Biogeochemical research with an Autonomous Underwater Vehicle: Payload structure and arctic operations

In modern polar marine research, Autonomous Underwater Vehicles (AUV) gain increasing importance due to their ability to take on high risk or time-consuming tasks in the harsh physical environment of the polar seas. They represent a relatively new group of robots that can investigate large areas in all three dimensions without the direct need of human surveillance. The most common objective for these vehicles that are currently operated worldwide is to gather sonar data and create high-resolution maps of the sea floor. A relatively new task is their deployment within the framework of water column studies dealing with the investigation of biogeochemical processes in the open sea or in ice-covered areas. The Alfred Wegener Institute for Polar and Marine Research (AWI) in Bremerhaven, Germany operates a 21-inch class AUV by the American manufacturer Bluefin Robotics (Quincy, Massachusetts) named PAUL. The main objectives of the herein described AUV project at AWI are the investigation of biogeochemical processes in the surface water and to analyze the stratification of the upper water column in the marginal ice zone as well as the dynamic interaction between ice and ocean. For this reason, since 2008, PAUL has been equipped with a number of biogeochemical sensors (e.g. Nitrate, Oxygen, Fluorescence, etc.) and a newly designed water sample collector all specified for operations in icy waters. In the course of several Arctic expeditions onboard the RV Polarstern, the vehicle was deployed in the Fram Strait close to the AWI deep sea observatory “HAUSGARTEN”. In a first series of diving trials in ice covered areas, the first mission under ice accomplished by PAUL occurred at the HAUSGARTEN site in summer 2010. Here, the description of the vehicle, the mode of operations and especially the structure of the payload is detailed. The payload development process took advantage of complementary instruments: combining data of different sensors with water samples. The samples allow data to be obtained for parameters that cannot be measured in situ, for example the amount of microscopic plankton, and in addition, they can also be used for validation purposes. For example, by measuring the amount of chlorophyll a in distinct samples, the flow-through chlorophyll a fluorometer of the AUV can be calibrated. This provides more accurate values than a calibration with laboratory standards. Results of such measurements gathered from 2010 to 2012 (ARK 25/2 - ARK 27/2, RV Polarstern) are presented exemplarily for the scientific use of the vehicle. Additionally, a description will be given for the new technologies that were developed (remotely operated flying drones, GPS tracking of the ice edge) the new procedures (vehicle tracking, special dive maneuvers) introduced into the AUV project in order to acquire a comprehensive picture of the physical and biogeochemical conditions of the pelagic environment assessed. These steps were mandatory, not only to ensure the safety of the vehicle, but also to be able to place the measured data into the greater context of challenges in understanding the dynamics of ice-covered seas.