News - May 2002
NSF PR 02-36
Sidebar: Listening for Echoes from Krill
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
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
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
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
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.