How do oceanographers measure and sample from the surface to the bottom?

Author: Denis Volkov

Figure 1. A rosette onboard the Research Vessel “Roger Revelle” during a research cruise in the Pacific Ocean in September 2016 (photo by D. Volkov). Note that the gray sampling bottles are open prior to the deployment.

The primary “workhorse” of sea-going oceanographers is a so called rosette – a framework with 12 to 36 sampling bottles (in our project we have 24 bottles with a volume of 12 liters each) clustered around a central cylinder, where a CTD and/or other sensor package can be attached. A CTD is an instrument that measures the conductivity, temperature, and pressure of seawater (the D stands for “depth,” which is closely related to pressure). The conductivity measurements are used to determine salinity. These are essential physical properties of seawater that determine its density and to a large extent ocean circulation. Usually, a rosette also houses Acoustic Doppler Current Profilers (ADCP) that measure the horizontal velocity, and oxygen sensors that measure the dissolved oxygen content of the water.

In order to take measurements, the ship stops and a CTD cast is carried out. The location where measurements are taken is called an oceanographic station. The rosette is lowered on a cable down to just above the seafloor with the sampling bottles opened at both ends, so that water can freely circulate through them. The CTD is connected to a computer onboard the ship, and scientists can monitor changing water properties in real time. When the instrument ascends, the sampling bottles are closed selectively at predefined depths by a remotely operated device.

During the I07N project, we are planning to complete 132 stations and collect water samples that will be analyzed for oxygen, nutrients, salinity, dissolved inorganic carbon, alkalinity, pH, chlorofluorocarbon, dissolved organic matter, dissolved organic radiocarbon, particulate organic matter, and some other parameters.

Figure 2. A rosette being lowered into the water by science technicians on board the Research Vessel “Ronald H. Brown” during the first I07N test station in the Indian Ocean on Apr. 25, 2018 (photo by D. Volkov).