Seabed images from Southern Ocean shelf regions off the northern Antarctic Peninsula and in the southeastern Weddell Sea

Recent advances in underwater imaging technology allow for the gathering of invaluable scientific information on seafloor ecosystems, such as direct in situ views of seabed habitats and quantitative data on the composition, diversity, abundance, and distribution of epibenthic fauna. The imaging approach has been extensively used within the research project DynAMo (Dynamics of Antarctic Marine Shelf Ecosystems) at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research Bremerhaven (AWI), which aimed to comparatively assess the pace and quality of the dynamics of Southern Ocean benthos. Within this framework, epibenthic spatial distribution patterns have been comparatively investigated in two regions in the Atlantic sector of the Southern Ocean: the shelf areas off the northern tip of the Antarctic Peninsula, representing a region with above-average warming of surface waters and sea-ice reduction, and the shelves of the eastern Weddell Sea as an example of a stable high-Antarctic marine environment that is not (yet) affected by climate change. The AWI Ocean Floor Observation System (OFOS) was used to collect seabed imagery during two cruises of the German research vessel Polarstern, ANT-XXIX/3 (PS81) to the Antarctic Peninsula from January to March 2013 and ANT-XXXI/2 (PS96) to the Weddell Sea from December 2015 to February 2016. Here, we report on the image and data collections gathered during these cruises. During PS81, OFOS was successfully deployed at a total of 31 stations at water depths between 29 and 784 m. At most stations, series of 500 to 530 pictures ( > 15 000 in total, each depicting a seabed area of approximately 3.45 m2 or 2.3× 1.5 m) were taken along transects approximately 3.7 km in length. During PS96, OFOS was used at a total of 13 stations at water depths between 200 and 754 m, yielding series of 110 to 293 photos (2670 in total) along transects 0.9 to 2.6 km in length. All seabed images taken during the two cruises, including metadata, are available from the data publisher PANGAEA via the two persistent identifiers at https://doi.org/10.1594/PANGAEA.872719 (for PS81) and https://doi.org/10.1594/PANGAEA.862097 (for PS96). Published by Copernicus Publications. 462 D. Piepenburg et al.: Seabed images from Southern Ocean shelf regions


Introduction
The research project Dynamics of Antarctic Marine Shelf Ecosystems (DynAMo) of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research Bremerhaven (AWI), aimed to comparatively assess the pace and quality of the dynamics of Southern Ocean benthos and endotherms. It is a contribution to the international Scientific Research Program Antarctic Thresholds -Ecosystem Resilience and Adaptation (AnT-ERA) of the Scientific Committee on Antarctic Research 5 (SCAR). Applying a comparative field study approach, the geographical focus of DynAMo was on an area with aboveaverage warming of surface waters and sea-ice reduction around the tip of the Antarctic Peninsula (Gutt, 2013;Gutt et al., 2016) and a stable high-Antarctic marine environment that is not (yet) affected by climate change in the southeastern Weddell Sea (Schröder, 2016).
Special emphasis has been on the study of spatial distribution patterns of shelf megabenthic epifauna. According to an often 10 used pragmatic definition proposed by Gage and Tyler (1991), this seabed community fraction comprises all organisms that are large enough to be visible in seabed images and/or to be caught by towed sampling gear (i.e., organisms of body sizes larger than approximately 1 cm). They are of ecological significance for Southern Ocean shelf ecosystems (Gutt, 2006), as they affect the small-scale topography of seafloor habitats and, hence, the structure of the entire benthic community (Gili et al., 2006). In addition, they contribute to biogeochemical cycling, e.g. recycling of nutrients and sequestration of carbon and 15 silicate. Moreover, some megabenthic species are sensitive to environmental change, due to their slow growth, specific reproduction mode, high degree of environmental adaptation, and narrow physiological tolerances, and can thus serve as early indicators of ecosystem shifts in response to environmental change (Barnes et al., 2009).
Based on several investigations performed during previous cruises of the German research vessel Polarstern to the Antarctic Peninsula and the Weddell Sea, we have carried out comparative follow-up field studies in austral summers of 2013 and 20 2015/16, the main objectives of which were: • Complement surveys of mega-epibenthic assemblages of the shelf off the northern Antarctic Peninsula (cruise PS81 2013) and of the southeastern Weddell Sea (cruise PS96 2015/16), providing further data comparable with those gained in earlier studies in these regions (Biebow et al., 2014) • Identify spatial distribution patterns of epibenthic megafauna across multiple spatial scales (10-m, 100-m, 10-km, 100-25 km) • Contribute to the standardization of the classification of Antarctic megabenthic communities (Gutt, 2007;Gutt, 2013) Seabed images are used for different purposes: (1) to assess the large epibenthos as a whole, (2) to carry out quantitative community and diversity analyses, (3) to include environmentally relevant (e.g., CTD data if CTD sensors are integrated in 30 OFOS) and visible seabed parameters (e.g., amount of gravel, debris, ripple marks) at exactly the same spots from which the biological information originates, (4) to allow analyses with high spatial resolution (patterns within and between adjacent photographs, e.g. to survey the impact of iceberg scouring), (5) to acquire information of biological interactions such as epibiotic life mode.

Ocean Floor Observation System
The Ocean Floor Observation System (OFOS) of the AWI was used for seabed imaging surveys along drift profiles (transects) to investigate the epibenthic megafauna and its seafloor habitats. The setup and mode of deployment of OFOS was similar to that described by Bergmann and Klages (2012). OFOS is a surface-powered gear (Fig. 1), equipped with two 40 downward-looking cameras installed side-by-side: a high-resolution, wide-angle still camera (CANON® EOS 5D Mark III; Earth Syst. Sci. Data Discuss., doi:10.5194/essd-2017-18, 2017  The system was vertically lowered over either the starboard side (PS81) or the stern of the ship (PS96) with a broadband 5 fibre-optic cable, until it hovered approximately 1.5 m above the seabed. It was then towed after the slowly sailing ship at a Three lasers, which are placed beside the still camera, emit parallel beams and project red light points, arranged as an 15 equilateral triangle with a side length of 50 cm, in each photo, thus providing a scale that can be used to calculate the seabed area depicted in each image and/or measure the size of organisms or seabed features visible in the image. In addition, the seabed area depicted was estimated using altimeter-derived height above seafloor and optical characteristics of the OFOS still camera.
In automatic mode, a seabed photo, depicting an area of approximately 3.45 m 2 (= 2.3 m x 1.5 m; with variations depending 20 on the actual height above ground), was taken every 30 seconds to obtain series of "TIMER" stills distributed at regular distances along the profiles that vary in length depending on duration of the cast. At a ship speed of about 0.5 kn, the average distance between seabed images was approximately 5 m. Additional "HOTKEY" photos were taken from interesting objects (organisms, seabed features, such as putative iceberg scours) when they appeared in the live video feed, which was recorded, in addition to the stills, for documentation and later analysis. 25

Field sampling
Within the context of the overall ecological DynAMo working programme, OFOS was used to collect seabed imagery during two cruises of RV Polarstern: the ANT-XXIX/3 (PS81) cruise to the Antarctic Peninsula in January-March 2013 (for cruise report see Gutt, 2013) and the ANT-XXXI/2 (PS96) cruise to the Weddell Sea from December 2015 to February 2016 30 (for cruise report see Schröder, 2016).

Polarstern cruise ANT-XXIX/3 (PS81)
During cruise PS81, OFOS was successfully deployed at a total of 31 stations at water depths between 29 and 784 m (Table   1), delivering a total of more than 15,000 seabed photos (Gutt, 2013). At most stations, series of 500 to 530 pictures were taken along transects of approx. 3,700 m (2 nautical miles) length during net wire times (with OFOS at the bottom) of four 35 hours per transect. At some stations, OFOS had to be deployed for shorter periods of time due to high wave heights and strong winds.
The stations were placed in three regions off the Antarctic Peninsula between 61°S and 64°S: the northwestern Weddell Sea off Joinville Island, the southern Bransfield Strait and the southern Drake Passage west of South Shetland Islands (Fig. 2). benthos. Within these regions, regional habitat blocks were selected, which comprised bathymetric intermediate-scale shelfslope gradients (Gutt et al., 2016;Dorschel et al., 2016). In the Bransfield Strait, such bottom-topography structures were most obvious, and three distinct habitat blocks were sampled. Within the blocks, single stations were associated to the following defined, primarily depth-related habitats: bank, upper slope, slope, and deep/canyon. In the Drake Passage, where the bottom topography was similar, albeit less pronounced as in the Bransfield Strait, also three habitat blocks were sampled. 5 In the northern Weddell Sea, stations were selected to cover a range of habitats that are as comparable as possible to those

Polarstern cruise ANT-XXXI/2 (PS96)
During cruise PS96, OFOS was deployed at a total of 13 stations at water depths between 200 and 754 m ( Table 2). During the casts of 0:54 to 2:14 hours on-ground duration, series of 110 to 293 photos (2,670 in total) were obtained along OFOS 15 transects of 0.9 to 2.6 km length. In addition, more than a total of 14:50 hours of video footage (available from first author on request) were recorded. OFOS stations were distributed over several regions in the southeastern and southern Weddell Sea (Fig. 4)