The backbone of Oceanography: The CTD

If there is one instrument that can be considered to be the core of sea-going oceanographic research, it’s the CTD package.  CTD stands for Conductivity, Temperature and Depth.  It’s deployed on a long wire from a winch into the depths of the ocean.  The measurements of temperature, salinity (estimated from conductivity) and depth allow us to study the stratification, or layering, of the ocean.  From this we can derive the currents, estimate mixing, and define how close to the surface the plankton are likely to be confined.

But our CTD, or “package” is special.  In addition to the core measurements, we are measuring optical properties of plankton to estimate their biomass and how happy they are.  We also have a little robot that take pictures of all the little beasties that fly by as the “package” goes down to 1,500 m, and then back to the surface.

Deploying the package is quite an ordeal.  The video below shows the process, sped up by 2000%.  Depending on how deep we are doing the cast, or deployment, the whole process takes from 1-3 hours and involves a total of 6 people directly, plus the crew on the bridge who make sure we are always facing into the waves and not in danger.  A crewmember mans the winch, a technician on deck orchestrates the whole ordeal, and four scientists tend the lines and trip the bottles from the computer lab.

CTD_small from Peter Gaube on Vimeo.

Oh yeah, the bottle, argh, those bottles.  In total, there are 24 bottles on the CTD that collectively are called the “rosette.”  These bottles are held open until an electrical signal is sent down the cable to the CTD telling them to snap shut, trapping 10-liters of water in each bottle.  The water is brought to the surface and quickly sampled by a swarm of scientists who analyze the samples for a multitude of measurements, including the rate at which viruses attack plankton, how many plankton there are, how fast they are growing, how green they are, and who is around eating them and at what rate.

Overall, on this field program, and most oceanographic cruises, the backbone of our sampling comes from the CTD package.  I’m just the lucky guy who gets to send it overboard and make sure it comes back on deck safely and full of water.