Deep-sea megabenthos communities of the Eurasian Central Arctic are influenced by ice-cover and sea-ice algal falls

Benthic remineralization under future Arctic conditions and evaluating the potential for changes in carbon sequestration in warming sediments

Benthic (seafloor) remineralization of organic material determines the fate of carbon in the ocean and its sequestration. Bottom water temperature and labile carbon supply to the seafloor are expected to increase in a warming Arctic and correspondingly, benthic remineralization rates. We provide some of the first experimental data on the response of sediment oxygen demand (SOD), an established proxy for benthic remineralization, to increased temperature and/or food supply across a range of Arctic conditions and regimes. Each factor significantly increased SOD rates (with different degrees of variability); however the largest increases were seen with both factors combined (50% to ten-fold increases), consistently across the four seasons and the spatial gradient covering shelf to deep basin included in our study. This ability of the Arctic benthos to process increased pulses of carbon suggests that increased sedimented carbon under warming conditions is likely to be utilized and processed, not accumulated, impacting carbon storage and decreasing the Arctic's role as a global carbon sink.

Habitat types and megabenthos composition from three sponge-dominated high-Arctic seamounts

Seamounts are isolated underwater mountains stretching > 1000 m above the seafloor. They are identified as biodiversity hotspots of marine life, and host benthic assemblages that may vary on regional (among seamounts) and local (within seamounts) scales. Here, we collected seafloor imagery of three seamounts at the Langseth Ridge in the central Arctic Ocean to assess habitats and megabenthos community composition at the Central Mount (CM), the Karasik Seamount (KS), and the Northern Mount (NM). The majority of seafloor across these seamounts comprised bare rock, covered with a mixed layer of sponge spicule mats intermixed with detrital debris composed of polychaete tubes, and sand, gravel, and/or rocks. The megabenthos assemblages consisted of in total 15 invertebrate epibenthos taxa and 4 fish taxa, contributing to mean megabenthos densities of 55,745 ind. ha^-1 at CM, 110,442 ind. ha^-1 at KS, and 65,849 ind. ha^-1 at NM. The faunal assemblages at all three seamounts were dominated by habitat-forming Tetractinellida sponges that contributed between 66% (KS) and 85% (CM) to all megabenthos. Interestingly, taxa richness did not differ at regional and local scale, whereas the megabenthos community composition did. Abiotic and biogenic factors shaping distinct habitat types played a major role in structuring of benthic communities in high-Arctic seamounts.

Giant sponge grounds of Central Arctic seamounts are associated with extinct seep life

The Central Arctic Ocean is one of the most oligotrophic oceans on Earth because of its sea-ice cover and short productive season. Nonetheless, across the peaks of extinct volcanic seamounts of the Langseth Ridge (87°N, 61°E), we observe a surprisingly dense benthic biomass. Bacteriosponges are the most abundant fauna within this community, with a mass of 460 g C m⁻² and an estimated carbon demand of around 110 g C m⁻² yr⁻¹, despite export fluxes from regional primary productivity only sufficient to provide <1% of this required carbon. Observed sponge distribution, bulk and compound-specific isotope data of fatty acids suggest that the sponge microbiome taps into refractory dissolved and particulate organic matter, including remnants of an extinct seep community. The metabolic profile of bacteriosponge fatty acids and expressed genes indicate that autotrophic symbionts contribute significantly to carbon assimilation. We suggest that this hotspot ecosystem is unique to the Central Arctic and associated with extinct seep biota, once fueled by degassing of the volcanic mounts.

This study reports the discovery of dense sponge gardens across the peaks of permanently ice-covered, extinct volcanic seamounts of the Langseth Ridge and on the remnants of a now extinct seep ecosystem. Using approaches to sample and infer food and energy sources to this ice-covered community, the authors suggest that the sponges use refractory organic matter trapped in the extinct seep community on which they sit.

The Sub-Ice Algal Communities of the Barents Sea Pack Ice: Temporal and Spatial Distribution of Biomass and Species

This work summarizes ice algal studies, presented as biomass and species temporal and spatial distribution, during 11 cruises conducted between 1986 and 2012. The majority of the biomass was found as loosely attached sub-ice algal layers, and sampling required diving. A maximum of 40 mg chlorophyll m−2 and 15.4 × 109 cells m−2 was measured in May. The species diversity was separated in zones based on ice thickness, with the highest biodiversity in the medium-thick ice of 30–80 cm. Nitzschia frigida was the most common species. There was a significant positive relationship between the dominance of this species and ice thickness, and it dominated completely in thick ice. Other common species, such as N. promare and Fossulaphycus arcticus reacted oppositely, by becoming less dominant in thick ice, but the positive correlation between total cell numbers and number of these three species indicated that they would most likely dominate in most populations. Melosira arctica was found several times below medium-thick annual ice. Algae occurred from top to bottom in the ice floes and in infiltration layers, but in very low numbers inside the ice. The bipolar dinoflagellates Polarella glacialis inhabited the ice, both as vegetative cells and cysts. The algal layers detached from the ice and sank in late spring when melting started. The cells in the sediments form an important food source for benthic animals throughout the year. Fjord populations survive the winter on the bottom and probably form next year’s ice algal inoculum. A few ‘over-summer’ populations found in sheltered locations might provide supplementary food for ice amphipods in late summer. The future faith of the ice flora is discussed in view of a warmer climate, with increased melting of the Arctic ice cover.

Vertical zonation of the Siberian Arctic benthos: bathymetric boundaries from coastal shoals to deep-sea Central Arctic

The bathymetric distribution of species of Annelida, Crustacea and Echinodermata from the region including the Kara, Laptev and East Siberian seas and the adjacent region of the deep-sea Central Arctic was analysed. We focused on vertical species ranges revealing zones of crowding of upper and lower species range limits. Using published data and in part the material obtained during the expeditions of the P.P. Shirshov Institute of Oceanology, we evaluated species vertical distribution from 0 m to the maximum depth of the Central Arctic (~4,400 m). The entire depth range was divided into smaller intervals; number of upper and lower limits of species depth ranges was counted and plotted to visualize the range limits crowding. Several zones of crowding of vertical species range limits were found for all analysed macrotaxa. The most significant zones occurred at depths of 450–800 m and 1,800–2,000 m. The first depth zone corresponds to the boundary between the sublittoral and bathyal faunas. The last one marks the boundary between the bathyal and abyssal faunas. Depths of these boundaries differ from those reported from other Ocean regions; possible explanations of these differences are discussed.

Climate-driven benthic invertebrate activity and biogeochemical functioning across the Barents Sea polar front

Arctic marine ecosystems are undergoing rapid correction in response to multiple expressions of climate change, but the consequences of altered biodiversity for the sequestration, transformation and storage of nutrients are poorly constrained. Here, we determine the bioturbation activity of sediment-dwelling invertebrate communities over two consecutive summers that contrasted in sea-ice extent along a transect intersecting the polar front. We find a clear separation in community composition at the polar front that marks a transition in the type and amount of bioturbation activity, and associated nutrient concentrations, sufficient to distinguish a southern high from a northern low. While patterns in community structure reflect proximity to arctic versus boreal conditions, our observations strongly suggest that faunal activity is moderated by seasonal variations in sea ice extent that influence food supply to the benthos. Our observations help visualize how a climate-driven reorganization of the Barents Sea benthic ecosystem may be expressed, and emphasize the rapidity with which an entire region could experience a functional transformation. As strong benthic-pelagic coupling is typical across most parts of the Arctic shelf, the response of these ecosystems to a changing climate will have important ramifications for ecosystem functioning and the trophic structure of the entire food web. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Vertical distribution of megafauna on the Bering Sea slope based on ROV survey

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.