cpr sampling: the technical background, materials and methods, consistency … · 2003-11-12 · is...

Progress in Oceanography 58 (2003) 193–215www.elsevier.com/locate/pocean

CPR sampling: the technical background, materials andmethods, consistency and comparability

S.D. Battena,∗, R. Clarkb, J. Flinkmanc, G. Haysd, E. Johnd, A.W.G. Johna,T. Jonasa, J.A. Lindleya, D.P. Stevensa, A. Walnea

a Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel Hill, Plymouth, PL1 2BN, UKb Centre for Environmental, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, UK

c Finnish Institute of Marine Research, P.O. Box 33, FIN-00931, Helsinki, Finlandd Department of Biological Sciences, University of Wales, Swansea, UK

Abstract

The Continuous Plankton Recorder has been deployed for 70 years. Although modifications to the machine havebeen relatively minor, there has been a steady increase in the speed at which it is towed, creating a need to quantifywhat effects this may have had on its sampling characteristics. Additionally, because the CPR database is one of thelongest and most geographically extensive biological time series in the world, and scientists are currently focusing ongaining understanding about climate-induced ecological changes, there is increasing pressure to quantify the samplingperformance and relate the CPR data to data collected by other plankton samplers. Many of these issues of consistencyand comparability have been investigated throughout the decades of the CPR survey. The primary aim of this studyis to draw together the results of those investigations, updating or integrating them where applicable. A secondary aimis to use the CPR database to address other previously unexamined issues. We show that the increase in speed of towhas had no effect on the depth of sampling and the mechanical efficiency of the internal mechanism, but that at thehighest tow speeds there is some evidence that flow may be reduced. Depth of tow may also be dependent on the shipoperating a particular route. We describe the processing procedures used to ensure consistency of analysis and detailthe changes in taxonomic resolution that have occurred through the course of the survey. Some consistency issuesremain unresolved, such as the effects of adding heavy instrumentation to the attitude of the CPR in the water andpossible effects on sampling performance. The reduction of flow caused by clogging of the filtering mesh has nowbeen quantified through the addition of flowmeters and each CPR sample can now be calibrated for measured, orderived, filtered volume. Although estimates of abundances for large areas have been shown to be unaffected byrecalibration, absolute quantification of plankton abundance is necessary to enable comparisons with other samplingdevices. Several studies have now been undertaken that compare plankton abundances obtained with the CPR withthose obtained using vertical nets at specific locations on the European continental shelf. Although catches by the CPRare almost always lower, seasonal cycles are replicated in each comparison, and interannual variability generally agreesbetween time series. The relative catch rates for an individual species by each device appear to be consistent, probablybecause of the organisms’ behaviour and attributes of the sampling device. We are now able to develop calibrationfactors to convert CPR catches to absolute abundances that can be integrated with other data sets where appropriate,which should increase the applicability and utility of CPR data.

∗ Corresponding author. Tel. and fax:+1-250-756-7747.E-mail address: [email protected] (S.D. Batten).

0079-6611/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.pocean.2003.08.004

194 S.D. Batten et al. / Progress in Oceanography 58 (2003) 193–215

2003 Elsevier Ltd. All rights reserved.

Keywords: Continuous Plankton Recorder; Zooplankton; European shelf; Consistency

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

2. CPR design and operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1952.1. Collection of samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1962.2. Sample processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1962.3. The gauze advance system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1982.4. Changes to the design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

2.4.1. Internal mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1992.4.2. External body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

2.5. Effects of the design changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2002.6. The sampling mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

3. Consistency of the CPR time series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2003.1. Effects of ship’s speed on depth of sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2013.2. Effects of ship’s speed on filtered volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2023.3. Effects of ship’s speed on retention of organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2033.4. Mechanical efficiency of the gauze transport mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2033.5. Consistency of taxonomic analysis and taxonomic resolution changes . . . . . . . . . . . . . . . . . . . . 203

4. Comparability with other data sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2044.1. The proportion of organisms retained . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2064.2. Effects of clogging on filtered volume and calculation of absolute abundances . . . . . . . . . . . . . . . 2074.3. Seasonal and interannual cycles of zooplankton in shallow coastal waters . . . . . . . . . . . . . . . . . . 2074.4. Other indices derived from CPR data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

5. Other issues whose effects require quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2135.1. Adding instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2135.2. Avoidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

1. Introduction

The Continuous Plankton Recorder (CPR) has been routinely operated for 70 years, and for at least the50 most recent years of sampling, the materials used and procedures applied have hardly varied. However,over that time there have been changes, outside the control of the agencies operating the CPR survey,which may have influenced the sampling characteristics. The most evident change has been the increasein the mean operating speed of the ships of opportunity used to tow the CPR. Hays and Warner (1993)calculated the mean annual towing speed and showed that after an initial decline between 1946 and 1952from 11.8 to 10.5 knots, there was a steady increase until 1991 to 14.2 knots. A significant amount ofwork has been carried out over the years to investigate possible effects of this increase in speed on the

195S.D. Batten et al. / Progress in Oceanography 58 (2003) 193–215

consistency of the CPR time series. One of the purposes of this paper is to bring together the results ofthese studies and to describe some new analyses that have been undertaken to quantify other issues oflong-term consistency.

A second, and related aim, is to describe to what extent the results from the CPR can be compared withthe results from other sampling devices, i.e. how well does the CPR sample the plankton and how confidentcan we be that abundances recorded by the CPR are representative of the actual abundances in the watercolumn. Our intention is to describe and quantify our current knowledge of the limitations and strengthsof CPR sampling.

2. CPR design and operation

The CPR consists of two main parts, an outer body and an internal removable mechanism, which werefully described by Hardy (1939) and are summarised here. The outer body has a rectangular cross-section,with a box-like central section that tapers to the front and rear. A towing eye and shock absorber areattached to the top surface. In earlier versions (Fig. 1) a diving plane was fitted at the front on the undersideand a rudder to the rear tapered section. In the most recent versions the diving plane has been removedand a box tail is fitted instead of a rudder (these modifications are discussed later). A propeller linked toa gearbox is fitted in the roof of the box section at the rear of the body. The internal mechanism is fitted

Fig. 1. A schematic longitudinal section of the CPR internal mechanism and external body. Top panel shows version with divingplane on the lower front, in use until late 1970s/early 1980s and lower panel shows current version with box tail. A, Water andplankton entering front aperture; B, gauze; C, filtered sea water exiting CPR; D, diving plane; E, towing cable; F, shock absorber;G, gear box; H, driving rollers; I, formaldehyde storage tank and spool; J, impeller turned by passing water; K, instrument payload area.


inside the outer section so that the gearbox engages with cogs of the mechanism to rotate the storage spool.Thus during the tow the filtering mesh is drawn steadily from the preloaded spools, through the internalmechanism and onto the storage spool. A fusee mechanism ensures a steady tension is maintained andalso compensates for the increasing diameter of the storage spool during the tow. Guide rollers with greaterdiameters at each end compress the edges of the mesh holding the sample, but not the central section onto which the plankton is deposited.

2.1. Collection of samples

The operation of the CPR and the processing procedures applied are described in Rae (1952); Colebrook(1960) and Warner and Hays (1994), and are repeated here because the issues of consistency and compar-ability that we discuss below, rely on an understanding of the operating procedures.

The CPR is towed behind the volunteer operating vessel, usually a fast moving (~15–20 knots) merchantvessel (also known as a ship of opportunity). The length of the towing cable is designed to produce atowing depth of about 10 m at the operating speed of the vessel. Water enters through the front 1.27 cm2

aperture in the nose cone of the CPR. The increase in the cross-sectional area of the cone slows the speedof the water flow by ~1/30, which reduces the damage to the organisms as they impact the filtering mesh.The water is filtered through a continuously moving band of silk filtering mesh, which has a leno weave(a single thread in one direction and a double twisted thread in the other) and a mesh size of ~270 µm.The interlocking nature of the weave ensures that unlike a simple square mesh, under operational tensionsthe mesh apertures do not distort significantly and retain their shape and filtering characteristics (Fig. 2).A second band of silk covers the filtering layer forming a sandwich with the plankton trapped betweenthe two layers of mesh. This sandwich is wound onto the storage spool in a tank that contains a dilutesolution of borax-buffered formaldehyde (~4%) that fixes the plankton. At the end of the tow the entiremachine is returned to the laboratory for unloading and sample processing.

2.2. Sample processing

The crew of the towing ship routinely complete a form that logs in detail the navigational data fromthe tow. These data are used to calculate the midpoints of sections of the gauze that represent individualsamples. In the laboratory the full length of the silk is marked out and then sectioned into the correspondingsamples. These samples are then allocated in a pseudo-random way to the team of analysts. This randomallocation (which began in 1957) ensures that each person processes samples scattered along the entirelength of the tow, but never receives consecutive samples. This reduces any analytical bias that may resultfrom variations in experience or subjectivity between the individual analysts. For most tows (except thoseshorter than 180 km, or when more detailed sampling is required) only alternate samples are distributedand processed. The remaining unprocessed samples are archived by soaking them with buffered preservativeand wrapping then in plastic film to prevent dehydration, and are stored in airtight containers. The processedsamples are also archived once their processing has been completed.

The first step in processing, before the silk is cut, is to assess the Phytoplankton Colour Index (PCI) ofeach sample. Each marked sample is compared to a standard colour chart and its colour recorded as 0 (nocolour), 1 (very pale green), 2 (pale green) or 3 (green). Acetone extraction experiments (Colebrook &Robinson, 1965; Hays & Lindley, 1994) have shown that these categories represent a semi-logarithmicscale of increasing colour intensity. On average PCI 2 samples were found to have twice as much colouras PCI 1, and PCI 3 samples 6.5 times as much as PCI 1.

Three separate microscopic procedures are then carried out on the cut samples and identifications arecarried out to the highest practical level. Thecate dinoflagellates and copepods, which are abundant andretain their features after being sampled by the CPR, are usually identified to species, whereas those less


Fig. 2. A schematic image of a CPR sample showing the separate stages of microscopic processing (scales are only approximate).Lines indicate the area of the sample exposed in the tunnel. (A) Phytoplankton. 20 fields of view are examined (a single meshbordered by strands of the silk) and presence of species in each recorded. (B) Staggered traverse where all small zooplankton (�~2mm) are identified and enumerated (note the covering mesh is not shown, but a mirror image traverse is carried out on the coveringsilk also). (C) Large zooplankton (�~2 mm) are separately identified and counted. (D) An image of the weave structure, showinghexagonal appearance.

rigid groups whose identification features are not readily evident under a stereo light microscope are onlyidentified to higher taxonomic levels, e.g. chaetognaths and larvaceans. A schematic diagram of the micro-scopic processing stages is shown in Fig. 2.

The first stage is a semi-quantitative identification and count of phytoplankton cells made by viewing


20 fields of view (diameter 295 µm) across each sample under high magnification (×450). This representsabout 1/8000 of the sample. Each phytoplankton taxon present in each field is recorded so that an abundanttaxon may be recorded in most of the fields, whereas a less common one occurs in only one or two.

The second stage is a staggered microscope ‘ traverse’ across both the filtering and covering portions ofthe silk (planktonic material may be transferred to the covering silk during sampling or processing) using×54 magnification. Each zooplankton organism �~2 mm in length encountered in the field of view (2.06mm in diameter) is identified and counted. This traverse represents a subsample of about 1/49 of the sample.The third and final stage identifies and enumerates all zooplankton individuals �~2 mm. Usually individualsare removed from the sample and viewed separately so their key features can be seen.

The method of counting zooplankton is a compromise between precision of enumeration and speed ofprocessing. Abundances are estimated in categories, shown in Table 1, and an accepted mean for eachcategory is taken to be the abundance of that organism. These accepted means were derived from calcu-lations of the mean number in each category derived from detailed counts (Rae, 1952). The limits of theaccuracy of these accepted values are discussed in Rae (1952) and it was concluded that ‘ if the intentionis to find a value for the mean population density of an organism on any recorder line or in any monthby averaging all available observations, it will be found that little accuracy has been lost through usingthese arbitrary categories instead of finite estimates’ . Nevertheless, the category system of recording abun-dances imposes some statistical limitations on the data, which users need to be aware of.

It should also be noted that because the CPR samples continuously, of the plankton carried on each cutsection of silk representing a 18.5 km sample length, 75% comes from the section of tow to which thesample is assigned, and 12.5% comes from each of the preceding and succeeding 9.25 km lengths ofthe tow.

2.3. The gauze advance system

Occasionally the length of filtering gauze that passes through the recorder is longer or shorter than thestandard tow length of 18.5 km. Prior to despatch each CPR is set up so that the pitch of the propellerblades is appropriate to the individual machine and to the characteristics of the towing vessel. If debrisgets wrapped around the propeller during a tow or the propeller is damaged during deployment, then therate at which the gauze passes through the machine may deviate from the standard 10.16 cm length per18.5 km sample. If when a machine is returned after a tow, the transport rate is found to have varied too

Table 1The categories employed in CPR sample processing, and the accepted values of abundance. For organisms recorded in the traversestage of processing the accepted values are further multiplied by 49 to give the actual abundance per sample

Actual abundance Category Accepted value

1 1 12 2 23 3 34–11 4 612–25 5 1726–50 6 3551–125 7 75126–250 8 160251–500 9 310501–1000 10 6401001–2000 11 13002001–4000 12 2690


much from the standard (e.g. �6 cm per 18.5 km) the whole set of samples is discarded. Occasionally itis operationally necessary to increase the distance covered by each sample length, reducing the gauzeadvance rate so that a tow length of 925 km can be sampled rather than the standard 830 km. The silk issectioned into samples representing a tow length of 18.5 km, but then the microscopic subsampling pro-cedures represent a greater or lesser proportion of the sample than is standard, because the diameter of thefield of view is fixed by the microscope objectives. In these cases the counts of each organism (in thephytoplankton and traverse zooplankton stages) are adjusted by the appropriate factor (0.7–1.2 dependenton the actual amount of silk that has passed through the machine relative to the standard 10.16 cm) beforethe data are entered into the database.

2.4. Changes to the design

2.4.1. Internal mechanismThe internal mechanism currently in use remains unchanged from that described by Hardy (1939). How-

ever, during the 1960s and 1970s a slipping clutch mechanism was developed to replace the fusee mech-anism on selected tows. There were several reasons for this replacement; the fusee mechanism is relativelyvulnerable to damage during loading and limits the length of a tow from one mechanism to 830 km. Theslipping clutch mechanism was more rugged, simpler to load, and allowed tows of �1100 km. However,if the slipping clutch mechanism was loaded incorrectly, or salt crystals or scratches occurred on thecomponent surfaces then the tow failed completely. Thus the fusee system regained favour proving to bemore tolerant of initial adjustment errors and so returning data more reliably. By 1985 all internal mech-anisms had reverted to the fusee system.

During the latter part of 1999 the cork gaskets, which lined the entrance to the formaldehyde storagetank were removed. They were found to absorb formaldehyde, which created problems with the storageof the mechanisms that were not in use. From 2000 onwards, the gaskets were made from butyl rubber,but it is most unlikely to have resulted in any changes to the device’s sampling characteristics.

2.4.2. External bodyHardy (1939) originally imposed an upper tow speed limit of 15 knots, owing to fears of instability.

Over time, the actual upper limit increased and Colebrook (unpublished) showed that between 1948 and1972 the mean speed increased from 11 to 14 knots and the maximum speeds rose from 16 to 20 knots.The vast majority of tows, which exceeded Hardy’s upper limit, have proceeded without incident, butreports of unstable performance, albeit infrequent, increased and prompted a series of investigationsthroughout 1975 to define the instabilities and correct them (Aiken, unpublished data). In adverse seaconditions it was found that the threshold speed at which the towing performance became unstable couldfall as low as 14 knots. It was this that stimulated the replacement of the front diving plane with a boxtail (Fig. 1), which successfully reduced the incidents of towing instabilities, stable towing at 10 m depthwas then possible at up to 20 knots. So between 1975 and 1986 all CPRs were modified removing thediving plane, but adding a box tail. In 1993 some machines were further modified by removing the gun-metal tails of the old body to provide space for attachment of instrumentation (P. Pritchard, personalcommunication).

At about the same time that the box tails were introduced, towing wires began to fail and several CPRswere lost. The wires then in use were 8 mm diameter steel with a 6 × 7 wire construction. It was decidedthat a more flexible and stronger wire was needed to cope with the demands of higher towing speeds andso between 1976 and 1980 towing wires were replaced with cables of 10 mm diameter, 6 × 36 construction.


2.5. Effects of the design changes

No record of which type of internal mechanism, whether fusee or slipping clutch, was used for a giventow is stored in the database, because it is believed the change had no affect on the sampling characteristics.The sensitivity of the slipping clutch system meant that if it was not set up correctly, the tow failedcompletely so no data were collected. When either system was set up correctly then the expected amountof mesh was wound through the machine.

During the diving plane trials, sensors were fitted to CPR’s to record (1) depth; (2) flow through theCPR, and (3) pitch and roll. Aiken (unpublished data) found no evidence that the removal of the divingplane affected the sampling performance as flow measurements were similar either with or without theplane. The replacement of the failing 8 mm cable with the heavier 10 mm cable also resulted in no detectablechange in the towing characteristics.

Potentially sampling depth will influence the type and quantity of plankton caught, and to some extentdepth is determined by the speed of the tow and the design of the CPR body (see Section 2). Hays andWarner (1993) showed that differences in towing speeds are unlikely to introduce any systematic variationin towing depth in CPRs fitted with box planes. Fitting of depth sensors has been a relatively recentinnovation, so the precise sampling depths for the majority of tows made by CPRs with diving planesremains unknown. There is no evidence that either flow or depth of tow has been affected by the modifi-cations. Only species that are markedly patchy in abundance in the near-surface waters will be susceptibleto these possible differing sampling efficiencies, so any studies involving such species will need to treatpre-box tail data with an element of caution.

2.6. The sampling mesh

The mesh used in the CPR to retain the plankton has remained unchanged in terms of mesh size, weaveand fibre, throughout the survey’s history. Suppliers of the silk have changed periodically, and in the 1996annual report of the Sir Alister Hardy Foundation for Ocean Science, comparisons of the characteristicsof different silks were reported. Measurements were made of the mesh and fibre diameter when the silkwas both dry and wet. Test samples from several previous and current suppliers were compared. Althoughthere was some variability in dimensions of the silks between batches from the same supplier, the variabilitybetween suppliers was similar. It was concluded that the changes in the source of the silk had not led toalterations in the filtering characteristics of the mesh.

3. Consistency of the CPR time series

During the time that the CPR survey has utilised ships of opportunity, the shipping industry has evolvedand ships have travelled at ever increasing speeds. Hays and Warner (1993) calculated that the annualmean speeds of the ships that have towed the CPRs steadily increased from the mid-1950s until 1991. Wehave updated these data to include tows to the end of 1999 (Fig. 3) by dividing the distance covered bythe time taken to give a mean speed for the tow. Since most commercial vessels travel between ports asquickly as possible, significant variations in speed during most tows are likely to be minimal. At times ofbad weather the ships may not have travelled at full speed for a particular tow. Bad weather occurs mostfrequently in winter months and winter tows have a lower mean speed than summer tows (Fig. 3) withsignificant variability among months (ANOVA, F = 6.19, p � 0.001). Mean ship speed is still increasingand in 1999 it rose to 14.8 knots (SD = 2.35), compared to 10.5 knots in 1953 (SD = 2.05) and 14.2 knotsin 1991 (SD = 2.74). Potentially there are numerous ways in which these increases in speed may have


Fig. 3. The mean annual tow speed for CPR tows from 1946 to 1999 (top panel) and mean monthly tow speed with standard errors(lower panel).

affected the sampling performance of the CPR. To quantify how the consistency of the CPR time seriesmay have been compromised we now address the factors most likely to have been affected.

3.1. Effects of ship’s speed on depth of sampling

The usual towing practice is to set the length of towing wire prior to despatch dependent on the knownspeed of the particular vessel. At higher tow speeds it was expected that a longer length of tow-wire isneeded to achieve the same depth of tow. However, Hays and Warner (1993) demonstrated that in practicetowing depth does not vary with towing speed, at least for the box tail CPR, between speeds of 7.7 and16.4 knots. For 77 tows carried out between 1987 and 1991, they also showed that variations in depthwithin a tow were generally independent of the ship’s speed. The earlier assumption that the CPR towsat a depth of 10 m was found to be incorrect, and that the true depth is 6.7 m on average. Towing depthwas also found to vary significantly between different vessels, probably owing to the different heights ofthe towing points. Subsequent to their study, depth sensors have continued to be fitted, and we have furtherexamined the relationship between ship’s speed and towing depth from 122 tows carried out between 1994and 2000 (Fig. 4). The towing speeds ranged from 8.7 to 17.1 knots, and again the mean depth of towwas found to be 6.7 ± 1.34 m. All the tows examined in this study were carried out by just three vessels(City of Manchester, n = 22; Godafoss, n = 79; Selfoss, n = 21). It was again evident that towing depth


Fig. 4. The relationship between tow speed and tow depth of the CPR for 121 tows between 1994 and 2000. Different symbolsindicate different vessels: �, City of Manchester (n = 22); �, Godafoss (n = 79); +, Selfoss (n = 21).

is vessel-dependent (ANOVA, p � 0.01), but ship’s speed had no effect either within vessels or betweenvessels, on the depth of tow (ANOVA, p � 0.05, Power �80%). Although different vessels may tow theCPR at different depths it is by no means certain that a change of vessel along a particular route willnecessarily affect the towing depth. In this analysis, for example, a route in the west Atlantic was towedby the Godafoss until May 1999 and then by the Selfoss for the remainder of the tows. Although the meanspeeds of the two vessels were significantly different (means of 12.78 ± 1.22 knots and 15.34 ± 1.04 knots,respectively, ANOVA, p � 0.01) the mean depth of tow did not change significantly (means of 6.94 ±1.30 m, and 7.29 ± 1.01 m, Power � 80%).

We can thus be confident that mean depth of towing has remained consistently at 6.7 m, since theaddition of the box tail. We cannot comment, however, on whether towing depths prior to the period ofmodification in 1975–1986 were also shallower than 10 m. Since CPRs fitted with depth sensors are onlyused on a small proportion of tows, if a time series from one route is to be examined, then any vesselchanges during that time series should be noted.

The effects of any changes in towing depth on the data are hard to quantify. As suggested earlier, datafor species which have a distinct near surface distribution may be influenced by a change in towing depthof ~3 m. However, the water immediately behind a large, fast-moving vessel is likely to be mixed downand hence homogenised, to below the towing depth.

3.2. Effects of ship’s speed on filtered volume

To quantify the volume of water filtered per sample and the effects of clogging, electromagnetic flowme-ters have been fitted to some CPRs in recent years (Walne, Hays, & Adams, 1998). The relationshipbetween plankton abundance and sample volume is described in more detail in Section 4.2 and John etal. (2002) but it is also possible to use these data to assess the effect of ship’s speed on the volume ofwater per sample. Data from 69 tows on two routes have been examined (Jonas, unpublished data) whichencompassed almost the full range of towing speeds experienced through the survey history (Fig. 3). Along


the two routes (operated by a total of eight ships over a 5-year period) mean speeds were 16.5 ± 1.59and 13.9 ± 1.63 knots, respectively. Preliminary analysis of the data suggests that at faster speeds thevolume of water filtered per sample may be reduced, but as yet this reduction cannot be quantified, becausethe data have yet to be corrected for plankton abundance and CPR instrument. Lowest plankton abundances,and therefore least clogging and higher flow, generally occurs in the winter months when ship speeds arelower because of bad weather (Fig. 3). As yet the implications for consistency within the time series areunclear, but if a significant relationship is found the necessary data (ship speed and estimated filteredvolume) are available to correct the time series data.

3.3. Effects of ship’s speed on retention of organisms

The CPR mesh width of 270 µm retains larger zooplankton with a high efficiency. But for the smallerspecies, whose minimum dimension is less than the mesh width, retention efficiencies are lower. It isconceivable that at higher tow speeds, despite the flow reduction in the nose cone, greater water pressureswill be applied to the plankton on the mesh so that more organisms will be extruded through the mesh.However, when Hays (1994) compared the retention of copepods by the CPR at a slow speed (5.85 knots)and a faster speed (9.6 knots), he found no significant change in retention efficiency. However, the fastestspeed examined by Hays was similar to the slower speeds of the current tow ships. The widening of theconical tunnel immediately behind the CPR aperture does significantly reduce the speed of the incomingwater (to 1/30), and hence the increase in pressure on the mesh at higher speeds will be much less. Theeffect on retention at tow speeds �9.6 knots remains unquantified.

3.4. Mechanical efficiency of the gauze transport mechanism

It is possible that the efficiency of the silk gauze transport system will be reduced at higher tow speeds;the increase in water pressure against the gauze may increase friction, and hence slow down the rate atwhich the silk is wound on through the mechanism. Although the plankton counts are corrected for vari-ations in sample size (see Section 2) the correction is applied as only one of six factors, each of whichrepresent a range of sample sizes, and so it is only an approximate correction. The slower the gauze movesthrough the machine the shorter the length of mesh across which the samples will be spread, since thesilks are always cut so that each sample represents 18.5 km of tow. Since tow speeds have increasedsteadily with time, there is some potential for an effect on the consistency of the time series. We have,therefore, examined the records of sample size and tow speed to determine whether or not the range oftow speeds used in the CPR survey has affected the efficiency of the gauze transport mechanism.

Although wind-on speeds influence the amount of gauze per sample they are independent of the speedof the tow, and so if efficiency is dependent on tow speed then for the large number of tows that havetaken place there should be a detectable relationship. Data for the length of silk per sample from 9278tows conducted between 1946 and 1994 were regressed against the speed of the ship. There was no declinein sample length with increasing speed (p � 0.05, Power = 100%) and so we conclude that at least forthe operational speeds of the survey, the rate of gauze transport per km of tow is unaffected by any increasein the water pressure against the gauze.

3.5. Consistency of taxonomic analysis and taxonomic resolution changes

Throughout the history of the CPR survey, except for the very first (pre-war) years, there has alwaysbeen a team of taxonomic processors so that expertise has been mixed. As some staff left the survey,others arrived, but there has been an overlap with newcomers receiving training from existing experiencedstaff. This means that although individual strengths and weaknesses have been present, there has been no


sudden or consistent change in the quality of taxonomic processing. The system of processing, as describedin Section 1, has remained unchanged since 1958, although many aspects of the procedures were similarbefore this time and in many cases the time series can be extended backwards to the 1940s.

Taxonomy, however, is an evolving discipline and throughout the course of the CPR survey (and presum-ably in the future) new species have been discovered, and old ones have been split or even merged, asnew information comes to light. One important example has been the splitting of Calanus finmarchicusinto C. finmarchicus and C. helgolandicus (Mauchline, 1956). CPR analysis has generally been able torespond to such changes, and from 1958 the two species of Calanus were identified and counted separately.Other species have begun to be recorded either through scientific interest or as a result of the increasingexpertise of the team. A distinction needs to be made between the first time an organism was recordedand the date at which it was first ‘ looked for’ , and counted, in CPR samples. A good example of theformer is the diatom Coscinodiscus walesii, which is not indigenous to the northeast Atlantic. It was firstrecorded in CPR samples in the English Channel in 1977, but since then has increased in abundance andexpanded the range of its distribution. Its absence from the CPR database prior to 1977 is significant interms of the organism’s ecology. In contrast the absence of the tintinnid Parafavella gigantea from thedatabase prior to 1996 is because this species was not separated from other tintinnids before this date. Thedegree of taxonomic resolution has never declined, but for some organisms, such as certain tintinnids, thetaxonomic resolution has increased. Table 2 lists these changes in taxonomic resolution and when theyoccurred. All taxa not on this list, but included in the list of taxa on the CPR database are those whichwere recorded as soon as they were encountered. Such additions may have occurred either through thesurvey being extended into new areas, or through the organism having extended the range of its distribution.

4. Comparability with other data sets

Combining different data sets can be a useful way to improve our ability to test a particular hypothesisor to maximise the information that can be gained. Before attempting to combine data sets, it is firstnecessary to be confident that they either measure the same things in the same way, or that one is consist-ently different from the other (and therefore one data set can be converted to match the parameters of theother). The previous section dealt with the consistency of the CPR data with time, which is of courseessential to combining or comparing with other plankton abundance data. No plankton sampling systemcompletely replicates the abundance of the organisms in the water column. Understanding just how rep-resentative a CPR sample is of the ambient concentrations of particular organisms is also a fundamentalrequirement if data sets are to be combined. In this section, comparisons that have been made with otherdata sets are discussed and how these comparisons have contributed to our understanding of what the CPRcatches. Several studies have compared CPR zooplankton data with abundance data acquired from watersaround the European coast. One challenge in comparing such data sets lies in the different mesh sizes usedin the CPR and other sampling devices. The standard mesh size for WP2 nets, for example, is 200 µm(UNESCO, 1968) and so retains a higher proportion of smaller specimens and species than the CPR mesh(270 µm). A second challenge is to account for the sampling bias of each device since each is designedto sample a particular aspect of zooplankton distribution and so direct comparisons may not be appropriate.The CPR’s strength lies in its horizontal coverage (albeit coarse), but it provides no information on verticaldistributions. The Longhurst–Hardy Plankton Recorder (LHPR), by contrast, was designed to providedetailed vertical resolution of plankton distributions at discrete locations (Sameoto, Wiebe, Runge, Postel,Dunn, Miller et al., 2000), and generally only provides horizontal information if many hauls are takenalong transects. Even when the LHPR has been towed horizontally, it only discriminates relatively finehorizontal scales.

The LHPR was used intensively at Ocean Station India (59°N, 19°W) in the North Atlantic between


Table 2Taxonomic resolution changes that have occurred during the last 60 years. All taxa not on this list, but included in the CPR databasehave been counted from their first occurrence on CPR samples

Taxon Date of change Description of change

Calanus copepodites I–IV Jan 1958 Counted separately and also in total Copepods categoryCalanus finmarchicus Jan 1958 Now recorded as a separate speciesCalanus glacialis Jan 1958 Now recorded as a separate speciesCalanus helgolandicus Jan 1958 Now recorded as a separate speciesCopepod eggs Jan 1993 Counted rather than just recorded as presentCoccolithophores Jan 1993 Counted rather than just recorded as a presenceDictyocysta spp. Jan 1996 Counted separately and included in Tintinnids categoryDinoflagellate cysts Jan 1993 Counted rather than just recorded as presentEchinoderm larvae Jan 1949 Counting system changed to match other small zoopl.Euphausiids Jan 1960 Now recorded as total Euphausiids

1968 to 1988 Also separated into juveniles and adultsEvadne spp. Jan 1958 Counted separately from other cladoceransFavella spp. Jan 1996 Counted separately & included in Tintinnids categoryForaminifera Jan 1993 Counted rather than just recorded as presentHalosphaera Jan 1993 Counted rather than just recorded as presentLamellibranch larvae Jan 1949 Counting system changed to match other small zoopl.Navicula planamembranacea May 1962 First recorded as a speciesParafavella gigantea Jan 1996 Counted separately and included in Tintinnids categoryPodon spp. Jan 1958 Counted separately from other cladoceransPtychocylis spp. Jan 1996 Counted separately and included in Tintinnids categoryPseudocalanus spp. (adults) Jan 1958 Counted separately and also included in Para-Pseudocalanus spp.

categoryRadiolaria Jan 1993 Counted rather than just recorded as presentSergestes larvae March 1962 Counted separately and also in Decapod larvae categorySilicoflagellates Jan 1993 Counted rather than just recorded as presentTintinnids Jan 1993 Counted rather than just recorded as presentTintinnopsis spp. Jan 1996 Counted separately and included in Tintinnids categoryUmrindeten cysts Jan 1993 Counted rather than just recorded as presentZoothamnium pelagicum Jan 1993 Counted rather than just recorded as present

1971 and 1975. Weekly samples down to 500 m were collected between March and October. A comparisonof CPR data from the same period has been made (John, Irigoien, Harris & Hays, in press) for a largearea, centred on Station India and extending 10° to the north, south, east and west. The mesh sizes of thetwo devices were almost identical (280 µm for the LHPR), although the LHPR used a nylon single weavemesh. Seasonal cycles based on the CPR data for common taxa were found to be similar to those recordedby the LHPR at the surface down to depths of 500 m. However, with the exception of Acartia, there wereclear differences in absolute abundances, with the CPR underestimating surface abundances by a factor ofbetween 5 and 40. This difference was attributed to timing of sampling, instrument design and avoid-ance reaction.

Comparisons between the CPR and an Undulating Oceanographic Recorder (UOR), which used the samemesh size, recorded similar abundances of many planktonic organisms both to the west of theShetland/Orkney Islands (Aiken, Bruce, & Lindley, 1977) and during the FLEX experiment in the NorthSea (Williams & Lindley, 1980). The significant differences recorded during both studies could be attributedto the depth sampling characteristics of the two devices; the UOR collected higher numbers of species thateither occupied deeper layers, or exhibited vertical migration, whereas the CPR recorded higher abundancesof the near surface dwelling species. The UOR was designed to operate in a similar way to the CPR


(Bruce & Aiken, 1975) and to act as a potential replacement. It, too, is towed behind the vessel and hasa small frontal aperture that channels plankton through a length of gauze. The plankton community issampled in a similar fashion to the CPR, but more information is gathered on depth distributions andphysical parameters.

4.1. The proportion of organisms retained

The mesh size of 270 µm was chosen as an appropriate size to retain a reasonable proportion of zooplank-ton such as copepods and cladocerans, and to give an indication of those areas in which there are highabundances of larger phytoplankton forms (Hardy, 1939). However, the efficiency with which an organismis retained depends not only on its size, but also on the abundance of other planktonic organisms. As moreand more organisms are filtered onto the mesh the open apertures are progressively clogged and reducethe effective mesh size. So as more large organisms are retained, smaller organisms, which at the start ofthe sampling would have been extruded, will be retained progressively more effectively. This effect is hardto quantify since the ambient concentrations of organisms (needed to determine the true proportion retained)will never be known for a specific patch of sea water at a specific time.

A simple experiment was carried out whereby a mixed zooplankton assemblage was split into equalhalves and each portion poured through a piece of CPR mesh. A finer mesh (80 µm) was placed beneathto catch organisms that passed through the 270 µm mesh and so calculate the proportion retained (Table3). More than 98% of the larger copepods and cladocerans were retained by the mesh, but a reasonableproportion (�30%) of species small enough to pass through the mesh, such as the cyclopoid copepodOithona, were also retained. Although these results give some indication of the retention properties of theCPR mesh during operations, as the CPR is towed behind a ship, there is a pressure exerted by the waterflowing through the aperture, which is likely to increase the extrusion of organisms through the mesh.

Robertson (1968) examined the proportion of various zooplankton organisms captured by the CPR meshduring towing and concluded that organisms with a width of �300 µm were not fully retained and thatthe width at which 50% of the organisms were retained was 287 µm. In terms of copepod life stages thistranslates to copepodites stage II and later of Calanus helgolandicus and stages IV or V and later of Temoralongicornis and Pseudocalanus spp. being fully retained. Some zooplankton that were too large to passthrough the mesh were found to escape retention; it was assumed that because the CPR is not an absolutelysealed instrument some leakage occurs around the edge of the mesh. Hays (1994) showed that this wasnot a significant source of error since the measured relationship between copepod width and retention didnot differ significantly from the expected relationship. The Hays study also broadly substantiated the find-

Table 3Mean retention (and range) of organisms by 270 µm mesh when water with a mixed zooplankton assemblage was poured throughthe mesh (n = 2)

Taxonomic group Mean number individuals (range) Mean retention (range) %

Temora longicornis 529 (17.0) 99.9 (0.1)Evadne spp. 83 (14.1) 98.2 (0.6)Podon spp. 57 (14.1) 100 (0)Acartia clausii 24.5 (3.5) 91.8 (1.2)Calanus helgolandicus CV-VI 26.5 (6.4) 100 (0)Calanus helgolandicus CI-IV 23.5 (0.7) 100 (0)Para-Pseudocalanus spp. 691 (75.0) 91.3 (2.6)Oithona spp. 570 (22.6) 37.6 (4.7)Oncaea spp. 316 (55.2) 44.4 (18.9)


ings of the earlier work by Robertson, although he found slightly higher rates of retention. Although onlya portion of smaller zooplankton is retained, this portion has been found to be consistent (Broekhuizen,Heath, Hay, & Gurney, 1995) and so the seasonal cycles of the smaller species can be reconstructed.

4.2. Effects of clogging on filtered volume and calculation of absolute abundances

To make a comparison with other sampling devices it is first necessary to know the efficiency of theCPR and to estimate the absolute abundance per sample. Historically, CPRs were not fitted with flowmetersand the assumption was made that with 100% efficiency, the CPR towed for 10 nautical miles (18.5 km)filtered a volume of 3 m3 of seawater (Robinson & Hiby, 1978). As Walne, Hays and Adams (1998)indicated, it seems likely that clogging of the filtering mesh by the retained organisms will reduce thevolume filtered. They demonstrated that the volume of water filtered by the CPR is variable, thus promptingthe fitting of flowmeters to several CPRs and detailed studies were carried out to examine the relationshipbetween quantity and type of plankton retained and the actual filtered volume per sample.

The flow rate data measured since 1995 indicate that the volumes filtered per sample are normallydistributed with a mean of 3.11 ± 0.8 m3 (John et al., 2002). Furthermore, although the mean flow ratedecreases with increasing plankton abundance, even at the highest plankton densities recorded cloggingonly decreased the flow rate by ~20% and the relationship obtained was always linear. These data wereused to derive a relationship that can adjust the filtered volume of historical samples, based on planktonabundance. A comparison between annual mean abundances of Calanus finmarchicus based on the assump-tion of a constant filtered volume per sample (3 m3) and the recalculated absolute abundances showed non-significant differences (p � 0.05 in a paired-sample t-test). Although inter-sample differences in filteredvolume may be large owing to the effects of clogging, because CPR data are usually presented as monthlyor annual means these time series remain robust. The inter-sample variation in filtered volume is also trivialwhen compared to the inter-sample variation in the abundance of the taxa (because of patchiness andseasonal changes in abundance). Although the ability to calculate absolute abundances per sample has littleeffect on the results of the CPR time series, it is important when trying to compare the CPR results withthose from other data sets.

4.3. Seasonal and interannual cycles of zooplankton in shallow coastal waters

Tows were carried out in the Baltic Sea in 1998 and 1999 as a pilot test to determine the suitability ofthe CPR for zooplankton monitoring of the Baltic Sea. There the zooplankton communities consist of amixture of neritic copepod species, and limnic copepod and cladoceran species (Segerstrale, 1969). Theroute, from Lubeck, Germany or Trelleborg, Sweden to Hanko, Finland covered the Baltic Basin Proper.The Baltic Sea environment is strongly seasonal, and because of its large drainage area, shallow connectionto the North Sea, and the basin’s geophysical properties, it is strongly stratified. A sharp thermocline isusually found at 15–20 m in summer in the northern Baltic proper, and a halocline at 60–70 m in the samearea (c.f. Voipio, 1981). These layers have a significant effect on zooplankton distributions (e.g. Hernroth &Ackefors, 1979). Additionally, the light summer nights and intensive planktivory by schooling pelagic fishcause strong diel vertical migration (DVM) in zooplankton (e.g. Flinkman, 1999).

A comparison of the CPR catch and WP-2 samples taken in the same area was carried out in 1999 andshowed that the abundances derived from CPR samples were considerably lower than those determinedfrom the WP-2 vertical net samples. The WP-2 hauls were taken with 100 µm mesh (HELCOM standard)from 10–15 m depth to the surface with the CPR running at its usual depth of approximately 7 m. Differ-ences in abundance (Table 4) varied by an order of magnitude or more, which can be partly explained bythe larger mesh size of the CPR. However, the higher abundance of zooplankters in the WP-2 samplesmay also be attributed to the deeper sampling depth. Since zooplankton in the Baltic undertake significant


Table 4Comparison of mean abundances (m�3) of plankton retained by a 100 µm mesh WP-2 net and the CPR in the Baltic Sea in sum-mer 1999

Species CPR WP-2

Acartia spp. 202 2895Eurytemora affinis 11 1158Evadne nordmanni 1 3892Fritillaria borealis 62 92Pleopsis polyphemoides 0 7359Temora longicornis 4 254Centropages hamatus 5 307Limnocalanus macrurus 0 100Podon intermedius 8 284Podon leuckarti 0 201Bosmina coregoni maritima 109 268

DVM, just a few metres difference in depth will greatly affect the observed species composition andabundance. This emphasises the need to account for the sampling strengths and weaknesses of the devicethat may be relevant to specific sampling areas.

In summer 2001, comparisons were carried out in the Baltic between the CPR and a U-Tow towed bodyequipped with a Valeport Ltd plankton sampling mechanism (PSM) using 200 µm mesh. CPR sampleswere processed using the standard methods described in Section 2. The U-Tow samples were processedby washing the plankton off the mesh, and then following standard HELCOM procedures. Although thesedifferences in processing may have contributed to the significantly different abundances obtained (Fig. 5),it seems more likely that mesh size, and possibly also the design of the whole sampling vehicle, significantlyaffected the estimates of plankton. So careful prior consideration needs to be given to choose the mostappropriate mesh size, particularly in non-oceanic environments. Further comparisons between CPRs fittedwith the standard mesh and those equipped with finer mesh would help to identify the effects of differentsampling vehicles on abundance estimates.

Fig. 5. Comparison of catch of adult copepods in 1 m�3 of water using a standard CPR (270 µm mesh) and a U-Tow PlanktonSampling Mechanism (200 µm mesh).


Perhaps a more robust comparison is given by two studies in relatively shallow waters where the wholewater column has been sampled and compared with CPR sampling. Clark, Frid and Batten (2001) compareda time series of monthly WP-2 net samples (200 µm mesh) from the northwest North Sea, just off theNorthumberland coast (55°7�N, 1°20�W), with CPR data from the surrounding region (54°7�N–56°5�N andfrom the coast out to 0°4�W). The time series of WP-2 sampling was started in 1968 and the data werecompared from 1968 to 1996. Similar species were captured by both the net and the CPR, and six of themost abundant taxa were given similar rankings. The greatest differences in abundance rankings wereobserved for very small species (probably because of the smaller mesh size of the net) and in the larvaeof benthic species (the net samples were taken vertically from about 4 m above bottom to the surface);both groups ranked higher in the net samples. Year-to-year fluctuations of total zooplankton abundanceand community composition changes with time were correlated significantly between the two time series(r = 0.64, p � 0.001 for abundance data, probability of there being no relationship = 1% for communitycomposition changes; see original paper for detailed description of methods). However, the net samplescontained, on average, 15 times as many individuals as the CPR samples.

John, Batten, Harris and Hays (2001) compare a time series of weekly WP-2 net samples (200 µm mesh)from the north western English Channel (50°15�N, 4°13.1�W) with CPR data from the western EnglishChannel (50°30�N–48°N and 2°W–6°W). The time series for the net samples began in 1988 and so isrelatively short, but 11 years were available for comparison. Both devices recorded the same species asbeing most abundant, and seasonal cycles for the most common copepods agreed closely. Interannualvariability in total copepod abundance only correlated significantly if two anomalous years were removed.Catches of most taxa, especially smaller species, were much higher in the net samples than in the CPRsamples.

The comparisons between the WP-2 net time series and the CPR catches draw broadly the same con-clusions. We selected the five copepod species or species groups that were most abundant in both theEnglish Channel and the western North Sea time series and examined the mean monthly abundances (formethods of estimation refer to original papers) as determined by CPR sampling and WP-2 net sampling(Fig. 6, Channel data redrawn from John et al., 2001). It is evident that in each area both devices recordsimilar seasonal cycles, even though the seasonal patterns for a given species or group may be quitedifferent between each area. In each of the 10 comparisons shown in Fig. 6, catches by both samplingsystems are correlated significantly (p � 0.01, except for Centropages typicus, where p � 0.05). Corre-lations between the WP-2 data and the CPR data from the other region were also sometimes significant(as is often the case with unrelated seasonal data), but only when the seasonal cycle had the same shape.The Para-Pseudocalanus group, for example, shows a bimodal pattern in the English Channel, but a singlesummer peak in the western North Sea. The English Channel WP-2 data did not significantly correlatewith the North Sea CPR data, nor did the North Sea WP-2 data correlate with the English Channel CPRdata. Calanus and Acartia also showed non-significant correlations in one of the two cross-region corre-lations. This supports our finding that either sampling method adequately describes the seasonal cycle ofa given plankton group.

As both comparison studies have already reported, the abundances recorded by each device vary mark-edly. Combining these studies reveals that this variability is consistent for the total catch (John et al., 2001)and also for a given species; for each month, a ratio of abundance between the WP-2 catch and the CPRcatch was calculated and then a mean for the year (Table 5). For the five species groups considered, ratiosvaried by a factor of 10 depending on the species; estimates derived from the WP-2 nets were between2.2 and 22 times greater. However, ratios calculated for a particular species group were not significantlydifferent between the two regions (ANOVA, p � 0.05, note however that the Power of this test is low,�20%). This implies that whatever factors are causing the difference in catches they operate consistently,at least across shallow coastal seas.

There are at least five separate factors that may contribute to the different catches. Firstly, and most


Fig. 6. Mean monthly abundances (number m�3) of five copepod groups (A ,Calanus spp.; B, Para-Pseudocalanus spp.; C, Temoralongicornis; D, Acartia spp.; E, Centropages typicus) from CPR samples (solid lines) and WP-2 net samples (dashed lines) in theEnglish Channel (left column) and western North Sea (right column).

Table 5Mean monthly catch ratios for 5 copepod groups for CPR samples versus WP-2 net samples (from data shown in Fig. 5)

Copepod group Mean (SD) channel: CPR Mean (SD) W. North Sea: CPR

Calanus spp. 2.66 (1.46) 2.24 (0.92)Para-Pseudocalanus spp. 16.92 (14.22) 14.86 (5.21)Temora longicornis 20.91 (9.33) 21.86 (27.58)Acartia spp. 8.95 (10.43) 10.36 (5.41)Centropages typicus 2.28 (2.63) 2.79 (3.71)


obviously, is the mesh size, which is probably the reason why the ratios for the largest species (Calanus)are lower than the ratios for smaller species such as Acartia (Table 5). However, the results in Table 5show that even similarly sized copepods, such as Temora and Centropages may have markedly differentcatch ratios, being about 21 and 2.5, respectively. Both Clark et al. (2001) and John et al. (2001) havenoted that organism size was not strongly related to catch ratio and have suggested that because individualspecies occupy different layers in the water column they are, therefore, caught with different efficienciesby the two devices. The consistency of the ratios between the two areas does suggest that the depthpreferences of organisms are another factor that influences the catching efficiency of samplers. Diel verticalmigration behaviour is a third factor that may further restrict the numbers caught by the surface samplingCPR. One cannot, therefore, assume catch efficiency is based purely on the size of the organisms, but mustexamine the catch ratios for each taxon before comparing data sets. The proportion of night to day samplingmust also be considered, so that vertical migration effects can be accounted for, if such data are to becombined. Fourthly, our estimates of CPR abundances from the North Sea (and the Baltic Sea) have takenno account of the effects of clogging of the mesh and are based on the mean filtered volume of 3 m3 persample (John et al., 2002). Although the reduction in flow was shown not to be important for the CPRdata set as a whole (see Section 3.1) if the abundances used in Clark et al. (2001) were recalculatedaccording to flow, higher abundances m�3 would probably result, and ratios would be lower. The abun-dances calculated for the English Channel CPR samples were re-calibrated for actual (or estimated) flow.Since the ratios between the two regions are so similar, such recalculation is likely to have only a minoreffect. In general the North Sea ratios were higher than for the English Channel, which may have resultedin abundances derived from the CPR catches being slightly underestimated. A fifth factor that may haveinfluenced the catch ratios is the relative avoidance of each device by the plankton, however, Clark et al.(2001) found no evidence for this.

Interannual patterns of abundance as recorded by the two sampling methods do not compare quite sofavourably (Fig. 7). The English Channel WP-2 net time series is relatively short and so although similaritiesare clear for some species the correlations are not significant. All five species groups in the North Seashowed positive correlations that were significant (p � 0.05) for Calanus, the Para-Pseudocalanus group,and Temora. Both studies found good agreement between sampling devices in recording interannual varia-bility of the total number of organisms caught, although the English Channel comparisons were only sig-nificant if the years 1988 and 1997 were removed from the time series. The lack of convincing correlationin interannual variability of individual species most probably stems from the difficulty in extrapolatingfrom the results of point sampling to areal estimates. Even minor interannual hydrometeorological fluctu-ations may mean that sampling at a specific site does not encounter an identical water mass structure atthe same time in different years, whereas the wider scale of the CPR samples may incorporate at leastsome of this interannual variability. CPR data would not normally be used to determine annual abundancesat point positions and so we conclude that both devices record interannual variability, but that the appropri-ate area needs to be defined in each case.

4.4. Other indices derived from CPR data

Although for most species, the abundance as recorded by the CPR is lower than the abundance recordedby net sampling, other indices of planktonic abundance have proven to be more closely comparable. Com-parisons made between zooplankton biomass values derived from CPR abundances and those derived fromoblique Longhurst–Hardy Plankton Recorder tows in the Celtic Sea (Batten, Hirst, Hunter, & Lampitt,1999) showed that the estimates were not significantly different. This allowed the data sets to be combinedto produce a three-dimensional view of zooplankton biomass, rather than the two-dimensional view avail-able from each device on its own.

The Phytoplankton Colour Index (PCI) is a visual assessment of the green colour of CPR samples (as


Fig. 7. Mean annual abundances (number m�3) of five copepod groups (A, Calanus spp.; B, Para-Pseudocalanus spp.; C, Temoralongicornis; D, Acartia spp.; E, Centropages typicus) from CPR samples (solid lines) and WP2 net samples (dashed lines) in theEnglish Channel (left column) and western North Sea (right column).

one of four colour categories) and has long been used to represent phytoplankton biomass, since it hasbeen assumed that the green coloration gives an index of the chlorophyll concentration (Reid, 1975).Comparisons between the PCI and fluorometrically measured chlorophyll have been undertaken (Hays &Lindley, 1994) and have shown a good relationship between PCI and chlorophyll, when the number ofcells retained by the CPR mesh was small. More recent comparisons between PCI, phytoplankton cellabundance, fluorometrically measured chlorophyll and satellite derived chlorophyll estimates (Batten,Walne, Edwards & Groom, 2003) show that this simple index reproduces the seasonal cycle of chlorophyllabundance. Monthly measurements of chlorophyll for the Iberian margin (between 39° North and 45°North) using three methods of estimation (PCI, fluorescence from a Chelsea Instruments Aquapack


attached to the CPR, and from SeaWiFS satellite imagery) were correlated. The three time series all corre-lated significantly (p � 0.05) with correlation coefficients of 0.67 (PCI and fluorometric chlorophyll), 0.79(fluorometric and SeaWiFS chlorophyll) and 0.70 (PCI and SeaWiFS chlorophyll).

5. Other issues whose effects require quantification

5.1. Adding instrumentation

In the last decade, further instruments have been added to some CPRs (CTD/chlorophyll sensors andflowmeters). These have been attached at the rear end under the box tail, and so may influence the flightattitude of the CPR and affect the sampling characteristics. No efforts have yet been made to assess ifthere are any such effects. Records are kept of every tow on which one of these instruments has beenfitted and the data can be used and if necessary corrected retrospectively.

5.2. Avoidance

It has been suggested that CPR data can be used to detect diel vertical migration (DVM) in many speciesof copepods (Hays, Proctor, John, & Warner, 1994), and inferences have been drawn about long-termchanges (Hays, 1995a; Hays, Warner, & Lefevre, 1996). The issue of avoidance has not been consideredin detail, mostly because it has been assumed that behind a fast moving ship of the size that tow CPRs,turbulence and the speed of the tow, will prevent active avoidance. Hardy (1939) described how the towingcable meets the top surface of the CPR at a steep angle, so that there is no disturbance of the water aheadof the aperture of the machine that might warn of the instrument’s approach. Most towed nets have a bridleand cables ahead of the net opening. There is some evidence to suggest that turbulence does reduce theability of an organism to escape (Singarajah, 1975). Furthermore, Hays (1995b) compared the typicaltowing speed of the CPR (�6500 mm s�1) with the modelled speed at which herring larvae of about 10mm showed no differential day and night avoidance (250 mm s�1; McGurk, 1992) and concluded thatcopepods are unlikely to avoid the CPR at the high tow speeds. Some evidence suggests, however, thatthe size of the aperture is more important than the tow speed (UNESCO, 1968). Clark et al. (2001) showedthat catching efficiency of the CPR declines rapidly compared to a towed net, with increasing escape abilityof the plankton because of the small aperture size. Even at quite low escape abilities the catching efficiencyof the CPR is low. Passive avoidance, whereby particles are pushed away from the aperture through thebow-wave generated by the passage of the CPR through the water has been mentioned (Clark et al., 2001)but not determined.

6. Conclusions

Most conclusions are included in the relevant section, but the main conclusions are summarised below:

� The operating speed of the vessels that tow CPRs has increased over the duration of the survey. Thishas not affected the depth at which the CPR is towed. However, towing depths do vary between vessels.We confirm that mean towing depth is 6.7 m. There is no effect of increased speed on the mechanicalefficiency of the sampler, but at highest speeds flow through the machine may be reduced.

� The effects of clogging of the mesh by retained organisms on the volume filtered have now been quant-ified so that abundances can be recalibrated. For large areas, such as the North Sea, this calibration doesnot significantly alter the mean organism abundance.


� Studies have been undertaken that compare plankton abundances obtained with the CPR with abundancesobtained by vertical net hauls at specific locations on the European continental shelf. Catches by theCPR are almost always lower (and often considerably so), but the patterns of the seasonal cycles ofabundance were significantly replicated for all comparisons made. Interannual variability generallyagrees between time series for overall abundance and for some species. The relative catch rates for anindividual species by each sampling device appear to be consistent, probably because of the organisms’behaviour and the attributes of the devices. Integration of data sets may be possible because of theconsistency of the catching performance. However the application of the sampling devices must beappropriate to the study in question.

We have shown that although there are issues that affect the consistency of the CPR time series severalof these effects can be quantified. Other issues need further evaluation and this is a central part of theongoing work of the Sir Alister Hardy Foundation for Ocean Science, which now manages the CPR survey.Recent work to establish the absolute abundance of organisms retained by the CPR and to compare thecatches of zooplankton with other time series demonstrates that the CPR data set is robust.

Acknowledgements

The contribution of the vessels, their owners, operators, officers and crew who have towed, and continueto tow, CPRs cannot be overstated. We are grateful for their support. All past and present members of theCPR team are also gratefully acknowledged. Special thanks are due to Roger Harris and Chris Frid formaking available the L4 and Dove time series respectively, and to Jim Aiken for comments on the CPRoperations. Anthony Richardson and an anonymous reviewer made suggestions, which significantlyimproved the manuscript and the authors wish to thank them for their input.

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