Use of a long endurance solar powered autonomous underwater vehicle (SAUV II) to measure dissolved oxygen concentrations in Greenwich Bay, Rhode Island, U.S.A

As hypoxic water masses increase worldwide in duration and extent due to coastal eutrophication, advanced technology water quality monitoring by autonomous vehicles can increase our capability to document and respond to these environmental perturbations. We evaluated the use of a long endurance autonomous underwater vehicle (AUV) to map dissolved oxygen levels to determine the extent of hypoxia in a small bay (Greenwich Bay, USA.) known to experience summertime hypoxic episodes that have resulted in massive fish kills. During a 7.5 hr nighttime/early morning period in September 2004, the AUV collected dissolved oxygen, conductivity, temperature and depth data along three transects in the Greenwich Bay. The AUV utilized was a new solar powered vehicle designed for continuous survey operations. This vehicle, referred to as the SAUV II, was recently developed for the US Office of Naval Research for long endurance missions requiring monitoring, surveillance, and/or station keeping with real-time bi-directional communications to shore. The vehicle was pre-programmed to transit to designated waypoints at various depths. Following each transect, the vehicle surfaced, maintained a watch circle, and transmitted data to shore via an RF link. The survey provided the participating scientists with near real-time access to the physical characteristics of the water column with improved spatial and temporal resolution over traditional sampling for dissolved oxygen concentrations. This pilot study served as a test of the SAUV II to collect high resolution, near real-time water quality data in a nearshore, shallow embayment. Water column temperature and conductivity was relatively uniform over the survey path (temperature varied from 22-23 /spl deg/C; conductivity varied from 43030-43981 /spl mu/S). The depth of the vehicle varied from near-surface to a maximum of 9.5 m. Depth and dissolved oxygen concentrations were correlated. Dissolved oxygen concentrations ranged from approximately 2.4 to 6.0 mg/L over the course of the survey; mildly hypoxic conditions were encountered. The lowest concentrations of dissolved oxygen occurred at the deeper depths (>6 m). As the pilot study was performed in September, the late stage of a hypoxic event was most likely observed. Using the SAUV II to increase spatial sampling of dissolved oxygen in Greenwich Bay and conducting more frequent SAUV II surveys during the summer months will improve our understanding of hypoxia in this complex coastal estuary.