Water Column Samplers

Water sampling devices range from a bucket dropped over the side of a ship to large water bottles sent thousands of meters toward the seafloor on a wire. Probably the most commonly used water sampler is known as a CTD/rosette, a collection of water bottles and sensors that record conductivity, temperature, and depth information. A transmissometer may be attached to a CTD/rosette, sent down a wire, or attached to a mooring line to measure the number of particles in the water. Aboard ship, a flow cytometer may be used to measure the size and abundance of tiny particles in water samples for research that is often aimed at identifying single-celled plankton.

The CTD is one kind of profiler - a device that descends through the water column making continuous measurements. Another is the eXpendable BathyThermograph (XBT), a temperature probe attached to a thin, spooled copper wire. Additional profilers can measure sound velocity and micro-turbulence.

Other water sampling and incubation instruments - either deployed on a wire or free-drifting - can measure the nutrient uptake of plants, the incorporation of dissolved organics into marine bacteria, or plankton feeding rates. Specially designed water bottles can enclose a sample in a chamber in which experiments may be performed. Undulating vehicles are towed behind ships to provide a three-dimensional image of physical, chemical, and biological properties in the upper few hundred meters of the water column.

Collecting nets come in a wide array of sizes. The smaller ones, perhaps a meter long, may be towed briefly in near-surface waters. The largest, towed for many hours, have great metal frames carrying as many as 20 nets, with multiple opening and closing devices and an environmental sensing array that sends information back to the ship's laboratory. Aboard ship, biologists signal the nets to open in sequence as they observe temperature, depth, salinity, and other physical, chemical, and biological characteristics of interest in the water column.

When the samples come aboard the ship, some may be examined immediately under microscopes in the ship's lab. Animals may be dissected or analyzed for clues to their food sources or exposure to pollutants. Other samples may be preserved for further work in shore-based laboratories.

In other techniques, a silhouette photographic system may take pictures of the animals collected or video cameras may record animal life in its natural environment. Some animals, such as gelatinous zooplankton, are collected by scuba divers, who gently place specimens in glass jars to avoid damaging their fragile bodies. These are maintained in aquaria in the ship's laboratory and studied at sea, as they are very difficult to grow or preserve intact. Many techniques developed by divers to handle fragile plankton have been redesigned for use with submersibles and remotely operated vehicles.

The ocean is largely opaque to visible light, but it is an excellent medium for transmitting sound waves over large distances in order to "see" into the ocean's interior and into seafloor sediments and the rock layers below them. The most common type of sonar device is an Acoustic Doppler profiler (ADCP), used to measure ocean currents and animal biomass.

Records of sound transmission and reception over long distances can be analyzed to reveal information about ocean currents, temperature, and density, which all affect the sound waves' travel time. Acoustic tomography, for example, employs the effect of temperature on the speed of sound in seawater (sound travels faster in warmer water) to estimate temperature structure over long ocean transects.

Sound is also used for geophysical exploration of the seafloor and the layers beneath it. Sidescan sonar offers profiles of rock outcroppings and sediment surfaces at ranges up to several kilometers. Some sidescan sonar can even be used to distinguish rock or sediment types.

Multibeam bathymetric systems consist of two instrument arrays attached to a ship's hull: one for transmission of sound, the other to receive the sound energy reflected from the seafloor. These arrays are used to map seafloor topography in real time.

In other uses of sound, ocean bottom hydrophones record earthquakes, and scientists study sound emitted by marine mammals to learn more about these fascinating creatures.

(Modified from: University-National Oceanographic Laboratory System (UNOLS), The Research Fleet, edited by Vicky Cullen; Moss Landing Marine Laboratories, Moss Landing, CA; 2000.)

Acoustic Doppler Current Profiler (ADCP)

Scientists use the ADCP to measure how fast water is moving across an entire water column.

Source: Ocean Instruments

Bongo Paired Zooplankton Net

One of the simplest biological samplers, zooplankton nets are made in a wide variety of styles and sizes.

Conductivity, Temperature, Depth (CTD) Sensors

A CTD—an acronym for Conductivity, Temperature, and Depth—is the primary tool for determining essential physical properties of sea water.

Source: Ocean Instruments

Rosette Sampler

Probably the most commonly used water sampler is known as a rosette. It is a framework with 12 to 36 sampling bottles clustered around a central cylinder, where a CTD or other sensor package can be attached.