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CTD: The Oceanographer's Work Horse

posted Jul 1, 2012, 7:49 PM by Shannon Bresnahan   [ updated Jul 1, 2012, 8:06 PM ]

Good Morning, San Diego!

As Chief Scientist Christina Frieder mentioned in her blog on June 28, we are running 24-hour operations, and our team, the Pelagic Environment Team, woke up at sunrise to conduct our first CTD cast! Today, the surface of the ocean is glassy calm, but beneath the air-sea interface, it is dynamic and full of life. Looking out at the San Diego coastline, I spot Black’s beach, the Scripps Institution of Oceanography Pier, and Del Mar Fairgrounds. Dr. SungHyun Nam and I are inside the control room talking with the crew outside. At the same time, our expedition volunteers from all over the world are helping deploy perhaps the most important and most reliable oceanographic instrument ever, the CTD.

Photo: A CTD is prepped for deployment on the deck of R/V Melville.

CTD is short for conductivity, temperature, and depth, and the CTD instrument is an invaluable tool that oceanographers have used for decades. The CTD is strapped to a weighted frame to keep it stable underwater during most weather conditions. The sensors that measure CTD are surrounded by a rosette of water sampling bottles that individually close shut by an electronic trigger onboard the ship. Thanks to this technology, we are able to lower the CTD to the bottom of the ocean by a winch and crane system onboard research vessels. These casts deliver real-time data to our television screen, just like any everyday communications cable. Except our cable is 6,000 meters long.

Photo: The underbelly of the CTD shows a cluster
canisters for collecting seawater from ocean depths.

indirectly measures the salinity of water. Think of salinity as the amount of salt or ions in a cup of water. When salt is added to the cup or if the water evaporates, the water becomes more saline and more like the ocean. When two cups of water with different salinities are mixed into one, the salinity changes, but not as much as one might think. Interestingly, different water masses retain some of their properties, like salinity, and it requires a lot of energy to mix two water masses of different salinities.

Seawater temperature is a pretty straightforward measurement, but it is an important concept to think about when measuring various chemical parameters. Temperature is also important because it influences some of the most basic biological processes like metabolism, photosynthesis, and respiration. We are able to maintain our body temperature at about 98.6 degrees Fahrenheit, right? Other animals, however, are dependent on the temperature of the environment in which they live. This makes for a chilled out lifestyle in the deep sea, especially since light energy can only reach so far.

Another measurement that must be made when at sea is depth because pressure affects several water properties, particularly temperature. Because water is compressible, the pressure can compress seawater and actually increase its temperature. With the CTD, however, we can correct for the compressibility of seawater and ultimately calculate the true or “potential temperature” of a water sample at depth. With these measurements, we may be able to identify and sample from specific bodies of water that formed from various regions around the world.  

Take a breath, please.


Just like we need oxygen to fuel our bodies, marine organisms need it for similar metabolic processes and can sometimes be starved for it, especially in Oxygen Minimum Zones (OMZs). It took many years for oceanographic engineers and chemists to figure out how to measure oxygen throughout the ocean’s water column with a sensor, but they eventually got it, and Eureka! Now, my generation is blessed to have these capabilities at our fingertips because it helps us address new research questions. What sort of organisms are found in and out of the OMZ? Is the OMZ a biogeographic barrier to dispersal of aerobic life?

Photo: Dr. SungHyun Nam, Kirk Sato and Yui Takeshita monitor real-time salinity, temperature, oxygen and chlorophyll readings from a Martz Lab sensor attached to the CTD rosette.

In addition to the CTD, team members in the Martz Lab have developed the instruments capable of measuring other parameters such as pH, dissolved inorganic carbon, oxygen, and calcium.  These very precise measurements will be useful in the future when monitoring changes in ocean chemistry caused by ocean acidification. Although today was our first day of working with the CTD aboard the R/V Melville, it feels like we’ve been working as a team for years. I’d like to thank the Pelagic Environment Team and the crew of the Melville for making our first few CTD casts successful ones!

--Kirk Sato, Scripps Graduate Student