Description: This is the first study to present the patterns and environmental controls of benthic biomass size spectra, carbon demand, and production along the entire bathymetric gradient from the shelf to the abyssal depths in the Arctic Ocean. The materials were collected at 17 stations (76 - 5561 m) in the eastern Fram Strait, in the Atlantic passage to the Arctic Ocean, in the vicinity of the productive Marginal Ice Zone, with concentrations of sediment-bound chloroplastic pigments (indicating food availability from phytodetritus sedimentation) higher than in other deep-sea localities at similar depths. Meiobenthic and macrobenthic individuals were measured using image analysis to assess their biovolume, biomass, annual production, and carbon demand. Benthic biomass in the area was clearly higher than that in the High Arctic locations and comparable to that in the lower-latitude North Atlantic. Biomass and annual production were significantly negatively correlated with water depth, with stronger bathymetric clines in macrofauna than in meiofauna and the increasing dominance of meiofauna with increasing depth. A bimodal shape in the size spectra was observed only at the shallow stations, while at depths below 2000 m, an additional trough was present in the macrofaunal part of the spectrum. The entire range of the spectra (i.e., the number of size classes) decreased with increasing depth, especially in the macrofaunal part of the spectrum. Similar slope values in the normalized spectra indicated that the distribution of the biomass across the present size classes was consistent from the shelf to the abyssal depths, irrespective of the decreasing amount of food availability. The fragmented macrofaunal size spectra documented at the two stations were probably due to physical disturbances at the sediment-water interface (e.g., intense bioturbation of holothurians and strong near-bottom currents). Benthic carbon demand declined from 50.7 gC m-2 y-1 at the shelf to 11.5 gC m-2 y-1 at the slope to 2.2 gC m-2 y-1 at the abyssal depths, and its partitioning among meiofauna and macrofauna changed with water depth, with meiofauna contributions increasing from 50 % at the shelf to over 90 % at the deepest station. The estimated total benthic carbon demand exceeded the vertical Corg fluxes, suggesting that the studied system can be particularly sensitive to future changes in productivity regimes and associated organic matter fluxes.

Description: Established in the Fram Strait in 1999, the LTER (Long-Term Ecological Research) observatory HAUSGARTEN enables us to study changes on the deep Arctic seafloor. Repeated deployments of a towed camera system (Ocean Floor Observation System) along the same tracks allowed us to build a time series longer than a decade (2004 - 2015). Here, we present the first time-series results from a northern and the southernmost station of the observatory (N3 and S3, ~2650m and 2350m depth respectively) obtained via the analysis of still imagery. We assess temporal variability in community structure, megafaunal densities and diversity, and use a range of biotic and abiotic factors to explain the patterns observed. There were significant temporal differences in megafaunal abundances, diversity and abiotic factors at both stations. A particularly high increase in megafaunal abundance was recorded at N3 from 12.08 (±0.39; 2004) individuals m-2 to 35.21 (±0.97; 2007) ind. m-2 alongside a ten-fold increase in (drop-)stones. At S3, megafaunal densities peaked in 2015 (22.74 ±0.61 ind. m-2) after an increasing trend since 2004 (12.44 ±0.32 ind. m-2). Holothurians showed particularly striking temporal differences: densities of the small sea cucumber Elpidia heckeri densities rose ten-fold from 0.31 ind. m-2 (±0.04; 2004) to 3.74 ind. m-2 (±0.14; 2015) at S3, coinciding with a sustained increase in phytodetritial matter (chloroplastic pigment equivalents) at the seafloor. Initially entirely absent from N3, densities of the larger holothurian Kolga hyalina peaked in 2007 (5.87 ±0.22 ind. m-2) and declined continuously since then. Overall diversity (γ) increased at both stations over the course of the study, however, with varying contributions of α and β diversities. Our results highlight the importance of time-series studies as megafaunal community composition is characterised by continuous changes. This indicates that epibenthic communities from the deep seafloor are reactive and dynamic, with no “null” community state. To continue to monitor them is therefore crucial in understanding natural and anthropogenic impacts in an area exposed to the effects of climate change.

Description: The LTER (Long-Term Ecological Research) observatory HAUSGARTEN, in the eastern Fram Strait, provides us the unique ability to study the composition of benthic megafaunal communities through the analysis of seafloor photographs. This, in combination with extensive sampling campaigns, which have yielded a unique data set on faunal, bacterial, biogeochemical and geological properties, as well as on hydrography and sedimentation patterns, allows us to address the question of why variations in megafaunal community structure and species distribution exist within regional (60-110 km) and local (〈4 km) scales. Here, we present first results from the latitudinal HAUSGARTEN transect, consisting of three different stations (N3, HG-IV, S3) between 78°30’N and 79°45’N (2500 m depth), obtained via the analysis of images acquired by a towed camera (Ocean Floor Observation System) in 2011. We assess variability in megafaunal densities, species composition and diversity as well as biotic and biogenic habitat features, which may cause the patterns observed. While there were significant regional differences in megafaunal composition and densities between the stations (N3 = 26.74 ±0.63; HG-IV = 11.21 ±0.25; S3 = 18.34 ±0.39 individuals m-2), significant local differences were only found at HG-IV. Regional-scale variations may be due to the significant differences in ice coverage at each station as well as the different quantities of protein available, whereas local-scale differences at HG-IV may be a result of variation in bottom topography.

Description: Deep-sea benthic communities and their structural and functional characteristics are regulated by surface water processes. Our study focused on the impact of changes in water depth and food supplies on small-sized metazoan bottom-fauna (meiobenthos) along a bathymetric transect (1200–5500 m) in the western Fram Strait. The samples were collected every summer season from 2005 to 2009 within the scope of the HAUSGARTEN monitoring program. In comparison to other polar regions, the large inflow of organic matter to the sea floor translates into relatively high meiofaunal densities in this region. Densities along the bathymetric gradient range from approximately 2400 ind. 10 cm-2 at 1200 m to approximately 300 ind. 10 cm-2 at 4000 m. Differences in meiofaunal distribution among sediment layers (i.e., vertical profile) were stronger than among stations (i.e., bathymetric gradient). At all the stations meiofaunal densities and number of taxa were the highest in the surface sediment layer (0–1 cm), and these decreased with increasing sediment depth (down to 4–5 cm). However, the shape of the decreasing pattern differed significantly among stations. Meiofaunal densities and taxonomic richness decreased gradually with increasing sediment depth at the shallower stations with higher food availability. At deeper stations, where the availability of organic matter is generally lower, meiofaunal densities decreased sharply to minor proportions at sediment depths already at 2–3 cm. Nematodes were the most abundant organisms (60–98%) in all the sediment layers. The environmental factors best correlated to the vertical patterns of the meiofaunal community were sediment-bound chloroplastic pigments that indicate phytodetrital matter.

Description: Effects of increased near-bottom flow velocities on the sedimentary environment and its associated small biota were studied in a long-term in situ experiment at 2,500 m water depth at the Deep-Sea Observatory HAUSGARTEN in the eastern Fram Strait. In 2003, the Remotely Operated Vehicle (ROV) "Victor 6000" was used to install a stainless steel flume of about 8.5 m in length, consisting out of a 6 m long passageway with a cross-section of 50 x 50 cm and 3-4 m wide funnel-like doorways to increase bottom currents by a factor of approximately 6. Sediment sampling for biochemical sediment analyses, bacterial studies and meiofaunal investigations (with special focus on the nematode communities) was carried out four years after the installation of the flume using the ROV "Quest 4000". The data showed clearly reduced values for parameters indicating organic matter (food) availability in the sediments, and corresponding lower bacterial and meiofaunal densities inside the flume, compared to control sites outside the channel. Results suggest that increased near-bottom currents and food deficiency not only diminish sediment-inhabiting meiofaunal assemblages but also alter the meiobenthic composition. Compared to background sediments, the nematode community inside the flume evidently showed adaptations to the overall reduced food availability and a more heterogeneous environment due to generally increased and more turbulent flow velocities. The variable environmental conditions inside the flume have an effect not only on the number of genera present, but also on the identities of the genera and the functional composition of the nematode community.

Description: Spatial distribution patterns in macrobenthos were studied based on the material collected during the R.V. Polarstern expedition ARK-XXVII/2 in July 2012. Eleven stations along the latitudinal transect at the deep-sea observatory HAUSGARTEN in the Fram Strait were taken at depths of about 2500 m. Macrofauna was obtained using the USNEL box corer. A single core (0.25 m2) was taken at each station and four subcores (0.03 m2) were taken from each core and used for the quantitative analysis. The results suggest that the single highly variable macrobenthic community with the dominance of polychaetes Galathowenia fragilis and Myriochele heeri inhabits the studied area. The prevalence of a mosaic in the community structure with the grain size at least more than the size of a core was detected. However, several abundant species (e.g. the polychaetes Prionospio sp. and Galathowenia fragilis) tend to form patches at the scale less than a core (0.25 m2). Despite the lack of significant differences in species distribution patterns along the latitudinal transect, there is a slight difference in community structure between the northernmost and southernmost stations (~170 km apart), which can be explained by variations in environmental factors (e.g. higher food availability in the northern part of the transect).

Description: This study compares the macrofaunal communities along two bathymetric transects (1000 – 2500 m water depth) in predominantly ice-covered western (offshore Greenland) and generally ice-free eastern (offshore Svalbard) regions of the Fram Strait. Material was collected using an USNEL 0.25 m2 box corer and all sediment samples were processed through a 500-μm sieve. A total of 1671 organisms from 169 species were found. Densities off Greenland were generally lower than those observed off Svalbard. On both sides of the Fram Strait, density, biomass and biodiversity generally decreased with increasing water depth. An exception was observed at the deepest station off Greenland (2500 m water depth), which was located within the Marginal Ice Zone. At this station, macrofaunal density was elevated (992 ± 281 ind. m−2) compared to the adjacent shallower sampling areas off Greenland (272 ± 208 ind. m−2 to 787 ± 172 ind. m−2) and the deeper stations (2000 and 2500 m water depth) off Svalbard (552 ± 155 ind. m−2 and 756 ± 182 ind. m−2). The most abundant species along both transects was the polychaete Galathowenia fragilis (off Greenland: 288 ind. m−2, off Svalbard: 740 ind. m−2). Sea ice coverage and water depth, as well as the associated food availability at the seafloor, seem to be crucial factors driving the macrofaunal community patterns. A strong pelago-benthic coupling is observed to be typical in Arctic deep-sea ecosystems, and is also confirmed by our study.

Description: Larval dispersal is a fundamental process responsible for colonization and connectivity of benthic invertebrate populations. It is difficult to study larval dispersal in polar environments because weather and climate conditions restrict sample collection to certain seasons. In this study, we leveraged oceanographic moorings as long-term scientific platforms for collecting larvae and recruits of benthic invertebrate species in the Fram Strait and along the continental slope north of Svalbard in 2017–2021. Larval traps and fouling panels were deployed at various depths on 15 moorings at 8 locations, and additional specimens of biofouling were obtained opportunistically from moored instruments. Our results showed a significant difference in species composition between samples collected in Atlantic Water in the West Spitsbergen Current (WSC) and samples collected in Arctic Water near the seafloor and in the East Greenland Current (EGC) in the western part of the Fram Strait. There was also a stark difference between Atlantic Water species in the Fram Strait and on the north Svalbard slope. Most specimens collected in the WSC belonged to species with long-duration planktotrophic larvae, such as the ubiquitous bivalve Hiatella arctica, the bryozoan Alcyonidium mamillatum, and two nudibranchs. Samplers exposed primarily to Arctic water at their given depth and location were dominated by hydrozoans. We observed medusae budding off of the hydroids Stegopoma plicatile and Rhizoragium roseum. Our study demonstrates that the WSC is an important vector for larval dispersal into the central Arctic Ocean. Integration of biological samplers on oceanographic moorings holds great promise for monitoring efforts as climate change progresses, especially in environments where research is challenging and seasonally limited, such as the Arctic. 1. Introduction For benthic invertebrates, especially those with sessile adult stages (e.g., sponges, anemones), larval dispersal is the primary mechanism of dispersal to new habitats (Pechenik, 1999). The patterns and mechanisms of larval dispersal are difficult to study in the Arctic Ocean, where weather and climate conditions restrict sample collection to summer months. As a result, larval dispersal and the subsequent processes of settlement and recruitment in benthic invertebrates are poorly understood in the Arctic Ocean, despite their importance. Oceanographic moorings provide excellent platforms for studying larval dispersal, recruitment, and growth of organisms (Chava et al., 2021; Schiaparelli and Aliani, 2019). Instruments and floats on a mooring are deployed in the water column by design, so the