CTD at sundown

2 September 2012 by Anonymous (not verified)

Guest post - Ben Barton (MSci. Oceanography - University of Southampton)
Having arrived on station at the site of the first mooring we deployed the CTD which is a instrument for measuring the Conductivity, Temperature and Depth of the water below us. We use conductivity and temperature from the CTD to measure salinity which becomes important when we are looking at physical processes in the ocean. Density of water is key for many ocean processes, from internal waves to eddies. Just as with oil floating on water because oil is less dense than water, water which is less dense 'floats' on denser water. Density is the simple stabilising parameter and can be found using the CTD.
CTD rising from the abyss
The CTD was lowered through 4.7 km of water along with a fluorometer, oxygen sensor and 24 large water bottles. It took several hours to reach the bottom the ocean and come back up, as you can imagine when you have to move almost 10 km at less than walking speed. As the CTD surfaced at dusk bearing packets of water in its bottles from throughout the water column, our work began. First an oxygen sample had to be taken from each bottle before it could become contaminated with air bubbles. This was followed by a sample that is used to calibrate the salinity sensor and samples for the dissolved nutrients; phosphate, silicate and nitrate. These nutrients are vital to the growth of phytoplankton. Phytoplankton are the plants of the ocean, just like trees and grass on land, they are everywhere and at the bottom of the food chain for nearly all sea creatures and fish. The remaining water from the three bottles that closed closest to the surface of the ocean were used for measuring a whole host of biological parameters. These include photosynthetic pigments in phytoplankton and the microscopic, biological silicate or calcium carbonate shells they build to protect themselves.
Danielle Waters in the lab
So what has all this biology got to do with eddies in the ocean and OSMOSIS? Well the eddies and physical processes control where the nutrients are found. Turbulence in the upper ocean can mix nutrient rich water up to the surface allowing phytoplankton to bloom. This means we can use the phytoplankton to help us understand what physical processes are taking place.