Date Published: July 16, 2019
Publisher: Public Library of Science
Author(s): Elena Rybakova, Antonina Kremenetskaia, Andrey Vedenin, Antje Boetius, Andrey Gebruk, Sébastien Duperron.
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.
Benthic megafauna comprises marine animals exceeding 0.5–1 cm in size visible on seafloor images. They play an important role in benthic ecosystems through active recycling of sedimented organic matter, bioturbation and food web linkages. Megabenthos is a dynamic component of deep-sea ecosystems able to react rapidly to environmental changes . Observations of benthic megafauna in the Central Arctic Basins are rare because of technical difficulties of sampling in the remote deep-sea region covered by permanent ice. Traditional sampling methods such as trawling are challenged by the ice-cover: vessels cannot keep steady speed and course in the ice. Hence, most previous studies of Arctic megafauna communities were confined to marginal seas [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13], the Fram Strait and areas around Svalbard [14, 15, 16, 17, 18, 19, 20]. Sampling of Central Arctic benthic megafauna from depths exceeding 2000 m began in the late 19th century [21, 22, 23, 24, 25, 26, 27, 28, 29]. Further contributions were made by Soviet expeditions [30, 31, 32] and by the drifting ice stations [33, 34, 32]. Quantitative studies of the Central Arctic megafauna are few and they are focused on the Canada Basin [35, 36, 37].
The present study aimed at providing quantitative data on the megafauna distribution and community structure in the ice-covered deep Nansen and Amundsen Basins of the Central Arctic. These basins show substantial change in sea ice cover owing to warming, yet little is known how these changes are reflected in phytodetritus export and responses by the deep-sea benthos.
Our study on the composition and structure of megabenthos communities in different areas of the Eastern Central Arctic Basin combining quantitative photographic surveys with trawl sampling provides quantitative information of the dominant megafauna of the ice-covered basins and key factors structuring the distribution of abyssal megafauna in the Central Arctic. Three types of megafauna communities were distinguished: dominated by 1) the actiniarian Bathyphellia margaritacea, 2) the holothurian Elpidia heckeri and 3) the holothurian Kolga hyalina. Variations in megafaunal abundance were first of all related to the proximity to the marginal ice zone. Stations closer to the ice margin under first-year ice were characterized by relatively high densities and biomasses of B. margaritacea and relatively high food supply to the seafloor indicated by several biogeochemical variables. Stations located closer to the North Pole under the multi-year ice showed relatively low food supply, but relatively high densities and biomasses of holothurians E. heckeri and K. hyalina feeding on fresh algal falls of the colonial sea-ice diatom Melosira arctica. In case extensive algal food falls to the seafloor become regular as a result of increasingly frequent sea-ice melt events, the abundance of mobile deposit-feeding megafauna, such as elpidiid holothurians and ophiuroids, in the abyssal Central Arctic may rise significantly. Our data provide a baseline for long-term monitoring of the seafloor of the changing deep-sea Arctic Ocean.