Article 33

Taking the Planktonic PulseBY KAREN YOUNG KREEGER AND JOHN GAMBLE

A pyramid is only as good as its base. Ask anyonewho’s ever tumbled from the top of a human one. Plank-ton, tiny drifting plants and animals, make up the foun-dation of the ocean’s food pyramid and are fundamentalto its health. Oceanographers have known this for de-cades. But capturing, preserving, and studying planktonis about as difficult and tedious as finding the needle in ahaystack. Enter the Continuous Plankton Recorder, orCPR for short.

A new research technique? Not quite. The CPR Surveyjust celebrated its 60th birthday in 1991. Operated since1991 by the Sir Alister Hardy Foundation for Ocean Sci-ence, the CPR Survey is a unique partnership between sci-ence and commerce. The concept is simple: So-calledships-of-opportunity such as ferries and merchant vesselstow the self-contained CPRs along established routeseach month, mostly in the North Atlantic Ocean and theNorth Sea.

For 60 years, using “a bread box with a nose cone” has enriched the scientific record about the base of

the oceanic food web

The CPR itself has a simple design—like a bread boxwith a nose cone. As it glides through the water about tenmeters below a ship, plankton enter through a small holein the nose cone. Once inside, plankton are trapped be-tween two bands of silk mesh to form a long “planktonand silk sandwich.” The sandwich is automaticallywound onto a spool, which is immersed in formalin to

preserve the newly caught creatures. This pickled spool isthen taken to the lab where the plankton are identified,counted, and logged into a computer database.

The design of the CPR has not changed much over theyears since it was invented by Hardy in 1931. The idea forit grew from a simple Hardy device of the late 1920s thatassisted herring fishermen to more easily find their elu-sive plankton-eating prey. In the early 1930s this device,

ILLUSTRATIONS: SIR ALISTER HARDYFOUNDATION FOR OCEAN SCIENCE

Animal members of the plankton family, like the decapods illus-trated above and next page, show many strategies for survival in adangerous world. Some eat plants, some eat other phytoplankton.

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or the Hardy Patent Plankton Indicator as it was calledthen, was patented, marketed, and sold to fishermen forabout $15. Early advertisements claimed it could cutdown “the waste of time and labor involved in unsuccess-ful hauls” and increase “catches on an average by 25 per-cent per night.”

The indicator recorded the abundance of plankton in apredetermined volume of water. However, Hardy real-ized that if he could capture the plankton on a movingband of filter mesh, he could also record changes in abun-dance of plankton along a track of a tow. Thus in the late1920s, he developed the first CPR to use in the South At-lantic Ocean near Antarctica from the RRS Discovery, aBritish Royal Research Ship first used by the explorerRobert Falcon Scott on his expeditions to Antarctica.

It also occurred to Hardy that if he deployed several ofthese CPRs simultaneously, he could gradually build upa map of plankton over wide areas of the ocean. With thismap, he could then trace seasonal and annual changes inabundance. Such information would provide a uniquebaseline to compare fluctuations in fish abundance andsuspected environmental changes.

Hardy was of course correct in his prediction. “As theCPR Survey grew over the decades, in terms of the routesit covered and the decades it spanned, the value of thedata has totally transcended the original concept,” ex-plains Mike Colebrook, former director of the survey, inits 60th anniversary report. Today, the survey is tacklingphenomena as large as global climate change.

An impressive list of statistics accompanies the sur-vey’s long history. As of 1992, CPRs have been towednearly 4,000,000 miles—that’s over 150 times around theearth—by ships from 10 countries to produce 165,000samples. The survey has even started its own Hall ofFame, of sorts. John Roskell, a researcher with the survey,holds the world record for the greatest number of sam-ples analyzed—11,797 between 1962 and his retirement inSeptember 1992. Last year CPRs were towed 226 separatetimes over 20 routes in the North Sea, North Atlantic,English Channel, and along the North American easternseaboard.

But, don’t let all these numbers cloud your under-standing of the true nature of the survey’s work. CPRshave been quietly plugging away on most of these routesvirtually unchanged since the 1930s, except for a breakduring World War II. But, it is this “routine,” long-termaspect that is the essence of the survey’s significance.Datasets that span decades are essential for understand-ing the slow process of change on earth.

Because of the survey’s duration and the wide area itencompasses, the dataset in its entirety seems rife withcomplex interactions—no simple x versus y relationshipshere. But, like a well-read novel, unforeseen subplotsemerge over the long haul to give many stories within astory.

A recurring theme links all of these stories—the CPRdataset is like a fugue, with overlapping motifs in timeand space. Only when you start breaking it down into di-gestible chunks—species, sample areas, year, season—can you see comprehensible relationships.

Many stories have emerged over the years. One of thefirst takes place in the North Sea and relates dramaticchanges in the abundance of commercially importantfish, such as cod and haddock. Using data from the CPRSurvey, David Cushing, a fisheries biologist from the De-partment of Fisheries Research, Lowestoft, United King-dom, discovered an interesting link between theabundance of fish and copepods (crustacean planktonwhich are a favorite food of juvenile fishes).

From 1962 to 1978, copepod production occurred inMay instead of April, compared with the previous 15years. This delay coincided with the most ravenous timein a young fish’s first year, a time when it grows at an in-credible rate of 10 percent per day. Because the CPR couldprovide consistent data in this area for almost three de-cades, researchers were able to compare copepod abun-dance during different decades.

More food at the right time of year meant that moreyoung fish survived to grow and eventually be caught infishermen’s nets. This might also explain why fish stickswere a favorite lunch in every school cafeteria in the1960s.

Later stories add more layers of complexity. On alarger scale, the survey contributes to understanding

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Article 33. Taking the Planktonic Pulse

where plankton ecology fits into the link between theocean and the atmosphere. These relationships may evencontribute to our understanding of climate change. Byjoining its long-term dataset with that of other programs,survey researchers and their colleagues have been able tolink some of these processes with patterns of changingabundance in marine life.

One of the most remarkable relationships in long-termtrends spans four tiers in a North Sea marine food web.Working with scientists from the United Kingdom GameConservancy and the University of Durham, MichaelColebrook discovered a five-step connection betweendaily weather patterns over the British Isles and thebreeding success of kittiwakes, an oceanic seabird. Overthe 33 years studied, three separate measures of kittiwakebreeding have been correlated with deviations from thelong-term average for the usual weather patterns in theNorth Sea, and phytoplankton, zooplankton, and herringabundance.

What is the link between plankton and climate?

All of the indicators started high in the late 1950s,dipped similarly, reaching a trough around 1980, thenshowed a striking upswing until the end of the dataset in

1987. As the researchers put it in Nature, “It is tempting toascribe the similarity in patterns in annual variation be-tween plankton, fish, and bird to straight causal relation-ships up the food chain, the base of the chain being drivenby weather.… “

There’s the catch. Many other factors such as overfish-ing of herring, predator-prey relationships, and nutrientavailability all interact to affect trends in abundance. De-spite all of this, though, the trends hold up.

A more recent discovery—by Arnold Taylor, an ocean-ographer from the Plymouth Marine Laboratory in Ply-mouth, United Kingdom—has shown another strikingcorrelation between changes in plankton abundance andoceanic processes spanning the breadth of the AtlanticOcean. From 1966 to 1990, in surveys from waters aroundthe British Isles, total numbers of copepods have fluctu-ated in the same way as the north-south position of theGulf Stream in the North Atlantic. The Gulf Stream is ameandering current of warmer water that starts in theCaribbean, flows north and veers east near the GrandBanks off Newfoundland.

When the Gulf Stream dips further south, planktonnumbers fall. But why? Because currents take months tocross the North Atlantic, Taylor had to look for anotherexplanation besides a direct link between the Gulf Streamand plankton thousands of miles away.

N. T. NICOLI

Plankton takes kaleidoscopic shapes with chains of diatoms and irregular single-celled dinoflagellates, above.

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He suggested changes in atmospheric circulation maybe the answer. During extremely cold winters over thesoutheast United States, the winter storm track over theNorth Atlantic is pulled southwest of its normal positionover Iceland and Greenland. This, in turn, causes the GulfStream to shift south. At the same time the abnormal win-ter weather patterns may affect plankton abundance.

One of the most far-reaching sets of relationships in-volves several species of phytoplankton and zooplank-ton. Though at first the explanation may seem quitecomplex, it is really an elegant and relatively simplestory. Researchers within the CPR Survey team have dis-covered that since 1948, the abundance of several speciesof plankton in the North Sea has steadily declined, but inthe mid-1970s to early 1980s, the trend reversed andplankton numbers are now on the increase. At the sametime, coastal upwelling (an indicator of localized produc-tivity) on both sides of the Atlantic, sea surface tempera-ture in the North Atlantic, and global air temperaturehave increased. Are these simply re-occurring cycles orsymptoms of a chronic change in the earth’s climate?

The link between climate and plankton abundance hasyet to be explained in detail. One hypothesis, though, isthat plankton are responding to global warming by an in-crease in overall abundance. However, as the survey re-

searchers warn, “It is too soon to be dogmatic aboutthis.… this is probably due to complex climatic factorsand not simply the increase in temperature as such.”

Research with the CPR has much more to tell. Howplankton abundance changes with ocean conditions andclimate is only one chapter. The survey is continuallygaining more information about plankton, so researchersare not only learning about their distribution patterns butalso about important aspects of their behavior, such asdaily and seasonal migration patterns.

The CPR survey is ready to expand to other areas of theworld’s oceans and use new techniques. A new genera-tion of CPRs will be developed that will also carry elec-tronic sensors to gather information about the physicalenvironment, such as temperature and salinity. This newinformation will relate both to the plankton collected bythe CPR and to remote measurements made by orbitingsatellites. As the survey researchers highlight, “this willcreate a dual approach to our increasingly vigilant watchon the health of the world’s oceans.”

Karen Young Kreeger, a Philadelphia-based science writer, works at thescience trade publication The Scientist. Formerly a marine biologist,Kreeger worked for one year at the Plymouth Marine Laboratory in En-gland. Plankton biologist John Gamble is project director of the Con-tinuous Plankton Recorder Survey.

From Sea Frontiers, Vol. 41, No. 2, Summer 1995. © 1995 by the International Oceanographic Foundation. Reprinted by permission.

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