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First Looks: Hydrography

about CTD

ctd sensor

Above: Collecting samples from the CTD array. (Jillian Worssam)

The CTD sensor records the Conductivity of the water that when adjusted for Temperature gives us salinity. The Depth of each sample is recorded because the ocean is not static; it is constantly moving both vertically and horizontally, and changing as it moves.

During the “CTD cast” at each station, 24 data points are collected each second, giving an excellent representation of each water column.

The sensors can employ accessory sensors to measure other ocean parameters such as oxygen, salinity, temperature, pressure, fluorescence, and turbidity.

On this cruise, researchers are also collecting data on micro-zooplankton, nitrates, iron, and radon.

Thanks to Jillian Worssam, PolarTREC

Hydrographic studies focus on the measurement of the physical characteristics of water. During this summer’s Healy research cruise, NOAA/EcoFOCI scientists are sampling oceanic temperature, salinity, nutrients, chlorophyll, and oxygen.

ned cokelet and ctdHydrographic conditions affect the entire food chain and are reflected in the health of fish populations and marine mammals. Summer conditions differ from spring.

A CTD instrument (shown at right) is sent from the surface to near-bottom and back, and uses sensors to record values. The data appear as profiles, from surface to near-bottom, that are averaged to a value at every meter of depth. At the same time, water samples are collected from various depths to look at nutrients and other constituents.

Above: BEST-BSIERP researcher Ned Cokelet obtains water samples from the CTD instrument. (Tom Van Pelt)

contoursA slice of ocean

A series of CTD casts are planned pre-cruise to obtain a line of profiles within a continuous area and a short time period. From this, we prepare a contoured view of a slice of the Bering Sea (right).
See larger image

Typical CTD lines

mapWe repeat these lines of profiles over cruises and seasons for interseasonal and interannual comparison.

A good illustration of a CTD line is the 70-meter isobath line (left). See larger image

“Underway” data

The “sea chest,” or flow-through system, provides additional data as the ship makes its way through the ocean. Water flowing through a pipe on the ship is sampled, keeping a record of changing seawater characteristics along the ship’s path. These along-track data show temperature, salinity, oxygen concentration, nitrate concentration, and chlorophyll levels at a depth of approximately 8 m.

Why are we doing this?

We seek to further understand the Bering Sea shelf ecosystem, beginning with climate’s influence on sea ice, currents and water properties, and its impact on nutrient availability and productivity of phytoplankton and zooplankton. In collaboration with other BSIERP scientists, we seek to detect a seasonal evolution in this ecosystem as it reflects changing physical and biological conditions.

Preliminary conductivity-temperature-depth (CTD) data are now available for the waters of the region.

Thanks to Peggy Sullivan, NOAA/EcoFOCI