In the Arctic, under-ice primary production is limited to summer months and is restricted not only by ice thickness and snow cover but also by the stratification of the water column, which constrains nutrient supply for algal growth. Research Vessel Polarstern visited the ice-covered eastern-central basins between 82° to 89°N and 30° to 130°E in summer 2012, when Arctic sea ice declined to a record minimum. During this cruise, we observed a widespread deposition of ice algal biomass of on average 9 grams of carbon per square meter to the deep-sea floor of the central Arctic basins. Data from this cruise will contribute to assessing the effect of current climate change on Arctic productivity, biodiversity, and ecological function.

During a survey of the Håkon Mosby mud volcano (HMMV), located on the Bear Island fan in the southwest Barents Sea at ∼1250 m water depth, different habitats inside the volcano caldera and outside it were photographed using a towed camera platform, an Ocean Floor Observation System (OFOS). Three transects were performed across the caldera and one outside, in the background area, each transect was ∼2 km in length. We compared the density, taxa richness and diversity of nonsymbiotrophic megafauna in areas inside the volcano caldera with different bacterial mat and pogonophoran tubeworm cover. Significant variations in megafaunal composition, density and distribution were found between considered areas. Total megafaunal density was highest in areas of dense pogonophoran populations (mean 52.9 ind. m⁻²) followed by areas of plain light-coloured sediment that were devoid of bacterial mats and tube worms (mean 37.7 ind. m⁻²). The lowest densities were recorded in areas of dense bacterial mats (mean ≤1.4 ind. m⁻²). Five taxa contributed to most of the observed variation: the ophiuroid Ophiocten gracilis, lysianassid amphipods, the pycnogonid Nymphon macronix, the caprellid Metacaprella horrida and the fish Lycodes squamiventer. In agreement with previous studies, three zones within the HMMV caldera were distinguished, based on different habitats and megafaunal composition: "bacterial mats", "pogonophoran fields" and "plain light-coloured sediments". The zones were arranged almost concentrically around the central part of the caldera that was devoid of visible megafauna. The total number of taxa showed little variation inside (24 spp.) and outside the caldera (26 spp.). The density, diversity and composition of megafauna varied substantially between plain light-coloured sediment areas inside the caldera and the HMMV background. Megafaunal density was lower in the background (mean 25.3 ind. m⁻²) compared to areas of plain light-coloured sediments inside the caldera. So the effect of the mud-volcano environment on benthic communities is expressed in increasing of biomass, changing of taxa composition and proportions of most taxonomic groups

Permanent sea-ice cover and low primary productivity in the mostly ice-covered Central Arctic ocean basins result in significantly lower biomass and density of macrobenthos in the abyssal plains compared to the continental slopes. However, little is known on bathymetric and regional effects on the macrobenthos diversity. This study synthesizes new and available macrobenthos data to provide a baseline for future studies of the effects of Arctic change on macrofauna community composition in the Arctic basins. Samples collected during three expeditions (in 1993, 2012 and 2015) at 37 stations on the slope of the Barents and Laptev Seas and in the abyssal of the Nansen and Amundsen Basins in the depth range from 38 m to 4381 m were used for a quantitative analysis of species composition, abundance and biomass. Benthic communities clustered in five depth ranges across the slope and basin. A parabolic pattern of species diversity change with depth was found, with the diversity maximum for macrofauna at the shelf edge at depths of 100–300 m. This deviates from the typical species richness peak at mid-slope depths of 1500–3000 m in temperate oceans. Due to the limited availability of standardized benthos data, it remains difficult to assess if and how the significant sea-ice loss observed in the past decade has affected benthic community composition. The polychaete Ymerana pteropoda and the bryozoan Nolella sp. were found for the first time in the deep Nansen and Amundsen Basins.

Video surveys were carried out during the 75th cruise of the RV Akademik M.A. Lavrentyev (June 2016) along the northern slope of the Volcanologists Massif, in the south-western Bering Sea. The seafloor was explored using the ROV Comanche 18. Seven dives were performed in the depth range from 4,278 m to 349 m. Overall, about 180 species of megafauna were recognised. Fifteen types of megafauna communities corresponding to certain depth ranges were distinguished based on the most abundant taxa. Dominance changed with depth in the following order: the holothurian Kolga kamchatica at the maximum depth (4,277–4,278 m); the holothurian Scotoplanes kurilensis at 3,610–2,790 m; the ophiuroid Ophiura bathybia at 3,030–2,910 m; benthic shrimps of the family Crangonidae at 2,910–2,290 m; the holothurian Paelopatides solea at 2,650–2,290 m; benthic jellyfish from the family Rhopalonematidae at 2,470–2,130 m; the enteropneust Torquaratoridae at 2,290–1,830 m; the holothurian Synallactes chuni and the ophiuroid of the genera Ophiura and Ophiocantha at 1,830–1,750 m. At depths 1,750–720 m most of the megafauna was associated with live or dead colonies of the sponge Farrea spp. Depths 720–390 m were dominated by the coral Heteropolypus ritteri and/or Corallimorphus pilatus. At 390–350 m depth, the shallowest depth range, the dominant taxon was the zoantharian Epizoanthus sp. Soft sediment megafauna communities dominated by torquaratorid enteropneusts to our knowledge have not been observed before in the deep-sea, the same as communities with a dominance of benthopelagic rhopalonematid jellyfish. The depths of the largest community changes, or the largest turnover of dominant species, were revealed at ∼2,790 m between the bathyal and abyssal zones and ∼1,750 m and ∼720 m within the bathyal zone.

Quantitative camera surveys of benthic megafauna were carried out during the expedition ARK-XXVII/3 to the Eastern Central Arctic Basins with the research icebreaker Polarstern in summer 2012 (2 August-29 September). Nine transects were performed for the first time in deep-sea areas previously fully covered by ice, four of them in the Nansen Basin (3571-4066m) and five in the Amundsen Basin (4041-4384m). At seven of these stations benthic Agassiz trawls were taken near the camera tracks for species identification. Observed Arctic deep-sea megafauna was largely endemic. Several taxa showed a substantially greater depth or geographical range than previously assumed. Variations in the composition and structure of megabenthic communities were analysed and linked to several environmental variables, including state of the sea ice and phytodetritus supply to the seafloor. Three different types of communities were identified based on species dominating the biomass. Among these species were the actiniarian Bathyphellia margaritacea and the holothurians Elpidia heckeri and Kolga hyalina . Variations in megafaunal abundance were first of all related to the proximity to the marginal ice zone. Stations located closer to this zone were characterized by relatively high densities and biomass of B . margaritacea . Food supply was higher at these stations, as suggested by enhanced concentrations of pigments, organic carbon, bacterial cell abundances and nutrients in the sediments. Fully ice-covered stations closer to the North Pole and partially under multi-year ice were characterized by lower concentrations of the same biogeochemical indicators for food supply. These stations nevertheless hosted relatively high density and biomass of the holothurians E . heckeri or K . hyalina , which were observed to feed on large food falls of the sea-ice colonial diatom Melosira arctica . The link between the community structure of megafauna and the extent and condition of the Central Arctic sea-ice cover suggests that future climate changes may substantially affect deep ocean biodiversity.

This study was aimed at comparing the composition and structure of macrofauna assemblages living on mytilid foundations from different hydrothermal vent areas and depths and comparing their composition with that of other foundation species (alvinellids, siboglinids and alvinocaridid shrimps) from various ecotopes within the same vent field, attempting to identify unique features of mytilid assemblages. Areas investigated were located at different depths on the Mid-Atlantic Ridge (MAR; Menez Gwen -850 m, Lucky Strike -1650 m, Rainbow -2300 m, Broken Spur -3000 m, Snake Pit -3480 m, Logatchev -3000 m) and at the depth of 2500 m on the East Pacific Rise (EPR; 9°N, 11°N, 17°S). Mussels occur at temperatures from 2 to 20°C, whereas shrimps and alvinellids inhabit warmer zones with temperatures ranging from 20 to 40°C. Siboglinids may occur together with mytilids, but more often they settle closer to hydrothermal vent emissions and live at temperatures exceeding 20°C. Four main geographic groups of mytilid assemblages were distinguished: the northern EPR (9°N and 11°N) and southern EPR groups (17°S) and the more northern MAR (Lucky Strike and Rainbow) and more southern MAR groups (Broken Spur, Snake Pit and Logatchev). The most northern and shallow Atlantic area, Menez Gwen (850 m), differed from other Atlantic and Pacific areas by specific taxa composition and dominance of one taxon, the lepetodrilid gastropod, Lepetodrilus atlanticus (59%). Family richness was higher in mytilid assemblages than in shrimp and alvinellid assemblages: 2 and 2.5 times, respectively. Species composition in shrimp and alvinellid assemblages was poorer than in mytilid assemblages from the same field: 35% of species penetrated from mytilid to shrimp assemblages and 22% from mytilid to alvinellid assemblages. The percentage of species unique to shrimp or alvinellid assemblages was relatively low: 0% and 32%, respectively. On the MAR, similarity among assemblages of different foundation species within the same hydrothermal field was higher than among assemblages of the same foundation species from different fields and depths. On the EPR, similarity was higher among assemblages of the same foundation species from different fields at the same depth than among assemblages of different foundation species from the same field.

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