Journal of Plankton Research Vol.12 no.6 pp.1173-1187, 1990
On the trophic fate of Phaeocystispouchetii (Hariot). HI. Functionalresponses in grazing demonstrated on juvenile stages of Calanusfinmarchicus (Copepoda) fed diatoms and Phaeocystis
B.Hansen1, K.S.Tande and U.C.Berggreen2
Department of Aquatic Biology, The Norwegian College of Fishery Science,University of Troms0, 9001 Troms0, Norway1 Present address: Marine Biological Laboratory, University of Copenhagen,Strandpromenaden 5, DK-3000 Helsing0r, Denmark2Present address: Greater Copenhagen Council, Gl. K0ge Landevej 1-3, DK-2550 Valby, Denmark
Abstract. The objective of this study was to quantify the functional response in feeding rate in thevarious developmental stages of Calanus finmarchicus to different concentrations of the diatomsThalassiosira nordenskioeldii and Porosira glacialis, and the haptophysean Phaeocystis pouchetii.Grazing of copepodite stage I-V C.finmarchicus was measured using two different approaches.Feeding rates were obtained from either incubation experiments, estimating the rate of removal ofparticles from suspension, or by quantifying the turnover rate of the plant pigments in the gut.Clearance as a function of algal concentration (1-30 (ig plant pigment 1 ) was described in juvenilestages of C.finmarchicus fed the diatoms T.nordenskioeldii [~20 p.m equivalent spherical diameter(ESD)], P.glacialis (~40 n.m ESD), and two size categories (30-100 |un and MOO n.m ESD) of thegelatinous alga P.pouchetii. When the copepodite stages were fed T.nordenskioeldii, the gut contentof plant pigments was in general higher than when fed P.glacialis. Rates obtained were variablewhen the same copepodite stages were offered the two size categories of P.pouchetii, but within thesame order of magnitude as those obtained for the larger diatom. At unialgal diets, diatoms weremore readily consumed than the larger size fraction among colonies of P.pouchetii by copepoditestage I—III C.finmarchicus. But given an appropriate prey size, C.finmarchicus grazed both diatomsand colonies of gelatinous algae at equal rates. A linear relationship between gut content and foodconcentrations <10 u,g chlorophyll I"1 was found. This indicates that the ingestion rate inC.finmarchicus is directly proportional to the ambient food concentration during the mostproductive period in May and June in high latitudes irrespective of algal species present.
Introduction
Colony-forming phytoplankton species are commonly found in aquatic systems,but the availability of these forms to suspension feeders is still unclear. Colony-forming phytoplankton can be divided into non-gelatinous and gelatinous forms.The feeding of copepods on the marine autotrophical gelatinous algaeThalassiosira partheneia and the arctic mixotrophic Dinobryon pellucidum,representatives of non-gelatinous colony forms, has recently been investigated(Schnack, 1983; Hansen et ai, 1989). Among the gelatinous algae, Phaeocystispouchetii appears to be of quantitative importance in the marine environment,but to what extent this species is available to grazers is still under debate (e.g.Huntley et al., 1987; Tande and Bamstedt, 1987; Verity and Smayda, 1989).Results from northern latitudes indicate that copepodites of both Calanusfinmarchicus and C.hyperboreus, when offered P.pouchetii, obtained dailyrations well within the range previously reported for the same species or
sympatric copepods of similar size (Huntley et al., 1987). These results haveencouraged more systematic studies related to the dynamic interactions withinthis plant-herbivore community, which is a consistent feature during theproductive period in high latitudes (Eilertsen et al., 1981, 1989; Eilertsen andTaasen, 1984).
In coastal and fjord areas in northern Norway and in the Atlantic regions ofthe Barents Sea, C.finmarchicus is probably the most quantitatively importantherbivorous macrozooplankton species (Tande, 1982; Tande et al., 1985). Thespecies spawns during the spring diatom increase. Although P.pouchetii is alsopresent, it is not until May and June that this phytoplankter dominates in thesewaters (Eilertsen et al., 1981; Eilertsen and Taasen, 1984). The annualrecruitment and the most productive period of C.finmarchicus in May and Juneoften takes place under conditions where P.pouchetii constitutes the major partof the standing crop (Tande, 1988). It is therefore of interest to quantify thefunctional response in feeding rate to the concentration of P.pouchetii in thevarious developmental stages of C.finmarchicus. Here we report on determi-nations of stage-specific functional responses in copepodites of C.finmarchicuson different size categories of P.pouchetii as well as on two species of diatoms.
Materials and methods
Phytoplankton
Colonies of P.pouchetii were isolated from the Balsfjorden area and grown in150 1 PVC containers in f/2 medium (Guillard and Ryther, 1962) at 5°C. Thecultures were grown in a 16 h light:8 h dark cycle at ~175 u.E m~2 s - 1 and weregently aerated to keep the colonies suspended. The colonies were separated intosize fractions of 30-100 u.m, >50 (xm and >100 \im equivalent sphericaldiameter (ESD). The diatoms Thalassiosira nordenskioeldii (~20 \txa ESD) andPorosira glacialis (~40 \xm ESD) were cultured in f/2 medium in Erlenmeyerflasks. Grazing experiments were performed with phytoplankton concentrationsin the range of 1-30 u.g plant pigment I"1 (plant pigment is chlorophyll a andphaeophytin a). The phytoplankton bloom reaches maximum chlorophyllconcentrations of ~20 p-g chlorophyll a I"1 in fjords and coastal waters ofnorthern Norway, whereas ambient concentrations of <5 jtg chlorophyll a I"1
are more typical during the period of culmination in May and June (E.N0st-Hegseth, personal communication). The concentrations in the experiments wereobtained by diluting the algae cultures with 0.45 u.m filtered seawater, and thefinal phytoplankton concentrations were measured on a Turner model 111 and aTurner Design fluorometer following Strickland and Parsons (1972). Carbonand nitrogen contents of the algae were measured on filtered algal culture in aPerkin Elmer CHN-instrumental analyser at 240°C (see Table I).
Zooplankton
Copepods were collected in Malangen just outside Troms0 in vertical hauls froma depth of 50 m to the surface with a 200 u.m mesh size WP-2 net equipped with
Table I. Size, chlorophyll a content (Chla), chlorophyll arphaeophytin a (Chla:Phaeo), C:N,carbon:chlorophyll a (CrChla) and carbon:plant pigment (C:P1. Pig) ratios for the experimentalalgae
Species Size Chla Chla:Phaeo C:N C:Chla C:P1. Pig. nOunESD) (pgcelT1)
n — number of determinations.- = no observations.° = H.C.Eilentsen, unpublished.
a 25 1 nonfiltering cod-end. On deck the copepods were immediately dilutedwith surface water and put into black plastic bags. The bags were stored in acontainer with running surface sea water while being transported to thelaboratory (3 h). In the laboratory the samples were further diluted by gentlytransferring them to 100 1 PVC containers filled with 50 u.m screened seawater.The containers were placed in a temperature-controlled room (4°C) in dim light.Every day —50% of the water volume was replaced with prescreened sea water,and the copepods were fed a mixture of the three alga species. The copepodswere used for experiments within 3 days of collection.
Gut pigment experiments
The functional relationship between gut fullness and ambient food concentrationwas investigated in all copepodite stages of C.finmarchicus. Separate exper-imental series were performed on three algal species. T.nordenskioeldii,P.glacialis and colonies of P.pouchetii. The various copepodite stages weresorted out for each experimental bottle. The prosome length of copepodite stageI was 0.78 ± 0.045 mm, stage II 0.94 ± 0.058, stage III 1.37 ± 0.065, stage IV1.93 ± 0.104, stage V 2.57 ± 0.112 and adult females 2.67 ± 0.250. Each stagewas identified in subsamples and picked out with a Pasteur pipette under abinocular microscope (Wild M-5). Each group of animals was retained inbeakers with 25 ml filtered seawater on ice, until all animals for the experimentwere sorted out. This procedure lasted several hours. Each group was thentransferred to individual 550 ml Kilner jars containing algae at pread justedconcentrations. In order to keep the algae in suspension, the jars were eitherinverted every 15 min, or fixed on a slowly rotating wheel (1-2 r.p.m.). Theexperiments lasted for 3 h and were performed in dim light. Algal concen-trations were never reduced by > 10-20%.
The experiments were stopped by gently pouring the contents of each jarthrough a submerged plankton gauze filter. The copepodite stages I—III wererinsed with filtered seawater, checked under a dissecting microscope for anydead individuals, and concentrated on a plankton gauze for subsequent gut
pigment analysis. Copepodite stages IV and V and adult females wereindividually taken with forceps and placed in the extraction solvent. Furtherinformation concerning the procedure for measuring the gut plant pigment isgiven in Huntley et al. (1987). All gut contents are given in ng chlorophyllequivalents, which is the sum of chlorophyll a and phaeophytin a asrecommended by Wang and Conover (1986).
Gut clearance experiments
To obtain estimates of grazing, gut clearance experiments were performed oncopepodite stage IV C.finmarchicus fed either T.nordenskioeldii, P.glacialis orP.pouchetii size fraction >50 ptm ESD. Approximately 150 copepodites weresorted into a 2200 ml bottle with preadjusted algal concentrations, and wereallowed to graze for 3 h. The animals were directly transferred to filtered seawater, and the gastric clearance was determined by periodically (every 5 minduring the first hour) measuring the gut content by sampling the copepodpopulation during 1.5 h. The gut clearance rate was described by an exponentialequation and the rate constant (R) was determined (see Ki0rboe et al., 1982;Tande and Bamstedt, 1985; Ki0rboe and Tiselius, 1987). The ingestion (/) wascalculated as gut content (G) x R. The clearance (F) was calculated as //initialambient plant pigment concentration.
Particle clearance experiments
To obtain separate estimates of clearance and ingestion rates of copepodites,experiments were conducted to monitor the reduction of the particle concen-tration in the ambient medium. Stage IV copepodites were introduced into300 ml glass jars, with either P.glacialis or colonies of P.pouchetii (30-100 \vmand >100 urn ESD) at preadjusted concentrations. Four to five individuals wereadded to duplicate screw-cap bottles containing the different algal species. Theconcentration was recorded at the start and the end of the experiment bymeasuring the plant pigment concentrations. Two control bottles withoutcopepods were run simultaneously. The copepods were incubated for 24 h on aplankton wheel in dim light at 4°C. Clearance (F) and ingestion (I) werecalculated by the equations in Frost (1972) and fitted by the following formulas:
I = /mM e-uc and F = ( W C ) * " ^
where A: is a constant and C is ambient concentration of plant pigment. Datafrom these experiments are given in Figure 4.
Results
Copepodite stages fed T.nordenskioeldii attained higher amounts of gut contentscompared with copepodites fed P.pouchetii colonial size >50 nm ESD orP.glacialis (Figure 1). The differences tended to decrease with each increasingstage of development, where the gut plant pigment content in copepodite stage
Fig. 1. Calanus finmarchicus: the functional relationship between gut content and food concentrationin copepodite stages I-V fed either the diatoms T.nordenskioeldii (O), P.glacialis (A) or colonies ofP.pouchetii, >50 \im ESD (•) .
V varied from ~25 to 1 ng chlorophyll equivalents individual"1 (equiv. ind."1)at plant pigment concentrations <10 u.g I"1 of all three phytoplankton species.The gut content in the copepodite stages II-V were in the same order ofmagnitude when fed the large diatom P.glacialis or the colonial algaeP.pouchetii at the same range of plant pigment concentrations.
The relationship between gut content and food concentrations in copepoditestages of C.finmarchicus is described by:
where Gm^ = maximum gut fullness of plant pigments, k = a constant, C =initial plant pigment concentration. The fitted curves were significant (P < 0.05)for all situations, except copepodite stage II and P.glacialis (Table II). Thecurvilinear model illustrates the above functional relationship better than alinear (y = ax + b) model.
The algal concentrations at which maximum gut fullness (Gmax) was attainedwere in the range of 3-26 u,g plant pigment I"1, highest for copepodite stage IV.The maximum values of Gmax were found in copepodites fed T.nordenskioeldii,increasing from 2.2 ng chl. equiv. ind."1 in copepodite stage I, to 18.0 ng chl.equiv. ind."1 in copepodite stage IV. Except for copepodite stage II, the Gmax
values were slightly lower in copepodite stages fed colonies of P.pouchetii thanin those feeding on the larger diatom P.glacialis.
The instantaneous rate constant (R) of gut evacuation was estimated to be0.035 and 0.038 in copepodite stage IV fed T.nordenskioeldii or P.pouchetii>50 n-m ESD respectively (Figure 2). When measurements <15% of the initialgut fullness were excluded from the regression analysis (cf. Ki0rboe andTiselius, 1987), the probability level of the exponential model (F-test) did notincrease. A mean of these two estimates was adopted in the estimations offeeding rates.
Ingestion and clearance rates of copepodite stages C.finmarchicus have beencalculated from gut content, gut clearance rate and initial plant pigmentconcentration (Table II). It is obvious that ingestion is a reflection of gut fullnessof the various copepodite stages given in Figure 1. Calculated clearances forcopepodite stage IV (Figure 3) were based on a non-linear regression model.They were in the range 7.9-1.0, 2.2-0.5 and 1.8-1.0 ml ind."1 h"1 fedT.nordenskioeldii, P.glacialis and P.pouchetti >50 u.m ESD respectively, in thechlorophyll range 1-22 u,g plant pigment I"1. Maximum ingestions (/max) were45.6, 20.4 and 13.0 ng chl. equiv. ind."1 h"1 fed the same phytoplankton speciesrespectively.
The data obtained from 24 h incubation experiments indicate that ingestionand clearance in copepodite stage IV C.finmarchicus fed P.glacialis were bestdescribed by the curvilinear model (Figure 4). Ingestion increased withincreasing food concentrations, and reached a maximum rate of 4.4 ng chl.equiv. ind."1 h""1 at a plant pigment concentration of ~5 jig I"1. These rateswere in general lower compared with those obtained by the gut pigment method.Clearance was negatively related to food concentration, and decreased from 3 to0.5 ml ind."1 h"1 at mean plant pigment concentrations from 0.5-8 u.g I"1.Copepodite stage IV attained a maximum ingestion rate of ~5 ng chl. equiv.ind."1 h"1 at ~8 jig chla I"1 when fed colonies of P.pouchetti in the size range30-100 u,m ESD colonies, with maximum clearance of ~2 ml ind."1 h"1 at lowfood concentrations of both the small and large colonies offered.
Discussion
The relationship between gut content and algal concentration appears to
Fig. 2. Calanus finmarchicus: the gut clearance of plant pigment content in copepodite stage IV fed(A) Tnordenskioeldii and (B) P h t i i (>50 ESD) function of time. The relationship
d (B) 209 °°37' (^ 0 )
g fi(A) T.nordenskioeldii and5, = So e~Rl is calculatedrespectively.
g a n c e of plant pigment conten(B) P.pouchetii (>50 u.m ESD) as a functiofor (A) as 7.23 e'003^ {f- = 0.90) and (B)
relationshipas 2.09 e - °° 3 7 ' (^ = 0.88)
describe grazing in C.finmarchicus both on diatoms and on P.pouchetiiaccording to the curvilinear functional response curve. The present data indicatea saturation level which has been described previously in numerous studies (seefor example Parsons et al., 1967; Frost, 1972). However, some of the data areobtained at chlorophyll concentrations above those normally found duringbloom conditions (>15 p,g \~l) in the sea. Recent studies have documented thatan upper limiting threshold in feeding activity need not occur in boreal speciesfeeding on natural paniculate material in the range 1-15 p,g chl. I"1 (Conoverand Huntley, 1980; Huntley, 1981). In the present study, the functionalrelationship between gut content and food concentrations <10 txg chl. I"1 wasequally well described linearly. This suggests that the ingestion rate inC. finmarchicus could be directly proportional to the ambient food concentrationduring the most productive period in May and June in high latitude environ-ments irrespective of algal species present.
In the present study the variation in gut fullness among groups of individualsmay have been increased through possible endogenous rhythms in the feedingbehaviour. A substantial range in copepod feeding patterns in marineenvironments has been well documented in the published literature (Huntley,1988). The observed behavioural patterns indicate that continuous, intermittent,and diurnal feeding are operative among copepods during the productive periodin marine environments (Cowles and Strickler, 1983; Nicolajsen et al., 1983;
Fig. 3. Calanus finmarchicus: ingestion and clearance rates in copepodite stage IV fedT.nordenskioeldii, P.glacialis and P.pouchetii (>50 \im ESD). The data are calculated from gutcontent of plant pigment during 3 h feeding experiments, gut clearance and ambient plant pigmentconcentrations.
Baars and Oosterhuis, 1984; Simard et al., 1985; Mackas and Burns, 1986;Stearns, 1986). These sets of behavioural patterns are believed to be mediatedby both internal and external factors. For instance, the sibling species C.glacialisand C.hyperboreus appear to display diurnal feeding rhythms with the highestrates occurring at night during periods with night-time darkness in late summerand autumn in the Canadian Arctic (Head et al., 1985). Light intensity has beenfound to have a negative effect on feeding activity in calanoids (Head, 1986;Head and Harris, 1987; Stearns, 1986), but increasing hunger appears to reducethe sensitivity to light (Head and Harris, 1987). The present experiments wereconducted after the culmination of the spring phytoplankton bloom, during themidnight sun period with minimum differences between night and day levels ofincident light intensity. Thus C.fmmarchicus were preconditioned to continuousdim light for at least 24 h on low phytoplankton concentrations prior to theexperiments. To reduce the effect of satiation on feeding activity (Conover,
Fig. 4. Calanus finmarchicus: ingestion and clearance rates for copepodite stage IV fed P.glacialisand two size categories (30-100 and >100 \Ltn ESD) of P.pouchetii. The data are obtained bymeasuring differences in initial and final plant pigment concentration in the ambient medium during24 h incubation experiments.
Table HI. Calanus finmarchicus: ingestion and clearance rates of copepodite stage IV fed P.pouchetii30-100 (Jim ESD and >100 \x.m ESD respectively, and P.glacialis (see text for further details)
1968; Pearre, 1973, 1979) the duration of the feeding experiments with the gutfluorescence method in the present study was standardized to 3 h, well above thetime needed for calanoids to fill their guts (Head and Harris, 1987).
A comparison of clearance and ingestion in copepodite stage IV C.fin-marchicus from the two experimental procedures shows the same trends, but notidentical rates (Figures 3 and 4). The gut pigment method generally yieldedhigher ingestion rates. Clearance obtained when fed P.pouchetii >50 n-m ESDin gut pigment experiments is comparable with clearance obtained duringincubation experiments both with colonies in size fraction 30-100 (xm ESD andin >100 p.m ESD at comparable chlorophyll concentrations. These experimentsindicate maximum clearance rates of ~2 ml ind."1 h"1. It must be emphasizedthat the two series of experiments were conducted with P.pouchetii in differentyears and had different chlorophyll a:phaeophytin a ratios. The large diatomP.glacialis was grazed by copepodite stage IV at rates comparable to those foundfor P.pouchetii in the same range of chlorophyll concentrations.
The differences in the actual rates of ingestion and clearance obtained by thegut fluorescence method and the incubation method in copepodite stage IVC.finmarchicus fed either P.pouchetii or P.glacialis could be related to variousfactors. The effect of hunger is more likely to be detected by short-term feedingexperiments. Thus the short incubation time in these experiments may explainwhy they yielded higher feeding rates. Head (1986) found that daily estimates ofingestion could vary considerably depending on the method adopted. Hourlyingestion and filtration of C.glacialis and C.hyperboreus measured by the 'gutfluorescence' method in short-term (<3 h) feeding experiments ranged from 45to 70% above rates obtained during long-term (30-60 h) incubation bottleexperiments (Head, 1986). Although pigment destruction during digestionwould underestimate algal ingestion in the gut fluorescence method (Conover etal., 1986; Ki0rboe and Tiselius, 1987), these two methods have recently provedto give comparable feeding estimates for calanoid copepods (Ki0rboe et al.,1985). Differences in digestibility appear not to mediate algal-specific differ-ences in copepod ingestion of diatoms and gelatinous phytoplankton, since theinstantaneous rate constant (R) was equal in copepodite stage IV fed eitherT.nordenskioeldii or P.pouchetii.
Based on the derived equations, clearance for copepodite stage IV C.fin-marchicus was estimated at chlorophyll concentrations of 5 \ng I"1. The gutfluorescence and incubation methods gave 1.7 and 0.7 ml ind."1 h"1 respec-tively, when fed colonies of P.pouchetii. The same trend is found for P.glacialiswhere clearances are estimated to be 1.7 and 0.9 ml ind."1 h"1 respectively. Theincubation experiments with P.pouchetii in 1988 were conducted with a differentstrain from that used in those conducted in 1989, when most of the gutfluorescence data were obtained. It is unlikely, however, that the abovedifferences are related to changes in algal physiology or to colony size, sinceeven larger discrepancies in clearance are found when the diatom P.glacialis,which has a more narrow size distribution, is offered as food. Thus, thediscrepancies in feeding activity yielded by these two methods are more likely tobe caused by differences in the duration of incubation than by any other source
of variation. However, which of these levels of clearance is closest to the in situfeeding rates of C.finmarchicus is still open to question.
The stage-specific differences in the feeding response to the algal speciessuggest that particle selection in Calanus s.l. appears to be determined to a largeextent by particle size (Frost, 1972, 1977; but see also Price, 1988). For Acartiatonsa maximum clearance was defined mainly by particle size (Berggreen et al.,1988), and an optimum in particle size spectra was found to increase withdevelopmental stage. Evidence that particle selection in copepods could bemodified by the composition of available food has been found for Paracalanusparvus (Paffenhofer, 1984a,b) and for Pseudodiaptomus marinus (Uye andKasahara, 1983). The colony size category (>50 jim ESD) of P.pouchetii waspresent in a large variety of shapes and sizes. The largest differences in feedingwere found among copepodite stages I—III when fed colonies of P.pouchetii andthe diatom T.nordenskioeldii. Since this difference is not apparent in copepoditestages IV and V, particle size 5*50 n-m ESD is probably on the upper end of theparticle size spectra for copepodite stages I—III C.finmarchicus. However, thehigher carbon:chlorophyll ratio in P.pouchetii compared with that in T.norden-skioeldii reduces the differences in carbon intake of copepodite stages (TableIV). When fed colonies of P.pouchetii both copepodite stages IV and V consumeequivalent or higher amounts of carbon compared with the diatom T.norden-skioeldii.
This study emphasizes that, when single algal species are offered, diatoms aremore readily consumed than the larger size fraction among colonies ofP.pouchetii by copepodite stages I—III C.finmarchicus. Unpalatability of colonyalgae has been advanced as an explanation of the large variety of observationsconcerning the trophic fate of this phytoplankton (Schnack, 1983; Tande andBamstedt, 1987; Huntley et al., 1987). Suspension-feeding copepods in thenorth-west African upwelling are found to graze readily on solitary cells fromdisintegrated colonies of T.partheneia, while healthy colonies were rejected(Schnack et al., 1985). In the present study, C.finmarchicus were offeredexponentially growing colonies of P.pouchetii, but in a large variety of colonyshapes. Recently, ingestion of colonies or solitary cells of P.pouchetii was foundnot to sustain the metabolic demand in female Acartia spp. from NarragansettBay, while nauplii of A.hudsonica grew well on diets of solitary cells (Verity and
Table IV. Calanus finmarchicus: calculated carbon ingestion of copepodite stages fed T.norden-skioeldii and P.pouchetti at plant pigment concentrations of 10 (ig I"1
Smayda, 1989). These results could have been explained equally well as an effectof size, where both solitary cells and colonies of P.pouchetii were outside theparticle size range which adult Acartia could graze.
The present study demonstrates that C.finmarchicus graze both diatoms andcolonies of P.pouchetii at similar rates given a proper prey size. Despite thedifferences in clearance, copepodite stages IV and V appear to obtain carbonratios in the same order of magnitude when fed T.nordenskioeldii, P.glacialisand various size categories of P.pouchetii. In the coastal areas of northernNorway and in the Barents Sea, P.pouchetii and diatoms coexist during thespring bloom (Eilertsen et al., 1981). During the culmination period inMay and June, P.pouchetii prevails with diatom cell numbers generally <0.2 x106 F 1 (Tande, 1988; K.Unstad and K.S.Tande, unpublished data). Activelygrowing P.pouchetii are found as regular globular colonies, whereas the increasein the proportion of irregular, disintegrating colonies is associated more withperiods of culmination of blooms (H.C.Eilertsen, personal communication). Inthe feeding experiments with C.finmarchicus, P.pouchetii was present both asregular and irregular colonies. The experiments do not answer questions relatedto selective grazing on diatoms, or whether differences in palatability of 'new'and senescent colonies exists. Nevertheless, these aspects are important tomediate the observed dynamic changes in the phytoplankton community inspring and early summer. The most intensive growth period in C.finmarchicusappears to coincide, at least in certain regions, with a nearly monospecificphytoplankton community with only P.pouchetii at superabundant concen-trations. Since copepodites of C.finmarchicus are found to graze on the largercolonies at progressively higher rates with increasing stages of development, thenutritional value of P.pouchetii emerges as a cue for a more elaborateunderstanding of the productivity aspect related to these components in high-latitude environments.
Acknowledgements
We would like to thank the staff at the Marine Biological Station, University ofTroms0 for their support during the experiments. We also wish to thankT.Ki0rboe for his comments on the manuscript. Ms H.Falkseth and G.Granaasare acknowledged for their help with the figures. This research was supported byThe Norwegian Fishery Research Council through PRO MARE, and TheNordic Council for Marine Science.
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Received on January 12, 1990; accepted on June 25, 1990
