Europ.J. Protistol. 27, 249-263 (1991)September 9, 1991

European Journal of

PROTISTOLOGY

Short-Term Variations of the Abundance andBiomass of Planktonic Ciliates in a Eutrophic Lake

Ielesphore Sime-Ngando and Hans Julian Hartmann'Laboratoire de Zoologie et Protistologie, Universite Blaise Pascal deClermont-Ferrand II, Aubiere, France

SUMMARY

Short-time spatio-temporal variations of planktonic ciliates in a eutrophic lake were examinedfor evidence of diel vertical migration in relation to food supply (bacteria, nanoplankton, anddetritus) and physico-chemical parameters. Two campaigns were conducted during successivesummers in Lake Aydat, France.

Ciliates were less abundant during the first campaign (July 1988; global means 1500 cells/Iand 61.0 f.tgC/I) than during the second (July 1989; 5000 cells/I; 110.8 f.tgC/I). On both dates,ciliate abundance decreased from surface to depth, while biomass increased. Each layer(epilimnion, metalimnion, hypolimnion) was inhabited by a separate ciliate community,generally dominated by bacterivores/detritivores.

There was no clear evidence of vertical migrations across major physical-chemical boundar­ies. Circadian variations in each layer occurred independently of light-dark rhythms, partly dueto micropatchiness. Temporal variability increased from the vertically mixed epilimnion(C.Y. = 32%-41%) to the well stratified deeper zones (C.Y. = 41%-100%).

Ciliate biomass was negatively and significantly correlated to temperature and dissolvedoxygen, and to most principal food parameters. The vertical differences in ciliate biomass werelinked to the long-term (seasonal) evolution of the lake system (correlations with temperature,oxygen) and its larger-sized food resources (correlations with chlorophyll, nanoplankton,detritus). By contrast, circadian variations of ciliates were linked to availability of bacteria, animportant food resource for many ciliate species of this study.

Introduction

Seasonal studies of ciliates in freshwater lakes havedemonstrated spatial and temporal correlations betweenpopulations of pelagic ciliates (i.e. changes in speciescomposition, cell counts, and biomass) and their potentialfood sources, primarily nanoplanktonic algae [10,37,38]and bacteria [55, 56, 63]. These relationships tend to bemost pronounced in eutrophic and hypereutrophic lakes,especially during summer. Ciliates shunt energy andmatter from the basic trophic web (i.e. the microbial loop)to higher levels[9,61,66], and turnover rates are relativelyfast. Furthermore, ciliates are motile, chemosensory andphotosensitive [7, 25]. Since their potential predators

1 Present address: School of Fisheries, University of Washing­ton, Seattle, USA

© 1991 by Gustav Fischer Verlag, Stuttgart

migrate vertically on a diel cycle[e.g. 14,72], and quantityand metabolism of their prey, such as algae [4,30,31] andpelagic bacteria [39, 48, 59] show spatial and circadianchanges, we hypothesized that correlations between cil­iates and their food sources should occur not just on aseasonal, but also on a diel scale. However, except forstudies done by Dale [19] and Heinbokel [43] in the marineenvironment, researchers have not yet looked at corre­sponding short-term variations of microzooplanktonicciliates. Finlay et al. [27] have monitored, over a 24-hourperiod, the vertical distribution of ciliates in a smallproductive pond (Priest pot, Cumbria, U.K.). However,this study only concerned two specieswith algal symbionts(Frontonia vernalis and Euplotes daidaleos).

The aim of the present study was to investigate thesmall-scale temporal and spatial (vertical) variations ofabundance and biomass of pelagic ciliates in relation to

0932-4739/91/0027-0249$3.50/0

Table 1. Morphometric characteristics of Lake Aydat [53]

250 . T. Sime-Ngando and H. J. Hartmann

their potential food and to physical factors in a eutrophiclake. The work was carried out in Lake Aydat, whereseasonal variations of the pelagic bacteria [51,52], algae[2,3] and metazoan zooplankton [50] have recently beeninvestigated.

Research was done during two summers (1988, 1989)when Lake Aydat was stratified, has an anoxic hypolim­nion, and develops a high concentration of detritus likeother lakes of similar trophy [10].

AltitudeSurfaceVolumeMaximum depth (Zm)Mean Depth (Z)Z/ZmSurface of the catch basin

825 m60.3 . 103/m1

4.14 . 106/m3

15.50 m7.40 m0.4830·106/m1

Material and Methods

Site and Sampling

Lake Aydat (Table 1) is a small eutrophic lake in the French"Massif Central". Samples were taken at the deepest point of thelake, during two day-night cycles:

- 20th-22nd]uly, 1988, with 7 sets of samples taken over 48 hat uneven intervals, i.e. at 10 h (20th), 10 h, 13 h, 16 h, 22 h(21st), and 04 h, 07 h (22nd); and

- 20th-21st]uly, 1989, with 9 sets of samples taken over 24 hat 3 h intervals, i.e. at 10 h, 13 h, 16 h, 19 h, 22 h (20th), and01 h, 04 h, 07 h, 10 h (21st).

For the first series of samples the levels of sampling wereintegrated over depth-ranges (0-4 m, 5-9 m, and 10-14 rn); forthe second series, samples were taken at fixed depths (2, 7, and14 m). These depths are situated in the epilimnion (0-4 m or2 m), metalimnion with oxycline (4-7 m or 7 m), and anoxichypolimnion (10-14 m or 14 m). For the first series, epilimneticsamples were collected by using a flexible plastic tube (diameter =3 em), provided with a rope connecting the ballasted bottom ofthe tube with the surface. With this system, water samples arecollected (in triplicate) by simple capillarity. This technique ofsampling is rapid, easy, and inexpensive (Hartmann, unpublisheddata). The metalimnetic (5-9 m) and hypolimnetic (10-14 m)samples are composite samples of an equal amount of watersampled with a Van-Dorn bottle at 1 m intervals.

On board of the boat, samples were immediately subsampled: 2to 3 I were stored in insulated dark boxes for live ciliate countingand for chlorophyll a (particles retained on a 1.2 urn meshglassfiber filter, measured according to S.C.O.R.-U.N.E.S.C.O.[65]) filtration in the laboratory. To preserve ciliates, 200 ml werefixed with mercuric chloride (final concentration 2.5% v/v) andstained with a drop of 0.04% v/v bromophenol blue [11, 56,69].One 30 ml sample was fixed for bacterial and nanoplanktoncounting (buffered, alkaline formaldehyde, final concentration2% v/v, from commercial formaldehyde 37%), another wastreated with nalidixic acid for determining viable bacteria [47]living in the oxygenated depths.

Other parameters (temperature; dissolved oxygen; pH duringthe second series only) were measured with automated in situprobes, and, in the case of Aydat 1, averaged over the appropriatedepth ranges.

Numerical Abundance and Biomass of the Ciliates

For the first series (Aydat 1), ciliates were counted by twomethods: in vivo counting, after concentration by passive filtra­tion, according to Sime-Ngando et al. [69], and fixed counting, byUtermohl's [69] inverted microscope method, following a 24 hsedimentation period of 25 to 100 ml subsamples. The twomethods gave comparable results with minor qualitative and

quantitative differences [69]. Hence, in the 2nd series (Aydat 2),only fixed samples were counted, since there was not sufficienttime to count all samples in vivo immediately within the 3 hsampling intervals. All fixed samples were counted within 1month, because of significant loss occurring during longer storage[70].At least 200 cells of each representative taxon were countedin one or, if necessary, several subsamples.

Ciliate biomasses were calculated from the mean cell volume ofeach taxon (Table 2) which was determined by measuring celldimension with a micrometer and approximating the cell shapesto geometrical figures. The dry weight biomass was obtained withthe conversion factor of Gates et al. [33], i.e, 1 Itm3 = 0.279 pg.The carbon biomass was calculated according to Finlay [24],assuming that carbon represents 47.1 % of the dry weight.

Ciliates were identified either in vivo under a phase-contrastmicroscope, or by various cytological techniques. The infracilia­ture was observed by protargol staining after Bodian [13], asmodified by Tuffrau [76] and Croliere [36]. This technique alsoreveals the nuclear apparatus and the excretory pore of thecontractile vacuole.

Bacteria, Nanoplankton and Detritus

Before counting, 2.5 to 10 ml subsamples were filter-collectedand treated with 4'6'-diamino-2-phenylindole (DAPI), accordingto Porter and Feig [60]. For each set of filtered subsamples, ablank was prepared. Observations were made with an Olympusepifluorescence (model HB 2) microscope (neofluar objectivelens, 100/1.25). Bacteria were counted and sized in 15 to 35 fields.At least 1000 cells were counted per subsample and their shapewas noted (c.f, Sime-Ngando et al. [68]). Free and attached cellswere counted separately. Biovolumes were calculated accordingto ]assby [45], and transformed into carbon biomass withBratbak's conversion factor [16], i.e. 1 Itm3 = 5.6 x 10-13 gc.

The use of DAPI permitted us to distinguish cyanobacteria,phototrophic nanoplanktonic cells (2-20 urn in size, [67]), anddetritus particles. With UVexcitation (365 nm), the DNA-DAPIcomplex fluoresces blue (wavelength> 390 nm) which allows tocount total bacteria and nanoplankton [17, 18, 60], while thedetritus fluoresces pale yellow [60, 75]. With blue excitation(390-450 nm), the cyanobacteria and the phototrophic nano­planktonic cells auto fluoresce yellow and red, respectively, due tophycoerythrin and chlorophyll a [17, 75]. The numerical differ­ence between total cells (U.V.) and those with fluorescentpigments (blue light) was considered as heterotrophic cells[75].

We also determined the frequency of dividing cells (FDC), arelative measure of bacterial production [40]. From the secondbacterial subsample taken at oxygenated depths (i.e. epilimnionand metalimnion), viable bacteria were determined and countedaccording to the method of Kogure et al. [47].

At least 300 items of cyanobacteria, viable oxybacteria,dividing cells, nanoplankton and detritus were counted persubsample.

Short-Term Varia tions of Plankton ic Ciliates · 251

Results

Physical and Chemical Data

During both campaigns, the water column of the lakewas well stratified. No deep wind-driven vertical mixingoccurred, since the weather was calm and sunny. Thewater mass of the lake was war mer in 1989, due to morestab le summer weather during the second year (Fig. 1).Temperatures in the epilimnion fluctuated around18 ± 2 °C in Aydat 1 and 21 ± 1 °C in Aydat 2 (Fig. 1); inthe hypolimnion, temperat ures were below 8°C. Due tonight-time cooling (d. Fig. lA, D), the epilimnionappeared to be well mixed through convective circulation .The deeper layers did not appear to be influenced byconvective mixing.

The hypolimnion was anoxic, as typically record ed inLake Aydat during summer [e.g. 1, 52]. In the epilimnion ,the dissolved oxygen concentration fluctuated between 9.3and 10.1 mg/I during Aydat 1, and 7.9 and 12.6 mg/Iduring Aydat 2; the concentration declined rapidly in themetalimnion (to < 1.5 mg/I) and was undetectable in thehypolimnion. Hydrogen sulfide, detected by smell, occur­red below 10m during both campaigns. Oxygen levels inthe epilimnion declined dur ing the night, up to early

morning (07 h) during both campaigns, parallel with anight-time drop in pH (during Aydat 2), in relation withrespiration processes.

The pH during Aydat 2 was alkaline in the epilimnion(7.4- 9.9), acidic to basic in the metalimnion (6.1- 8.2),and remained mostly acidic in the hypolimnion (e.g. above7.0 only during a 6 h period from 19 h to 1 h, Fig. 1C).The acidic pH in the hypolimnion routinely occurred withthe presence of hydrogen sulfide, indicating a prevalence ofbiological and chemical reduction processes [58]. The pHwas a very sensitive circadian parameter. A daylightincrease in all layers was followed by a night-time decline.The range between extremes was more pronounced in thesurface, due to photosynth esis.

More chlorophyll a (Fig. 2) was in the water columnduring Aydat 2 (23.0 ~g/I weighted, integrated mean),compared to Aydat 1 (4.4 ~g/I weighted mean) . Thisdifference primarily occurred in the epilimnion duringAydat 2, where the average concent ration was nearly9-fold higher (62.6 ~g/l) than during Aydat 1 (7.2 ug/l), Inthe deeper layers, the mean values were comparable, i.e.4.8 and 3.4 ~g!l in the metalimnion and hypolimnion ofAydat 2, versus 4.3 and 1.7 ~tg/I in Aydat 1. The chloro­phyll a levels showed highly variable circadian patterns(Fig. 2). During Aydat 1, the chloroph yll a concentrat ion

AYDAT 1 (JULY 1988) AYDAT 2 (JULY 1989)

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Fig. 1. Spatia-temporal variations of temperature,dissolvedoxygen, and pH.ThepHwas notmeasuredduring thecampaign Aydat 1.

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252 . T. Sime-Ngando and H. J. Hartmann

AYDAT I (JU LY 19 88 ) AYDAT 2 (JULY 1989)

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DAY: HOUR

varied relatively little over 48 h in the upper two layers(Figs.2A, B).Bycontrast, in the hypolimnion, a night-timeincrease (from zs 1 to > 3 ug/l)was seen (Fig. 2C). DuringAydat 2, a large night-time peak occurred in the epilim­nion (Fig. 2D), while in the two deeper layers, the peakswere delayed until early morning (Figs. 2E, F).

Horizontal Patchiness

The effect of horizontal heterogeneity on ciliate countswas investigated during Aydat 1. For this, nine sampleswere taken at 2 m depth around a 5 m radius of thesampling site. Ciliate counts from those samples were donein triplicate. Variation was relatively low (mean =3.9 ± 0.8 x 103 cells/l, C.V. = 21%), ranging from 3.7 to4.9 x 103 cells/l, Pairwise comparisons between the sam­ples revealed no significant differences of counts betweensampling points. We concluded that the individual samplestaken during the campaigns represented horizontalpatches of at least 10-20 m diameter. The patch sizeexceeded the range within the boat drifted around thesampling site.

Ciliate Species Composition

We classified the ciliate taxa by feeding preference intocategories proposed by Pratt and Cairns [62] (Table 2).

Bacteria- and detritus feeders (B) prevailed (34 taxa); theywere dominated by scuticociliates, which also representedthe greatest number of speciesper group (6 taxa), as well asby Urotricha sp., Coleps hirtus, and Vorticella spp.Algivores (A) were also present (11 taxa), such as Fron­tonia spp., Stokesia vernalis, Strombidium and Strobilid­ium. This group also includes the genus Loxodes, accord­ing to Finlay and Berninger [26] and Finlay and Fenchel[29]. The non-selective omnivores (N) were almost exclu­sively represented by tintinnids (Tintinnidium fluviatileand Tintinnopsis lacustris) and were found only duringAydat 1.

Numerical Abundance and Biomass of Ciliates

(i) Global comparisons and vertical patterns. The abun­dance of ciliates was lower during Aydat 1, compared toAydat 2. However, during Aydat 1, the total ciliate densitywithin one layer fluctuated over 40 fold within 48 h,between 0.25 X 103 and 10.6 X 103 cells/l (in the meta­limnion, Fig. 3B). Since the extreme peak in the metalim­nion occurred during the first morning, and no othersamples were taken during that day, the first sampling isexcluded from comparisons of 24 h averages. Hence, the24 h mean cell density during Aydat 1 was 1.5 ± 1.3 X 103

cells/l.Mean abundance in the epilimnion (2.7 ± 1.1 X 103

Table 2. List of different groups [62] of planktonic ciliate taxacounted in the samples (Aydat 1 and 2). Greatest average lineardimensions and biovolumes of 20-30 preserved individuals.IAbsent in [62] classification; 2Absent at Aydat 2 sampling

90 47.765 140.0

100 170.080 100.040 28.380 100.064 55.860 110.030 14.150 65.435 22.442 14.551 13.7

300 935.3210 696.0

75 76.360 48.290 72.4

120 386.0150 754.0120 386.080 100.0

900 8550.0430 1380.0550 2120.0120 386.0105 170.021 4.212 0.940 100.025 8.132 4.845 8.070 32.3

120 850.0290 3170.0110 830.0171 2300.0310 2200.0230 1430.0124 99.770 87.920 4.230 7.145 15.3

300 1150.01500 2450.0140 194.068 54.7

Ciliate Taxa

Bacterivores-Detririvores (B)

Askenasia [aurei!Askenasia volvoxCaenomorpha sp.Chilodonella cucullulusChilodonella uncinataCodon ella sp.?Coleps hirtusCyclidium sp.Cyclidium spp.Dexiotricha plagiaDexiotricha spp.Drepanomonas sp.Epistylis rotans?Hemiophrys aselliHemiophrys fusidensLagynophrya rostrataMetacystis sp. 11

Metacystis sp. 21

Metopus sp.Metopus spiralisPlagiopyla nasutaPlagiopyla sp. 2Spirostomum ambiguumSpirostomum filumSpirostomum intermediumSpirostomum teresStichotricha aculeataUronema nigricansUronema spp.Urotricha saprophilaUrotricha sp.Vorticella sp. 1 (attached)Vorticella sp. 2 (colonial)Vorticella sp. 3 (free)

Algivores (A)

Frontonia atraFrontonia leucasFrontonia sp.Stokesia vernalisLoxodes magnusLoxodes rostrumLoxodes striatusStrobilidium gyransStrobilidium spp.Strombidium spp.Strombidium viride

Nonselective Omnivores (N)

Paruroleptus caudatusStentor coeruleusTintinnidium fluviatile2

Tintinnopsis lacustns-

Length(urn)

Biovolume(.103ftm3)

Short-Term Variations of Planktonic Ciliates' 253

cells/l) was about 1.5 times higher than in the metalimnion(1.1 ± 1.1 X 103 cells/I), and 4.5 higher than in thehypolimnion (0.6 ± 0.3 x 103 cells/l).

During Aydat 2, ciliate abundance was higher (weightedand integrated mean = 5.0 ± 2.8 X 103 cellsll), butaltogether ranged only about 20 fold, from 0.6 to12.7 X 103 cells/l. Ciliate density also declined with depthfrom a mean of 7.6 ± 2.4 X 103 cells/l (epilimnion) to4.7 ± 1.6 X 103 cells/l (metalimnion) and 2.2 ± 0.9 X 103

cells/l (hypolimnion).During both cycles, B-type ciliates dominated numeri­

cally and prevailed in all layers: 94, 81, and 63% (Aydat 1)and 53,92, and 97% (Aydat 2) of total cell counts in epi-,meta- and hypolimnion, respectively. N-type ciliates werepresent only during Aydat 1, particularly in the metalim­nion, while algivores were especially abundant in theepilimnion during Aydat 2.

Similar to numerical abundance, the total carbon bio­mass of ciliates (Fig. 4) was lower in Aydat 1, where thebiomass ranged over 20 fold between 7.1 and 200.7 ftgC/l,with a 24 h mean of 61.0 ± 53.3 ftgC/l. In Aydat 2,biomass ranged about 14 fold, from 22.7 to 306.6 ftgC/l,the weighted and integrated mean (110.8 ± 80.9 ftgCIl)being nearly double that of Aydat 1.

As opposed to cell abundance, ciliate biomass duringboth campaigns increased with depth (Fig. 4). It waslowest near the surface (means = 30.2 ± 25.0 ftgCIl and38.3 ± 10.6 ftgC/l for Aydat 1 and 2, respectively), peakedin the metalimnion (102.1 ± 70.8 ftgC/l and 168.2 ± 83.9ftgC/l) and declined at depth (50.8 ± 27.1 ftgCIl and126.0 ± 65.0 ftgC/l). B-type ciliates usually made up thebulk of the biomass, due to the dominance of scuticocil­iates and oligotrichs near the surface (79% and 80%B-type ciliate biomass for the epilimnia of Aydat 1 and 2,respectively), and the presence of larger B-type grazers(Coleps hirtus, Spirostomum spp., Table 2) at depth (81and 95% B-type ciliate biomasses in the hypolimnion). Bycontrast, algivores and tintinnids together dominated thebiomass in the metalimnion during Aydat 1 (i.e. only 38%B-type ciliates in Aydat 1, but 95% in Aydat 2).

(ii) Circadian variations. The short-term fluctuations ofcell abundance and total biomass of ciliates within eachlayer were considerable over the 24 h cycle. Occasionally,the 24 h extremes succeeded each other within less than6 h (e.g, Figs. 3 F, 4 A, 4 B, 4 F). Overall, these within­layer variations clearly exceeded the mean vertical varia­tions between layers, which were maximally 4.5 fold forabundance (Aydat 1), and 4.3 fold for biomass (Aydat 2).Differences between die! extremes were generally greaterduring Aydat 1, from 3-4 fold in the epilimnion to 5-6fold in the hypolimnion and 11-12 fold in the metalim­nion. During Aydat 2, the values in the epilimnion variedonly about 2-3 fold, were higher in the metalimnion(3-6 fold) and highest in the hypolimnion (6-8 fold), dueto very low counts in the 13 h sample (Figs. 3 F, 4 F).

The circadian patterns differed markedly betweenAydat 1 and Aydat 2. During Aydat 1, the ciliate abun­dance was higher during the day (water column mean =1.7 X 103 cells/l) than during the night (0.9 X 103 cells/l)

254 . T. Sime-Ngando and H. J. Hartmann

AYDAT I (JU LY 1988) AYDAT 2 (JU LY 1989)

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while the biomass was slightly highe r at night (59.6 [lgC/1and 63.7 [lgC/1 during day and night, respectively). DuringAydat 2, mean abundance and biomass were $ 20%lower during the day (0.4 x 103 cells/l and 108 .3 f.lgC/l),compared to night (0.5 X 103 cells/l and 131.1 [lgC/I). Thediffer ences between day and night were never signifi­cant.

Within each layer, during Aydat 1, ciliate abundance sdeclin ed rap idly during the afternoon (13 h or 16 h), thenincr eased during the night until the following morning(Fig. 3A, B, C). The ciliate biomasses in the epilimnion andhypolimnion followed a similar cycle. In the meralimnionho wever, their biom ass was extremely variable betweensuccessive samplings. Tintinnids and larger bacterivoresstrongly contributed to total biomass in three out of 6samplings, and no clear trend was evident.

During Aydat 2, the circadian patterns in the upper twolayers differed from Aydat 1. In both epilimnioh andmetal imnion, ciliate abundance and biom ass increased intwo phases (10- 16 h, 19-01 h) from morning until pastmidnight. The night-tim e peak in the epilimnion coincidedwith the chlorophyll a peak (d . Figs. 2D, 3 D, 4D). Duringlate night and the following morning, abundance andbiomass decl ined or leveled off, generally to a high er valuethan 24 h earlier. By contrast, in the hypolimnion abun-

dance and biomass declined from afternoon until midnightand then increased again during late night and earlymorning (d . Figs. 3 F, 4F). The steady increase in ciliatebiomass in the hypolimnion resembled that of Aydat 1, butbegan later during the night.

Numerical Abundance and Biomass of Bacteria

The abundance of bacteria was higher during Aydat 1,compared to Aydat 2 (Ta ble 3). During Aydat 1 (whereonly 4 sets of samples were analyzed), bacterial densityfluctu ated nearl y 4 fold within 48 h, between 0.96 X 106

and 3.69 X 106 cells/ml (Fig. 5 A). Similar to ciliates,bacterial abundance declined with depth, from a mean of3.0 3 ± 0.82 X 106 cells/ml (epilimnion), to 2.34 ±0.56 X106 cells/ml (metalimnion) and 1.54 ± 0.42 X 106 cells/ml(hypo limnion).

Bacterial abundance of Aydat 2 fluctu ated from0.99 X 106 to 3.28 X 106 cells/ml (Fig. 5B). Mean abun­dance was lowest in the metalimnion (1.41 ± 0.26 X 106

cells/nil), and nearly equally high in the epilimnion(2.43 ± 0.61 X 106 cells/ml) and hypolimnion (2.7 ± 0.41X 106 cells/ml).

During both cycles, free heterotrophic bacteria domi­nated (Table 3). They prevailed in all layers, i.e. 79 , 77,

Short-Term Variations of Planktonic Ciliates' 255

£PILIMNION

AVDAT 2 (JULV 1989)AVDAT 1 (1988)

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Fig. 4. Spatio-remporal variations of thebiomass ofdifferent groups ofciliates. The ciliates ofgroup(N)were absentduring thecampaign Aydat2. !1.I0 !I ;1j 21.16 !112 22Jl'o 22fJ1 !O.IO!o1j !OI6 !Olq !O!! ! I.o1 !U), 11.fJI 21.10

DAV : HOUR

and 91 % (Aydat 1) and 60, 79, and 89% (Aydat 2) ~n epi-,meta- and hypolimnion, respectively. Heterotrophic bac­teria altogether contributed 82 % and 84% ~uringAydat 1and Aydat 2, respectivel y, to total bacterIal. abundance(Table 3). Furthermore, the h.igher proportlo~s of thedividing (FDC), attached, and vlab.le~eterotrophlc bacter­ia in Aydat 2 (Table 3), seems to indicate that heterotro­phic bacteria were more active during Aydat 2.

Conversely to the abundance, the bacterial biomass waslower during Aydat 1 (by about 55 % ) than during Aydat 2(Table 3). It must be noted, however, that biomass ",:asextremely variable during Aydat 2. In Aydat 1, the bIO­mass in the water column fluctuated roughly 3 fold,between 54.6 and 168.3 /lgC/1 (Fig. 5 C), while in Aydat 2,it fluctuated about 27 fold, between 17.3 ~lgC/1 and 468.0/lgC/1 (Fig. 5D). Similar to the ~bundance, the bacterialbiomass declined with depth during Aydat 1 from a meanof 115.0 ± 41.6 /lgClI (epilimnion) to 95.3 ± 20.0 /lgClI(metalimnion) and 7004 ± 15.3 /lg~1l (hypolimnion): Dur­ing Aydat 2, mean biomass again was ~owest . III themetalimnion (126 .0 ± 88.6 /lgC/I), and higher III bothepilimnion (226.0 ± 109.7 /lgC/I) and hypolimnion(178.6 ± 80.1 /lgClI).

Abundance of Nanoplankton and Detritus

The abundance of the nanoplankton was lower duringAydat 1 (17.8 X 103 cells/ml) than Aydat 2 (33.3 X 103

cells/ml; Table 3). Phototrophic cells prevailed (Table 3),due to their relatively higher abundance in the upper layers88 62 and 25 % (Aydat 1), and 90, 72, and 60%(Aydat 2) in epilimnion, metalimnion, and hypolimnion,respectivel y. . .

Detritic particles were about 20 fold higher duringAydat 1 (58.7 X 103 particles/ml), compared to Aydat 2(2.9 X 103 particles/ml; Table 3). They were most abun­dant in the epilimnion and the metalimnion but fluctuatedconsid erably (Fig. 5 G, H). Picodetritus dominated, espe­cially during Aydat 1 where the 0.2-2 urn d.etritic particlesrelatively constituted 99% of the total detritus abundancein the water column (Table 3).

Relations between Ciliate Biomass and OtherParameters

Significant linear correlations between ciliate biomassand other parameters differed between Aydat 1 andAydat2 (Table 4). During Aydat 1, the biomass of ciliates

256 . T. Sime-Ngando and H. J. Hartmann

Table 3. Mean values and percentages compared to total (% Tot) of abundance and (or) biomass of microbial parameters and detriticparticles of the ~~ter~olumn, obtai~ed .during the Aydat 1 and Aydat 2 regimens. Viable bacteria have been sampled in oxygenatedwater layers (epilimnion and metalimnion) only. Means for Aydat 2 are weighted by depth-range

Aydat 1 Aydat 2Mean ± s.d. % Tot Mean ± s.d. % Tot

Bacterial Abundance (: 105 cells/ml)

Free heterotrophic bacteria 18.3 ± 6.0 79 14.3 ± 3.5 74Free cyanobacteria 3.6± 1.5 16 2.5± 2.1 13Attached heterotrophic bacteria 1.1± 0.3 4 1.9± 0.9 10Attached cyanobacteria 0.1 ± 0.1 1 0.6± 0.3 3Total 23.1 ± 6.0 100 19.3 ± 4.1 100

Viable heterotrophic oxybacteria 0.7± 0.7 3 4.6 ± 2.2 24Viable oxycyanobacteria 0.5 ± 0.3 2 0.6 ± 0.4 3

Bacterial Biomass (!!gC/l)

Free heterotrophic bacteria 70.5 ± 18.7 75 130.2 ± 78.0 76Free cyanobacteria 15.6 ± 6.5 17 19.5 ± 16.0 11Attached heterotrophic bacteria 5.8± 3.8 6 19.1 ± 11.9 11Attached cyanobacteria 1.7± 0.4 2 2.8± 3.0 2Total 93.6 ± 25.6 100 171.6 ± 93.2 100

Frequency of Dividing Cells (%)

Heterotrophic bacteria 1.7± 0.3 3.0± 1.2Cyanobacteria 0.2± 0.1 0.5 ± 0.4

Nanoplankton Abundance (x 103 cells/ml)

Heterotrophic nanoplankton 4.8 ± 1.5 27 7.8 ± 2.4 23Phototrophic nanophytoplankton 13.0 ± 4.8 73 25.5 ± 13.4 77Total 17.8 ± 5.4 100 33.3 ± 14.4 100

Detritus Abundance (x 103 particles/ml)

Picodetritus 58.2 ± 67.7 99 2.1 ± 1.8 72Nanodetritus 0.5± 0.4 1 0.8 ± 0.4 28Total 58.7± 67.6 100 2.9 ± 2.1 100

was closely and negatively correlated to the physico­chemical parameters and to chlorophyll a. Ciliate biomassalso was negatively correlated to phototrophic nanoplank­ton. No significant correlations existed with bacterialparameters, but nonselective ciliates were significantly andpositively correlated to the FDC of cyanobacteria (r =0.591, Table 4). No significant relation appeared betweenciliates and detritus.

During Aydat 2, all the significant correlations betweenthe ciliate biomass and other parameters found duringAydat 1 also appeared, except that the correlation betweenthe ciliate biomass and cyanobacterial FDC was inversedand total nanoplankton was not correlated. In addition,significant negative correlations existed between ciliatebiomass and most bacterial parameters, as well as withdetritus (Table 4). All correlations of Aydat 2 were due tothe bacterivorous-detritivorous (B) ciliates, since the algi­vorous ciliates (A) were not consistently present (d. Fig. 4,Table 4).

Discussion

The results illustrate that the abundance and biomass ofciliates can vary considerably over a 24 hour period withina single depth-zone. The coefficients of variation for the24 h means within each layer range between 28 and 100%(Table 5), which is considerably higher than spatial varia­tions observed around a single sampling point (C.V. =21%). In some cases, 24 h maxima succeeded minimawithin 3-6 hours. Some sharp increases in ciliate abun­dance and biomass would require community doublingtimes of < 3 h (e.g. Aydat 1 Fig.4E 10-16 h; Aydat 2Fig. 4F 13-16 h), which appears biologically unrealistic,even for rapidly growing ciliates [5]. Therefore, duringthose instances, ciliates must have been imported andexported through advection, even in the vertically inte­grated samples of Aydat 1.

In the epilimnion, abundances varied the least (e.g,lowest C.V.'s, Table 5) and had relatively small variations

Short-Ter m Variations of Planktonic Ciliates' 257

Aydat I (JULY 1988) AYDAT 2 (J ULY 1989 )ce 5....

A B! -+-

'"~ .. -£>--0 -~

i 3

I 2

-e

~~ 0.. 20:10 21:10 21:22 22:0i 20:10 20:13 20:1 6 20:19 20:.22 21:01 21:Oi 21m 21:10p 0,5e....~

0,"a~ 0,3~..-e 0 ,2~~ 0,1..p 0 ,0e....! 70

"'~0...; 56

i ..2

i 28z

i ... o--«

I0-- --0 ""0-

0

p 300 15 - -e G H....,; 2..0 12

t",II. 1800

...;

i 120

I 60 .

Fig. 5. Spatia -temporal variations of the bacter-~ 0

ial abundance and biomass, and of the abundanceof nanoplankton and detritus . § DAY : HOUR

between successive samples (Figs. 3A, D; 4A, D). General ­ly, due to the night- time cooling, the epilimnia in bothyears were relatively well mixed compared to the deeperzones. At depth , the strong physical stratification andchemical gradients of dissolved oxygen, pH, and otherparameters, such as HzS, redox potential, and Fe2+/Fe3+

(as recorde d by Philippe [58]) increased the pro babilities ofciliates forming tight aggregations within chemical micro­layers [6, 7,24,37]. These peaks in abundance may havebeen sampled (extremely high abundances) or missed (verylow abundances) during our sampling cycles.

Furthermore, in the well stratified metalimnion, physi­cal and chemical isolines can be expected to shift verticallyon a diel cycle and to be displaced vertically and horizon­tally with the passage of internal waves, observed to occurin Lake Aydat even during calm weather [49]. Comparisonof temperature and ciliate data of Aydat 2 suggest internaloscillation on the order of 6-9 h (cf Figs. 1E, 3E, 4E).

Mor eover, in the meta limnion, there was a significantpositive corr elation of total ciliate abundance with tem­perature (r = 0.741, df = 9, p :::; 0.05). The periods ofoscillation are consistent with observations from othersmall lakes (e.g. [54]).

Thu s, the variable patterns of change within each layergive no clear evidence for large-distance diel vertica lmigrations of the entire ciliate population across majorchemical or physical boundaries. Nor did an examinationof individual taxa show evidence of vertical migration.

Furthermore, we found that vertical sample-integration,as done for Aydat 1, did not dampen between-samp levariations within a layer. Rather, the integrated samples ofAydat 1 had greater coefficients of variation than thepoint- samp lesof Aydat 2 (Table 5). Moreover, in Aydat 1,there was a drop in ciliate abundance in the metalimnionduring the first 24 hours, followed by strong oscillationsdue to the periodic appearance of tintinnids. Besides

258 . T. Sime-Ngando and H. J. Hartmann

Table 4. Coefficients of correlation (r) between the biomass of different groups of ciliates (abbreviations same as in Table 2), andphysico-chemical, microbial, nanoplanktonic, and detritic parameters, during both studies

Ciliate Biomass Aydat 1 Aydat 2B A N Total B A Total

Physico-Chemical Data

Temperature -0.678* -0.961* -0.686* -0.973* -0.742* 0.249 -0.741*Dissolved oxygen -0.664* -0.959* -0.695* -0.968* -0.753* 0.268 -0.751 *pH nd nd nd nd -0.186 -0.093 -0.195Chlorophyll a -0.761 * -0.771" -0.553 -0.927* -0.677* 0.121 -0.682*

Bacterial Biomass

Free heterotrophic bacteria 0.309 O.oll 0.116 0.269 -0.235 -0.169 -0.249Free cyanobacteria 0.107 0.135 0.137 0.165 -0.431* 0.046 -0.435*Attached heterotrophic bacteria -0.135 -0.173 -0.052 -0.144 -0.584* 0.282 -0.577*Attached cyanobacteria 0.331 0.091 0.061 0.231 -0.483* 0.119 -0.484*Total bacteria 0.206 0.021 0.109 0.199 -0.471* -0.041 -0.481*

Viable Oxybacteria

Viable heterotrophic oxybacteria 0.185 0.576 0.185 0.311 -0.519* 0.127 -0.521 *Viable cyanobacteria 0.041 0.666 0.411 0.343 -0.178 -0.209 -0.194Viable total bacteria 0.146 0.611 0.251 0.324 -0.544* 0.111 -0.548*

Frequency of Dividing cells

Heterotrophic bacteria 0.435 -0.163 -0.369 0.057 -0.194 0.129 -0.189Cyanobacteria 0.066 0.447 0.591* 0.439 -0.411* -0.061 -0.422*Total bacteria 0.484 -0.031 -0.203 0.201 -0.407* 0.108 -0.407*

Nanoplankton Abundance

Heterotrophic nanoplankton 0.086 0.404 0.318 0.291 0.136 0.345 0.161Phototrophic nanoplankton -0.411 -0.751* -0.637* -0.727* -0.433* 0.131 -0.432*Total nanoplankton -0.376 -0.676* -0.577* -0.668* -0.294 0.316 -0.281

Detritus Abundance

Picodetritus 0.043 -0.159 -0.038 -O.ol8 -0.416* -0.011 -0.424*Nanodetritus -0.147 -0.231 -0.148 -0.214 -0.475* -0.029 -0.485*Total detritus 0.041 -0.161 -0.039 -0.019 -0.438* -O.ol5 -0.447*

Aydat 1: df = 10, r = 0.576* for p :0=:; 0.05 (for viable oxybacteria df = 6, r = 0.707* for p :0=:; 0.05); no data collected forpH. Aydat 2: df = 25, r = 0.381* for p :0=:; 0.05 (for viable oxybacteria df = 16, r = 0.468* for p =s 0.05).

Table 5. Mean values of the diel coefficientof variations (± X %)of ciliate abundance and biomass during the Aydat 1 and Aydat 2regimens

Epilimnion 41 83 32 28Metalimnion 100 69 34 50Hypolimnion 50 53 41 52Integrated mean 87 87 56 74(Watercolumn)

vertical/horizontal advection, the initial decline is difficultto explain in the absence of other samplings during the first24 h, and it was excluded from calculations of means. Ourresults thus leave open to debate which sampling method(integrated - Aydat 1 versus fixed depth - Aydat 2) better

Ciliates Aydat 1Abundance Biomass

Aydat 2Abundance Biomass

represents ciliate abundance and biomass in a smallstratified lake. Only samplings at very short depth inter­vals (0.5 m) and time intervals (0.5-2 h) can eventuallyresolve this question.

The mean densities of the ciliates which we observedduring the two die! cycles, 1.9 x 103 cells/l (Aydat 1) and5.0 x 103 cells/l (Aydat 2), were lowerthan those reportedby Beaver and Crisman [8] for eutrophic lakes: 18 to71 x 103 cellsJ1 during an annual cycle. These authors notehowever, together with others [15, 57, 71], that thedensities observed in subtropical lakes (Florida) wherethey worked are higher than those in temperate lakes of thesame trophic state. Besides, our study concerned only two24 h periods and the result cannot be directly comparedwith investigations involving seasonal and annualcycles.

In both years, the densities of ciliates were higher in theepi- and metalimnion than in the hypolimnion (Fig. 3);

Short-Term Variations of Planktonic Ciliates' 259

Table 6. Ratios of means of bacterial biomass (Bac/C), chlorophyll a (ChI/C), nanoplankton abundance (Nan/C) and detritus (Detlc)abundance to ciliate biomass (C) as a function of depth zone

Depth Zone BadC (f.lgC/f.lgC) Chl/C (f.lg/f.lgC) Nan/C DetiC(numbers x 106/f.lgC) (numbers x 1Q6/f.lgC)

A1 A2 A1 A2 A1 A2 A1 A2

Epilimnion 3.00 5.90 0.24 1.63 1.06 1.24 3.88 0.13Metalimnion 0.93 0.75 0.04 0.03 0.15 0.11 0.57 0.02Hypolimnion 1.39 1.41 0.03 0.03 0.11 0.31 0.02 0.01

A 1 = Aydat 1, A 2 = Aydat 2.

inversely, their biomass was greater in the two lower layers(Fig. 4). This type of vertical pattern of the abundanceversus biomass of ciliates, already observed by Pace [55] ina lake with a trophic state and morphometric featuresresembling those of the Lake Aydat, can be explained bythe numerical prevalence of small-sized ciliates, essentiallyscuticociliates and small oligotrichs « 50 urn, Table 2), inthe euphotic zone of the lake. These two groups of ciliatesformed 65% of the total abundance of ciliates in the epi­and metalimnion. In the metalimnion, higher abundancesof large-sized ciliates, essentially Loxodes spp., Spirosto­mum spp., and Coleps hirtus were responsible for theincrease of the total ciliate biomass. In the hypolimnion,the intermediate-sized Metopus spiralis, Plagiopyla nasu­ta, and Caenomorpha sp. dominated the ciliate biomass,together with Cyclidium spp. and Uronema spp.Undoubtedly, the spatial distribution of some of thespecies (e.g. Loxodes, Frontonia) is partly linked to thedissolved oxygen concentration gradient [6, 7, 27, 37].

The abundance and biomass of the ciliates, as well aschlorophyll and other microbial parameters tended to behigher during Aydat 2 (d. Figs. 2, 3, 4, 5; Table 3).Apparently, the planktonic communities were at differentstages of development, which could partly be explained byatmospheric conditions. In 1989, the summer (e.g. June­July) was warmer and the weather more stable than in1988. Water temperatures were higher throughout thewater column, and there was less night-time cooling (d.Figs. lA, 1B). The water column of July 1988, as well asquantities of chlorophyll a and biomasses of bacteriaactually resembled those of mid-June 1989, when the lakewas in a spring-summer transition phase (recorded byAleya [1]), while our values of July 1989 were very similarto Aleya's values recorded during the same month. Thegreater abundance and biomass of algivorous ciliates (e.g.Stokesia, oligotrichs) in the epilimnion and the absence oftintinnids during Aydat 2 provide further indirect evidencethat the lake microplankton community was at a season­ally more advanced stage in Aydat 2 than in Aydat 1.Tintinnids, which develop optimally at lower temperatures(10-12 "C) [34,64] generally reach peak abundances inLake Aydat during the colder and unstable periods ofspring and late autumn (Sime Ngando, unpublished data).Their waning presence during Aydat 1 indicates that lakeconditions preceding July 1988 had been less stable thanthose preceding July 1989.

The concentrations of bacteria, being of the order of 106

cells/ml (Fig. 5), are near those reported by Marvalin et al.[52] for Lake Aydat in 1986, and by Pace [55] in othereutrophic lakes. The relative importance, in carbon bio­mass, of the phycoerythrin-containing bacteria (16% ofthe total bacterial biomass for the two cycles, d. Table 3)was considerable, although lower than that (20%)reported for marine picocyanobacteria [46]. The concen­trations of nanoplanktonic cells (of the order of 10L104

cells/ml, Fig. 5) were mainly formed by the phototrophicorganisms (dominated by blue-green algae [2, 51, 52])which constituted, on average for the two periods, 75% ofthe total abundance of the nanoplankton, near thosereported by Caron [17]. The higher concentrations of thephototrophic nanoplankton in Aydat 2 as compared toAydat 1 (Table 3) contributes partly to the much higherconcentrations of chlorophyll a during Aydat 2, especiallyin the epilimnion (Fig. 20).

The concentrations of these potential food sourcesappeared to be sufficiently high to support the observedpresence of ciliates [23,28, 66]. In particular, we noted inboth years an excess of bacterial over ciliate carbon in theepilimnion, but more equal proportions of both in thedeeper layers (Table 6). Other potential food sources(chlorophyll a, nanoplankton, detritus) were also highestin the epilimnion. Apparently, the small ciliates in theepilimnion were unable to fully exploit the availableresources. Three explanations are plausible: unsuitabilityof food, restrictions of ciliates due to competition by smallmetazoans (e.g., rotifers, small cladocera) or due topredation.

The first hypothesis could account for the excess ofepilimnetic chlorophyll a (Table 6). Indeed, from earlysummer on, many large-sized and colonial phytoplanktonspecies prevail in the epilimnion of Lake Aydat (e.g.,Aphanizomenon (los-aquae, Microcystis aeroginosa,Gomphosphaeria lacustris, Coelastrum microporum,Staurastrum pingue, and Fragilaria crotonensis [1, 52]),which are unsuitable food species for most small ciliates.However, it would not explain the relative excesses ofnanoplankton or detritus in the epilimnion (Table 6).

The second explanation, competition, also appearsinsufficient, since it would require much more reducedconcentrations of the nanoplankton and bacteria foodresources, which are most suitable to all microzooplankt­ers. Finally, predation appears to be the most likely factor.

260 . T. Sime-Ngando and H. J. Hartmann

It has been shown to influence fluctuations of pelagicciliate populations [12, 71]. Potential predators during thesummer in Lake Aydat, e.g. the calanoid copepod Acan­thodiaptomus denticornis, which readily prey on small andmedium-sized ciliates [42] are largely restricted to oxygen­ated water, e.g. they are most active in the epilimnion andmetalimnion [22]. They effectively feed on ciliates atconcentrations even below 10 !!gC/1 in the presence ofother food (Hartmann, unpublished data) and have thusthe capacity to control ciliate populations. Predation maythus be an important factor explaining the inability ofepilimnetic ciliates to optimally take advantage of availa­ble food resources in the epilimnion.

The strong negative correlations between ciliate bio­masses and temperature and dissolved oxygen essentiallyreflect the result of a summer-adaptation of the ciliatepopulation to the vertical stratification of the watercolumns. Biomasses tended to increase vertically withdepth while temperature and oxygen levels decreased.Within-layer correlations were found to be insignificant,with few exceptions, such as the positive correlationbetween temperature and ciliate abundance in the meta­limnion noted earlier. Since we nored earlier some distinctdifferences in the ciliate community composition of eachlayer, this result confirms our observation that in astrongly stratified water column, the planktonic ciliates donot migrate vertically over large distances on a dielcycle.

The significant negative correlations obtained betweenciliate biomass and most of the potential food sourcesindicate the possibility of rather strong trophic coupling ofpelagic ciliates on several types of food, including phyto­plankton and bacteria, as already noted by other authors[10, 28, 38, 44, 55]. During Aydat 1, the relationshipswere restricted to indicators of autotrophs, includingphytoplankton (chlorophyll a, phototrophic nanoplank­ton), while during Aydat 2, bacteria and detritus were alsoincluded. Similar correlations with detritus have not beenreported previously in the literature.

The apparently broader food base of ciliates in Aydat 2(extending to bacteria and detritus) may be explained bythe relatively more advanced community status. Thebacterial cells of Aydat 1 were smaller, there were fewerattached cells and a lower proportion of dividing cells(Table 3). The size of bacteria and the proportion ofattached cells have been shown to increase with theseasonal maturation of Lake Aydar's nanoplankton com­munity (52]. It is conceivable that the larger, attachedbacteria provided a more suitable prey for ciliates living inthe seasonally more mature plankton community ofAydat 2 than the smaller, free-living bacteria of Aydat 1.The ciliates of Aydat 2 were thus exploiting a larger varietyof food sources, feeding not only on free bacteria but alsoon bacteria degrading dead algae [55], and on detritus.This illustrates the great plasticity of feeding preference inciliates observed by several other authors [10, 20, 21].

In Aydat 2, there were no significant within-layercorrelations between ciliate biomasses and chlorophyll a,nanoplankton, and detritus. Those negative correlationsthus reflect, similar to temperature and oxygen, the

long-term evolution and vertical stratification of the ciliateand nanoplankton communities, i.e. an increase of ciliatebiomass from epilimnion to hypolimnion with depth (e.g.,Fig. 4) coincided with decreases of nanoplankton anddetritus (Table 6). Due to an insufficient number of pointsof comparison (2 degrees of freedom), no conclusions weredrawn on within-layer correlations between ciliates andfood resources of Aydat 1.

By contrast, the significant negative correlations ofciliate biomass with bacteria in Aydat 2 apparently derivedfrom circadian variations. The biomasses of total ciliatesand total bacteria were comparable in the meta limnionand hypolimnion, and differed by less than one order ofmagnitude in the epilimnion (e.g., Table 6). Within-layercorrelations were negative in all three layers, and signifi­cant in the hypolimnion (r = -0.303, -0.301, -0.752* (p~ 0.05 for df = 7J for epilimnion, metalimnion, andhypolimnion, respectively). This is especially apparentthrough a coupling of the night-time peak and subsequentdecline of bacterial biomass in the hypolimnion (Fig. SF)with a simultaneous increase of ciliate biomass (doublingtime = 5.3 h; Fig. 4F). Since bacterial abundance did notdecline during the same period (Fig.5b), the inversecoupling illustrates a rapid intrinsic growth of ciliates, e.g.[5], feeding selectively on larger bacterioplankton cells[35J.

Moreover, despite the noise from physical variations, itappears obvious that daytime declines in bacterial biomassand abundance (Fig. 5B, D, 13 h-22 h) coincided withincreases of ciliate biomass (Fig. 4D, E). It is thus likelythat a fairly close link existed between available bacterialfood and observed ciliate population growth during the24 h period of Aydat 2.

In conclusion, it is clear that circadian variations ofpelagic ciliates in a stratified eutrophic lake vary indepen­dently in different layers, due to the absence of major dielvertical migrations. The vertical differences are essentiallylinked to the seasonal evolution of the lake system (e.g.towards a strong physical and chemical stratification) andare inversely related with the larger-sized food resources(phytoplankton, nanoplankton, detritus).

Circadian variations of ciliates within a stratum varyindependently of day-night cycles, due to physical advec­tion and microdisplacements of ciliates that could not beaccounted for during this study. A much tighter samplingprogram than ours would be required to better observegeneral patterns of diel changes, and comparisons wouldbe needed from exclosure experiments [e.g. 41]. Neverthe­less, ciliates appear to quickly be able to take advantage ofand influence bacterial biomass in seasonally evolvedsystems (e.g. Aydat 2) in the absence of predation (e.g. inthe hypolimnion).

Clearly, our study was inadequate to distinguish allfactors relating to circadian variations of pelagic ciliates.Thus, it leaves unanswered the nature of competition bysmall, rapid-growing metazoans, such as rotifers, and therole of a variety of sources of predation (41, 74], as well asthe role of ciliates in the recycling of organic matter andnutrients [9, 73]. Nevertheless, we believe that short-termin situ models of study constitute a rather promising

approach to the elucidation of the functional role of theciliates in aquatic ecosystems, a role far greater thangenerally assumed because of the high metabolic activity ofthese organisms related to their small size relative to themacrozooplankton [5, 73].

Acknowledgements

We thank Dr. C. A. Groliere for help with identification ofciliates, and Dr. I. B. Raikov and an anonymous reviewer forhelpful suggestions to improve the manuscript.

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Key words: Planktonic ciliates - Bacteria - Nanoplankton - Detritus - Circadian cycle

Telesphore Sime-Ngando, Laboratoire de Zoologie et Protistologie, Universite Blaise Pascal de Clermont-Ferrand II, U.A. CNRS 138,63177 Aubiere Cedex, France