A CTD is so named as it measures conductivity, temperature and depth. This particular model aboard the R/V Rutgers also has sensors that measure turbidity and chlorophyll a.
Why we collect this data
These measurements provide us with basic information about the ecosystem as well as providing insight into the source of the water (freshwater from runoff or saltier water from tides). The conductivity, temperature, and pressure (depth) can also be used to determine density. The density differences between different masses of water can cause currents.
Turbidity gives us information on how much “stuff” is in the water. For example, how much sediment was mixed up into the water column during a storm.
Phytoplankton are primary producers as well as the base of the food chain. By measuring the primary photosynthetic pigment in phytoplankton called chlorophyll a, we can estimate the amount of phytoplankton and learn more about the ecosystem.
How it works
The CTD determines conductivity by measuring how well the water conducts an electric current. This information can then be used to determine salinity.
Temperature is determined using a thermistor, which measures temperature based upon changes to the electrical resistance of a metal.
Since depth and pressure are directly related, the CTD uses a pressure gauge to measure the pressure and this information can be used to calculate depth.
Turbidity is measured using an optical backscatter, which measures the intensity of light that is transmitted through the water.
When chlorophyll is irradiated with light, it emits (sends out) light of a higher or lower wavelength. This is called fluorescence, the absorption of light energy at one wavelength and its re-emission at another. A fluorometer measures fluorescence. It sends out a certain wavelength of light and measures the the intensity of light that is returned when it is re-emitted by the phytoplankton. The greater the intensity of light that the fluorometer reads, the more chlorophyll and phytoplankton in the water.