Roy Moffitt: Measuring Ocean Properties with the CTD, August 19, 2018


NOAA Teacher at Sea

Roy Moffitt

Aboard USCGC Healy

August 7 – 25, 2018

 

Mission: Healy 1801 –  Arctic Distributed Biological Observatory

Geographic Area: Arctic Ocean (Bering Sea, Chukchi Sea, Beaufort Sea)

Date: August 19, 2018

 

Current location/conditions:

Evening of August 19 – Edge of the Barrow Canyons in the Beaufort Sea

Air temp 32F, sea depth  185m , surface sea water temp  32F

 

Measuring Ocean Properties with the CTD

Scientists have a tendency to use acronyms to refer to select processes and measures.  The acronym heard the most, if not constant, on this trip has been CTD.  So here is my best attempt to give you a brief overview of what that “CTD” means and some of the measurements scientists are taking.

CTD Deployment

Deploying the CTD (Conductivity, Temperature, and Depth) probe, which is suspended in a metal “package” with Niskin water bottles

The acronym CTD stands for conductivity, temperature, and depth of the ocean water. This instrument, which takes a measurement 24 times a second, is attached to a large frame that includes big plastic bottles know as Niskin bottles. Nearly every time we stop the ship the CTD package (shown in the image above) is slowly lowered to just above the sea floor (or less depending upon the scientific interest at the site).   On the way back up, the Niskin bottles are filled with seawater from different pre-determined depths.  An electronic switch is triggered for each bottle at different depths so that the containers are sealed closed trapping water from that depth.  Once the package is back on board the scientists measure various properties of the water, including its salinity and oxygen content which will be used to verify and calibrate the electronic sensors on the CTD.

The three main measurements of the CTD represent fundamental characteristics of seawater. Conductivity (C) determines the salinity or the amount of salt in the water.  Electrical conductivity or how well an electric current can flow through the water gives an instant real time measurement of water salinity.  When combined with temperature (T) and depth (D) this gives a measure of the density of the water, and even tells us something about how the water is moving.

In addition to these physical properties, other sensors attached to the CTD provide information on the underwater marine life.  Phytoplankton is the base of the underwater food web and is an important indicator for the overall health of the local marine environment. Phytoplankton is too small to be seen individually without the aid of a microscope; however, scientists have found a way to test for its presence in water. Phytoplankton gets its energy, as all plants do, from the sun using the process of photosynthesis. One of the sensors on the CTD tests for chlorophyll fluorescence, a light re-emitted during the process of photosynthesis.  The amount of fluorescence measured can be used to determine the amount of living phytoplankton at different depths in the ocean.  Another sensor measures the levels of sunlight in the water.

The water samples from the Niskin bottles are used to determine many other properties of the water. One such property is dissolved carbon dioxide.  Just like the atmosphere, the ocean has its own carbon cycle.  We might hear of increased atmosphere CO2 levels associated with global warming.  Some of this CO2 is absorbed from the atmosphere at the surface of the ocean and some of the carbon from the ocean is also exchanged into the atmosphere. This carbon exchange rate between the air and sea helps regulate the pH of the ocean.  Tracking dissolved carbon dioxide measurements over time gives scientists additional physical measurements to track the overall health of the marine environment.  Scientists have been seeing increasing amounts of dissolved carbon dioxide in the ocean which can decrease pH levels making the ocean more acidic.  Small changes in the ocean pH can affect some marine life more than others upsetting the balance in the marine ecosystems.

 

The Exiting Pacific Ocean

At the moment scientists are doing even more CTD casts with a focus on ocean currents.   We are at the edge of the Chukchi Sea where the Pacific-origin water exits the shelf into the deep Arctic Ocean. Much of this happens at Barrow Canyon, which acts as a drain for the water to flow northward. Scientists are still uncertain what happens to the water after it leaves the canyon, so the survey we are doing now is designed to track water as it spreads seaward into the interior Arctic.

 

The Pressure of the Deep Sea

Most of the CTD casts during our time on the Healy have not exceeded 300 meters.  Lowering and raising the CTD from deeper depths takes a lot of precious time, and on this cruise the emphasis is on the upper part of the water column.  However, on August 18, we completed a cast 1000 meters deep.  In addition to collecting data, we were able to demonstrate the crushing effects of the deep ocean pressure by placing a net of styrofoam cups on the CTD to the depth of 1000 meters.  Styrofoam cups contain significant amounts of air. This is why the styrofoam cup is such a good insulator for a hot drink.  At 1000 meters deep, much of the air is crushed out of the cup. Since the pressure is equivalent around the cup, it is crushed in a uniform way causing the cup to shrink. Here are some images demonstrating the crushing power of the sea.  *Note: The big cup with no drawing is the original size.  This will be a great visual tool to bring back to the classroom.

shrunk cups

Styrofoam cups shrunken by the increased pressure of the deep ocean

 

Today’s Wildlife Sightings

A highlight today was not seeing but hearing.  I was able to listen in live on Beluga whales with the help of  deployed sonobuoys.  The sonobuoys  are floating hydrophones that transmit back what they hear with their underwater microphones.  Today they picked up the Beluga whales and their short songs.  I thought their calls sound like the songbirds from home and little did I know, this is why they are called the canaries of the sea!

 

Now and Looking forward

Tonight we saw 100s of Walruses mostly on the ice.  On Monday we will have a presentation about walrus from one of the scientists on board.  I look forward to sharing pictures and what I learned in the next blog.

One response to “Roy Moffitt: Measuring Ocean Properties with the CTD, August 19, 2018

  1. Oh my goodness, Roy: this is sooooo interesting. I had no idea what sort of scientific studies were being done near the Arctic Circle until you became a TAS passenger on the Healy! And those poor cups! I’m guessing that each member of your crew designed one, maybe putting their name on one. I’m so amazed at the work you have done and I’m proud to be a U.S. citizen with a great Coast Guard doing important scientific research.

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