IMAGINE trying to study the foraging habits of bumblebees by flying low over
a meadow in a light aircraft while hanging, blindfolded, out of the window with
a butterfly net. You can count the bees you catch and learn which flowers are
blooming by the debris at the bottom of the net. But you鈥檇 be out of luck if you
wanted to learn much more. Are the bumblebees visiting all the flowers, or only
one or two species? Do the bees wander the meadow at random, or do they head for
the richest floral displays? You can鈥檛 answer questions like these from the
passenger seat of a Cessna.
Biologists who study plankton, the tiny plants and animals that drift about
the world鈥檚 oceans, face just this problem all the time. For decades, they have
been stuck with learning about plankton by tossing fine-mesh nets into the sea
and then picking through whatever organisms they catch鈥攖he oceanographic
equivalent of the flying bee survey. But while they can identify the species in
their soggy, mangled hauls, they cannot learn much about the behaviour of
individual organisms or the conditions under which they live.
鈥淓verybody can relate to a fish or a whale, but I have a hard time relating
to a copepod,鈥� says Jules Jaffe, of Scripps Institution of Oceanography in La
Jolla, California. Copepods, distant relatives of crabs and lobsters, are the
most important group of animals in marine plankton.
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But better times are just around the corner for those who really want to
understand what makes plankton tick. And this is no idle aim: phytoplankton, the
planktonic plants, account for 40 per cent of global photosynthesis, and
planktonic animals, zooplankton, form the next several levels of the food chain
in the world鈥檚 vast oceans. Jaffe and a small group of his colleagues and
competitors are developing new technologies that use sonar, video cameras and
lasers to give them a copepod鈥檚 view of the miniature world of plankton. For the
first time, oceanographers can look at a litre of undisturbed ocean water,
record the position of all the organisms within it, and track their movements
for several seconds or minutes. This new-found ability is already helping to
answer questions that have vexed plankton biologists for decades. 鈥淚t鈥檚 given us
a new view of the ocean,鈥� says Peter Franks, one of Jaffe鈥檚 colleagues at
Scripps.
Jaffe uses two instruments to track plankton. The first, which he calls 鈥淔ish
TV鈥�, consists of an array of eight sonar transmitters that emit ultrasonic
impulses in a tight beam only 16 degrees wide, roughly the field of view of a
150-millimetre telephoto lens. The sound waves bounce off all objects in the
beam, and an array of eight receivers picks up the echoes. Through computer
analysis of these sonar signals, Jaffe can determine the position of every
object in the beam to within 2 degrees (about 15 centimetres, at the system鈥檚
normal range of 4 metres) and its distance to within 4 centimetres. Right now,
only animals larger than about 5 millimetres show up on Fish TV鈥檚 sonar screens.
These include krill, fish larvae, and the largest copepods, but not the smaller
and more numerous animals.
鈥淭hey want us to spot a pubic hair on a copepod at 300 metres,鈥� grumbles
Jaffe. Higher-frequency sonar pulses鈥攁t 1.5 megahertz instead of the
present 450 kilohertz鈥攚ould pick out smaller organisms, but the
instruments are harder to build. In any case, the sonar system will never detect
the microscopic phytoplankton that form the base of the food chain.
To see these, Jaffe has designed a second instrument that relies on optics
rather than sound. The device, dubbed 鈥淪on of Fish TV鈥�, scans through the water
with a blue-green laser. Chlorophyll molecules, the coloured pigments of
planktonic algae and cyanobacteria, readily absorb light energy at this
wavelength, fluorescing red as they re-emit some of that energy. A camera鈥�
about a million times as sensitive as a good home video camera鈥攔ecords
these red flashes for later analysis. By measuring the intensity of the
fluorescence, Jaffe can calculate the concentration of chlorophyll, and from
that come up with an estimate of the density of the phytoplankton for each cubic
centimetre of water in a cubic metre of ocean.
Although Fish TV and its offspring sound straightforward in concept, it has
taken Jaffe and his colleagues several years to work out the practical details.
鈥淵ou鈥檙e doing this on a ship, and things are going up and down, and you鈥檙e maybe
not feeling all that well,鈥� he says. 鈥淎 lot of these oceanographic things sound
simple, but it鈥檚 a feat to be able to do that in the ocean.鈥� To complicate
matters still further, the phytoplankton detector works only in the dark because
sunlight overwhelms the faint red fluorescence.
The instruments first went out to sea together last summer for three nights
of observation from a floating platform off the California coast. 鈥淚t looks very
cool when it鈥檚 out there,鈥� says Franks. 鈥淵ou look off the stern of the ship and
you see this green line of the laser shooting upwards. It鈥檚 the most
surreal-looking thing.鈥�
The results were equally exciting. Even on this first brief cruise, the
researchers glimpsed the answer to a long-standing paradox about how
plankton-eating organisms make a living. Oceanographers can easily measure the
average density of phytoplankton in the ocean with a net, and they can also
observe how often plankton-eaters encounter and capture their prey in laboratory
tanks. The problem is that when they do the sums, the researchers end up
concluding that planktivores can鈥檛 catch enough prey to survive at the average
phytoplankton density found in nature. In other words, if the figures were
right, the herbivores in the world鈥檚 oceans should have starved to death long
ago.
For decades, many biologists have assumed that in the real world, predators
avoid this fate by seeking out patches where plankton are denser than average.
After all, the average density of food in London or New York is rather low; we
avoid hunger by seeking out high-density patches called supermarkets and
restaurants. This explanation sounds plausible, and oceanographers know that in
some places there are more phytoplankton than others. This unevenness generally
takes the form of stacked layers, as well as large horizontal patches on a scale
of hundreds of metres or more that form along water flow boundaries. But are
there also variations at scales small enough to be relevant to the lives of the
tiny herbivores?
Thanks to Fish TV and Son, Jaffe and his colleagues have the answer. Even if
you zoom in to the smallest scale, you find that phytoplankton is patchy. 鈥淚t鈥檚
extremely heterogeneous, even over centimetre scales,鈥� says Jaffe. Dense patches
have between three and ten times the amount of plankton found in sparser zones.
鈥淪ome people say that鈥檚 surprising, some say it鈥檚 not surprising. But what鈥檚
certainly true is that nobody鈥檚 ever measured it before.鈥�
Jaffe and Franks think these patches form as a result of turbulence, which
leads to incomplete mixing of the water column. 鈥淚t鈥檚 like if you put cream in
your coffee and don鈥檛 mix it completely, you get little tendrils and blobs,鈥�
says Franks. They are now testing this idea by comparing the patterns of
variability they see in the ocean to those that are predicted by physical
theory.
The researchers have not yet got their two instruments working in synchrony
well enough to know whether planktivores actually seek out the richer pickings
of the patches. But if the planktivores do hang out in patches, then
conventional sampling with plankton nets fails to measure what the planktivores
actually see, which knocks much of oceanography on its ear. 鈥淚f zooplankton
really are staying in these coffee cup-sized patches of high fluorescence, then
we really don鈥檛 know how to measure food availability,鈥� says Franks.
Jaffe and Franks also saw an interesting pattern at a larger scale. Their
fluorescence detector found two layers where chlorophyll was especially abundant
within the uppermost layer of the ocean where sunlight penetrates. One was a
layer known as the 鈥渟ubsurface chlorophyll maximum鈥�, which is a well-known
feature of ocean waters. Phytoplankton gather in this zone, at a depth of about
55 metres, because it is richer in nutrients than waters nearer the surface
while still shallow enough to provide the light they need to photosynthesise.
But Jaffe and Franks also discovered a second peak, much narrower than the first
and about 8 metres deeper. 鈥淭hat was very surprising to us,鈥� says Franks. He
speculates that this second peak may have been a large shoal of phytoplankton
several kilometres long and around 2 to 3 metres thick. 鈥淭he whole purpose of
this pair of instruments is to look for features like this,鈥� he adds. 鈥淭hey
could be very important as snack bars for zooplankton and larval fish.鈥�
To the researchers鈥� further surprise, they found that zooplankton shunned the
deeper, better-stocked snack bar and aggregated around the shallower one. 鈥淵ou
don鈥檛 find the zooplankton in the zone with the most chlorophyll. So why are
they avoiding the McDonald鈥檚 of the ocean?鈥� asks Franks. Perhaps, he suggests,
they found the phytoplankton tastier in the shallower layer. 鈥淭hat tells us that
chlorophyll is not a good measure of food available to zooplankton,鈥� he says.
鈥淚t also tells us that we don鈥檛 really understand what zooplankton are doing out
there and what they choose to eat and why.鈥� Jaffe is not the only oceanographer
looking for a way to see the ocean at the scale of individual plankton. At Woods
Hole Oceanographic Institution in Massachusetts, Cabell Davis and Scott Gallager
have developed a video system that takes a continuous sequence of
high-resolution photos of a 10-centimetre field of view. Unlike Jaffe鈥檚 sonar
gear, Davis and Gallager鈥檚 cameras reveal so much detail that they can often
identify individual species of plankton. By towing their 鈥渧ideo plankton
recorder鈥� behind a ship or mounting it on a minisubmarine, they can make an
accurate census of a transect through the ocean, much as a botanist can census
trees by walking along a path through the forest.
Such a census, though one-dimensional, could give oceanographers their first
good view of horizontal patchiness at the hard-to-measure scale of tens to
hundreds of metres. But the job is harder than it looks, says Franks. To follow
a single horizontal layer of water, the camera must somehow 鈥渞ide the waves鈥�,
moving up and down with the natural motion of the water. This is a tough
technical challenge.
At the University of Rhode Island, biological oceanographer Percy Donaghay
has an even higher-tech method of watching plankton. Donaghay and his colleagues
are developing a technique that uses lasers to record a holographic image of a
cylinder of water 40 centimetres long and 7 centimetres in diameter. Back in the
lab, Donaghay projects the hologram鈥攁n exact 3D replica of the original
slice of ocean at the instant the image was taken鈥攁nd examines its
contents with a microscope. 鈥淵ou can freeze in space a chunk of water and see
everything in it at a scale of 10 micrometres,鈥� he says.
By comparing two images taken in quick succession, Donaghay hopes to measure
the speed at which plankton drift on ocean currents and how fast the swimming
members of the plankton can travel. Once oceanographers understand the balance
between these two kinds of movement, they may be able to predict, or at least
understand, what may happen if global climate change disrupts ocean flow
patterns. 鈥淓ven minor changes in the flow regime could have profound
implications for marine ecosystems, even at these very small scales,鈥� says
Thomas Powell of the University of California, Berkeley, who heads the National
Science Foundation鈥檚 global ecology initiative.
Oceanographers are watching eagerly to see what other surprises these three
new technologies will turn up in the next few years. 鈥淲e鈥檙e going to learn a
tremendous amount about how these organisms feed, how they reproduce鈥攁ll
those things that we鈥檝e never been able to do in a noninvasive fashion,鈥� says
Powell. One subject he is particularly interested in is the defecation behaviour
of zooplankton. Although it sounds an odd obsession, faeces are a key link in
the global carbon cycle.
鈥淔aecal pellets are a fast way to package carbon so it falls quickly into the
depths and is taken out of commission,鈥� he explains. This carbon originates as
carbon dioxide captured from the air by photosynthesising plankton. So, in
effect, the sinking faeces consign CO2 to long-term storage on the
ocean floor, possibly mitigating the greenhouse effect. Paradoxically, then, the
oceanographers who zoom in on microscopic plankton may end up learning about the
global-scale problem of climate change.