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At-Sea Detection: We Learn How It All Fits Together

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This morning, our first full day out of sight of land, we had a meeting of the entire scientific party. Kyle, our chief scientist, put together a group of speakers to explain how each component (CTD crew, observers, unmanned aerial system (UAS) crew) is an important part of the cruise mission. First up was Evan, describing the CTD casts and how they fit in with the marine debris mission. Evan described the oceanography of the area we’ll be visiting. What we’re looking for as we head north is the chlorophyll front that signals the Subtropical Convergence Zone (STCZ). This feature, which goes pretty much across the Pacific Ocean, signals a transition from the warm and nutrient-depleted waters of the North Pacific Gyre to the south and the colder, productive (nutrient-rich) waters of the Subarctic Gyre to the north. These gradients and the high concentration of nutrients attract fish and birds, and the convergence brings marine debris together in a relatively concentrated area (more another day on what we mean by “relatively concentrated”).

Evan and the rest of the CTD crew will be testing the water every 15 nautical miles once we hit 26 degrees north latitude. In addition to measuring conductivity, temperature, and depth down to 500 meters, they’ll collect water samples at 10 depths on the way back up, between 200 meters and the surface. After the bottles are recovered, they’ll analyze the nutrients and chlorophyll in the water. In some respects, this work is ground-truthing the data being gathered by satellites. Sensors on satellites can measure the temperature and the amount of certain colors in the water and use this as a way to measure chlorophyll (the green stuff in plants, including tiny photosynthesizing plankton). The CTD casts’ chlorophyll measurements will be a check on the accuracy of the satellite-derived information. In addition to the CTD casts every 15 nm, the ship has a flow-through fluorometer, which measures chlorophyll concentration on a continuous basis from water that’s sucked into the ship. This will also be important in helping to identify when we hit the STCZ. Right now the fluorometer is measuring about 0.07 mg per cubic meter of chlorophyll. The convergence zone generally shows a jump up to 0.6 or even 1.0 mg/m3.

Next up was Kyle describing the role of the Big Eyes crew. What we are spotting on the way up to the convergence zone may be thought of as a background; we’re looking now mainly to train our eyes how to look through binoculars for a long time and how to spot debris, as well as getting a baseline of sorts so we’ll be able to sense the increase we hypothesized we’ll find. Our position at the top of the ship makes us much more likely to see debris than the folks in small boats who will be in the water once we get to the STCZ. The UAS may be the best at spotting, but until we get up there and start testing, we just won’t know. So it’s important for us to develop our “chops” so we’re ready to start supporting the UAS and small-boat operations in a few days.

Finally, Tim briefed us on the unmanned aerial system (UAS) component that he’s leading. This entry is too long already, and the UAS story is a good one, so I’ll fill you in more on that once we get closer to the STCZ.


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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