The name CTD says it all (almost). This water quality profiler samples
Conductivity Temperature & Depth*
*Actually, the profiler samples pressure, and infers depth. From the Sea-Bird website:
“Despite the name, all CTDs actually measure pressure, which is not quite the same thing as depth. The relationship between pressure and depth is a complex one involving water density and compressibility as well as the strength of the local gravity field. The CTD data can be used to calculate salinity, density, sound velocity, and other parameters of interest. The term CTD is often used today to describe a package that includes the actual CTD as well as auxiliary sensors to measure other parameters (such as dissolved oxygen, pH, turbidity, fluorometer, altimeter, etc.) and a water sampler to collect water samples for later analysis in the lab. The term Sonde is sometimes used as an alternative to CTD.”
In 2008, John Halfman upgraded the Sea-Bird SBE-19 CTD he used in the Finger Lakes since the 1990s to the SeaBird SBE-25 shown above, which measures temperature, conductivity, dissolved oxygen, radiance (Photosynthetically Active Radiation, PAR), fluorescence (algal concentrations), and Turbidity vs. Depth.
A cool feature of the CTD is that it is programmed to collect data every 0.5 seconds during its deployment, thus collecting a data point every meter or so through the water column.