The CTD (Conductivity, Temperature, Depth) is the primary oceanographic instrument. The first instrument measures the conductivity of the water - how well electricity passes through it - from which we can work out the salinity of the water. Another sensor measures the temperature very accurately.

The final standard sensor measures depth. Although we have a fairly good idea how deep in the water the CTD should be when we lower it over the side of the ship by the amount of wire we let out, in strong currents, the CTD is often shallower than we expect as the drag on the instrument can cause it to be at an angle to the ship. It is important to know exactly how deep the instrument is to be able to build up an accurate picture of the oceanographic parameters through the water column.

Most CTDs have a rosette attached. This is a series of bottles which can be closed remotely wherever you wish to take a water sample. Such water samples are used to ensure that the sensors are working correctly and when you need to measure additional parameters which the CTD and ancillary sensors cannot measure, like the nutrient content of the water. In the pictures, the rosette is the grey plastic tubes around the outside.

Additional sensors which we usually have attached to the CTD include dissolved oxygen, a fluorimeter to measure chlorophyll concentration and a PAR (Photosynthetically Active Radiation) sensor which measures light. Dissolved oxygen sensors are particularly finicky; we always calibrate the results from that sensor with several water samples using the Winkler titration method, which you may have done in chemistry class.

These parameters tell us a lot about the water in which organisms live; each one of these parameters can have important influences on their physiology and behaviour.

The CTD we normally use is a Sea-Bird Electronics 911 unit.

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