Abundance of zooplankton during the polar night (cruise in January 2017) at 13 stations using a 64um-mesh Multinet

Study Extent

13 stations were sampled between 2017-01-06 and 2017-01-17: Six oceanic stations: TB1 and TB2 (located in the western Barents Sea), NS1, NS4 and NS10 (located on the shelf north of Svalbard) and NS6 (located off the shelf north of Svalbard). Seven fjord stations: VMF9 (located near Bellsund, at the opening of Van Mijenfjorden), KF1, KF2 and KF3 (located in Krossfjorden on the west coast of Svalbard), and R3, R3b and R4 (located in Rijpfjorden on the northern coast of Nordaustlandet).

Sampling

Zooplankton was sampled using vertically stratified net hauls with a multiple opening/closing net (MultiNet type Midi, Hydro-Bios, Germany, mouth opening 0.25 m2, 64-µm mesh, towing speed 0.4 m s-1). The four depth strata sampled were 3-50 m, 50-100 m, 100-200 m and 200-400 m, or to 10 m above the bottom at stations shallower than 400 m. A 64-µm mesh WP-2 net (Hydro-Bios, Germany, opening 0.25 m2) was used for the 0-50 m sampling at station TB2 due to a tear in the MultiNet net bag. A technical error resulted in only the upper 100 m being sampled at station KF1.

Method steps

1. Immediately after collection the samples were fixed in hexamethylenetetramine-buffered formaldehyde in seawater solution at 4% final concentration. The samples were later analyzed under a stereomicroscope (Olympus SZX7). Organisms with total length > 5 mm were sorted from the sample, identified and counted. Then aliquots were taken with a 2-mL pipette with the tip cut at 5-mm diameter to allow collection of mesozooplankton. The number of aliquots and subsamples analyzed was chosen so that at least 300 individuals were counted in each sample. The remainder of the sample was screened for rare species. Species were identified to the lowest taxonomic level and classified as holoplankton or meroplankton. For copepods, a detailed analysis of copepodid stage composition was performed for Calanus spp., O. similis, M. norvegica, Pseudocalanus spp., M. longa and Microcalanus spp. The CI to CIII stages were not differentiated for Microcalanus spp. at station TB1, and for O. similis and M. norvegica at all stations. Early stages (CI to CIII) classified as Microcalanus spp. probably included young copepodids of Paracalanus spp., Clausocalanus spp. and Ctenocalanus spp., because it is difficult to distinguish these species via visual identification. The CI to CIII stages of M. longa and M. lucens were not differentiated, and were designated M. longa. The three Calanus species were differentiated on the basis of size (Kwasniewski et al. 2003); this involves some uncertainty because prosome lengths of species of the genus can overlap (Gabrielsen et al. 2012; Choquet et al. 2018). Copepod nauplii were determined to order (Calanoida, Cyclopoida and Harpacticoida).

1. Bopyridae
2. Isopoda
3. Meganyctiphanes norvegica
4. Thysanoessa inermis
5. Thysanoessa longicaudata
6. Thysanoessa raschii
7. Sarsia
8. Hydrozoa
9. Hydrozoa larvae
10. Dimophyes arctica
11. Siphonophorae
12. Beroe cucumis
13. Mertensia ovum
14. Nemertea pilidium
15. Pelagobia
16. Tomopteris septentrionalis
17. Echinodermata larvae
18. Bryozoa larvae
19. Fritillaria borealis
20. Decapoda
21. Amphipoda
22. Ctenophora
23. Rotifera
24. Turbelaria
25. Acartia clausii
26. Acartia longiremis
27. Aetideidae
28. Aetideopsis minor
29. Bradyidius similis
30. Calanus finmarchicus
31. Calanus glacialis
32. Calanus hyperboreus
33. Candacia armata
34. Clytmenestra scutellata
35. Ctenocalanus
36. Gaetanus tenuispinus
37. Heterorhabdus norvegicus
38. Jashnovia brevis
39. Metridia longa
40. Metridia lucens
    rank: species
41. Paracalanus/Clausocalanus/Ctenocalanus CIV-CV
42. Paraeuchaeta
    rank: genus
43. Pleuromamma robusta
    rank: species
44. Pseudocalanus
    rank: genus
45. Scolecithricella minor
    rank: species
46. Spinocalanus
    rank: genus
47. Calanoida nauplii
48. Cyclopoida nauplii
49. Harpacticoida nauplii
50. Clausocalanus
51. Calocalanus styliremis
52. Cyclopina schneideri
53. Harpacticoida
54. Microcalanus
55. Microsetella norvegica
56. Microsetella rosea
57. Oithona atlantica
58. Oithona similis
59. Oithona nana
60. Oncaeidae
61. Paracalanus
62. Aglantha digitale
63. Bivalvia
64. Cirripedia
65. Clione limacina
66. Erythrops erythrophthalma
67. Eukrohnia hamata
68. Euphausiacea
69. Gastropoda veliger
70. Limacina helicina
71. Limacina retroversa
72. Oikopleura
73. Ostracods
74. Polychaeta
75. Sagitta elegans
76. Themisto abyssorum

13 stations were sampled: Six oceanic stations: TB1 and TB2 (located in the western Barents Sea), NS1, NS4 and NS10 (located on the shelf north of Svalbard) and NS6 (located off the shelf north of Svalbard). Seven fjord stations: VMF9 (located near Bellsund, at the opening of Van Mijenfjorden), KF1, KF2 and KF3 (located in Krossfjorden on the west coast of Svalbard), and R3, R3b and R4 (located in Rijpfjorden on the northern coast of Nordaustlandet).