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Near-bottom zooplankton communities have rarely been studied despite numerous reports of high zooplankton concentrations, probably due to methodological constraints. In Kongsfjorden, Svalbard, the near-bottom layer was studied for the first time by combining daytime deployments of a remotely operated vehicle (ROV), the optical zooplankton sensor moored on-sight key species investigation (MOKI), and Tucker trawl sampling. ROV data from the fjord entrance and the inner fjord showed high near-bottom abundances of euphausiids with a mean concentration of 17.3 ± 3.5 n × 100 m−3. With the MOKI system, we observed varying numbers of euphausiids, amphipods, chaetognaths, and copepods on the seafloor at six stations. Light-induced zooplankton swarms reached densities in the order of 90,000 (euphausiids), 120,000 (amphipods), and 470,000 ind m−3 (chaetognaths), whereas older copepodids of Calanus hyperboreus and C. glacialis did not respond to light. They were abundant at the seafloor and 5 and 15 m above and showed maximum abundance of 65,000 ind m−3. Tucker trawl data provided an overview of the seasonal vertical distribution of euphausiids. The most abundant species Thysanoessa inermis reached near-bottom concentrations of 270 ind m−3. Regional distribution was neither related to depth nor to location in the fjord. The taxa observed were all part of the pelagic community. Our observations suggest the presence of near-bottom macrozooplankton also in other regions and challenge the current view of bentho–pelagic coupling. Neglecting this community may cause severe underestimates of the stock of pelagic zooplankton, especially predatory species, which link secondary production with higher trophic levels.

Zooplankton are the link connecting primary producers to higher trophic levels, and knowing their distribution and community is important for predicting the distribution of predator species, like fish, seabirds, and marine mammals. However, data from open Arctic oceans are still scarce. In autumn, tens of millions of the planktivorous little auks (Alle alle) (about 75 % of the world’s population) and millions of thick-billed murres (Uria lomvia) pass through the Baffin Bay. To investigate their potential food sources, we investigated the spatial and vertical distribution of zooplankton and small fishes in the upper 500 m of southern Baffin Bay in September 2009. The zooplankton community was dominated by copepods (55 % of abundance in the upper 500 m), primarily of the genus Calanus. Other important zooplankton taxa included Limacina helicina, Chaetognatha, and Cirripedia nauplii. On the Greenland Shelf, most Calanus were late copepodite stages and most were found at the depths of >200 m, suggesting they were in diapause. On the Canadian Shelf, there were relatively more Calanus in the near-surface layers, which were probably still actively feeding and which were available to visual predators such as seabirds and fish. The acoustic survey showed the highest density of polar cod Boreogadus saida in the upper 50 m on the western part of the Greenland Shelf. A particularly high biomass of both zooplankton and polar cod was found in the central part of the basin in association with a local relatively shallow area.

This paper presents the first comprehensive insight into the structure of planktonic communities, from nanoprotists to mesozooplankton in the waters north of Svalbard during the dark Arctic winter. Our investigations were carried out in two contrasting locations comparing a shallow fjord, Rijpfjorden, and a deep ice-covered region, Sofiadjupet, in January 2012. They revealed low chloro- phyll a concentrations (\0.05 mg m -2 ), as a result of the low protist biomass (0.54-2.04 mg C m -2 ). Overall, 27 plastidic protist taxa were observed including 14 auto- trophic taxa and one typical early Arctic spring bloom species (Entomoneis sp.). The presence of these photo- trophic organisms, mostly dinoflagellates, showed that some pelagic autotrophs are able to survive the dark winter period. The high mesozooplankton biomass (6-8 g C m -2 ) indicated a healthy community despite the scarcity of pri- mary producers. Three Calanus species dominated the mesoplanktonic biomass (79 %) in Rijpfjorden, whereas in Sofiadjupet carnivorous Chaetognatha (72 %) were domi- nant. Vertical distribution/profiles of the different trophic categories showed that herbivores were concentrated in the surface water layers (20-0 and 100-10 m) at both sites, whereas predators were occupying the deeper waters (600-100 m) of Sofiadjupet. Our results indicate that high mesozooplankton biomass is concentrated in surface waters. This contradicts the paradigm that mid-January, Arctic zooplankton communities overwinter in deep waters in a dormant state. Moreover, this work fills an evident gap in existing knowledge of Arctic plankton standing stocks and the relationships between its various size fractions during the polar night.

Chaetognaths are among the most abundant predators in the Southern Ocean and are potentially important components in the biological carbon pump due to the production of large, fast-sinking fecal pellets. In situ S. gazellae abundance, fecal pellet production, sinking rates, carbon content, and vertical carbon fluxes were measured at the Lazarev Sea between December 2005 and January 2006. Sagitta gazellae produce fecal pellets that sink at speeds of 33–600 m day−1 and have carbon contents of 0.01–0.8 mg C pellet−1. Vertical carbon flux was later compared with the total carbon flux measured at 360 m depth at the study area. Rough estimates using published seasonal abundance of S. gazellae indicate that, at 360 m depth in the Lazarev Sea, this specie may contribute 12 and 5% of the total vertical carbon flux in winter (ice-covered) and summer (ice-free), respectively. Thus, the role of chaetognaths in the downward transport of organic matter may be far more important than previously thought.

In the Lazarev Sea, Southern Ocean, I studied the population structure and reproduction of Eukrohnia bathypelagica and E. bathyantarctica during winter and summer. A lack of unimodality in their population structures indicated at least two generations building one population and a life cycle longer than 1 year. During both seasons all maturity stages were present; therefore, continuous reproduction is very probable. Extensive breeding seasons with several releases of eggs by one generation are assumed, as adults with empty marsupial sacs continue to build new ova. E. bathypelagica carried between 86 and 128 eggs in both marsupial sacs. E. bathyantarctica had only between 8 and 13 eggs. Although self-fertilization seemed at least to be possible in E. bathyantarctica, cross-fertilization appears to be more common in both species, as most individuals developed testes and ovaries consecutively.

We conducted multinet sampling during winter and summer in the Southern Ocean (Atlantic sector) to investigate the effect of water mass, season and water depth on abundance and species composition of meso- and bathypelagic chaetognaths. Eukrohnia hamata (mean 115 ind. 1,000 m−3) and Sagitta marri (mean 51 ind. 1,000 m−3) were dominant, complemented by E. bathypelagica (mean 19 ind. 1,000 m−3) and E. bathyantarctica (mean 19 ind. 1,000 m−3) below 1,000 m. A further six species were identified, among them the rare bathypelagic species Heterokrohnia fragilis and the subtropical Eukrohnia macroneura that is new to the Antarctic. Water depth and season were the principal determinants of abundance and species composition patterns, indicating vertical seasonal migration and vertical segregation of species. The life cycles of E. hamata and S. marri were studied additionally. Their maturity stages were vertically segregated and prolonged reproductive periods are suggested for both species.

The shape and surface structures of the grasping spines and teeth of the Antarctic chaetognaths Eukrohnia hamata, Sagitta gazellae and S. marri collected in Bransfield Strait, were studied by scanning electron microscopy. The characters of arrangement, shape, curvature, surface, edges, and tips of hooks and teeth are described and discussed.

Trophodynamics and predation impact of the 2 dominant chaetognaths Eukrohnia hamata and Sagitta gazellae were investigated at 19 stations in the vicinity of the Prince Edward Islands and at a 24-h station occupied at the sub-Antarctic Front in late summer (April/May) 1996. During the entire investigation, the zooplankton assemblages were numerically dominated by copepods with densities ranging from 21 to 170 ind. m−3. Amongst the copepods, Clausocalanus brevipes, Metridia gerlachei and M. lucens dominated accounting for >90% of the total. Generally, chaetognaths were identified as the second most important group composing at times up to 30% (mean = 14.7%) of total zooplankton abundance. Of the two chaetognath species, E.␣hamata was generally numerically dominant. Gut content analysis showed that both chaetognath species are opportunistic predators generally feeding on the most abundant prey, copepods. No feeding patterns were evident during the 24-h station, suggesting that both species feed continuously. The feeding rates of E. hamata ranged from 0 to 0.50 prey ind. day−1 and between 0 and 0.90 prey ind. day−1 for S. gazellae. The maximum total predation impact of E. hamata was equivalent to 5.2% of the copepod standing stock or up to 103% of copepod production per day. For S. gazellae the predation impact was lower, reaching a level of 3.2% of the copepod standing stock or 63% of the daily copepod production. Chaetognaths can, therefore, be regarded as an important pelagic predator of the Prince Edward Islands subsystem.

Two new species of the genus Heterokrohnia, H. longidentata and H. fragilis, are described and compared with the other three known Heterokrohnia species, H. mirabilis Ritter-Zahony 1911; H. bathybia Marumo and Kitou 1966 and H. involucrum Dawson 1968. The species have been found at great depths (1,000 m–2,000 m) near Elephant Island, north of the Antarctic Peninsula.

The vertical distributions of species of chaetognaths are described to a depth of 1043 m in Baffin Bay during the summer. The dominant species in the southern region of the Bay were Eukrohnia hamata and Sagitta maxima and, in the northern region, E. hamata and S. elegans. The younger stages of all chaetognath species were found at depths above the older more mature stages. There was no evidence of any diurnal vertical migration by any species. The population structure of E. hamata was bimodal suggesting two sub-populations and a two year life cycle. Sagitta maxima showed a multimodal population suggesting a life cycle of four or more years. The population structure of E. elegans was unimodal. Gut contents showed that E. hamata fed at a higher rate than did S. maxima. E. hamata fed at a higher rate during the night but there was no significant difference between day/night feeding rates for S. maxima. It was estimated that E. hamata consumed up to 1.8 prey items day-1 and S. maxima 0.5 prey items day-1. The daily ration was influenced by the water temperature. Calculations showed chaetognaths are major predators on copepods consuming between 1.2 and 1.3% of the standing crop of copepods per day. Generally the maximum concentration of chaetognaths was found at a depth below the maximum concentration of copepods.

A new bathypelagic species of the genus Heterokrohnia, H. longicaudata, is described and separated from the other five previously described Heterokrohnia species, H. mirabilis Ritter-Zahony 1911, H. bathybia Marumo and Kitou 1966, H. involucrum Dawson 1968, H. longidentata Kapp and Hagen 1985 and H. fragilis Kapp and Hagen 1985. The new specimen has been found at great depths (2,350m–1,000m) near Elephant Island, in the atlantic sector north of the Antarctic Peninsula.