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News - May 2002

NSF PR 02-36

Media contact:

 Peter West

 (703) 292-8070

Sidebar: Listening for Echoes from Krill

Dezhang Chu
Dezhang Chu
Credit: Kristin Cobb

the Acoustical Properties Of zooPlankton device
The Acoustical Properties Of zooPlankton (APOP) device.
Credit: Kristin Cobb

Listening for Echoes from Krill: Though you'll find Dezhang Chu in the biology lab aboard the Nathaniel B. Palmer, he's really a physicist. "I love physics," Chu smiles. "In physics, you can explain things so perfectly; everything fits; the world is in harmony. Not like biology."

Chu, an acoustician from Woods Hole Oceanographic Institution, would like to bring some of the certainty of physics to biology. Chu's goal aboard the Palmer is to measure two material properties of krill -- density and sound speed -- more directly and accurately than anyone before him.

These values are notoriously tricky to measure in small, fluid-filled animals like krill. However, they are critical for interpreting acoustical data, such as the data from BIOMAPER-II.

The echo, or scattered sound, that a marine animal returns to the Bio-Optical Multi-frequency Acoustical and Physical Environmental Recorder (BIOMAPER-II) relates directly to the animal's density and sound speed (the speed at which sound travels through the animal) relative to water. These are such important values to acousticians, they have special shorthand names: "g" and "h," respectively.

Scientists use g and h in mathematical models that help translate raw, "fuzzy" acoustical data into estimates of krill abundance. But scientists have been using crude, inconsistent values of g and h for years.

"If you play with the model for a long time, you see that the prediction of biomass is very sensitive to g and h," Chu says. Chu found that biomass estimates changed by as much as 100-fold depending on which values of g and h he tried. In other words, one group could be estimating 1000 krill when another group was estimating 100,000.

Chu, along with Woods Hole scientist Peter Wiebe, realized it was imperative to improve estimates of g and h.

They also realized that these measurements had to be made in the open ocean, not in the lab or in shallow water, as was done in the past. Lab conditions may dry or kill krill, changing their material properties. Material properties might also vary at different depths of the ocean; krill might be more compact at higher pressures or more fatty at colder temperatures, for example.

As part of this year's SO GLOBEC cruise, Chu is making the first krill g and h measurements ever in open ocean. The first time doing any new science experiment is thorny--the first time in the Antarctic is just plain hard.

"The big challenge was that I got notice of funding very late before the cruise," Chu says. "I had to pack everything up very quickly. It was very stressful. Some parts of hardware and some of the software had to be developed here on ship."

But Raytheon marine technicians and scientist Karen Reiner of Oregon State University banded together to help Chu trouble-shoot, and soon Chu was in business.

Weighing Krill: No Easy Task

Measuring density seems straightforward: measure the animal's weight, measure its volume, and divide. But it's not that simple with tiny sea animals. Krill are mostly composed of water. If you take them out of water to measure them, they lose volume and weight due to drying. But if you keep them moist, they carry extra water, adding weight.

The trick is to measure density without ever measuring the volume and weight of the animal directly, Chu says.

To demonstrate the method, he holds up a small jar of amphipods--tiny, fly-like creatures of the sea, with translucent skin and dark beady eyes. Suspended in their native seawater, they form a thick, raw stew of paralyzed zooplankton. "They're pretty ugly, aren't they?" Chu says with a grin.

Chu weighs the jar of amphipods on a sensitive balance beam scale that has been cleverly rigged to function when rocking. Then Reiner carefully adds distilled water to the jar -- to just the right volume. Chu re-weighs the new mixture. Together they filter out the little water bugs and measure the density of the remaining water mixture.

Armed with the information he needs, Chu does a little algebra, and out pops the density of amphipods. He'll do the same with fresh samples of krill later.

Chu measures h, or sound speed contrast, with APOP (Acoustical Properties Of zooPlankton), a device he designed. The APOP system is housed in a stainless steel pail that can be lowered into the ocean so that h can be measured at many depths. Currently, APOP contains a pair of cylindrical tubes with transducers at the ends and a live animal chamber in between. Chu sends sound waves from the transmitter to the receiver with and without live krill in the chamber.

Thin-shelled and diaphanous, the krill blend in with the water except for their tiny dark eyes and the reddish outlines of their shrimp-like forms. As the sound waves travel through the water and hit the krill, they are slightly accelerated. When the krill are absent, the sound waves take longer to arrive at the receiver.

The difference in arrival time is directly related to the sound speed difference between krill and water, and can be used to calculate h.

"So far, so good," Chu says after the fifth APOP cast. To this point, Chu has observed that h depends on the life stages of krill, but not on water depth. "That's very encouraging. At least it's consistent with what people thought."

And, for Chu, it's all about making biology more consistent.




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