Blog by Isabelle Schmitz
The CTD is a crucial part of research in the field of oceanography. It can measure Conductivity, which converted to salinity tells you how salty the water is, Temperature, and pressure which can be converted to Depth. You can use measurements of salinity and temperature to calculate a lot of important things like density or transports which highlights the necessity of the CTD in general. And there is also always space to attach additional sensors e.g. for oxygen or turbidity. During the measurement (a.k.a. cast) you have a live overview of the water column beneath you. The colored lines show different variables plotted against depth (see the photos below).
The properties we measure also give clues about where different water masses might origin from. Depending on the location there are various different water masses which have predefined values of temperature and salinity. And if you would find water which is very cold and salty, so very dense, in regions where there would be no way to cool water down like that (like the tropics) then it gets obvious that this water mass origins from someplace else. It probably moved to our measuring station then through surface or bottom currents for example.
The measurements are taken at a fixed location ( so the ship doesn’t move ) and we lower the CTD rosette over the side of the working deck into the water. After that a winch driver is responsible for lowering it. The first 100 m we go slower with a speed of 0.5 m/s to get higher resolution of datapoints in the upper layer. Because often this is where the interesting things happen. After that we accelerate to 1 m/s. So if you have a water depth of around 4500m it easily can take 1 1/2 hours until the CTD reaches the bottom. There we also slow down again to not risk the CTD touching the bottom, so nothing breaks.
Apart from slowing down, several other mechanisms prevent us from touching the seafloor with the CTD. First we have an altimeter, which can measure the distance to the bottom so we have a nice curve which tells us exactly when to stop. And just to be sure we also attached a bottom weight (which is a stone and a 10m long rope) to the CTD Rosette. When the stone touches the bottom, the tension in the rope changes and an alarm is triggered. After that we have to go up again (for another 1 1/2 hours :) ). But this upcast allows us to take water samples. In addition to our CTD, we also have water sampling bottles (or niskin bottles) on our Rosette, which can be closed from the Lab through a special mechanism.
We take water samples for various different reasons. The first is about calibrating sensors. If we have real samples we are able to investigate them more precisely on the ship. Then if we compare the water sample and what the sensor actually measured, we can estimate how accurate the sensor is. And if it’s not, then we can recalibrate. The same goes for salinity or rather conductivity, where we also compare the measured variable with the sensor and the processed water sample. Furthermore we also measure CFC tracers in the water column, for which we also need water samples from different depths but more on that later.
The results of this CTD measurement are mostly displayed as casts, where you see the chosen variables plotted against pressure. So you get datapoints for each depth level. This allows you to have an overview of the characteristics of the water column right beneath you.
Scientists preparing the “Niskin” bottles on the CTD rosette. Images by K.Scheliga