Description: In a bid to further understand processes that influence deep-sea epibenthic megafauna, which fulfil critical roles in the global carbon cycle, we present data from the Arctic Long-Term Ecological Research observatory HAUSGARTEN, in the Fram Strait, showing significant temporal changes in total biomass of 3 key organisms (Kolga hyalina, Elpidia heckeri and Mohnia spp.) at stations N3, HG-IV and S3 during repeated deployments over a time series spanning 2004−2015. Overall, all species investigated displayed a similar reproduction/recruitment cycle, with increasing mean mass per individual leading to decreases in abundance, and vice versa. However, there were 3 ‘events’ that deviated from this pattern. The first was a mass reproduction event of E. heckeri at HG-IV from 2012 onwards, likely due to an increased carrying capacity. The second event involved migration of K. hyalina from HG-IV between 2004−2007, with a return in 2011. This coincided with a shift in the composition of the particle flux at the station. The final event was a mass migration of K. hyalina to N3 between 2004 (0 ind. m−2) and 2007 (4.765 ± 0.084 ind. m−2). This event coincided with a 4-fold increase in phytodetrital food availability at the seafloor at N3. Our results highlight the importance of time-series studies to ascertain the key factors that influence epibenthic megafaunal communities. It also highlights the fact that more needs to be done in understanding the life history of these organisms, as this understanding is, so far, widely lacking.

Description: The second leg of the Polarstern expedition PS93 to the Arctic started on 21st July 2015 in Tromsø (Norway) and ended on 15th August 2015, again in Tromsø (Fig. 1.1). The cruise led to the deep-sea observatory HAUSGARTEN in the Fram Strait (approx. 78°30’N - 80°00’N, 05°00’W - 11°00’E). The total duration of the expedition was 24 days; about 285 hours (almost 12 days) were spent for station work. The remaining time was used to reach the study area, for steaming between individual stations, and for the transit from 79°N back to Tromsø. More than 30 stations were sampled, thereby usually deploying several scientific instruments per site. The total length of the expedition was approx. 1,800 nautical miles. The scientific work during expedition PS93.2 supported the time-series studies at the LTER (Long-Term Ecological Research) observatory HAUSGARTEN, where we document Global Change induced environmental variations on a polar deep-water ecosystem. This work was carried out in close co-operation between the HGF-MPG Joint Research Group on Deep-Sea Ecology and Technology, the PEBCAO Group (Phytoplankton Ecology and Biogeochemistry in the Changing Arctic Ocean) at AWI, and the Helmholtz Young Investigators Group SEAPUMP (Seasonal and regional food web interactions with the biological pump), representing a joint effort between AWI and the MARUM - Center for Marine Environmental Sciences at the University of Bremen. The expedition was also used to accomplish installations for the HGF infrastructure project FRAM (FRontiers in Arctic marine Monitoring). The FRAM Ocean Observing System aims at permanent presence at sea, from surface to depth, for the provision of near real-time data on Earth system dynamics, climate variability and ecosystem change. It serves national and international tasks towards a better understanding of the effects of change in ocean circulation, water mass properties and sea-ice retreat on Arctic marine ecosystems and their main functions and services. Within the framework of a ‘Trans-National Access’ (TNA) initiative of the European project FixO3 (Fixed-point Open Ocean Observatories), the expedition provided access to the FRAM Ocean Observing System thereby supporting external and joint scientific projects logistically. Beside the more “traditional” instruments used for marine research (e.g. water sampler, plankton nets, sediment sampler, moorings, bottom-lander) we operated a deep-diving Work-Class Remotely Operated Vehicle (ROV) as well as an Autonomous Underwater Vehicle (AUV) and different autonomous Unmanned Aerial Vehicles (UAVs) during the overall technically and logistically very challenging expedition. The effective cooperation between the scientific party and the ship’s crew, in combination with perfect weather conditions during most times of the cruise, made this expedition a great success.

Description: Stones released by melting icebergs are called dropstones, and these stones constitute island-like hard-bottom habitats at high latitudes. In 2012, dropstone megafauna in the HAUSGARTEN observatory in the Fram Strait was sampled photographically. We tested the hypothesis that dropstones would have the same species distribution patterns as terrestrial islands, using 5 patterns commonly found in the classical island literature. Higher richness, diversity, and abundance of fauna occurred on larger stones and on stones near a deep-water rocky reef. These patterns can be explained by the greater surface area of larger stones, the exposure of larger stones to faster current higher in the benthic boundary layer, and increased larval supply from the rocky reef. Some pairs of morphotypes (12 pairs out of 56 morphotypes and 1540 possible pairs) co-occurred less often than expected by chance. While similar patterns have been attributed to interspecific competition in the classical island literature, we offer alternative mechanisms for dropstones. Non-random co-occurrence on dropstones may be explained by larval dispersal. Dropstone fauna had an overdispersed (clumped) distribution, so pairs of morphotypes may have negative non-random co-occurrence simply because short larval life and limited dispersal ability prevent them from having randomly overlapping distributions. In addition, we found 8 morphotype pairs that co-occurred more often than expected by chance because of epibiontism. The patterns found in dropstone communities resemble terrestrial islands, but different mechanisms may be responsible.