Correcting and validating moored oxygen time-series observations in the Gulf of Maine

The University of New Hampshire has been involved with an Open Ocean Aquaculture demonstration project for the last 10 years in the western Gulf of Maine. As part of an environmental monitoring program, a mooring has been deployed at the field site 10 km off the New Hampshire coast about 9 months each year. Since 2005, dissolved oxygen sensors have been deployed on the mooring. Oxygen was measured by Sea-Bird SBE-43 sensors powered and recorded by SBE-16 Plus SEACATs located ~22 m depth and ~2 m above the bottom. The sensors were calibrated and checked yearly by Sea-Bird. In-situ calibration of the moored oxygen sensors was also done with water samples and Winkler titrations about 6 times a year from 2005 to 2008. The difference between the moored and water sample oxygen concentrations showed a nearly linear drift with time. The drift rates during each of the years and for each sensor (at different depths) were not the same. As the SBE-43 sensors have no apparent zero drift, the drift was corrected by a time changing sensitivity which was applied to the data. The resulting time-series showed the dissolved oxygen variations with time over a year, and showed low values of dissolved oxygen in the deeper waters in late summer and fall. With the successful correction for the sensor drift, the climatology of the Western Gulf of Maine can be studied in more detail and related to the spring and fall plankton blooms, the biological cycles and health of the Gulf.