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Sampling water column properties

Knowing the physical properties of the water column is a critical component in understanding marine ecosystems. Scientists use a variety of instruments to observe and monitor essential seawater properties. One of the most widely-used piece of equipment used during research cruises is a CTD, which stands for conductivity-temperature-depth.

CtdA CTD (pictured right) gives us essential information about salinity and pressure (see graphs below), but auxiliary sensors can also be installed on the CTD, such as fluorometers, turbidimeters, transmissometers or oxygen sensors. These can increase the information collected about the physical and chemical properties of the water column.

Above: Parameters measured by the CTD. On the left vertical profiles of temperature (red line) and salinity (blue line) and on the right a T/S diagram.

When working aboard a research vessel, the CTD is normally mounted on to a rosette, a frame-like device that carries a series of sampling bottles. These bottles are used not only to verify the CTD measurements, but also to provide seawater samples for analyses back in the lab. 

The CTD rosette system is connected to the ship by a conducting wire, which transmits the seawater measurements in real time and allows the system to be controlled from the ship via a control unit known as the unit deck. The  sampling bottles, which are open whilst the instrument is lowered through the water column (known as the downcast) are automatically activated from the unit deck, so that they close during the upcast (the return upwards journey through the water column) at selected depths. This enables very precise sampling of the water column at specific depths, and allows a vertical profile of water properties to be constructed.

This information improves our knowledge of the oceanic circulation, the living conditions for sea organisms, and the effects of climate change in the ocean.

Left: The CTD rosette - the CTD instrument is mounted at the bottom, and the water sampling bottles in a ring. Above: the unit deck, from where the sampling apparatus is controlled.
Above: Typical plot showing the vertical sections of temperature (upper panel) and salinity (lower panel) across a seamount.
Written by Angela Mosquera

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EU

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 818123 (iAtlantic). This output reflects only the author’s view and the European Union cannot be held responsible for any use that may be  made of the information contained therein.