Under-shelf ice video footage and hydrographic data from ROV transects at Drescher Inlet

Transects of a Remotely Operated Vehicle (ROV) providing under-shelf ice video footage and hydrographic data were carried out during ANT-Land 2015/2016 via Neumayer Station III. The campaign was conducted in liaison with the research expedition Filchner Ronne Outflow System Now (FROSN) of RV Polarstern (PS96). The ROV deployment at the Drescher Inlet represents a follow-up study of seal investigations carried out during RV Polarstern's expedition PS82 at the Filchner Outflow System and at the Drescher Inlet in 2014. It also complements earlier investigations at these locations initiated in 1986 (PS65, PS48, PS34, PS20, PS17). The ROV was launched from sea ice within the Drescher Inlet. The establishment of an 1.5 to 1.5 m hole in the fast ice at -72.80279 (LAT) -19.23379 (LON) to allow the deployment of the ROV was by far the most time-consuming and physically exhausting effort. Opening of the 1.20 m thick sea ice close to a tidal crack and removing of a 6 m thick platelet ice layer beneath required more than five days. In addition, a full day after each of the two snow drift events was required to re-open the ice hole, and further daily maintenance of between one and four hours required to remove new platelet ice out of the hole. The V8Sii ROV (Ocean Modules) was equipped with two High Definition (HD) video cameras (Kongsberg oe14-502) in the front and one wide-angle camera (Bowtech L3C-550) in the rear. Both HD cameras were complemented with three parallel red lasers providing reference scales of 10 by 10 cm on the videos. The lighting is ensured by four LED lights (Bowtech LED- 2400 aluminium) in the front and one in the back. The ROV had a compass, an orientation sensor, an altimeter (Tritech Micron Echo Sounder), an obstacle avoidance sonar (Tritech Micron), and an Ultra Short Baseline (USBL) system (Easytrak Lite USBL, Applied Acoustics) to determine its exact underwater position relative to a GPS being installed on top of a crane over the ice hole. The USBL transducer was installed at the end of a six meter aluminum pole inside the ice hole. A sledge module bearing a CTD (SeaBird SBE19 plus) and sensors for pH, oxygen, light, fluorescence, as well as a Doppler Velocity Logger (Doppler Velocity Log (DVL) Teledyne RDI Instruments, Explorer PA) for bottom tracking and current measurements was mounted to the underside of the ROV. A mini-dredge and sampling box called Rowski's Bucket Broom (RBB) attached to the upper side of the ROV was available to obtain invertebrate specimens for post hoc morphological and genetic investigations. Setting out and retrieval of the ROV was achieved by an A-framed crane over the hole and coiling of the ROV cable was done by a hand winch, which was placed on a sledge next to the ice hole. All steering equipment was sheltered by a pop-up tent. Two successful dive operations under the ice shelf provided high resolution footage, hydrographic data, and samples of the under shelf ice community at depths of around 80 m. The video footage documents cryo-benthic isopod crustaceans attached head-down to the underside of the shelf ice at 80 m water depth, where they occur in dense aggregations and different life stages. The successful retrieval of invertebrate samples by the ROV allows the first genetic investigation of the corresponding species