 0 0 0 0.06

0 0

0

0.23

0 0.01

0

0.32

} }

H

} }

} } } } } } } } }

H

H H

H H

Present study

0.01 0.03 0.02 0.04 0.06 0 0 0 0.11

MB

Present study

}

}

} }

} } } } } } } } }

}

CB

Kosobokova, 1981 }

AB

Present study

0.42

MB

Present study

0.6

CB

Kosobokova, 1981

AB

Minoda, 1967 (AB)

AB#MB#CB

Virketis, 1959 (AB#MB#CB)

EUB

Grice, 1962 (AB#MB#CB)

Biomass, %

Kosobokova, 1981 (MB)

Abundance, %

Markhaseva, 1984 (MB)

Taxa CB

Hughes, 1968 (CB)

Dunbar & Harding, 1968 (CB) Johnson, 1963 (CB)

Brodsky &Nikitin, 1955 (CB)

Virketis, 1957 (AB#MB#CB)

Kosobokova et al., 1998 (NB)

Mumm, 1993 (NB)

Bogorov, 1946 (NB#AB)

Shirshov, 1938 (AB)

Sars, 1900 (NB)

Table 2 Relative abundance (%) and biomass (%) of the zooplankton taxa in the Amundsen (AB), Makarov (MB) and Canadian Basins (CB), and inventory of copepod species found in the deep basins during previous investigations. EU } Eurasian Basin.

Grainger, 1965 (CB)

2036 K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Ctenophora Polychaeta Pteropoda Clione limacina Phipps, 1774 Limacina helicina Phipps, 1774 Crustacea Copepoda Calanoida Acartia longiremis (Lilljeborg, 1853) Aetideopsis minor (Wolfenden, 1904) A. rostrata G.O. Sars, 1903 Aetideidae spp. CI-III Augaptilus glacialis G.O. Sars, 1900 A. polaris Harding, 1966 Calanus cristatus Kroyer, 1848 C. xnmarchicus (Gunnerus, 1765) C. glacialis Jaschnov, 1955 C. hyperboreus Kroyer, 1838 Chiridiella abyssalis Brodsky, 1950 Ch. reductella, Markhaseva 1996 Ch. sarsi Markhaseva 1983 Chiridius obtusifrons G.O. Sars, 1903 Disco triangularis Markhaseva & Kosobokova, 1998 Drepanopus bungei G.O. Sars, 1898 Euaugaptilus hyperboreus Brodsky, 1950 Eucalanus bungii bungii Geisbrecht, 1892 Gaetanus brevispinus (G.O. Sars, 1900) G. tenuispinus (G.O. Sars, 1900) G. variabilis (G.O. Sars, 1907) Haloptilus acutifrons (Giesbrecht, 1892) H. pseudooxycephalus Brodsky, 1950 Heterorhabdus compactus (G.O. Sars, 1900) H. norvegicus (Boeck, 1872) Jaschnovia tolli (Linko, 1913) Lucicutia anomala Brodsky, 1950 L. polaris Brodsky, 1950 L. pseudopolaris Heptner, 1969 Metridia longa (Lubbock, 1854) M. pacixca Brodsky, 1950

0.01 0.07 0.11 0.11

9.7 37.1 0.01 0.08 0.02 0.03 0.26 1.09 1.05 0 0.04

0 0.12 0.01 0.02 0.17 0 0.02 0 2.82

0.01 0.14 0.1 0.1

94.8 40.4

0.01 0.06 0.01 0.03

0.24 1.05 1.28

0.01

0.07

0.01 0.17

0.01

0.05 0.17

0 0.02 0 3.34

85.8 82.4

1.93

0.23 1.93

0.02

0.17 0.4

0.18 0.57 * 0 0.28 0.08 0 2.1 5.78

0 0.04

0

H 0.07

H

0.06

0.05

0.05 0 0.22

0 0.13 0 7.62

0.07 0.54

0.02

0.02 0.41

0.02 0 0.19

0.78 5.41 *

*

*

*

0 H 0.03 * *

H

H

0

H H

H

H 0.24 H

H

0.18 H

0.19

H

H

0.02 H

H

H

H

1.69 2.66 H 0.59 15.14 14.2 7.66 1.49 28.45 34.78 57.67 H

0.06 0.39 0.003 0.22 0.3

81.8 76.9

} 0.36 0.41 0.41

H

0.03 0.17 0.003 0.32

74.1 70.0

} 0.31 0.3 0.3

H

H H

98.9 39.1

H 0.18 0.27 H 0.27

*

H * *

H

H H

H

H H H

H

*

H *

H

H H

H

H H H

H

H

*

H * *

H

H H

H

H H H H

H

H H

* *

*

H *

H H H H H

*

*

H *

H

H H H H

H

H H

H

H

H

H H

H H H

H

H

H H

*

H *

H

H H H

H

*

*

H * *

H

H H

H

H

H H H

H

H H

*

*

H

H

* *

H * * * *

H H

H H H H

H

H

H *

H H

*

H H

H H

*

H * * * *

H

H H H H

H

H H H

H

*

H

H

H H H

H

* * (continued on next page)

*

*

H * *

H

H H H H

H

H H H H

H

H H

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060 2037

Microcalanus pygmaeus (G.O. Sars, 1900) Mimocalanus damkaeri Vyshkvartzeva, 1983 Onchocalanus cristogerens Markhaseva & Kosobokova, 1998 Pachyptilus pacixcus Johnson, 1936 Pareuchaeta barbata (Brady, 1883) P. glacialis (Hansen, 1886) P. norvegica (Boeck, 1872) P. polaris Brodsky, 1950 Pareuchaeta spp. CI-V Pseudaugaptilus polaris Brodsky, 1950 Pseudocalanus minutus (Kroyer, 1848) Ps. major G.O. Sars, 1900 Pseudochirella batillipa Park, 1978 Ps spectabilis (G.O. Sars, 1900) Scaphocalanus brevicomis (G.O. Sars, 1900) 0 0.01 H 1.38 0 H 0 0 H 0 0.07 0.32 0.77 0 0 0 0.46 0.03 0.02 0 0.04 H 0.26

0 0.02

0.24

*

Sars, 1900 (NB)

0.68

0.15

*

0.13 0.58

*

*

*

*

*

*

Shirshov, 1938 (AB)

*

Bogorov, 1946 (NB#AB)

0 1.47 0 * 0 0.55 2.21 0 0 0.03 0 * * 0 0 0.76 0.58 *

0

0

0.01 0 0 0.07 0 0.6

H

0

0

0.03

0.01

Present study 0.01

MB

Present study

0.01

CB

Kosobokova, 1981

3.97

AB

Present study 4.16

MB

Present study

18.26 17.19 27.68 4.37

CB

Kosobokova, 1981

AB

*

*

*

*

*

Minoda, 1967 (AB)

EUB

Mumm, 1993 (NB) *

* *

*

Kosobokova et al., 1998 (NB) *

*

* * *

* * * *

*

Virketis, 1957 (AB#MB#CB) *

*

*

* *

*

AB#MB#CB

*

*

*

* * *

*

Virketis, 1959 (AB#MB#CB)

Biomass, %

*

*

*

Grice, 1962 (AB#MB#CB)

Abundance, %

*

*

*

*

*

Kosobokova, 1981 (MB)

Taxa

Markhaseva, 1984 (MB)

Table 2 (Continued)

Brodsky &Nikitin, 1955 (CB) *

*

* *

*

*

Johnson, 1963 (CB) *

*

* *

*

*

CB

*

*

*

Grainger, 1965 (CB)

Dunbar & Harding, 1968 (CB) *

*

* *

*

* *

* * * * *

*

*

Hughes, 1968 (CB) *

*

2038 K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Sc. acrocephalus (Th. Scott, 1893) Sc. polaris Brodsky, 1950 Scolecitricella minor var. occidentalis Brodsky, 1950 Sc.laminata (Farran, 1926) Spinocalanus abyssalis Giesbrecht, 1888 Sp. antarcticus Wolfenden, 1906 Sp. elongatus Brodsky, 1950 Sp. longicomis G.O. Sars, 1900 Sp. longispinus Brodsky, 1950 Sp. polaris Brodsky, 1950 Spinocalanus spp. CI-V Temorites brevis G.O. Sars, 1900 Tharybis groenlandicus (Tupitzky, 1982) Undinella oblonga G.O. Sars, 1900 Xanthocalanus borealis G.O. Sars, 1900 X. polaris Brodsky, 1950 X. profundus G.O. Sars, 1907 Nauplli Calanoida Cyclopoida Hyalopontius typicus G.O. Sars, 1909 Lubbockia glacialis G.O. Sars, 1900 Mormonilla minor Giesbrecht, 1891 Oithona atlantica Farran, 1908 O. similis Claus, 1866 Oncaea borealis G.O. Sars, 1918 O. conifera Giesbrecht; 1891 O. minuta Giesbrecht, 1892 O. notopus (Giesbrecht, 1891) Oncaea spp. Nauplii Cyclopoida Harpacticoida Ectinosoma xnmarchicum Scott, 1896 Microsetella norvegica (Boeck, 1864) M. rosea (Dana, 1852) Tisbe furcata (Baird, 1850) Ostracoda Conchoecia sp. Euphausiacea Thysanoessa inermis (Kroyer, 1846) Th. longicaudata (Kroyer, 1846) 0

0

* }

}

3.42 3.42 }

1.28 0 0.01

*

1.55 *

*

*

*

*

*

0.14 3.07 0 0.09 * 0.15 *

0.02 * *

0.41 0.41

1.9 1.9 }

* 0.63 0.17 *

1.33

* 0.26 0.26

0.95 0.95 0

0.9 0.9 0

0.1 0.27 0.01 0.03

0.38 4.92 0.03 0.03 0.13 0.002 4.16 0.19

0

0.36 0.17 0.18 0.09 0.08 3.29 0.07 0.01 0

0.01

0

0.71 0.21 0.37 0.11 0.15 2.62 0.06 0.02 0.01

0.11 0.32 0.02 0.23

0.01

0.01

0.01

H 2.38 0.13

0.9 H H

1.33 *

0.001 0.002 0 0 0

3.45 0

1.75 1.47 H 12.63 12.9 31.27 2.15 1.39 0.58 0.15 0.04 0.16 0.14 0.02 0.01 0.02 0.16

0.21 0.15 0.41 0.07 0.19 7.32 0.03 0.02 0

0.32 0.14 0.75 0.06 0.21 4.9 0.02 0.04 0

0

3.58 0.07

0.07 3.85 0.17 0.02 0.14 0.002 2.9 0.17

0.01 0

0.02 0

0.22 H

8.48 4.6 2.78 54 59.8 59.2 0.01 0 0 0.12 0.09 0.35 1.06 1.25 2.4 0.03 0.02 32.56 40.0 25.19 3.64 2.71 H

0.28 0

0.27 0.01

*

* *

* *

* *

*

*

*

*

*

*

*

* *

* *

*

*

*

* *

*

*

*

*

*

* * *

*

*

*

* *

* * * * *

*

* * *

* * * * *

*

* *

* *

* *

* *

*

*

* * *

*

* *

*

* * * * *

*

*

*

* * *

*

*

*

* *

*

*

* *

*

*

*

*

* * * * * *

*

*

*

* * *

*

* *

*

*

* *

* *

*

*

* * *

*

* *

* *

*

*

H * *

*

*

* *

* *

*

* *

*

*

*

* *

*

*

*

*

*

*

(continued on next page)

*

* *

* *

*

* *

*

*

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060 2039

0

0

H 0.01 0.01 0 0.17 0.17 * H

0.68

} } 0.81 0.81 0 12.5 12.5

} } 4.11 3.91 0.19 18.1 18.09 0.04 0.03 0 7.35 7.35

4.7

Present study

1.13

Present study 0.06 H H H H H

CB

Kosobokova, 1981

0.03 H H H H H H 0 0 0.42 0.34 0.08 0.84 0.84

AB

Present study

0.03 H H H H H H 0.01 0.01 1.39 0.93 0.46 1.29 1.29

MB

Present study

MB

Minoda, 1967 (AB)

EUB

Virketis, 1957 (AB#MB#CB)

Kosobokova et al., 1998 (NB)

Mumm, 1993 (NB)

Bogorov, 1946 (NB#AB)

Shirshov, 1938 (AB)

Sars, 1900 (NB)

AB#MB#CB

H} taxa registered without abundance/biomass estimation; } biomass is not calculated. Values less than 0.01 are shown as 0.

Amphipoda Apherusa glacialis (Hansen, 1887) Cyclocaris guilelmi Chevreux, 1899 Lanceola clausi Bovallius, 1885 Themisto abyssorum Boeck, 1870 Th. libellula (Lichtenstein, 1882 Pseudalibrotus glacialis G.O. Sars, 1900 Decapoda Hymenodora glacialis (Buchholz, 1874) Appendicularia Oikopleura vanhoweni Lohmann, 1896 Fritillaria borealis Lohmann, 1896 Chaetognatha Eukrohnia hamata (Moebius, 1875) Sagitta elegans Verril, 1873 S. maxima (Conant, 1896)

CB

Kosobokova, 1981

AB

Virketis, 1959 (AB#MB#CB)

Biomass, %

Grice, 1962 (AB#MB#CB)

Abundance, %

Kosobokova, 1981 (MB)

Taxa

Markhaseva, 1984 (MB)

Table 2 (Continued)

CB

Grainger, 1965 (CB)

2040 K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Hughes, 1968 (CB)

Dunbar & Harding, 1968 (CB) Johnson, 1963 (CB)

Brodsky &Nikitin, 1955 (CB)

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2041

were also of importance to biomass, as was C. xnmarchicus (Table 2). The contribution of any other copepod species was less than 5%. Single species from other taxonomic groups were of very low importance in terms of abundance (Table 2). In terms of biomass, the chaetognath Eukrohnia hamata ranked second (18.1%) in the Amundsen and third (12.5%) in the Makarov X hBasin, and the appendicularian Oikopleura vanhoeweni ranked sixth (3.9%) in the Amundsen and only thirteenth (0.8%) in the Makarov Basin.

3.3. Vertical distribution of species Water depth at multinet stations along the transect ranged from 3800 m in the Amundsen Basin to 1200 m west of the tip of the Lomonosov Ridge (Fig. 2). To study the possible e!ect of varying water depth on the vertical distribution of species, the relative frequency was calculated for each species in each depth layer at all stations and compared along the transect. Most species inhabiting the epi- and mesopelagic maintained their vertical range and vertical distribution patterns along the transect, independent of the bottom depth, with just few exceptions. Thus, Calanus hyperboreus, which inhabited the uppermost layer almost exclusively, at two stations (47 and 57) was found more evenly distributed over the upper 1000 m (Fig. 3). The bathypelagic species with upper limit about 1000 m also maintained their position in the water column. As a consequence, when water depth in the ridge region was less than this, they were completely absent (Lucicutia spp., Fig. 3). The small variation in depth distribution of species along the transect allowed a generalized presentation of the abundance and depth range of each species. Mean concentrations were calculated from all stations and presented in tabular fashion according to Mumm (1993, Fig. 4a and b). Species were sorted according to their centers of vertical distribution. In general, most species inhabited at least two water masses of the Arctic Ocean (Arctic and Atlantic Waters or Atlantic and Arctic Bottom waters), or all three of them (Arctic Surface, Atlantic and Arctic Bottom waters). According to their vertical distribution patterns, species were assigned to two major groups. Each of them could be subdivided in subgroups based on the depth preferences of particular species:

3.3.1. Species with a wide vertical range Most of these were found from the surface to the bottom (Fig. 4a and b). Among them a number of species demonstrated a strong preference for the Arctic Surface Water, 0}50 m (Othona similis, Oncaea borealis, Calanus hyperboreus, C.glacialis). All these species are known to perform seasonal vertical migrations in the Arctic Ocean (Pavshtiks, 1977; Geynrikh et al., 1983; Kosobokova, 1980; Dawson, 1978). Their vertical distributions observed during the present study were typical of the summer season. Several species preferred the Arctic Subsurface water, 50}200 m (Microcalanus pygmaeus, Metridia longa, Chiridius obtusifrons), while others were centered in Arctic Subsurface water and the transition layer between Arctic and Atlantic waters,

Fig. 3. Relative vertical distribution (% abundance) of three copepod species along the transect across the Lomonosov Ridge.

2042 K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2043

100}300 m (Scaphocalanus acrocephalus, Spinocalanus antarcticus). Several species showed their preference for the depths from 100 to 500 (750) m, the transition layer between Arctic and Atlantic waters and the upper Atlantic water mass (Mormonilla minor, Heterorhabdus norvegicus, Gaetanus tenuispinus). 3.3.2. Species with restricted vertical range A large group of species was absent from the upper water layers (Fig. 4a and b). Thus, the copepods Scolecithricella minor and Scaphocalanus brevicornis were found deeper than 25 m, and the hydromedusans Aeginopsis laurentii and Sminthea arctica deeper than 50 m. The copepods Aetideopsis minor, Haloptilus acutifrons, Temorites brevis, Heterorhabdus compactus were found only below 100 m. The copepods Gaetanus brevispinus, Pareuchaeta norvegica, Pseudochirella spectabilis, Lubbockia glacialis, Aetideopsis rostrata, Augaptilus glacialis, Xanthocalanus groenlandicus, and the hydromedusae Botrynema ellinorae were found below 200 m. The copepods Spinocalanus polaris and Spinocalanus longispinus were recorded only below 300 m. Another group of species was strictly con"ned to the Arctic Bottom Water and was completely absent from the Arctic and Atlantic Water. It included the copepods Lucicutia anomala, L. pseudopolaris, L. polaris, Mimocalanus damkaeri, Scaphocalanus polaris, Pseudoaugaptilus polaris, Spinocalanus elongatus, Disco triangularis, Onchocalanus cristogerens, Hyalopontius typicus and the hydromedusae Botrynema brucei (Fig. 4a). The "rst nine copepods are endemic to the Arctic Basin (Brodsky, 1967; Dunbar and Harding, 1968; Heptner, 1969; Damkaer, 1975; Markhaseva and Kosobokova, 1998). In contrast, a few expatriate species formed a small group with highest concentration in the upper water layers and absent from the deep layers. The neritic expatriate Pseudocalanus minutus had a maximum abundance in the Arctic Surface water, and the Atlantic expatriates Oithona atlantica, Calanus xnmarchicus and Aglantha digitale showed highest concentrations in the Arctic Subsurface layer. None of the species listed were found below the lowest boundary of the Atlantic layer. 3.4. Zooplankton abundance and diversity Vertical distribution of abundance was studied for the whole zooplankton, and diversity was studied in detail for copepods only since they were by far the most important group of the zooplankton. The vertical pro"le of abundance showed a maximum near the surface and an exponential decrease with depth in both the Amundsen and Makarov Basins (Fig. 5a). There was little between-station variability of the pro"le pattern, although abundance varied considerably along the transect. In order to show the distribution of the species number, the number of copepod species found in a certain depth layer at all stations was pooled (Fig. 5b). The lowest number was observed in the Arctic Surface water layer (0}25 m), although the highest

Fig. 4. Generalized vertical distribution patterns of copepod (a) and non-copepod (b) zooplankton. Mean of 7 stations.

2044 K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2045

abundance was found there. Below 100 m the number of species increased linearly and reached a maximum between 500 and 750 m. Thereafter it decreased slowly in the deep basins. At the deepest stations 75 and 47 in the Amundsen Basin, it reached the minimum of 16 and 18 species between 2000 m and the bottom.

3.5. Zooplankton biomass along the transect Total biomass along the transect varied by a factor of 3 (Fig. 6a). It decreased from 5.1 mg m\ at the westernmost station on the Nansen-Gakkel-Ridge to 3.4 in the center of the Amundsen Basin (station 47) and increased from there to the east to a maximum at 9.5 just west of the crest of the Lomonosov Ridge. Thereafter it decreased continuously towards the Makarov Basin, where it was only 4.4 mg m\. The contribution to biomass of the major taxonomic groups followed closely the variation of total biomass (Fig. 6a and b). When copepods were separated into `large calanoidsa including Calanus hyperboreus, C. glacialis, C. xnmarchicus and Metridia longa (Fig. 6a) and the `resta, the overwhelming contribution of the large calanoids to total biomass became apparent. Within this group C. hyperboreus is the dominating component, making up between 15 (station 75) and 45% (station 51a) of total biomass. Most important `other groupsa of zooplankton, namely chaetognaths, ostracods and appendicularians, also followed closely the variation of total biomass (Fig. 6b). Vertical distribution patterns of biomass were rather constant along the transect (Fig. 7). At all stations except one (station 57), concentration peaked in the upper 25 m, and decreased rapidly below 50 m. In the upper two layers variability of biomass concentration along the transect was also highest. At the station with the biomass maximum (station 51a), it was higher by a factor of 13 in the upper 25 m, and by a factor of 3 between 25 and 50 m, than at the station with the biomass minimum (station 47). In the deep water layers biomass di!ered still by a factor of 2}3. For a detailed picture of the along-transect variability of the copepod species, the respective portion of a species to its total biomass on the whole transect was calculated, and a cluster analysis was then performed. Five major groups were identi"ed. They are shown for all but the rare species belonging to each group in Fig. 8. Group A is very homogenous and is characterized by a pronounced maximum at station 51a; from there biomass decreases steeply to the east and to the west to minima at the two extreme stations on each end of the transect. The 11 species according to their depth distribution (Fig. 4a) belong to two di!erent groups. Aetideopsis rostrata, Undinella oblonga and Xanthocalanus groenlandicus live below 100 m, while the remaining species are all very frequent in the upper 100 m. In Group B are combined two patterns that share a maximum at station 52 followed by a decrease. This decrease extends to the ends of the transect for Gaetanus tenuispinus and Heterorhabdus norvegicus, but it is followed by an increase at both ends for Aetideopsis minor and Oithona similis. Oithona is found throughout

2046

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Fig. 5. Vertical pro"les of zooplankton abundance (a) and copepod species number (b) along the transect across the Lomonosov Ridge (stations 47}75) and during NP-22 (mean of 10 stations; from Kosobokva, 1989). CB"Canada Basin, LR"Lomonosov Ridge.

the whole water column, but the other species have their depth preference below 100 m. Group C includes "ve species inhabiting the waters below 100 or 200 m. They all decrease from maxima in the basins towards a minimum at stations 51a and 52. There is a tendency in most species for higher biomass in the Amundsen Basin than in the Makarov Basin. Group D combines deep-water species (Spinocalanus elongatus, Sp. polaris, all three species of Lucicutia, Scaphocalanus polaris, Pseudochirella spectabilis, Mimocalanus damkaeri and Hyalopontius typicus) with a pronounced minimum at stations 52 and 55, partly also at 51a, and a sharp increase at station 57. In the western part of the transect, maxima are observed mainly at stations 47 and 49. Several very rare deepwater species not shown on Fig. 8 (Pseudoaugaptilus polaris, Onchocalanus cristogerens, Disco triangularis) are completely absent at stations 51a, 52 and 55. Obviously they are restricted to depths below 1000 m. The seven species in Group E belong to di!erent groups of vertical distribution. They do not show a clear trend or much variability at all.

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2047

Fig. 6. Composition of zooplankton biomass (dry weight) along the transect across the Lomonosov Ridge (LR) and during NP-22. Contribution of four copepod species to total biomass (a) and composition of non-zooplankton biomass (b). Figures on the top of the bars indicate total biomass (g m\) at each station. AB"Amundsen Basin, MB"Makarov Basin, CB"Canada Basin.

4. Discussion 4.1. Inventory Since the "rst descriptions of the pelagic fauna by Sars (1900), several reports of shipboard and drift expeditions have presented lists of the mesozooplankton species for di!erent areas of the Arctic Ocean (Table 2). In most of these reports, speci"c composition of copepods was studied in much more detail compared to other groups due to the large amount of work done on taxonomy of Copepoda Calanoida in the North Atlantic and the Arctic Ocean (Sars, 1900, 1903; Brodsky, 1967; Heptner, 1969; Damkaer, 1975; Markhaseva, 1984, 1998; Markhaseva and Kosobokova, 1998). In Table 2 all lists published so far are combined to compare the copepod faunas of the major basins of the Arctic Ocean.

2048

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Fig. 7. Vertical distribution of mesozooplankton biomass (dry weight) along the transect across the Lomonosov Ridge.

All copepod species encountered during our study were known before from the Arctic Ocean (Table 2) except for Disco triangularis and Onchocalanus cristogerens, new species described from the present material, and Scolecithricella laminata (Markhaseva and Kosobokova, 1998). These three species we found on both sides of the Lomonosov Ridge, in both the Amundsen and Makarov Basins. In addition, "ve copepods (Chiridiella reductella, Lucicutia anomala, L. polaris, Spinocalanus polaris and Hyalopontius typicus) were observed in the Eurasian Basin for the "rst time, and a copepod, Xanthocalanus groenlandicus, was registered for the "rst time in the Makarov Basin. Consequently, there is only one species, Xanthocalanus profundus, left reported exclusively for the Eurasian Basin, which is also known from the North

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2049

Fig. 8. Patterns of horizontal distribution of copepod species along the transect across the Lomonosov Ridge. Groups A}E were derived from cluster analysis.

2050

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Atlantic (Richter, 1994), and four species (Pachytilus pacixcus, Augaptilus polaris, Gaetanus variabilis and Haloptilus pseudooxycephalus) recorded in the Canada and Makarov Basins only (Table 2). However, each of the three latter species, A. polaris, G. variabilis, and H. pseudooxycephalus, was found in the Arctic Ocean only once as a single individual (Brodsky and Nikitin, 1955; Virketis, 1957) and was described as a new Arctic Ocean species by Brodsky (1967). Since that time they have not been observed again, which could point to a mistake in identi"cation of individuals of more common species due to a slight deviation in their morphology. Keeping in mind the low probability of existence of these species, one may conclude that the resident copepod faunas of the Eurasian, and Canadian Basins are very similar (Table 2). Moreover, most species collected in the present study were found from young copepodites to the adult stage at most stations, suggesting that they breed succesfully within the Arctic Ocean. Dunbar and Harding (1968) arrived at similar conclusions for the copepods encountered in the Canada Basin. In contrast, the few expatriated species regularly found in the Arctic Ocean, which were present only in late copepodite stages and adults, are con"ned mostly to their original hydrographic regime. Thus, the Paci"c expatriates, Calanus cristatus, Eucalanus bungii bungii and Metridia pacixca, were found almost exclusively in the Canada and Makarov Basins (Brodsky and Nikitin, 1955; Johnson, 1963; Harding, 1966; Huges, 1968; Pavshtiks, 1971a; Brodsky and Pavshtiks, 1976). The Atlantic expatriates Oithona atlantica and Calanus xnmarchicus are restricted mostly to the Nansen, Amundsen and Makarov Basins (Virketis, 1959; Minoda, 1967; Kosobokova, 1981; Kosobokova et al., 1998), and neritic expatriates from the Siberian shelf seas (Drepanopus bungei, Acartia longiremis and Pseudocalanus major) are restricted to the eastern Nansen and Amundsen Basins (Sars, 1900; Kosobokova et al., 1998). The absence of detailed information on other taxa in most of the previous publications does not allow a similar comparison between the non-copepod faunas of the di!erent basins. However, the di!erent contribution of the chaetognath Eukrohnia hamata and the appendicularians Oikopleura vanhoweni and Fritillaria borealis to the zooplankton abundance and biomass in the Eurasian and Amerasian Basins (Table 2) suggests a more basin speci"c distribution. The numerical importance of copepods and their dominant role in zooplankton biomass in the Arctic Ocean have been pointed out since the earliest publications (Sars, 1900; Jaschnov, 1940; Brodsky and Nikitin, 1955; Virketis, 1957, 1959; Grainger, 1965; Pavshtiks, 1971b; Brodsky and Pavshtiks, 1976; Kosobokova, 1980, 1982; Mumm, 1993; Wheeler et al., 1996; Thibault et al., 1999). Our data showed an even higher proportion of copepods in the three major basins: Amundsen and Makarov (abundance 95 and 97%; biomass 74 and 82%) and Canada Basin (abundance 99%; biomass 85%). In addition, we found a much higher number of small forms like Oithona, Oncaea and Microcalanus than earlier studies, most likely because of the "ner mesh. However, only a few estimates of the contribution of other taxa are available to date. According to Pavshtiks (1971b), copepods made up 75}80%, and appendicularians ca. 20%, of the total numbers near the North Pole in summer, while other groups were of very low importance. In the western Nansen Basin, Mumm (1993) reported copepods to contribute up to 85%, chaetognaths 3.7%, ostracods 4.5%, and appendi-

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2051

cularians 3.0% to the total numbers. So far no representative macrozooplankton samples are available, as large species are certainly under-represented in the relatively small nets used. 4.2. Vertical distribution The general concept of vertical distribution of zooplankton abundance and diversity in the Arctic Ocean was developed in early Russian studies based on the collections obtained in the Canada and Makarov Basins. From samples covering all three water masses of the Arctic Ocean, Brodsky (1957) and Virketis (1957, 1959) claimed that the highest abundance of plankton occurred in the uppermost 50 m, and decreased exponentially with depth. In addition they were the "rst to show the speci"c pattern of the vertical distribution of species, with a minimum number of species near the surface, a maximum in the Atlantic water and a decrease in the Bottom water. Later publications added more details obtained by higher sampling resolution (Brodsky and Pavshtiks, 1976; Kosobokova, 1989; Grainger, 1989), but the general picture has not changed during the last 40 years. Here we report the "rst complete vertical pro"les of zooplankton abundance and diversity for the Eurasian Basin. They are very similar to those observed previously in the Amerasian Basin. However, the abundance values in the Amundsen and Makarov Basins are at least one order of magnitude higher at each depth than in the Canada Basin (Fig. 5a). The vertical distribution of species diversity in the Amundsen and Makarov Basins shows a maximum in the Atlantic layer at depths between 500 and 750 m. In the Canada Basin it was found between 300 and 1000 m (Virketis, 1957, 1959; Dunbar and Harding, 1968; Kosobokova, 1989). This is in good agreement with the observation on deepening of the Atlantic water mass in the Amerasian Basin (Coachman and Barnes, 1963). The sampling scheme during our and earlier studies (Brodsky and Nikitin, 1955; Virketis, 1957, 1959; Kosobokova, 1989) does not allow a more detailed comparison of di!erences in species distribution between the Eurasian and Canadian Basins. Although only a few bathypelagic samples are available, there is an obvious decrease of the species number below 2000 m in both major basins (Fig. 5b). Thus, the three catches obtained hitherto below 2000 m in the Canada Basin contained twelve (Virketis, 1959), fourteen (Harding, 1966) and six copepod species (Kosobokova, 1989). In the three catches in our study the number of species varied between sixteen (Amundsen Basin, station 75) and 23 (Makarov Basin, station 57) below 2000 m. The most diverse zooplankton fauna inhabits more shallow waters in the Arctic Ocean compared to the deepest areas of the World Ocean, where it is generally con"ned to the depths between 500 and 1500}2000 m (Vinogradov, 1970). This seems to be a direct consequence of the `polar emergencea (Zenkevitch, 1963) of the pelagic fauna in the Arctic Ocean. It has been shown for the Canada Basin (Harding, 1966) and is now con"rmed for the Amundsen and Makarov Basins that many of the deep-water species are found at considerably shallower depths in the Arctic compared to the adjacent areas to the south. Many species demonstrate a clear upward shift of

2052

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

the layer of the maximum concentration and the upper limit of vertical distribution. Namely, the copepods Scaphocalanus acrocephalus, Scolecithricella minor, Spinocalanus antarcticus have their maximum concentration between 100 and 300 m depth in our study area, while in the Norwegian Sea they inhabit mostly layers below 600 m ("stvedt, 1955). One of the most striking examples is the copepod Temorites brevis. In the Arctic Ocean it is found up to 50 m depth (Kosobokova, 1989) with maximum concentrations between 100 and 750 m. In contrast, in the Norwegian Sea it inhabits depths well below 900 m ("stvedt, 1955). The present data on the vertical distribution patterns of zooplankton species con"rm that most species are not restricted to any speci"c water mass of the Arctic Ocean, but have a rather wide vertical range. This was explained by the absence of distinct boundaries between the three main water masses by Dunbar and Harding (1968). The only exception from this rule is a well-de"ned group of bathy-pelagic species con"ned exclusively to the Bottom Water. Most of the representatives of this group are endemic to the Arctic Ocean. The vertical patterns of particular species obtained during the present study coincide in principal with the data presented by Dunbar and Harding (1968) and Kosobokova (1989) for the Canada Basin. Where our results disagree with those studies, we obtain a wider depth range for particular species. Striking di!erences between earlier and present patterns were observed only for one species, the copepod Aetideopsis rostrata, which was especially abundant in the ridge area close to the bottom (depths between 750 and 1500 m, Fig. 4a). Earlier studies in the Canada Basin registered this species mainly as late copepodite stages at depths between 100 and 300 m (Dunbar and Harding, 1968; Kosobokova, 1989). The presence of all developmental stages in our study area including mature females and males close to the bottom in samples from benthopelagic sledge (Kosobokova, unpublished observation) suggests its benthopelagic life style. 4.3. Distribution of abundance and biomass The Arctic Ocean has long been considered as an area of very low biological production owing to its short summer and its ice cover. The extremely low biomass estimates of 0.1}0.4 g m\ dry weight by Hopkins (1969a, b) for the Canada and Amundsen Basins have for long been taken as evidence for an extremely poor zooplankton stock all over the Arctic Ocean. However, subsequent studies reported biomass values one order of magnitude higher for di!erent parts of the Ocean (Table 3). Kosobokova (1982) and Wheeler et al. (1996) reported biomass in the Canadian Basin to vary from 1.0 to 4.3 g m\. Hirche and Mumm (1992), Mumm (1993) and Mumm et al. (1998) presented a wider range of values between 1.1 and 5.4 g m\ and demonstrated for the "rst time regional variability within one of the deep Arctic basins, the Nansen Basin. The recent publication by Thibault et al. (1999) reported an even higher biomass up to 12.8 g m\ in the Makarov Basin, which should be due to the impact of amphipods and euphausiids included in their biomass calculations. The transect studied here covered two basins, Amundsen and Makarov, and a prominent ridge, the latter playing an important role in steering the circulation of the Atlantic layer (Rudels et al., 1994). Hence, a strong e!ect of bottom topography and

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2053

current regime on the horizontal distribution of zooplankton populations along the transect was expected in analogy to observations by Hirche and Mumm (1992) and Mumm (1993) in the Nansen Basin. Among these two factors, the bottom topography clearly controlled the distribution of bathypelagic species (Group D, Fig. ), which had their maxima in the basins and were almost or completely absent in the ridge area (stations 51}55). Reduction of abundance of a group of midwater species (Group C, Fig. 8) with distribution minima over the ridge and increasing biomass towards the basins may originate in a cut-o! of the deeper parts of their populations. Surprisingly, two species of midwater copepods were more abundant over the ridge. There is some evidence that they have a benthopelagic life style with a preference for shallower water depths: Aetideopsis rostrata has been mentioned above: Xanthocalanus groenlandicus belongs to a genus which is mostly represented by benthopelagic species (Sirenko et al., 1996). The distribution patterns of deep living copepods along the transect show clearly that there is reduced between-basin exchange across the Lomonosov Ridge, which is also indicated by di!erent temperature and salinity of the deep waters on either side of the ridge. However, living conditions seem to be adequate in both Eurasian and Amerasian Basins, re#ected also by the similarity of species composition and age structure in both basins. Also, Rudels et al. (2000) assume that intermittent deep exchanges between the basins in both directions are likely to occur. The dynamics of the Atlantic layer also strongly in#uenced the zooplankton distribution in the upper 1000 m. According to recent hydrographic studies, Atlantic water enters the Arctic Ocean through Fram Strait and over the Barents and Kara Shelfs (Rudels et al., 1994). It is trapped in a boundary current running counterclockwise along the perimeter of the Arctic Ocean (Perkin and Lewis, 1984; Aagaard, 1989). Recirculating branches of the Atlantic water are de#ected where mid ocean ridges meet the Eurasian Shelf, e.g., the Nansen-Gakkel Ridge (Anderson et al., 1989), the Lomonosov Ridge (Aagaard, 1989; Anderson et al., 1989), and the Alpha-Mendeleev Ridge (Rudels et al., 1994). This in#ow is the most important feature determining the distribution of zooplankton biomass within the Eurasian Basin, as it advects a large biomass from the Greenland and Barents Seas (Hirche and Mumm, 1992; Mumm, 1993; Mumm et al., 1998). During the present study the core of the Atlantic water was found somewhat west of the crest of the Lomonosov Ridge (Fig. 2; Rudels et al., 2000). The biomass maximum of 9.5 g m\ was located there (Table 3), formed mainly by species with centres of distribution in the upper 200 to 300 m, and Calanus hyperboreus and Metridia longa being the main copepod contributors (Fig. 6a). Of six mesopelagic Atlantic copepods, four species (Gaetanus tenuispinus, Heterorhabdus norvegicus, Aetideopsis rostrata and A.minor) also demonstrated a clear increase of abundance and biomass in the ridge area, while two others (Chiridius obtusifrons and Scaphocalanus acrocephalus) did not show any signi"cant trend. In the non-copepod zooplankton, biomass of chaetognaths and ostracods was also elevated (Fig. 6b). As Calanus hyperboreus and Metridia longa are very abundant in the Greenland Sea (Hirche, 1991, 1997; Hirche et al., 1994; Richter, 1994) and less important in the Barents Sea, the origin of the individuals at station 51a was most likely Fram Strait.

2054

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Table 3 Zooplankton biomass (g m\, dry mass) for di!erent regions of the Arctic Ocean and adjacent Northern Atlantic. Biomass was calculated for di!erent depths to facilitate comparison Region FRAM STRAIT West Spitsbergen Current NANSEN BASIN South-Western Nansen Basin Inner Western Nansen Basin Eastern Nansen Basin NANSEN-GAKKEL RIDGE AMUNDSEN BASIN north of Greenland Western Amundsen Basin Eastern Amundsen Basin

North Pole LOMONOSOV RIDGE 87}883N 81N

Sampling depth, m

Biomass, g m\

Source

6.7

Hirche et al. (1991)

500 500 1500 500 1500 500

4.5}5.4 1.2}1.8 2.4 5.3}7.2 4.1}4.4 0.9}1.3

Mumm et al. (1993) Mumm et al. (1998) Auel (1995) Kosobokova et al. (1998) Kosobokova et al. (1998) Mumm et al. (1998)

500 1500 500 500 1500 500 bottom 100 500

0.2}0.3 0.2}0.4 1.3}2.3 3.6}3.9 4.7}5.1 2.1}4.3 3.4}6.0 4.8
2.3

Hopkins (1969a) Hopkins (1969a) Mumm et al. (1998) Kosobokova et al. (1998) Kosobokova et al. (1998) This study This study Wheeler et al. (1996) Mumm et al. (1998)

500 500 bottom

1.0}1.9 5.0}7.5 6.6}9.5

Mumm et al. (1998) This study This study

1.0}3.0 3.0}4.3
1.4}1.8 3.5}4.3 4.4}5.2

Kosobokova (1982) Wheeler et al. (1996) Mumm et al. (1998) This study This study

0.1}0.2 0.2}0.4 1.2}2.0 1.3}2.1

Hopkins (1969b) Hopkins (1969b) This study This study

100

MAKAROV BASIN 80}833N 1000 75}873N 100 883N 500 Eastern Makarov Basin, 81 3N 500 bottom CANADA BASIN Beaufort Sea 500 Alpha-Mendeleev Ridge bottom 500 bottom

Calanus xnmarchicus, C. glacialis; C. hyperboreus and Metridia longa only.
Conversion from mesozooplankton carbon (C) after C"0.4;DW. Conversion from wet weight (WW) to dry weight (DM) after DW"0.16;WW.

Questions arise whether these animals travelled all the way from Fram Strait, or whether local growth and advection from other shelf seas also was contributing. Stage composition shows di!erent age structures for the most important species (Fig. 9). Calanus xnmarchicus is almost exclusively represented as CV and adults, a clear indication of reproductive failure. This was observed earlier in the Eurasian Basin by Hirche and Mumm (1992). C. hyperboreus in the Arctic Ocean reproduces between

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2055

April and May (Pavshtiks, 1976). Therefore the presence of CII and CIII can be taken as a sign of active growth of the new generation (Hirche, 1997). The high portion of young stages of Metridia longa and C. glacialis and the presence of all copepodite and adult stages points clearly to regular reproductive success. The Laptev Sea polynya may be one of the feeding grounds for advected and local populations, as indicated by high egg production rates of C. glacialis in this area (Kosobokova and Hirche, in prep.). In contrast to the similarities in abundance and biomass between the Amundsen and Makarov Basins, abundances there were at least one order of magnitude higher at each depth, and biomass was higher by a factor of 2 or 3, compared to historical data from the Canada Basin (Fig. 6, Table 3). The data of Hopkins (1969a, b) and Minoda (1967) on biomass are one order of magnitude lower than comparable studies and are ignored here. However, our measurements in the Amundsen and Makarov Basins agree well with other recent observations (Table 3). While the relative composition of zooplankton is similar in the Nansen, Amundsen and Makarov Basins (Table 2), in the Canada Basin the proportion of copepods was higher because of a dramatic decrease of non-copepod zooplankton. Appendicularians have almost disappeared, and the biomass of chaetognaths was less than half of that found in the Eurasian Basins (Fig. 6b). This was due mainly to the strong decrease of the proportion of the Atlantic species Eukrohnia. Within the copepods, the complete absence of the Atlantic indicator species Calanus xnmarchicus in the Canada Basin is noteworthy. On the other hand, C. hyperboreus makes up ca. 40% of zooplankton biomass. Combination of our data and historic measurements (Table 3) with present concepts on the circulation of the Arctic Ocean, allows one to draw a general picture of the distribution of zooplankton. The Arctic Ocean hosts two zooplankton communities: an autochthonous one of low biomass (ca. 1 g m\) consisting mainly of copepods, which in its purest form is found in the inner basins, and an allochthonous community of mostly Atlantic species, which has its biomass maximum in the boundary current. The proportion of this community decreases rapidly as the water from the boundary current spreads laterally within the Atlantic in#ow into the basin interior over the slopes and ridges that mark the basin margins (Carmack et al., 1997). Lateral advection may also cause reduction in the biomass of the boundary current as it recirculates. We can now track this community in the Nansen Basin north of the Barents Shelf (Hirche and Mumm, 1992), o! the Laptev Shelf (Kosobokova et al., 1998), and on top of the Lomonosov Ridge (this study). According to the circulation of the boundary current, maxima are also expected on the margins of the Canadian Basin and in recirculation branches over the Alpha-Mendeleev Ridge. Not much information is available for comparison of the inner basins. Where only the upper layers were sampled (e.g. Nansen Basin, Table 3) seasonal ontogenetic migration may bias the results. The low biomass in the Canada Basin, and especially the decrease in its allochthonous components, is explained by the fact that this basin is more isolated from the Atlantic in#ow and has a much longer renewal time than the other basins (Swift et al., 1997). This view is supported by the dominance of C. hyperboreus as a leftover of the allochthonous fauna. This is the species with probably the largest starvation potential and a multi-year life cycle, which is extremely

2056

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

adapted to a short growing season (Hirche, 1997). This does not necessarily mean that allochthonous species are not able to survive in the Arctic Ocean. Stage distributions on our transect (Fig. 9) showed the contrary to be true at least for some species. The reduction of Atlantic species in the inner basins, however, shows that the alloch-

Fig. 9. Relative stage composition (C"copepodite stage) of four copepods along the transect across the Lomonosov Ridge.

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2057

thonous community is dependent on advection and therefore is subjected to climatic variability. Thus recently reported changes in the intensity of the Atlantic in#ow and their e!ect, especially on the Makarov Basin, should have strong in#uence on the zooplankton biomass and distribution according to the concept laid out above. From past investigations it seems that the water mass structure characteristic of the Canadian Basin extended fully to the Lomonosov Ridge, where a front separated the two hydrographic domains (Kinney et al., 1970; Moore et al., 1983; Gorshkov, 1983). Now there is evidence for recent lateral displacement of the upper halocline water from the Chukchi-East Siberian Sea region by water from the Eurasian Basin, which was accompanied by a shift of the sharp Atlantic-Paci"c boundary to the Mendeleev Ridge, 600 km east of the Lomonosov Ridge (McLaughlin et al., 1996). The recent warming at intermediate depth of the Makarov Basin has resulted from in#ow of Atlantic waters (Carmack et al., 1995; Swift et al., 1997). It is suggested that this warming and water mass transition has occurred in the Arctic Ocean during the 1990s, with a rapid propagation of the temperature signal by topographically steered boundary currents into at least three of the four major basins (Carmack et al., 1997). As in the hydrographic data on our transect (Rudels et al., 2000), there was no indication in the zooplankton data of a front east of the Lomonosov Ridge. It is therefore very likely that this recent invasion of warm water was associated with an invasion of Atlantic zooplankton. As a consequence, the relatively high biomass found during our study in the Makarov Basin may have replaced the previous poor stock of the Canadian Basin. This would imply a tremendous increase of zooplankton carbon considering the size of the Makarov Basin. Also, the unusual biomass peak on the Lomonosov Ridge may be an indication of increased advective intensity, eventually in combination with good growth conditions for the advected fauna underway.

Acknowledgements We appreciate the help of T. Scherzinger with sample collection and the great support of K.K. in the computer work and H. Deubel for help with cluster analysis. U. Schauer and B. Rudels provided hydrography data. This work was supported by BMBF Grant 03F11GUS and RFBR grant 98-05-64526 to K. Kosobokova. This is publication no. 1814 from the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven.

References Aagaard, K., 1989. A synthesis of the Arctic Ocean circulation. Rapports et Process-verbaux des ReH unions Conseil International pour l' Exploration de la Mer 188, 11}22. Anderson, L.G., Jones, E.P., Koltermann, K.P., Schlosser, P., Swift, J.H., Wallace, D.W.R., 1989. The "rst oceanographic section across the Nansen Basin in the Arctic Ocean. Deep-Sea Research 36, 475}482. Bogorov, V.G. (1946). Zooplankton from the collections of the expedition of the ice breaker `G. Sedova 1937}1939 (in Russian). In: Trudy dr. eksp. Glavsevmorputi l/p `G. Sedova in 1937}1940. 3, 336}370.

2058

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

Brodsky, K. A., 1957. Fauna of Copepoda (Calanoida) and zoogeographic zonation of the northern part of the Paci"c Ocean and adjacent waters (in Russian). Akademiya Nauk, Moscow-Leningrad, 220 pp. Brodsky, K.A., 1967. Calanoida of the far eastern seas and Polar Basin of the USSR. In: Pavlovskii, E.N. (Ed.), Keys to the fauna of the USSR. Israel Program for Science Translation Jerusalem, 35, pp. 440. Brodsky, K.A., Nikitin, M.N., 1955. Observational data of the scienti"c research drifting station of 1950}1951. Hydrobiological work (in Russian). Izd Morsk Transp 1, 404}410. Brodsky, K.A., Pavshtiks, Y.A., 1976. Plankton of the central part of the Arctic Basin (in Russian). Polar Geography 1, 143}161. Carmack, E.C., Aagaard, K., Swift, J.H., MacDonald, R.W., McLaughlin, F.A., Jones, E.P., Perkin, R.G., Smith, J.N., Elis, K.M., Killius, L.R., 1997. Changes in temperature and tracer distributions within the Arctic Ocean: results from the 1994 Arctic Ocean section. Deep-Sea Research Part II: Topical Studies in Oceanography 44, 1487}1502. Carmack, E.C., Macdonald, R.W., Perkin, R.G., McLaughlin, F.A., Pearson, R.J., 1995. Evidence for warming of Atlantic water in the southern Canadian Basin of the Arctic Ocean: Results from the Larsen-93 expedition. Geophysical Research Letters 22, 1061}1064. Chislenko, L.L., 1968. Nomograms for determining the weight of aquatic organisms according to the size and shape of their body (marine mesobenthos and plankton) Nauka, Leningrad (in Russian). Coachman, L.K., Barnes C.A., 1963. The movement of Atlantic water in the Arctic Ocean. Arctic 16, 8}16. Damkaer, D.M., 1975. Calanoid copepods of the genera Spinocalanus and Mimocalanus from the central Arctic ocean, with a review of the Spinocalanidae. NOAA Tech. Rep. NMFS CIRC-391, Seattle, 88 pp. Dawson, J.K., 1978. Vertical distribution of Calanus hyperboreus in the central Arctic Ocean. Limnology and Oceanography 23, 950}957. Dunbar, M.J., Harding, G., 1968. Arctic Ocean water masses and plankton * a reappraisal. In: Sater, J.E. (coord.) (Ed.), Arctic Drifting Stations: a report on activities supported by the O$ce of Naval Research. Arctic Institute of North America, pp. 315}326. Eicken, H., Kolatschek, J., Evers, K.-U., Jochmann, P., 1997. Sea ice physics and remote sensing. Observations on sea-ise conditions in the Laptev Sea during the cruise. Berichte der Polarforsching 226, 40}43. Field, J.G., Clarke, K.R., Warwick, R.M., 1982. A practical strategy for analysing multispecies distribution patterns. Marine Ecological Program Series 8, 37}52. Geynrikh, A.K., Kosobokova, K.N., Rudyakov, Y.A., 1983. Seasonal variations in the vertical distribution of some proli"c copepods of the Arctic basin. Canadian Translation Fisheries and Aquatic Science 4925, 1}22. Gorshkov, S.G., 1983. World Ocean Atlas. Pergamon, New York 3 (Arctic Ocean), pp. 189. Gosselin, M., Levasseur, M., Wheeler, P.A., Horner, R.A., Booth B.C., 1997. New measurements of phytoplankton and ice algal production in the Arctic Ocean. Deep-Sea Research Part II: Topical Studies in Oceanography 44 (8), 1623}1644. Grainger, E.H., 1965. Zooplankton from the Arctic Ocean and adjacent Canadian waters. Journal of Fisheries Research Board, Canada 22, 543}564. Grainger, E.H., 1989. Vertical distribution of zooplankton in the central Arctic ocean. In: Rey, L., Alexander, V. (Eds.), Proceedings of the Sixth Conference ComiteH Arctique International, pp. 48}60. Gray, J.S., Aschan, M.R., Carr, K.R., Clarke, R.H., Green, T.H., Pearson, T.H., Rosenberg, R., Warwick, R.M., 1988. Analysis of community attributes of the benthic macrofauna of Frierfjord/Langesundfjord and in a mesocosm experiment. Marine Ecological Program Series 46, 151}165. Grice, G.D., 1962. Copepods collected by the nuclear submarine Seadragon on a cruise to and from the North Pole, with remarks on their geographic distribution. Journal of Marine Research 20, 97}109. Harding, G.C., 1966. Zooplankton distribution in the Arctic Ocean with notes of life cycles. M.S. Thesis. McGill University 134 pp. Heptner, M.V., 1969. Systematic status of Lucicutia polaris Brodsky 1950 (Copepoda, Lucicutiidae) and description of L. pseudopolaris sp.n. from the Polar Basin (in Russian). Zoology Journal 48, 197}206. Hirche, H.J., 1991. Distribution of dominant calanoid copepod species in the Greenland Sea during late fall. Polar Biology 11, 351}362. Hirche, H.J., 1997. Life cycle of the copepod Calanus hyperboreus in the Greenland Sea. Marine Biology 128, 607}618.

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

2059

Hirche, H.J., Hagen, W., Mumm, N., Richter, C., 1994. The Northeast Water Polynya, Greenland Sea. III. Meso- and macrozooplankton distribution and production of dominant herbivorous copepods during summer. Polar Biology 14, 491}503. Hirche, H.J., Mumm, N., 1992. Distribution of dominant copepods in the Nansen Basin, Arctic Ocean, in summer. Deep-Sea Research 39 (Suppl. 2), S485}S505. Hopkins, T.L., 1969a. Zooplankton standing crop in the Arctic Basin. Limnology and Oceanography 14, 80}95. Hopkins, T.L., 1969b. Zooplankton biomass related to hydrography along the drift track of Arlis II in the Arctic Basin and the East Greenland Current. Journal of Fisheries Research Board, Canada 26, 305}310. Huges, K. H., 1968. Seasonal vertical distribution of copepods in the Arctic water in the Canadian basin of the North Polar Sea. MSc. Thesis, University of Washington. Jaschnov, V.A., 1940. Plankton productivity of the northern seas of the USSR (in Russian). Moskovskoe Obchestvo Ispytatelei Prirody Press, Moscow. Johannessen, O.M., Miles, M., Bj+rgo, E., 1995. The Arctic's shrinking sea ice. Nature 376, 126}127. Johnson, M.W., 1963. Zooplankton collections from the high polar basins with special reference to the Copepoda. Limnology and Oceanography 8, 89}102. Kinney, P., Arhelger, M.E., Burrell, D.C., 1970. Chemical characteristics of water masses in the Amerasian Basin of the Arctic Ocean. Journal of Geophysical Research 90, 4911}4930. Kosobokova, K.N., 1980. Seasonal changes of the vertical distribution and age composition of Microcalanus pygmaeus, Oithona similis, Oncaea borealis and O. notopus populations in the central Arctic basin (in Russian). In: Biologiya Tsentral'nogo Arkicheskogo basseyna (Biology of the central Arctic Basin). Nauka, Moscow, pp. 167}182. Kosobokova, K.N. 1981. Zooplankton of the Arctic Ocean (in Russian). PhD Thesis, Institution of Oceanology, Akad, Science, Moscow, 150. Kosobokova, K.N., 1982. Composition and distribution of the biomass of zooplankton in the central Arctic Basin. Oceanology 22, 744}750. Kosobokova, K.N., 1989. Vertical distribution of plankton animals in the eastern part of the central Arctic Basin (in Russian). Explorations of the Fauna of the Seas. Marine Plankton, Leningrad 41 (49), 24}31. Kosobokova, K., Hanssen, H., Hirche, H.-J., Knickmeier, K., 1998. Composition and distribution of zooplankton in the Laptev Sea and adjacent Nansen basin during summer, 1993. Polar Biology 19, 63}76. Macdonald, R.W., 1996. Awakenings in the Arctic. Nature 380, 286}287. Markhaseva, E.L., 1984. Copepods Aetideidae (Copepoda Calanoida) from the eastern sector of the central Arctic Basin (in Russian). Okeanologiya 29, 511}514. Markhaseva, E.L., 1998. New species of the genus Xanthocalanus (Copepoda, Calanoida, Phaennidae) from the Laptev Sea. Journal of Marine System 15, 413}419. Markhaseva, E.L., Kosobokova, K.N., 1998. New and rare species of calanoid copepods from the central Arctic basin (Crustacea, Copepoda). Zoosyst. Rossica 7, 45}53. McLaughlin, F.A., Carmack, E.C., Macdonald, R.W., Bishop, J.K.B., 1996. Physical and geochemical properties across the Atlantic/Paci"c water mass boundary in the southern Canadian Basin. Journal of Geophysical Research 101, 1183}1197. Minoda, T., 1967. Seasonal distribution of Copepoda in the Arctic Ocean from June to December, 1964. Records of Oceanographic Works in Japan 9, 161}168. Moore, R.M., Lowings, M.C., Tan, F.C., 1983. Geochemical pro"les in the central Arctic Ocean: Their relation to freezing and shallow circulation. Journal of Geophysical Research 88, 2667}2674. Mumm, N., 1993. Composition and distribution of mesozooplankton in the Nansen Basin. Arctic Ocean, during summer. Polar Biology 13, 451}461. Mumm, N., Auel, H., Hanssen, H., Hagen, W., Richter, C., Hirche, H.-J., 1998. Breaking the ice: large-scale distribution of mesozooplankton after a decade of Arctic and transpolar cruises. Polar Biology 20, 189}197.

2060

K. Kosobokova, H.-J. Hirche / Deep-Sea Research I 47 (2000) 2029}2060

"stvedt, O.J., 1955. Zooplankton investigations from weathership `Ma in the Norwegian Sea, 1948}49. Hvalra detcs Skrifter 40, 1}93. Parkinson, C.L., Cavalieri, D.J., 1989. Arctic sea ice 1973}1987: seasonal, regional, and interannual variability. Journal of Geophysical Research 94, 14499}14523. Pavshtiks, E.A., 1971a. Hydrobiological characteristics of waters of the Arctic Basin in the drifting area of `North Pole 17a (in Russian). Trudy Arkticheskogo Antarkticheskogo Nauchro-Issledovatel'skego Institute 30, 63}69. Pavshtiks, E.A., 1971b. Seasonal variations of zooplankton abundance in the area of the North Pole in Russian). Dokladyi Akademiyi Nauk 196, 441}444. Pavshtiks, E.A., 1976. Biological seasons and the life span of Calanus hyperboreus Kr+yer in the central Arctic (in Russian). In: Nature and Economy of the North, 4, pp. 121}127 (in Russian). Pavshtiks, E.A., 1977. Seasonal changes in the stage composition of the populations of calanoid copepods in the Arctic Basin (in Russian). Issledovaniya Fauny Morei, Nauk St. Petersb 19, 56}73. Perkin, R.G., Lewis, E.L., 1984. Mixing in the West Spitsbergen Current. Journal of Physical Oceanography 14, 1315}1325. Richter, C. (1994). Regional and seasonal variability in the vertical distribution of mesozooplankton in the Greenland Sea. Thesis. University of Kiel, 101 pp. Rudels, B., Jones, P., Anderson, L., Kattner, G., 1994. On the intermediate depth waters of the Arctic ocean. In: The Polar Oceans and Their Role in Shaping the Global Environment Geophysical Monograph, 85, pp. 33}46. Rudels, B., Muench, R.D., Gunn, J., Schauer, U. The Arctic ocean north of the Siberian shelves: advection, lateral mixing, slope convection and intermediate and deep water characteristics. Journal of Marine Systems, in press. Rudyakov, Y.A., 1983. Vertical distribution of Calanus hyperboreus (Copepoda) in the Central Arctic basin. Oceanology 23, 249}254. Sars, G.O. (1903). An account of the crustacea of Norway. Copepoda (Calanoida)., Christiania and Copenhagen 4, 171 pp. Sars, G.O., Crustacea, V., 1900. In: Nansen, F. (Ed.), The Norwegian North Polar Expedition 1893}1896. Christiania 1, Jakob Dybwad. Schlosser, P., Swift, J.H., Lewis, D., P"rman, S.L., 1995. The role of the large-scale Arctic Ocean circulation in the transport of contaminants. Deep-Sea Research II: Topical Studies in Oceanography 42, 1341}1367. Shirshov, P.P., 1938. Oceanographical observations (in Russian). Dokllady Akademie Naukori SSR 17, 569}580. Sirenko, B.I., Markhaseva, E.L., Buzhinskaya, G.N., Golikov, A.A., Menshutkina, T.V., Petryashov, V.V., Semenova, T.N., Stepanjants, S.D., Vassilenko, S.V., 1996. Preliminary data on suprabenthic invertebrates collected during RV Polarstern cruise in the Laptev Sea. Polar Biology 16, 345}352. Swift, J.H., Jones, E.P., Aagaard, K., Carmack, E.C., Hingston, M., MacDonald, R.W., McLaughlin, F.A., Perkin, R.G., 1997. Waters of the Makarov and Canada basins. Deep-Sea Research II: Topical Studies in Oceanography 44, 1503}1529. Thibault, D., Head, E.J.H., Wheeler, P.A., 1999. Mesozooplankton in the Arctic Ocean in summer. Deep-Sea Research I 46, 1391}1415. Vinogradov, M.Y., 1970. Vertical distribution of oceanic zooplankton. Israel Program for Scienti"c Translations, Jerusalem, pp. 339. Virketis, M.A., 1957. Some data on the zooplankton from the central part of the Arctic Basin (in Russian). Materials of scienti"c observations of the drift stations `North Pole 3a and `North Pole 4a 1954/55. Moscow 1, 238}311. Virketis, M.A., 1959. Materials on the zooplankton of the central part of the Arctic Ocean (in Russian). Results of scienti"c observations on the drift stations `North Pole 4a and North Pole 5a, 1955}1956, Moscow, 132}206. Wheeler, P.A., Gosselin, M., Sherr, E., Thibault, D., Kirchman, D.L., Benner, R., Whitledge, T.E., 1996. Active cycling of organic carbon in the central Arctic Ocean. In: Nature, pp. 697}699. Zenkevitch, L., 1963. Biology of the seas of the USSR. Allen & Unwin, London, pp. 955.