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| Enlarge ImageOBSERVING THE SEA FROM SHORE—A variety of sensors on ocean observatories provide running data logs on changing conditions in the sea. At left, temperature and salinity data from the seafloor to the surface off Martha’s Vineyard over three weeks (day 274 to 296), collected by the Autonomous Vertically Profiling Plankton Observatory (AVPPO), show distinct water layers at the start that become less distinct. (Saltier and warmer waters are red; colder, fresher waters are blue.) At right, a video plankton recorder on the AVPPO captures images of tiny planktonic animals called copepods, while compiling a record of copepod abundance over three weeks (middle). The data shows that during a passing storm (days 277 and 278), the copepod population swam down to keep away from surface waves. (Image generated by Scott Gallager, WHOI) |
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| Enlarge ImageOBSERVATORY OVERBOARD—Scientists and crew aboard R/V Connecticut lower the Autonomous Vertically Profiling Plankton Observatory (AVPPO) to the seafloor. The AVPPO carries instruments that record changing conditions in the coastal ocean, including its temperature, salinity, motion, levels of chemicals and dissolved gases, and the numbers and kinds of organisms living in the area. Data are relayed via cable to the WHOI Martha's Vineyard Coastal Observatory. (Photo by Andrew Girard, WHOI) |
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Monitoring An Ecosystem
Illustration by Jayne Doucette, WHOI | » View Flash
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By Scott Gallager, Associate Scientist
Biology Department
Woods Hole Oceanographic Institution In science, the key to understanding any situation is careful
observations and measurements. The key to observing and measuring,
however, is being there—in the moment—and that
has always proved challenging for oceanographers.
It is difficult
and expensive to go to sea, hard to reach remote oceans and
depths, and impossible to stay long. Like scientists in other
fields, oceanographers use sensors to project their senses
into remote or harsh environments for extended time periods.
But the oceans present some unique obstacles: Instruments
are limited by available power, beaten by waves, corroded
by salt water, and fouled by prolific marine organisms that
accumulate rapidly on their surfaces.
The oceans also surpass
the limits of human observation at both extremes. It takes
a long and large perspective to measure the exchange of greenhouse
gases between Earth’s entire
atmosphere and oceans, over seasons or decades. On the other
hand, chronicling the transfer of gas molecules at the interface
between air and water requires a nanosecond-short, millimeter
view. Once again, sensors can extend observations to detect
phenomena beyond human capabilities. But it takes a wide
spectrum of sensors and platforms to survey whale populations
and their global migrations, while simultaneously collecting
information on the microscopic plants and animals that whales
eat.
Today, rapid advances in micro- and nanotechnology, biotechnology,
computing power, and sensor integration are fueling development
of a new generation of low-power, cost-effective, high-precision
sensors that will withstand extended deployments in harsh
environments and be able to relay data in real time. What’s
more, these sensors will be mounted on an expanding variety
of observatory platforms that provide unprecedented access:
satellite imaging systems, autonomous underwater vehicles
carrying sensors on wide-ranging surveys, and ocean observatories
with cables that continuously transmit power to instruments
and send their data back.
In July 2003, the WHOI Ocean Life
Institute and Deep Ocean Exploration Institute, along with
the National Science Foundation and the Office of Naval Research,
sponsored a workshop called “The
Next Generation of in situ Biological and Chemical Sensors
in the Ocean.” It brought together ocean scientists
and engineers with colleagues from the fields of biomedical
technology, nanotechnology, and electrical engineering to
explore new approaches and possibilities for ocean sensors.
The
workshop presented an exciting vision and road map for sensors
in the not-so-distant future that will allow quantum leaps
in what we can observe and discover in the oceans. Our decade-old
dream is now becoming a reality: to be able to observe phenomena
in the ocean continuously, on all scales and in real time,
and to be able to interact with sensors in the oceans—all
from shore.
Testing the waters
Oceanographic sensors come in all flavors: They measure light,
temperature, sound, mass, or chemical species. All of these
senses will be needed to gain the full picture of all the
interacting physical, biological, and chemical dynamics
going on in the oceans.
Scientists have a fairly good idea
of what we need to measure in the ocean. To study ocean
pollution, for example, ocean chemists require sensors
that detect synthetic compounds, such as those derived
from plastics and petroleum products, automobile exhaust,
storm and sewer runoff, pesticides, fertilizers, surfactants,
and chlorofluorocarbons (Freon). To understand how chemical
cues help organisms find food, or initiate mating or spawning,
we need sensors to identify complex organic molecules and
learn their concentrations and persistence in the environment.
To
determine whether the oceans can absorb excess greenhouse
gases, we need sensors that measure climatically and ecologically
important gases such as carbon dioxide, methane, hydrogen,
hydrogen sulfide, and radon. Other chemical sensors can indicate
how much carbon dioxide is converted by photosynthetic plankton
into organic carbon, and how much of this sinks to the deep
ocean—to mitigate the buildup of greenhouse gases,
or to feed hungry populations of deep-sea organisms. All
these sensors, along with others that measure seawater properties
such as temperature, salinity, and turbulence, will let biological
oceanographers begin to see how ecosystems work and how they
change over microseconds to decades.
Identifying the inhabitants
To learn how organisms respond to changing habitats and interact
with each other, oceanographers first need to determine
when and where species are present, from bacteria to whales.
To identify organisms over the scale of microscopic plankton
(micrometers) to a full ocean (thousands of kilometers),
scientists need systems that integrate optical and acoustic
sensors, which give complementary information.
Sound propagates
far in water, providing information over long distances.
But it travels in long wavelengths that yield only low
spatial resolution. Light, on the other hand, scatters
quickly in water, but travels in short wavelengths, giving
us high-resolution information on small organisms and their “spheres
of influence” —a few body lengths around them.
Some
integrated systems already exist. One is the Bio-Optical
Multifrequency Acoustical and Physical Environmental Recorder,
or BIOMAPER-II, developed at WHOI, which was used recently
to survey krill populations around Antarctica. (See “Voyages
into the Antarctic Winter”) Towed behind
a vessel, BIOMAPER-II carries an acoustic system to detect
small marine organisms such as krill or plankton, a video
plankton recorder to take pictures of them, and other sensors
to measure water properties.
But just knowing the locations,
concentrations, and types of species is still not sufficient.
Scientists also need information on organisms’ feeding,
growth, and reproduction. Integrated systems will soon carry
sensors that sample, analyze, and identify biological molecules—among
them DNA, proteins, enzymes, and lipids—that signal
biochemical activities.
The Environmental Sample Processor,
developed by Chris Scholin at Monterey Bay Aquarium Research
Institute, is a working example. Attached to a mooring on
the seafloor, it extracts nucleic acids from water samples
and detects specific organisms by their DNA. (See “Revealing
the Ocean’s Invisible
Abundance”)
An expanded toolkit
Exciting additions to our sensor arsenal are already being
developed. To begin to measure tiny “needles” of
dissolved gases, trace metals, elements, and nutrients
in the “haystack” of the oceans, several new
approaches show great promise.
Laser-Induced Breakdown Spectroscopy
(LIBS) uses a laser to vaporize tiny amounts of a material
and determine its elemental composition based on the light
spectrum it emits. WHOI scientists are collaborating with
the Army Research Laboratory to develop oceanographic sensors
using LIBS.
Raman spectroscopy uses laser light to cause
tiny samples of water to vaporize and the molecules in the
water to vibrate. That changes the spectrum of light scattered
from the molecules, thus revealing many high molecular weight
compounds in the water, including large organic molecules
such as lipids, proteins, and amino acids. Raman spectroscopy
can also be used to detect dissolved carbon dioxide.
It may
soon be possible to identify microorganisms in seawater by
scanning it with light and measuring the way they scatter
light at different wavelengths. Miniaturized equipment to
make this measurement already exists, and advances in mathematical
analysis techniques (known as spectral deconvolution) may
allow us to detect the species, concentrations, mass, chemical
compositions, and even nucleotides (components of DNA) in
seawater samples.
Scientists are just beginning to measure
chemicals in the extremely harsh conditions of hydrothermal
vents and seeps, where the high temperatures (up to 400°C
or 750°F)
and corrosive nature of hydrothermal fluids make them almost
impossible to sample directly with sensors. A promising technology
for these conditions, called voltammetry, simultaneously
detects a variety of chemical ions including oxygen, hydrogen
sulfide, iron, and manganese.
Voltammetry employs electrodes
to scan seawater with a range of voltages while measuring
the electrical current output occurring in response to
the voltage scan. This output is recorded as a spectrogram:
a graph of multiple peaks in which the location and height
of the peaks are proportional to the types and amounts
of ions in the seawater.
‘Wiring’ the oceans
But all these sensors are of little value unless they can
get out into the ocean and stay there. Autonomous underwater
vehicles (AUVs) are one way to accomplish that mission,
but oceanographers have also been developing exciting new
cabled observatories that provide continuous power to plugged-in
instruments and two-way communications to scientists ashore.
Developed and developing observatories are being located
to study various ecosystems, including productive coastal
areas, harbor entrances, or regions under polar ice.
At
WHOI, the Martha’s Vineyard Coastal Observatory
will soon become the homeport of a new observing platform
called the Autonomous Vertically Profiling Plankton Observatory
(AVPPO), which is designed to observe daily, seasonal, and
annual changes in the coastal Atlantic Ocean ecosystem (see interactive at left).
A winch system drives a platform on a 15-minute trip from
the seafloor to the surface. It is equipped with a range
of instruments—35 sensors in all—that measure
salinity, temperature, oxygen, water motion, water turbulence
and clarity, light, chlorophyll, organic matter, the amount
and types of zooplankton and phytoplankton present, along
with the platform’s own orientation in the water. These
measurements can be correlated with weather and storm events
and will help us monitor the coastal ecosystem’s response
to climate and other changes.
A similar instrument, the Polar
Remote Interactive Marine Observatory (PRIMO), will soon
be installed under the ice in the Southern Ocean and cabled
to shore from Palmer Station on the western peninsula of
Antarctica. It will be the first cabled remote observatory
in the harsh Antarctic environment and our first long-term,
real-time look at this fertile ecosystem that supports a
wealth of marine life.
PRIMO will transmit data via cable
and satellite and give researchers and students a direct
link to critical phenomena and events, including storms,
currents, sea ice formation, and the spring phytoplankton
bloom that fuels an entire food web. It will also provide
clues on how this delicately balanced ecosystem might respond
to the receding ice edge and other changes related to climate.
Like other observatories, PRIMO will be used in concert with
AUVs by including docking facilities for AUVs in the future.
We
have entered a new era with a changing paradigm of how we
sample the ocean. We soon will “wire the oceans” with
instrumental “eyes, noses,
and hands”—which can’t help but dramatically expand our understanding
of what’s going on in the oceans. Stay tuned, the best is yet to come.
Posted: June 1, 2005 [top] |
