Pam Schaffer: Oceanographers Toolbox: What is a CTD? July 7,2018


NOAA Teacher at Sea

Pam Schaffer

Aboard NOAA Ship Bell M. Shimada

[July 2-10, 2018]

Mission: ACCESS Cruise

Geographic Area of Cruise: North Pacific:  Greater Farallones National Marine Sanctuary, Cordell Bank National Marine Sanctuary

Weather Data from the Bridge

Date July 7 2018
Time 1200  (noon)
Latitude 37° 58.3’ N
Longitude 123° 06.4’ W
Present Weather/ Sky Cloudy
Visibility (nm) 10
Wind Direction (true) 341°
Wind Speed (kts) 18
Atmospheric Pressure (mb) 1018
Sea Wave Height (ft) 3-5
Swell Waves Direction (true) 330°
Swell Waves Height (ft) 3-5
Temperature  Sea Water (C) 13.2°
Temperature Dry Bulb (C)

Air Temperature

13.1°
Temp Wet Bulb (C ) 12.1°

 

Science and Technology Log

Marine life is not evenly distributed throughout the World’s oceans.  Some areas contain abundant and diverse life forms and support complex food webs whereas other areas are considered a desert.  This variation is due to environmental factors like temperature, salinity, nutrients, amount of light, underlying currents, oxygen levels and pH.  Some of these variables, such as temperature, oxygen levels, and pH, are experiencing more variability as a result of climate change.  In order to understand the health of marine environments, scientists explore the chemical and physical properties of seawater using a set of electronic instruments on a device called a CTD.   CTD stands for conductivity, temperature and depth and is the standard set of instruments used to measure variables in the water column.

Source: ACCESS www.ACCESSoceans .org

Source: ACCESS http://www.ACCESSoceans .org

The CTD is the bread and butter of oceanography research. It is primarily used to profile and assess salinity and temperature differences at varying depths in a water column.  But the device can also carry instruments used to calculate turbidity, fluorescence (a way to measure the amount of phytoplankton in the water), oxygen levels, and pH.  Conductivity is a way of determining the salinity of water. It measures how easily an electric current passes through a liquid.  Electric currents pass much more easily through seawater than fresh water.  A small electrical current is passed between two electrodes and the resulting measurement is interpreted to measure the amount of salt and other inorganic compounds in a water sample. Dissolved salt increases the density of water, and the density of water also increases as temperature decreases.  Deeper water is colder and denser.  Density is also affected by water pressure. Since water pressure increases with increasing depth, the density of seawater also increases as depth increases.

Optical sensors are used to measure the amount of turbidity, fluorescence, and dissolved oxygen at various depths in the water column.  Dissolved oxygen levels fluctuate with temperature, salinity and pressure changes and is a key indicator of water quality.  Dissolved oxygen is essential for the survival of fish and other marine organisms.  Oxygen gets into the water as gas exchange with the atmosphere and as a by-product of plant photosynthesis (algae, kelp etc.).

Photo Credit: Julie Chase/ACCESS/NOAA/Point Blue

Photo Credit: Julie Chase/ACCESS/NOAA/Point Blue

Typically, CTD instruments are attached to a large circular metal frame called a Rosette, which contains water-sampling bottles that are remotely opened and closed at different depths to collect water samples for later analysis. Using the information and samples collected, scientists can make inferences about the occurrence of certain chemical properties to better understand the distribution and abundance of life in particular areas of the ocean.

Scientist Carina Fish collects samples from CTD

Scientist Carina Fish collects samples from CTD

On our mission, scientists deploy the CTD to a depth of 500 meters at most stations. On the shelf break, the researchers deployed the CTD to 1200 meters (more than 3/4 of a mile below the surface) to collect samples.    The pressure is so great at this depth that a 1 foot by 1 foot square of Styrofoam is crushed to a quarter of its size(3″x 3″).

Retrieving the CTD Rosette

Retrieving the CTD Rosette

Personal Log

Around 01:30 last night we lost our Tucker Trawl net as it was being re-positioned.  The winds had picked up to around 20 knots and the sea height was around 5-8 feet according to the bridge log.   The sea state complicated the retrieval and as best we can conclude the wind and seas pushed the net bridle into a prop blade which swiftly and effortlessly cut the 1/3” thick metal wire cable and separated the net from its tether.  Mishaps at sea are part and parcel of working in a harsh and variable environments. Even the very best and most experienced captain and crew encounter unforeseen issues from time to time.   Dr. Jaime Jahncke quickly stepped into action and made contact with onshore colleagues to arrange for another net for the next research cruise.   In the meantime, we plan to use the hoop net to collect krill samples, weather permitting.

Did You Know?

According to NOAA scientists, only about 5% of the Earth’s oceans have been explored.

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