The Utility of Monitoring Estuarine Water Quality with ACROBAT: Assessing Spatial Profiles with a Multi-Sensor Towed Instrument Platform

Howard Kator, Leonard Haas, Iris Anderson, Hunter Walker and Lorraine Brasseur

Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, 1208 Greate Road, Gloucester Point, Virginia. 23062 USA

The ability to profile the hydrography, biology and chemistry of large, complex estuarine systems in real time and space provides distinct advantages over fixed moorings and surface data logging. We are testing a towed vehicle with multiple water quality sensors in the York River, Virginia to assess its utility to quantify water quality parameters in three-dimensional space. The ACROBAT (Sea Sciences Inc.) is a small, "winged" instrument platform towed from a small vessel and depth-controlled by computer. It is equipped with sensors for chlorophyll/turbidity (SCUFA, Turner Designs), conductivity, temperature and depth (pressure) (Falmouth Scientific), and dissolved oxygen (Analysenmeßtechnik GmbH). Of particular interest is the short response time of the oxygen sensor (milliseconds), a requirement for accurately mapping dissolved oxygen (DO) while moving rapidly in water with strong vertical oxyclines. The Acrobat has been successfully deployed in the York River in depths ranging from 5-20 m and at speed over the ground of 4 m/s with typically 10 top-to-bottom excursions/km in a 10 m water column. Using MATLAB software (The MathWorks, Inc), two-dimensional plots of DO along the estuarine axis depict the dimensions of sub-pycnocline, summer hypoxia. The ACROBAT also mapped widespread oxygen depletion in the river in the aftermath of hurricane Isabel. Hypoxic/anoxic water volume has been proposed as a critical water quality criterion for the Chesapeake Bay and will require intensive, real-time sampling for documentation. Aspects of ACROBAT design, calibration and use will be discussed using data collected during 2003.

Keywords: monitoring, estuarine water quality, towed instrument, hypoxia/anoxia, sensors,
3-D water quality mapping.