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.
A 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.
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.