Seabed photographs taken along OFOS profiles during POLARSTERN cruise PS81 (ANT-XXIX/3)

• Main Authors: Piepenburg, Dieter, Segelken-Voigt, Alexandra, Gutt, Julian
• Format: Article in Journal/Newspaper
• Language: English
• Published: PANGAEA - Data Publisher for Earth & Environmental Science 2013
• Subjects: Antarctic Thresholds - Ecosystem Resilience and Adaptation AnT-ERA; Antarctic; Antarctic Peninsula; Alfred Wegener Institute; Antarc*; Iceberg*; SCAR; Scientific Committee on Antarctic Research
• Online Access: https://dx.doi.org/10.1594/pangaea.872719; https://doi.pangaea.de/10.1594/PANGAEA.872719

As a contribution to the international Scientific Research Program "Antarctic Thresholds - Ecosystem Resilience and Adaptation" (AnT-ERA) of the "Scientific Committee on Antarctic Research" (SCAR) and to the ecological working programme "Dynamics of Antarctic Marine Shelf Ecosystems" (DynAMo) of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven (AWI) the Ocean Floor Observation System (OFOS) was deployed during expedition PS81 (ANT-XXIX/3) of RV Polarstern. Sea-bed photographs were taken along 30 drift profiles (=stations) at water depths between 35 and 780 m on both sides of the northern Antarctic Peninsula in January to March 2013. The setup and mode of deployment of OFOS was similar to that described by Bergmann and Klages (2012, doi:10.1016/j.marpolbul.2012.09.018). OFOS is a surface-powered gear equipped with a downward-looking high-resolution, wide-angle still camera (CANON® EOS 5D Mark III; lens: Canon EF 24 f/1.4L II, f stop: 13, exposure time: 1/125 sec; in-air view angles: 74° (horizontal), 53° (vertical), 84° (diagonal); image size: 5760 x 3840 px = 21 MPix; front of pressure resistant camera housing consisting of plexiglass dome port).OFOS was vertically lowered over the starboard side of the ship with a broadband fibre-optic cable, until it hovered approximately 1.5 m above the seabed. It was then towed with the slowly sailing ship at a speed of approximately 0.5 kn (0.25 m/s). The ship's POSIDONIA Positioning System, combined with the Inertial Navigation System (INS) and satellite-based Global Positioning System (GPS) of Polarstern, was used to gain highly precise underwater position data.During the profile, OFOS was kept hanging at the preferred height of 1.5. m above the seafloor by means of the live video feed and occasional minor cable-length adjustments with the winch to compensate small-scale bathymetric variations in seabed morphology. Information on water depth and height above the seafloor were continuously recorded by means of OFOS-mounted sensors (POSIDONIA transponder, Tritech altimeter).Three lasers, which are placed beside the still camera, emit parallel beams and project red light points, arranged as an 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 m² (= 2.3 m x 1.5 m; with variations depending 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 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 image feed via the fibre-optic cable.If any image from this collection is used, please cite the appropriate reference. : According to the manufacturers' specifications, the GPS receiver of Polarstern (Trimble SPS461) has an accuracy of ?better than 5 m 3DRMS? (without differential signal that was not available during the cruise), and the POSIDONIA system has an accuracy of 0.2% of slant ranges, which correlate best with cable lengths, which in turn were very close to water depths at the operational conditions of OFOS deployment during the cruise (e.g., small towing speed, so that OFOS was constantly hanging almost directly below the side of the ship during the entire cast). However, the actual accuracy is known to be always less than the manufacturer's specifications because shipboard hydroacoustic systems are highly influenced by a variety of factors (e.g., transducer installation and calibration, ship's movements and noise, acoustic water-column characteristics). Therefore, the actual accuracies of Polarstern's underwater positioning systems were found to be 1 to 1.5% of slant range for 95% of the data, based on the ship's data post-processing/filtering/validation history. This suggests that for the depth range of OFOS stations during PS81 (about 30 to 800 m), a range of 0.3 to 12 m should be added to Polarstern's GPS accuracy. The spatial offsets between positioning devices on board and in the water were corrected automatically. In conclusion, the overall accuracy of the spatial positions of OFOS-derived seabed images can be estimated to fall into the range of 5 to 15 m, depending on water depths. However, the relative precision of the determination of the spatial intervals between the images taken along a drift transect is assumed to be one order of magnitude higher, i.e., these distances can be assessed with a precision on a m-scale. Accordingly, the coordinates of the seabed images are given as decimal degrees (DD) expressed to five decimal places, corresponding to a spatial resolution in the range of 0.5 (for longitudes at latitudes of about 60°S) to 1.1 m (for latitudes).