Reid, PC

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118 The Marine Observer July 2001Climate change and the ContinuousPlankton Recorder surveyPhilip C. Reid(Sir Alister Hardy Foundation for Ocean Science, Plymouth)The Continuous Plankton Recorder (CPR) survey, which is operated by the Sir AlisterHardy Foundation for Ocean Science (SAHFOS) will be 70 years old in 2001 duringwhich time close to 4 million nautical miles of the oceans have been sampled. Thisremarkable achievement has only been made possible through a partnership with morethan 200 vessels and the voluntary assistance of many shipping companies, agents,stevedores, Masters and crew. All onboard operations including deployment andrecovery are carried out in totality by the ship’s crew. The survey and its products areunique as they comprise the only long-term operational monitoring programme forplankton, that extends in most ocean basins.In contrast to the meteorological sciences there are few long-term datasets ofbiological information in the world, and on land those that do exist are difficult tointerpret because of the development of agriculture and the effects of pollution. Theoceans and shelf seas are still relatively pristine and changes in the abundance andcomposition of the plankton are more likely to be a response to ‘natural’ variability. Asfree floating organisms they are highly dependent on their immediate environmentand can integrate effects of meteorological and hydrographic variability. Plankton hasa second more direct role with respect to climate change through the export of CO2, viawhat is known as the ‘biological pump’, to the deep ocean and in the production andexport to the atmosphere of chemicals such as Dimethylsulphide (DMS) that mayreinforce global warming. Clear evidence of pronounced trends and decadal change isseen in the results of the CPR survey. These changes may be acting as a ‘barometer’ orearly warning of the effects of climate change on the oceans. PlanktonThe free floating plant life of the sea (phytoplankton), at the base of the food web,provides food for the animal plankton (zooplankton) and in turn the fish and theirpredators. Many of these tiny organisms exhibit spectacular patterns of shape andcolour (Figure 1). For example, the dominant group in the phytoplankton, diatoms, areenclosed in two glass cups like an old fashioned pill box, often with attached spines,and under the microscope are often iridescent. Copepods are the dominantzooplankton group in the North Atlantic. They are small (typically 1 mm long)crustaceans with long antennae at the front of a muscular body that may be bright reddue to oily storage products. Light, nutrients and the degree towhich the water is mixed are the mainagents governing the growth ofphytoplankton. Many of these factorsin turn are dependent on, for example,wind strength/direction/frequency,cloudiness and precipitation whichexert a strong influence on the upper100 m of the water column. Even inclear tropical waters light onlypenetrates down to 100 m so mostFigure 1: PhytoplanktonNorman NichollThe Marine Observer July 2001 119phytoplankton are found in the upper 40 m. The majority of zooplankton are also foundin the upper layer of the water column although some show patterns of daily verticalmigration over hundreds of metres, apparently as a predator avoidance mechanism.The Continuous Plankton Recorder surveyThe CPR towed body (Figure 2) is deployed at a depth of approximately 10 m frommerchant ships, weather ships (to 1996) and more recently some naval hydrographicsurvey vessels on their normal routes of passage. Plankton is filtered on a band of silkthat moves past an aperture at a rate that is proportional to the speed of the ship. A rollof four metres of silk is equivalent to a tow of 400 nautical miles. To enable sequentialtows to be made on long routes a cassette system is used. The silks are cut into10 nautical mile samples on which phytoplankton and zooplankton are counted andidentified under a microscope into ~400 taxonomic categories.Figure 2: The Continuous Plankton Recorder and its componentsStarting in the North Sea in 1931 the survey extended out into the Atlantic in 1939.Currently, the core operational area reaches from the east coast of the USA to the northwest European shelf between approximately 37° N and 64° N. While most CPR sampleshave been obtained in this core area the database, which contains more than 2 milliondata points for the period from 1946 to present, includes samples from all the oceans ofthe northern hemisphere. In addition a small CPR survey is operated by the USNational Marine Fisheries Service in the Gulf of Maine and to Bermuda. In 2000 a newsynoptic survey, operated by SAHFOS, was initiated in the north-east Pacific with fivetows between Alaska and California using the s.s. Polar Alaska and s.s Polar Independenceand one between Canada and Kamchatka using the m.v. Skaubryn.A sister survey which operates in the Southern Ocean uses the research vessels of theAustralian Antarctic Division to tow CPRs between Tasmania and the Australian basesin Antarctica. Three parallel north/south CPR transects across the Southern Oceanalong approximately 25° E, 110° E and 158° E were completed by this survey incollaboration with the Japanese Polar Institute within a two-week period of the winterof 1999. The CPRs towed for this unique initiative used the Australian research shipAurora Australis and the Japanese vessels Kaiyo Maru and Shirase (an icebreaker). Theplankton of the Southern Ocean has been little studied and the samples generated by thisprogramme are giving a new insight into the dynamics of the major plankton group inthis region of the world, crustacean euphausiids known as krill.PropellerStorage tankGear boxTowing wireEntrance apertureCovering silkFiltering silkTunnelExit aperture120 The Marine Observer July 2001Logistics and operations of the CPR surveyThe one-metre long, 87-kg CPR is a well-proven technology that is rugged, reliableand capable of withstanding at times harsh handling experienced during transport anddeployment. Machines have even survived operation in up to force 12 storm conditionsand are rarely lost at sea, which is a compliment to the seamen that deploy them andthe skill of the SAHFOS workshop staff. Typically 90 per cent of tows return useableplankton samples. The length of the towing cable is adjusted according to the speed ofthe ship and may be up to 80 m astern. For most vessels a purpose-made davit or towpoint is installed from which to deploy CPRs.Maintaining a routine synoptic survey using ships of opportunity is a major exercise.Typically 22 routes are towed in the core region each month and more than 80,000nautical miles are sampled each year. Dispatch of CPRs in their distinctive yellow boxesby road carrier to a UK port and return to the Laboratory in Plymouth may takeanything from two weeks to many months. At present, trading patterns of ships arecontinually changing and it is proving increasingly difficult to maintain tows onstandard routes for long periods.By tradition the survey has employed an ex-merchant navy Captain as Marine SurveyManager, and much of the success of the current survey is down to the enthusiasticliaison of the present holder Captain Peter Pritchard. Peter regularly visits tow vesselsand supervises the design and installation of davits as well as assisting with trials of newinstrumentation. Our naval links were further strengthened in 2000 by theappointment of Commander Alan Johnson (RN Retd) as Assistant Director withresponsibility for administration.The North Atlantic Oscillation Strong associations have been demonstrated between the plankton sampled by the CPRand a major mode of atmospheric variability in the northern hemisphere known as theNorth Atlantic Oscillation (NAO). The NAO is an alternation in the atmospheric pressuredifference between the low pressure zone that is typically centred over Iceland and thesubtropical high pressure zone centred over the Azores1. The oscillation has a pronouncedeffect on temperature, wind and precipitation especially in northern Europe and has beenlinked to changes in current strength and direction as well as the formation of deep waterin the Greenland Sea and intermediate water in the Labrador Sea. Four layers of increasingdensity can be distinguished in the northern Atlantic from top to bottom. An upper layerequivalent to the depth of winter mixing by the wind (typically 0–600 m), anintermediate layer (normally down to 1,500 m), a deep layer down to 4,000 m) and, in thewest below this layer, northerly penetration of Antarctic bottom water.Formation of dense salty water at the surface in Nordic seas that sinks rapidly to topup the deep water layer is what pulls the warmer surface waters of the Atlantic towardsthe Arctic Ocean. This exchange is part of what is known as the ‘Global Conveyor Belt’and ensures that Europe has a much warmer climate than its equivalent latitude(Labrador) on the other side of the Atlantic. Some copepod plankton sampled by theCPR have undergone large vertical migrations as they are normally associated withintermediate or deep water or ‘hibernate’ in these waters during winter months. Theirpatterns of occurrence reflect some of the dramatic changes that have been observed byhydrographers in the formation and distribution of intermediate and deep water overthe last few decades. Since 1988 we have been in a strong positive phase of the NAO1 To characterise variability in the NAO, an index is calculated from the difference in sea-level pressurebetween the Iceland Low and Azores High (determined at standard met. stations). This index is ameasure of the strength of the westerlies especially in winter months. When the index is high thewesterlies are strong and vice versa.The Marine Observer July 2001 121index; in consequence, deep water formation has ceased in the Greenland Sea and, inthe early 1990s, the Labrador Sea became an important site of intermediate waterformation (Figure 3). Penetration of water from the North Atlantic current into Nordicseas appears to have reduced compared to a low NAO situation as found during the1960s to 1970s, at the same time as an apparent increased northerly penetration ofwarmer water in the shelf edge current at the eastern margin of Europe. These patternsof physical change are backed up by biological evidence from CPR plankton. Figure 3: Phases of the North Atlantic Oscillation. (LSIW — Labrador Sea Intermediate Water formation.NADW — North Atlantic Deep Water formation)Regime shift in the North SeaWhat has been termed a regime shift occurred in the North Sea around 1987/88,approximately when the NAO changed to a strong positive phase. Regime shifts arestep-wise alterations in the composition and productivity ofplankton and fish, at a regional scale, that reflect majorhydrographic change. The changes seen in the North Sea areclearly evident in the Phytoplankton Colour index of theCPR survey (Figure 4). When CPR silks are returned to thelaboratory the colour on the surface is visually characterised(based on a standard colour card) into three levels ofgreenness and zero colour. While only a coarse visual indexof chlorophyll, the many samples taken in the central NorthSea each month of the year over the last 50 years (>20,000)clearly define progressive changes in the abundance andtiming of the phytoplankton growing season. Large increasesalso occurred in the catches of some fish species and in thebiomass of organisms living on the bottom. Associatedchanges in the physical characteristics of the North Seaincluded an increased influx of oceanic water and higher seatemperature.Coinciding with the beginning of the regime shift a majorincursion of oceanic water into the North Sea from theeastern boundary shelf edge current to the west of the BritishIsles appears to have occurred. This water carried with itunusual southerly plankton more characteristic of the Bay ofBiscay and further south. These lusitanean species included agelatinous tube-like plankton species that sucks and filtersFigure 4: Phytoplankton Colour Index for the North Sea, 1949–1999122 The Marine Observer July 2001very large volumes of water to derive its food known as Doliolum nationalis. This specieshad not been seen in the North Sea since 1911 and then only occurred rarely. In contrast,in the autumn of 1989 and in 1998 when a second major incursion of oceanic waterappears to have occurred, this species was extremely abundant. Evidence that theeastern boundary current was stronger at these times has been derived fromhydrographic sections made across the Rockall Trough. Satellite measurements of seasurface temperature along the European shelf edge suggest that pulses of warm waterhave extended north along the shelf over hundreds of miles. A number of fish species,some subtropical, appear to have extended their northerly range as part of these eventspossibly as a response to the higher temperatures found in European waters in recent years.Plankton changes in the north-west AtlanticThe CPR routes operated by SAHFOS in the western Atlantic form an integral part ofthe monitoring programme of the Canadian Department of Fisheries and Oceans. Thecod fishery of the North-west Atlantic was of major economic and social importance toEastern Canada until it started to decline in the mid to late 1980s reaching a crisis in1990/91 when the fishery was closed. Unfortunately, the CPR survey in this region wasnot operating during the crucial period of change in the cod fishery because of a lack offunding. It is however possible to make a comparison between sampling that wasundertaken between approximately 1960 and 1980 and the new survey, which startedagain in 1991. Pronounced differences are evident between the two periods. Since theearly 1990s in the western Labrador Sea and over the Grand Banks there has been a sharpdecline in juvenile copepods and euphausiids at the same time as a marked rise in thePhytoplankton Colour index and changes in the composition of the phytoplankton.Small copepods form possibly the main food source for larval cod and the reduction intheir abundance is likely to be a major factor in the decline of cod recruitment.  The Biological Pump and DMSPhytoplankton play an important role in determining the atmospheric concentrationof CO2 by transporting CO2 from the surface ocean to the deep ocean. They uptake CO2by photosynthesis in the surface waters and export a fraction of this carbon to the deepocean in what is known as the ‘biological pump’. The operation of this pump means thatthe atmospheric CO2 concentration is lower than it would be if the ocean was devoid ofphytoplankton. Any substantial alteration, on a global scale, of the composition orfunctioning of the present-day phytoplankton distribution would have significantimplications for atmospheric CO2, in addition to anthropogenic carbon release. Thiscould act as a feedback mechanism altering the pace of climate change.A second potential ocean biological feedback to climate is through the natural sulphurcompound Dimethylsulphide (DMS). DMS is produced by many phytoplankton, andthe amount of DMS produced by different groups of phytoplankton can vary greatly. Afraction of DMS passes to the atmosphere where it can form aerosols that modulatecloud properties such as cloud lifetime and albedo. Simulations by the Met OfficeHadley Centre using their Global Climate Change model have indicated that climate issensitive to ocean DMS emissions (personal communication: Steve Spall). As little isknown about how ocean DMS emissions may alter in the future, this is a furtherpotential feedback mechanism that may influence climate change.ConclusionsObservations from the CPR survey and analyses of other long data sets have shownthat pronounced changes have taken place in the shelf sea and oceanic ecosystems ofthe world. The extent to which these are natural events as part of long-term cycles ormay be forced by global warming is still far from clear. What is evident is that thesechanges have major implications for the productivity of regional seas and theirharvestable living resources. Part of the reason for the collapse of the cod fishery inCanadian waters, and for the similar collapse of the herring fishery in the North Sea inthe late 1970s, is now believed to be linked to associated changes in the plankton,although over-fishing is clearly also implicated. The observed changes in the planktonappear to be closely associated with alterations in the circulation and strength of majorcurrents, which have implications for future weather patterns of adjacent land regions.If climate change is forcing these events, feedback mechanisms may reinforce or reducethe biological pump with possible major consequences for the CO2 cycle. Developingan improved understanding of plankton abundance and distribution is thus of keyimportance to understanding and forecasting future climate change scenarios.The CPR survey was incorporated in 1999 into the Initial Observing System of theGlobal Ocean Observing System (GOOS). The Inter-governmental OceanographicCommission and the World Meteorological Organisation jointly co-ordinate GOOS. Thesurvey, as a long running existing survey, fits well into the developing plans of GOOS. TheInitial Observing System includes measurements from ships, buoys, coastal stations andsatellites although up to the present the emphasis has been on physical measurements. Incollaboration with the merchant marine SAHFOS has pointed the way to a system thatcould be extended to form a series of regional scale programmes to monitor the biology aswell as the physics and chemistry of the oceans under the flag of GOOS.Philip (Chris) Reid is at the Sir Alister Hardy Foundation for Ocean Science, 1, WalkerTerrace, The Hoe, Plymouth, PL1 3BN. Tel: 01752 221112. E-mail: pcre@pml.ac.uk. The CPR survey is funded by a consortium that has recently comprised agencies fromCanada, Denmark, Faeroes, France, Iceland, Ireland, Netherlands, UK, USA, theEuropean Union, UNIDO and IOC. Referenceshttp://www.npm.ac.uk/sahfos/Beaugrand, G., Reid, P.C., Ibanez, F. and Planque, B., 2000: Biodiversity of North Atlanticand North Sea calanoid copepods. In Marine Ecology Progress Series, 204, 299–303Hardy, A.C., 1956: The Open sea: The world of plankton. J. Fisher, J. Gilmour, Sir J.Huxley and L. Dudley Stamp, (Editors). Collins, London.Mann, K.H. and Lazier, J.R.N., 1991: Dynamics of Marine Ecosystems: Biological-PhysicalInteractions in the Oceans. Blackwell Scientific Publications. Cambridge.Reid, P.C., de Borges, M.F. and Svendsen, E., 2001: A regime shift in the North Sea circa1988 linked to changes in the North Sea horse mackerel fishery. In Fisheries Research,50, 163–171Reid, P.C., Planque, B. and Edwards, M., 1998: Is observed variability in the observedlong-term results of the Continuous Plankton Recorder survey a response to climatechange? In Fisheries Oceanography, 7, 282–288.Sameoto, D., 2001: Decadal changes in the phytoplankton colour index and selectedcalanoid copepods in Continuous Plankton Recorder data from the Scotian Shelf. InCanadian Journal of Fisheries and Aquatic Sciences, 58, 749–761.IPCC, 2001: Working Group I Climate Change 2001: The Scientific Basis.The Marine Observer July 2001 123

Climate change and the Continuous Plankton Recorder Survey

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