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Taking oceanographical measurements: using a CTD

Scientist from the Bedford Institute of OceanographyJust as taking a person’s temperature tells something about their physiological status, so can taking the temperature in the ocean tell us something about the ocean. Other parameters, such as salinity, water density etc reveal information about water movements and productivity.

Oceanographers routinely measure a number of parameters in order to learn more about the ocean. In addition to temperature, they measure conductivity, or how easily electrical current is conducted through the water. This measurement gives an indication of the water’s salinity. The density of the water, or its weight per litre, is a function of the water’s salinity, temperature and pressure. Pressure, in turn, depends on depth.

Oceanographers use a number of different instruments in their work. Learn more.

Water sampling devices range from a bucket dropped over the side of a ship to large water bottles sent thousands of meters toward the seafloor on a wire. Probably the most commonly used water sampler is known as a CTD/rosette.

This rosette image comes from a NOAA cruiseTone Falkenhaug, a MAR-ECO scientist, tells us that the CTD instrument measures the conductivity and temperature at the depth where the instrument is situated. CTD, she says, stands for conductivity, temperature and density. A CTD rosette is a framework designed to carry 12 to 36 sampling bottles (typically ranging from 1.2- to 30-liter capacity) and a conductivity/ temperature/ depth sensor.

The CTD sensor measures the conductivity and temperature at the depth where the instrument is situated (and this information is sent directly to the scientists via computer aboard ship). The density of the water at a certain depth is then calculated from its conductivity (i.e. salinity), temperature and the pressure (i.e. depth).

The data from the CTD sensor is sent back to the ship continuously. Scientists generally pre-programme the water bottles in the rosette to open and sample at pre-determined depths, but they can also be used selectively at specific depths to confirm any interesting features indicated by the CTD data as the instrument ascends. A standard deployment of a rosette/CTD, depending on water depth, requires two to five hours of station time, which is when a ship remains stationary at a given location.

Falkenhaug explains that the effects of pressure on density (and temperature) have little importance in coastal areas where depths are relatively shallow. However, it has to be considered when the depth is large.

Rosette of Niskin bottlesExample:
If water of salinity 35 ppt (parts per thousands) and temperature 5 C at depth of 4000 m is brought to the surface, the temperature would cool to 4.56 C due to expansion (conversely, the temperature would rise again due to compression when returning to 4000m). In this example, says Falkenhaug, the temperature at 4000 m is called "the in-situ temperature", and the temperature 4.56 is called "potential" temperature. Similarly, scientists refer to in-situ density and potential density, depending on what kind of temperature value has been used for the density calculation. 
 
Such differences, says Falkenhaug, are most relevant for physical oceanographers, who deal with the distribution and movement of different water masses, because of their different salinities, temperatures and densities. While the change in temperature from 5 to 4.56 does not have much direct importance for biological life, the patterns of water mass movements and currents may be one of the most important factors explaining the distributions of organisms!

Researchers during the MAR-ECO cruises will record CTD data in order to gain as much information as possible about the environment under study.

 

Links

NOAA web page about oceanography techniques

Scientist from Bedford Institute of Oceanography using CTD

information about oceanography tools

Scientist from the University of Bergen using CTD

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