Cast away

With the advent of technology that allows us to measure conductivity, and by association salinity, with great accuracy, the CTD has become a standard oceanographic instrument. They come in all shapes and sizes, depending on the platform to which they are applied (ship, float, towed instrument, etc). Most, if not all, oceanographic research vessels have a ship-board CTD, but other instruments that get deployed from a ship also typically carry some form of a CTD.

The ship-board CTD suspended over the water from its crane. The red spring-loaded disk at the top protects the CTD cage from damage by the crane. The numbered bottles of the rosette are visible in gray, with the sensors mounted below. (Photo credit: Rosalind Echols)

 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.

 

Kate preserves samples by freezing them in liquid nitrogen. (Photo credit: RE)

On this trip, our biologist Kate is collecting water samples at depths ranging from 5 to 200 meters, using the density profile shown by the CTD on the way down to decide where to collect bottle samples on the way up. Once the CTD is back on board, she collects samples for two later analyses: flow cytometry (which will be used to look at chlorophyll levels) and DNA analysis. Some research cruises are equipped with more elaborate biological equipment on board, but in this case much of the space is taken up by the physics equipment, so she is preserving the samples for later use. The goal of the project she is working on with Dr. Anne Thompson of Portland State University is to look at the prevalence of different strains of the cyanobacteria Prochlorococcus, and to see how this relates to what is happening in the physical system (particularly how they adjust their chlorophyll levels after mixing events that move them to a new location in the water column). This is a great example of a project where physicists and biologists are collaborating to understand how two different aspects of a system relate, something that is incredibly important in a complex arena like the ocean.

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).