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At-Sea Detection: Ah, the CTD’s Not the Life for Me

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I had a light schedule of observing today, so after my last shift, I headed down to see how half of the other half lives. I got to see most of one CTD cast. Once the ship has come to a stop, they lift the metal-framed “rosette” of bottles and electronics with a winch (you can see the winch operator, Bruce, posing for his moment of fame), push it away from the ship, and lower it on a cable into the ocean. Because there’s a lot of tension on the cable and a break could send cable flying hazardously around, the CTD crew has to wear hard hats and life vests any time the winch is running.

CTD stands for conductivity (a measure of salinity), temperature, and depth. In all, a CTD “cast,” or toss over the side and down to 600 meters and back up, measures a lot more than salinity, temperature, and depth, but that’s the term they use. It’s a measure of a number of oceanographic factors that determine water masses and help describe circulation. We’re looking at CTD information to see when we cross over into the Subtropical Convergence Zone.

While the ship is still stopped, the cable is fed out. The whole time the instrument is in the water, the bridge keeps the ship as stationary as possible—if the cable got under and behind the ship, it could be cut by or ruin the ship’s propellers. Several real-time measurements are taken all the way down to 600 meters—you can see a graph of the readings of salinity, temperature, dissolved oxygen, and chlorophyll as the rosette descends. Once the CTD rosette reaches 600 meters, they bring it back up to a depth of 200 meters. Every 20 meters from there to the surface, one of the bottles is “tripped” to close up and bring a sample of water from that depth. After the last sample is taken, the rosette is brought up out of the water, lassoed with a rope attached to an overgrown crochet hook, and pulled back onto the deck. At that point, the ship can get under way again.

Next step is getting the water from the 10 bottles on the rosette into other bottles that can be taken into the lab. It reminded me a bit of farmers milking cows—the scientists are all squatting down and watching a liquid flow into another container. There are three bottles filled from each rosette bottle; the smallest one is used for measuring nutrients—this sample is just frozen in a plastic bottle. The medium bottle (which holds one liter) is filtered for chlorophyll, and the largest one (holding two liters) is filtered to analyze by HPLC (high-performance liquid chromatography) to determine the community of organisms that contribute to the chlorophyll reading (diatoms? foraminifera? dinoflagellates?). They filter the water by filling a funnel that sits on top of a quarter-sized filter, through which the water is sucked by vacuum. So they’re really interested in what very small particles are in the water rather than the water itself. When the entire bottle of water is filtered, they use a tweezers to pick up the filter and lay it on a piece of aluminum foil that’s labeled with the station, bottle number, and sample type (for chlorophyll or for organism type). These go into a deep freeze—a vat of liquid hydrogen—for storage until we get back to Hawaii.

Then it’s just a matter of making sure the rosette is ready to go for the next cast. All the filtering and preparation takes up most of the time to cruise to the next station, so the CTD crew gets just a few 20-minute breaks during their 12-hour shifts. Makes me feel particularly spoiled to get two hours off a couple times a day. However, I haven’t found any CTD crewmembers who are dying to look through the Big Eyes.

The Big Eyes crew is improving day by day. Very few nausea complaints, and we’re getting the hang of data entry, scanning with the binoculars, and even putting in longer shifts on the Big Eyes. It helped that the sea was extra-calm today. At one point we were seeing so much debris that the data entry person was back-logged. It was just one patch, and you couldn’t say it was debris-filled, but instead of seeing a piece of debris every half hour, we saw something every 5 or 10 minutes. You hate to be excited to see debris, but it’s nice to get the practice in.

Author: NOAA Marine Debris Program

The NOAA Marine Debris Program envisions the global ocean and its coasts, users, and inhabitants free from the impacts of marine debris. Our mission is to investigate and solve the problems that stem from marine debris, in order to protect and conserve our nation's marine environment, natural resources, industries, economy, and people.

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