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.

Christine Hedge, September 3, 2009

NOAA Teacher at Sea
Christine Hedge
Onboard USCGC Healy
August 7 – September 16, 2009 

Mission: U.S.-Canada 2009 Arctic Seafloor Continental Shelf Survey
Location: Beaufort Sea, north of the arctic circle
Date: September 3, 2009

Weather Data from the Bridge   
Latitude: 780 34’N
Longitude: 1360 59’W
Temperature: 290F

Science and Technology Log 

Ethan Roth shows me the inner workings of a sonobuoy.
Ethan Roth shows me the inner workings of a sonobuoy.

Low-Impact Exploring 

Some of my previous logs have talked about sound in the Arctic Ocean.  Sounds made by seals, whales, ice cracking and ridges forming, bubbles popping, wind, waves – these are the normal or ambient noises that have always occurred. As governments, scientists, and corporations explore the Arctic their presence will have an impact. Ships breaking ice and the seismic instruments they use to explore, add noise to the environment.  We call this man-made noise, anthropogenic noise.  Will these additional sounds impact the organisms that live here? Can we explore in a way that minimizes our impact on the environment?  The marine wildlife of the Arctic has evolved in an ocean covered by ice. But the ice is changing and the human presence is increasing.

Studies of other oceans have shown that more ship traffic means more background noise. In most regions of the Pacific Ocean the background noise has increased 3 decibels every 10 years since the 1960’s. The scientists on the Healy and the Louis are interested in minimizing their impact as they explore the Arctic Ocean.

Do No Harm – Step 1 Collect Data 

I am tossing the sonobuoy off the fantail of the Healy.
I am tossing the sonobuoy off the fantail of the Healy.

One of the ways we are listening to the noise that our own instruments make is with sonobuoys. These are devices that help us listen to how sound propagates through the ocean.  While the Louis is using airguns to collect seismic data – scientists on the Healy are throwing sonobuoys into the ocean to listen to the sound waves created by the airguns. Knowing how the sound waves from airguns travel through the water will help us to understand their impact on the environment. Sonobuoys are self-contained floating units. They consist of a salt-water battery that activates when it hits the water, a bag that inflates with CO2 on impact, a 400-foot cable with an amplifier and hydrophone (underwater microphone).

The data acquired through the sonobuoy are relayed to the ship via radio link. A receiving antenna had to be placed high up on the Louis in order to collect this data. Like many of the devices we are using to collect information, the sonobuoys are single use instruments and we do not pick them up after their batteries run out. After 8 hours of data collection, the float bag burns and the instrument sinks to the bottom. They are known as self-scuttling (self-destructing) instruments. The more we know about the sounds we make and how these sounds are interacting with the animals that call the Arctic home, the better we will be at low impact exploring.

Personal Log 

The float inflates as the sonobuoy floats away.
The float inflates as the sonobuoy floats away.

I’ve had lots of questions from students about the weather. For most of our trip, the air temperature has been around 270F and the visibility has been poor. A log fog has prevented us from seeing the horizon. We have also had quite a few days with snow and freezing rain.  Some of our snow flurries have coated the decks with enough snow to make a few snowballs and prompted the crew to get out the salt to melt the slippery spots. 

This past week we had some seriously cold days.  On September 1st, the air temperature was 160F with a wind chill of -250F. These cold days brought blue skies, sparkling snow, and beautiful crystals forming on the handrails, ropes and many other surfaces on the deck.

Ice crystals on a valve
Ice crystals on a valve

FOR MY STUDENTS: Why do you think it is foggier on warmer days? 

As we travel south we are starting to get some sunsets and sunrises.  There are a few hours of twilight between the times that the sun dips below the horizon – but no true night sky.  One of the things I miss the most is seeing stars.  I look forward to seeing the Indiana night sky in a few weeks. But until then, the gorgeous sun over the Arctic will have to do.

As the seasons change and we travel south, the sun gets lower in the sky

Arctic snowball
Arctic snowball