The value in a CTD, which is short for “conductivity-temperature-depth” sensor, is that it allows oceanographers to measure the three variables that affect the density of the water, and density is a primary driver of circulation in the ocean. Warmer water is less dense than cold water, fresh water is less dense than saltier water, and (even though water is mostly incompressible) water at great depth with a huge column of water stacked on top of it is denser than water at the sea surface. As a result, knowing each of these three values is of great interest in determining the density of the water. On our particular project, we are using CTDs on SWIMS, the floats, the buoy, and the ship. Each of them is a different size and has a different sampling frequency according to the limitations of the platform. Ultimately, the measurements each CTD provides allow us to calculate density and then look at how this changes in time and space, which in turn governs how water can mix and move.
|Marine techs Jen and Bern and Bosun John guide the CTD back into its home inside the ship. You know what's awesome? A door in the side of the ship opens when it's time to deploy the CTD. (Photo credit: RE)|
Ship-board CTDs are often accompanied by additional instruments, such as a fluorometer (which enables subsurface chlorophyll measurements) and a dissolved oxygen sensor. This expands the range of the CTD from purely physical properties to those of interest to biologists as well. Many CTDs are also equipped with a “rosette” (seen in the picture here), which is a set of large bottles that can be closed on demand to gather water from various depths. Once the water is on board the ship, the water can be tested for pH (acidity, effectively), nutrients, and various other chemical species (like CFCs, heavy metals, and so on), and samples can be collected for biological analysis. Each of these tests allows oceanographers to say something about the ocean: who lives there, what nutrients it uses, how long since a particular chunk of water last interacted with the atmosphere, how the water is changing over time, how organisms are effected by physical changes to a system, and so on.
|The CTD is visible below the surface in the super clear water. (Photo credit: RE)|
(Deploying the CTD off the ship is called a "cast", hence the title of the post).