Hydrographic Surveys (CTDs)

Conductivity, Temperature, and Depth

Motioning temperature and salinity helps track changes in water properties over time. During the field experiment, this information was collected in two ways: ocean moorings, and underway measurements. These surveys will be complemented by similar measurements collect by autonomous underwater vehicles.

Using Ocean Moorings

Each of the subsurface moorings deployed in the Sound had three conductivity-temperature recorders (CTDs) mounted at three different depths: 30 m depth, 100 m depth, and 5 m above the bottom.  These instruments periodically sampled temperature and salinity and tracked changes in water properties over time.  Used in conjunction with the ADCP current measurements, they helped identify periods of deepwater exchange (which tends to be colder and saltier) between PWS and the Gulf of Alaska.  The data also helped to determine the amount of freshwater coming into the Sound from the Copper River Delta and the many glaciers around the Sound.  Data from these moored instruments were being combined with hydrographic data collected by NOAA in the summer of 2007 to improve forecasts of tides, tidal currents, and circulation patterns.

Underway Measurements: Temperature, Salinity, Chlorophyll, and Turbidity

Ship based CTD provided additional data as well as an opportunity to collect water for nutrient and plankton analysis. Since 2007, the F/V Alena K has collected underway measurements of near-surface temperature and salinity, chlorophyll, and turbidity.  From spring 2007 to January 2008, the F/V Alena K also conducted CTD casts at seventeen locations during each survey with the goal of estimating the seasonal cycle of freshwater content in the sound.

During the Experiment…

Three vessels were used to run CTD transects.  the Prince William Sound Science Center’s SeaBird 19+ CTD instrument measured pressure, conductivity, temperature, chlorophyll fluorescence, and turbidity.  Depth, salinity and density were consequently derived from the measurements.  The data was processed at the end of each day to provide 1 m vertical binned information at each station.  It was then transmitted to AOOS each evening by 10 p.m. to ensure it was available for incorporation into the evening model run.

The primary survey areas originally panned for the field experiment included a north-south (NS) line in the central portion of the Sound, and lines across Hinchinbrook Entrance. Both lines followed historical sampling locations.

However, the initial sampling plan for the main hydrographic ship was abandoned the second day of the experiment due to weather.  Scientists worked hard to continue sampling, and juggled the schedule to sample lines which were protected from wind and waves.

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