Prince William Sound Field Experiment

why care about ocean moorings?

We have only a general understanding of how much and how often water is exchanged between the Sound and the Gulf of Alaska. Data from moored instruments will provide more detail and help improve forecasts of tides, tidal currents, and circulation patterns.

moorings in prince william sound

Go to AOOS Mooring data

measuring ocean currents

We measure current velocity with an Acoustic Doppler Current Profiler (ADCP). ADCPs measure Doppler shifts to calculate the speed of moving water. We experience Doppler shifts in everyday life; think, for example, of the change in sound of passing cars when walking on the side of the road.

ADCPs emit a sound at a given frequency and listen to the echo of this sound after it has bounced on small particles naturally present in seawater. We can then estimate the distance between the instrument and the reflecting particles, using the delay between the emission of the sound and the arrival of the echo.

If the echo from a given distance arrives with a higher frequency than the emitted sound, then the volume of water at this distance is moving toward the instrument; should the echo arrive with a lower frequency, then this volume of water is moving away. Larger shifts in frequency indicate larger current speeds.

Ocean Moorings

Principal Investigators:
Carter Ohlman, University of California Santa Barbara
mark halverson, prince william sound science center

Researchers believe that water exchange between the Gulf of Alaska and Prince William Sound is the main physical process influencing the abundance and distribution of microscopic plants and animals, called plankton, that form the base of the marine food web.

Moored instruments are anchored to the ocean floor. This prevents the instruments from drifting in currents, and enables them to take measurements repeatedly at the same location. Moorings are often equipped to measure temperature, salinity, and current velocity.

The Prince William Sound Observing System employs ocean moorings as well as nearshore moorings.

Water Properties and Exchange

To help us learn about water exchange, we deploy moored Acoustic Doppler Current Profilers (ADCPs) in the two main entrances of the Sound -- Hinchinbrook Entrance and Montague Strait -- to obtain measurements that allow volume estimates of water transport.

salinity measurements in prince william soundConductivity, Temperature, and Depth

Each mooring has a conductivity-temperature recorder (CTD) mounted at three different depths. CTDs sample temperature and salinity, and thus track changes in water properties over time.

Above: Salinity profile for a part of the Sound. See larger image

CTDs help identify periods of deepwater (colder and saltier) exchange between the Sound and the Gulf of Alaska. The data also help to determine the amount of freshwater coming into the Sound from the Copper River Delta, or from any of the many glaciers around the Sound.

Underway measurements: temperature, salinity, chlorophyll, and turbidity

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.  

Surface salinity mapping will help quantify fresh water inputs to the Sound for assimilation by the ROMS model.  Chlorophyll fluorescence was also measured in the Sound to provide an estimate of interannual and seasonal phytoplankton variability.  Additional surveys are funded by AOOS and will continue through 2008 and 2009.