Ring Around the Rosette: The Science of Seawater Sampling

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Taking CTD Samples offshore from Baja California Sur, Mexico. Photo credit: Melissa Ward

In the open ocean, water can be thousands of feet deep. When oceanographers need to test seawater from such depths, how do they get their samples? The most experienced divers can only go to a few hundred feet, and even then, a diver can only bring back as much seawater as he or she can carry.  Therefore, specialized equipment is needed in order to obtain samples from the deep ocean. A CTD rosette is often the equipment of choice for such a task.

The CTD rosette, sometimes called “the package,” is comprised of two main, complementary parts: the CTD, which continuously measures the conductivity (i.e., salinity), temperature, and depth of the water; and the rosette, an arrangement of large, cylindrical bottles, called Niskin bottles, that capture seawater samples.

The CTD rosette is attached by a steel cable to a winch aboard the ship.  When the ship arrives at a sampling station, survey technicians lower the CTD rosette into the water. Once the package is submerged, the winch gradually unfurls the steel cable, and the package sinks deeper into the water to its maximum sampling depth.  Inside the winch cable, a communications cable is also connected to the CTD rosette.  This communications cable allows oceanographers on board the ship to control the package via computer.

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Viewing the live data from the CTD as it drops to 2500 meters from the lab on board. Photo Credit: Melissa Ward

Before deployment, each Niskin bottle is cocked open at its top and bottom, allowing water to flow through the body of the bottle until the reaches its sampling depth. Caps tethered to the top and bottom of the bottle snap into place and close the bottle when the “close” command is given.  After the package reaches its maximum depth, sometimes as deep as 3,000 m (9,500 ft.), the winch begins to pull the package back toward the surface.  As the package ascends, the oceanographers controlling the package from the ship close the Niskin bottles one at a time to capture samples, noting the salinity, temperature, and depth for each sample grabbed. The rosette houses 24 Niskin bottles, each of which is closed at a different depth on the package’s ascent.

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Taking an oxygen sample from the CTD (and making sure there are no bubbles!).         Photo Credit: Emma Hodgson

Once the CTD rosette has been raised to the surface, the survey technicians secure it on board the deck of the ship.  At this time, the scientists measuring the seawater begin sampling from the Niskin bottles.  Each Niskin bottle contains 11 liters of water, enough to meet the needs of the scientists on board.  Because some properties of the seawater are more sensitive to air exchange than others, the scientists follow a strict sampling order:

  1. oxygen concentration
  2. pH
  3. carbonate ion concentration
  4. dissolved inorganic carbon concentration
  5. total alkalinity
  6. chlorophyll concentration
  7. nutrients concentration
  8. salinity
  9. domoic acid concentration (the harmful algal bloom toxin responsible for amnesic shellfish poisoning)
  10. bacterial diversity, metabolism, and interactions with harmful algae

After sampling, the scientists return to their shipboard laboratories to take their measurements, and the ship begins steaming to the next station.  This procedure will be repeated at each of the 151 planned stations, and operations will run around the clock. The suite of measurements will then be compiled, giving scientists a broader understanding of the ocean’s changing chemistry.

Author: Katie Douglas

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