Hydrobiologia 161: 75-87 (1988)R. . Jones and V. Ilmavirta (eds.). Flagellates in Freshwater Ecosystems© Kluwer Academic Publishers

Vertical distribution and diel migration of flagellated phytoplankton in asmall humic lake

R. I. Jones

75

Department of Human Sciences (Environmental Biology Unit), Loughborough University of Technology,Leicestershire LEII 3TU, England

Key words: flagellated phytoplankton, vertical distribution, diel migration

Abstract

The vertical distributions and migrations are described of the most abundant flagellated phytoplankton speciesfrom the summer community of a small forest lake in southern Finland. The lake showed a steep and stablethermal stratification with a shallow oxygenated epilimnion. Horizontal variation of phytoplankton distribu-tion within the lake was tested on two scales and found to be statistically significant only in the case of Mallomo-nas reginae. The vertical distribution of flagellated phytoplankton was assessed by reference to the distributionof a non-motile, neutrally buoyant species Ankyrajudayi. Statistically significant, active vertical positioningwas demonstrated for all the flagellates examined with the exception of Spiniferomonas bourrellyi. Diel verticalmigrations were apparent for all species showing active positioning and the pattern of an evening descent anda morning ascent was ubiquitous. The extent and timing of diel migrations varied between species. The mostextensive migrations were by Cryptomonas marssonii which crossed a temperature gradient of 14°C andpenetrated far into the anoxic hypolimnion. Several categories of competitive advantage can be gained by spe-cies undertaking such diel vertical migrations.

Introduction

Individuals within a phytoplankton community arefree to move in both the horizontal and verticalplanes. Such movement may be passive, the result ofbeing suspended in a turbulent, fluid medium. How-ever, some organisms also possess the capacity to in-fluence their spatial location, in which case their ac-tive movements are superimposed upon theunderlying passive distribution. As understandingof the functional ecology of phytoplankton has de-veloped, it has become increasingly necessary to takeaccount of the spatial distribution of different spe-cies as well as of the whole community (e.g. Platt &Denman, 1980; Reynolds, 1984).

Considerable attention has rightly been directedtowards phytoplankton spatial heterogeneity in the

horizontal plane. Knowledge about horizontal spa-tial variability is necessary for the design of effectivewhole-lake sampling programmes which aim to esti-mate the average phytoplankton density (Irish &Clarke, 1984). Horizontal 'patchiness' is known tooccur on several scales and a variety of statisticaltechniques have been used to detect it (Reynolds,1984). Except in small water bodies, where edge ef-fects may become pronouced, this variability isprobably more a function of passive, than of active,movement of phytoplankton.

Studies on the vertical distribution of phytoplank-ton include those concerned with populations oc-cupying fixed, but restricted, depth intervals (e.g.Klemer, 1976; Fee, 1976). However, of equal or great-er interest are the observations that certain speciesperiodically change their vertical distribution, fre-

76

quently with a diel periodicity. Such diel verticalmigrations have been reported for decades (see Rey-nolds, 1984) and may involve changes in cell buoyan-cy (e.g. Ganf & Oliver, 1982) but more usually con-cern actively motile, flagellated phytoplankton (e.g.Nauwerck, 1963; Soeder, 1976; Happey-Wood, 1976;Heaney, 1976; Frempong, 1981.

Vertical migrations may be a direct response to achanging environment (e.g. Heaney & Talling, 1980)or be affected by endogenous rhythms (Sournia,1974). In either case, the ability to select favourableconditions can clearly confer a considerable com-petitive advantage on motile algae, especially inmarkedly heterogeneous environments. Possible ad-vantages to be gained include selection of optimal ir-radiance conditions (e.g. Tilzer, 1973; Ilmavirta,1974), retrieval of nutrients from below the depth ofwater circulation (Salonen et al., 1984b) and reduc-tion of loses due to grazing and hydraulic washout.Better understanding of these behavioural responseswill be necessary for the improvement of currentmodels to explain phytoplankton community com-position (Reynolds, 1980).

The study of vertical migrations within mixedpopulations of phytoplankton invariably involvessampling several depths down a water columnrepeatedly over selected time intervals, followed bymicroscopic enumeration of individual species. Avariety of graphical plots have been used to representthe temporal sequence of the resulting cell density-depth profiles. Interpretation of such plots is con-founded by the possibility that changes in verticaldistribution may result from horizontal patchinessand water movements as well as from any be-havioural response of the phytoplankton. Unfor-tunately, decisions as to whether a particular set ofobservations indicates active migration or not havemainly been subjective, with little attempt to assessthe statistical significance of any changes; a notableexception is the more careful analysis by Happey-Wood (1976). While subjective analysis may lead tocorrect interpretation of the more dramatic exam-ples of migrations, some form of objective distinc-tion between active migrations and other mechan-isms of vertical redistribution is clearly needed. Thispaper explores some possible approaches to thisproblem using data for phytoplankton distribution

in Lake Nimet6n, a small, steeply stratified, humic,forest lake in southern Finland.

The study site

Samples were collected during summer 1982 fromLake Nimet6n (Fig. 1), a small dystrophic lake in theEvo district of southern Finland. The sheltered posi-tion of the lake together with its overdeepened mor-phometry (surface area 0.4 ha, max. depth 11 m)minimizes turbulent mixing and at the time of thestudy, shortly after clear-cutting of the catchment,the lake was apparently still meromictic (Salonen etal., 1984a). The high humic content of the water isindicated by the water colour (c. 300 mg Pt 1-1 inthe surface layers) and restricts the euphotic zone tolittle more than 1 m (Jones & Arvola, 1984). The lakeis mesotrophic with summer phlrtoplankton domi-nated by chrysophytes and cryptophytes. Further de-

AFig. 1. Bathymetric map of Lake Nimet6n showing the locationof the sampling sites.

77

tails of the seasonal characteristics of the lake andits plankton are given by Arvola (1983) and Salonenet al. (1983).

Methods

On 16 July 1982, between 10.00 and 11.00, a seriesof samples was collected from around the lake inorder to assess the horizontal variability of thephytoplankton. The position of the 10 sampling sitesis shown in Fig. 1. Samples of the outflowing water(site A) were taken from the overflow of a V-notchweir directly into a polythene bucket. At the lakesites integrated samples of the upper 2 m were col-lected with a perspex tube of 5 cm diameter. Fromeach site, 5 such primary samples were collected, andat sites F and J subsamples of 100 ml were reservedfrom each primary sample. At all sites 500 ml por-tions from each primary sample were mixed togetherto give a bulked sample of 2.5 1, and a 100 ml sub-sample was then reserved from each bulked sample.Particular care was taken to thoroughly mix samplesprior to any subsampling. All the 100 ml subsampleswere preserved with Lugol's iodine.

The vertical distribution of the phytoplanktonwas investigated by sampling at site F (Fig. 1) in earlyAugust. Previous information indicated that themain changes in vertical distribution patterns in thisand similar small forest lakes could be expectedaround sunset and sunrise (Arvola, 1984; Salonen etal., 1984b). On 2 August samples were collected at17.00, 19.00, 21.00 and 23.00 and on 4 August at03.00, 05.00, 07.00 and 09.00. On each occasiondetailed depth profiles were obtained using a close-interval syringe sampler (Blakar, 1979) operatedpneumatically from a boat at 2-3 m distance fromthe point of sampling. The sampler was fitted with50 ml syringes located at 10 cm intervals along a to-tal depth interval of 1 m. TWo sets of samples thuscovered the upper 2 m of the water column. Addi-tional samples from 2.5 and 3 m were obtained witha I I Patalas-type sampler. All samples werepreserved immediately with Lugol's iodine.

For enumeration of phytoplankton, between 10and 50 ml of thoroughly mixed sample wassedimented for at least 24 hours in a 2.6 cm di-

ameter, compound chamber (Leitz Instruments).Cells of selected, more abundant species were thencounted in 100 randomly selected fields of view ata magnification of x 400 using a Zeiss IM35 invertedmicroscope with phase contrast illumination.Counting of random fields rather than patternedtransects is strongly recommended when usingsedimentation chambers with an inverted micro-scope (Sandgren & Robinson, 1984). Although someof the species in this study tended towards a conta-gious distribution on the bottom of the sedimenta-tion chamber, an adequate estimate of the mean celldensity within the chamber was still obtained by theuse of a large number (n > 30) of replicate fields ofview. In this case the central-limit theorem is applica-ble (Elliott, 1977) permitting use of the normal ap-proximation; counts of any species from a singlesample generally showed a standard error of10-15% of the mean. Since the variance of countswas found to be strongly dependent of their means,a square root transformation was applied prior toany parametric statistical tests; the adequacy of thistransformation was confirmed graphically and byBartlett's test for homogeneity of variances (Sokal& Rohlf, 1981).

Temperature and oxygen profiles for the watercolumn were obtained with a combined temperatureand oxygen probe (Yellow Springs Instruments). Thetime course of irradiance increase at sunrise wasmonitored above and below the water surface on 4August using an underwater sensor (Lamda Instru-ments, Model Li2190) responsive to photosyntheti-cally active radiation (PAR) between 400 and700 nm.

Results

During the period of study the lake was stronglystratified (Fig. 2) and the vertical stratificationshowed very little variation through July and Au-gust. The main source of turbulence in the upperwater layers appears to be from convectional mixinginduced by night-time cooling at the surface. Themaximum and minimum temperatures obtainedfrom continuous monitoring equipment show thatthe effect of such mixing was negligible by I m

- 1000

500

a)

04 06 08 10

Time

1

Fig. 2. Depth profiles of temperature ( - ) and dissolved oxy-gen ( o -- o ) in Lake Nimeton on 4 August 1982 at 06.00. Horizon-tal bars show maximum and minimum temperatures on that date.

(Fig. 2). Oxygen concentration decreased rapidly be-low 0.5 m to complete anoxia below 3 m (Fig. 2).

The increase in incident irradiance after 05.00(Fig. 3a) occurred at sunrise, while the more sub-stantial increase seen from 08.00 occurred when thesun rose above the surrounding forest so that directsunlight was then incident on the lake surface. Irradi-ance changes at different depths (Fig. 3b) reflect theincrease in incident irradiance and the rapid attenua-tion of light by the highly coloured water. Less than0.1% of subsurface irradiance penetrated to 2 m.

Counts of phytoplankton in the integrated 0 - 2 msamples from 16 July (Table 1) showed the commu-nity to be dominated by flagellated species. The onlyrelatively abundant non-flagellate was the chloro-phyte, Ankyrajudayi (G. M. Smith) Fott. The com-munity was composed mainly of chrysophytes (Mal-lomonas akrokomos Ruttner, Mallomonas reginae

Fig. 3. Surface incident irradiance (a) and isopleths of under-water irradiance (b) as AE m-2 s-I in Lake Nimet6n during earlymorning on 4 August 1982.

Teiling and Spiniferomonas bourrellyi Takahashi)and cryptophytes (Chryptomonas marssonii Skujaand C. ovata Ehrenberg). One flagellated chloro-phyte (Chlamydomonas sp.) was also counted and inAugust also the colonial, flagellated chrysophyte,Dinobryon divergens Imhof. Uroglena americanaCalkins was also a substantial part of thephytoplankton biomass, but the large colonies werenot sampled effectively and disintegrated duringpreservation, so were not counted for this study.Other species were present in only very low numbers.

Table 1. Densities (cells ml- ) in the upper 2 m water column of the seven most abundant phytoplankton species at the ten sites sampledon 16 July. Values are means of three replicate counts from five bulked samples.

Site A B C D E F G H I J

Ankyra judayi 24 12 13 29 11 19 19 13 16 13Chlamydomonas sp. 11 13 20 4 11 8 24 16 17 15Cryptomonas marssonii 733 227 213 231 139 117 231 195 305 175Cryptomonas ovata 4 197 121 201 193 138 144 172 195 199Mallomonas akrokomos 262 1003 1061 629 707 903 829 648 553 845Mallomonas reginae 88 73 151 19 47 145 213 199 217 141Spiniferomonas bourrellyi 37 32 56 41 28 49 55 53 23 17

78

S 10 mg 02110

0.

0 i

;

79

Horizontal variability in phytoplankton density

For sites F and J, two replicate counts were made oneach of the preserved five primary samples. ANO-VAR revealed no significant (p > 0.05) variation be-tween the primary samples for any of the species.Partitioning the variance into its components (Sokal& Rohlf, 1981) showed the 'counting' variation to begenerally equal to or greater than the 'sampling' var-iation. Hence the procedure adopted of bulking theprimary samples had little effect in this particularcase.

Variation between the 10 sites (Table 1) was alsotested by ANOVAR for each species using the repli-cate counts of the bulked samples. No significant be-tween site variation was found for Ankyra judayi,Chlamydomonas sp., Mallomonas akrokomos orSpiniferomonas bourrellyi. For the three specieswhich did show significant (p < 0.05) between sitevariation, the differences were further investigatedby calculating the minimum significant difference(MSD) for p < 0.05 (Sokal & Rohlf, 1981). Crypto-monas ovata showed significantly lower density atthe outflow (site A) but no difference between theother sites; this is explained by this species avoidingthe surface water (see later) and therefore being un-der-represented in the outflow samples. Conversely,Cryptomonas marssonii showed significantly higherdensity at site A, being concentrated in the surfacewaters of the lake at the time of sampling and there-fore becoming over-represented in the outflow sam-ples. Mallomonas reginae was the only species whichshowed significant variation in density between thenine lake sites. This species showed a distinct gra-dient of increasing abundance from SW to NEacross the lake.

Vertical distribution of phytoplankton

The vertical distribution of the seven most abundantphytoplankton species at each interval during thetwo sampling periods in August is shown in Fig. 4.Of these seven species, only Ankyra judayi is notflagellated. This species can be considered approxi-mately neutrally buoyant (Reynolds, 1984) and dur-ing this study was effectively restricted to the upper

I m of the water column (Fig. 4) which correspondsto the maximum depth of mixing (Fig. 2). The distri-bution of this species is evidently the product of theinteraction between any tendency of the cells to sinkand the limited water circulation. Therefore it can beused as a reference distribution against which toevaluate the distributions of the other species which,being flagellated, all possess the potential for somedegree of self determination in their vertical distri-butions.

If possible changes in vertical distributionsthrough time are to be detected, it is essential thataccount is taken of any variation in the total popula-tion of cells in the water column. The vertical pro-files of cell density for each species (Fig. 4) wereplanimetrically integrated to yield values for the to-tal population (cells m - 2) within the 0-3 m watercolumn at each sampling interval. For each samplingperiod the coefficient of variation (CV) was then cal-culated (Table 2). Variation is produced by: counting'error'; real population changes (cell division, graz-ing etc.); passive horizontal movement; activehorizontal movement; and loss to, or recruitmentfrom, the unsampled water column below 3 m. Sincelosses from sedimentation below 3 m were clearlynegligible, only the first three factors will be applica-ble to non-motile species (Ankyra judayi) or weakswimmers restricted to the epilimnion (e.g.Spiniferomonas bourrellyi). Moreover, it has alreadybeen shown that horizontal variability within thelake was small during the period of study. Thereforethe much higher CV associated with certain species,particularly C. marssonii (Table 2), confirm the im-pression gained from inspection of the vertical pro-files (Fig. 4) that some species were not restricted tothe 0- 3 M water column. In view of this it was notconsidered appropriate to express the density of cellsat any depth as a percentage of the total populationin the water column in order to standardize the depthprofiles from each sampling interval (cf. Happey-Wood, 1976).

In order to compare the shapes of the profiles, thecounts for each depth were ranked within each sam-pling interval. The four sets of ranks from each sam-pling period were then subjected to Friedman'smethod for randomised blocks (Sokal & Rohlf, 1981)to test for the effect of depth on cell density for each

2.V111.1982

17.00 1900 2t0 2300

Ankyra judayi

760 736 504

00 600 0 800 0 600

r~~~~~ I 2

646 348 S190

0 600 0 800 0 6000

1

2

222 254 292

2as

Cryptomonas marssonii0 800 0 800

I0 00 0 600

2 4 420

4.VII1.1982

0300 05oo 07.00 0900

100JpK)

280 352 344

0 600 0 800 0 600

274 S0o 632

0 600 0 800 0 800

368 392 404

Dinobryon divergens0a IB a 1OOn

2

- 42 84

0 1600

82

0 160

I

L7a

species. The resulting X2 values and their level ofsignificance are shown in Table 3. Also shown arevalues of W, Kendall's coefficient of concordance(Sokal & Rohlf, 1981) which indicates the degree ofsimilarity between the four profiles from each sam-pling period on a scale from 0 (no similarity) to 1 (ex-act similarity). It can be seen that, as expected fromexamination of the profiles, the effect of depth on

1600 0 1600 1600 0 1600I

320 408 464

C

168

the distribution of Ankyrajudayi was highly signifi-cant (p < 0.001) and the distributions showed aremarkably high degree of similarity over the sam-pling intervals within each sampling period. Sincethe distribution of Ankyra is assumed to be passive,it follows that any species whose distribution showedno significant effect of depth was not passively dis-tributed and must therefore have been influencing its

80

0 1600 0 1600 0 1600I'I

E

.

a

0F - -' i

3

F V

2.VII1.1982

17.00 1900 2100

0

2

0 o00 0 oo

rl asL

r

98

0

72

0

0 600 0 G00 0 6000

I

°i

3 2

216 200 156

23.00 03.00

Spiniferomonas bourrellyiI 0 0 000

F- 7 '

10W 4 2

Chlamydomona

000

120

4.VIII.1982

ooo 0 07.00 000

300 a am a a0

r

40

r

40 40

a p.oo 0 6 0 o a

I I

196 260 212 40

Mallomonas reginae0 00 0 oo00 0 0o 0 oo00 00 0 00 0 0 00

50 476 378 320 16 216 240 236

Fig. 4. Depth profiles of densities (cells ml-') of seven selected phytoplankton species at the four sampling intervals during each sampleperiod. The integrated cell content of the 0-3 m water column (106 cells m- 2 ) is shown for each profile.

Table 2. Coefficient of variation, determined from the foursampling intervals on each of the two sample periods, for the in-tegrated number of cells within the 0- 3 m water column of theseven most abundant phytoplankton species.

CV %/O

2 August 4 August

Ankyra judayi 22 11Chlamydomonas sp. 25 44Cryptomonas marssonii 57 53Dinobryon divergens 28 38Mallomonas akrokomos 13 6Mallomonas reginae 25 11Spiniferomonas bourrellyi 16 3

own distribution pattern. Several species showed adeparture from a passive distribution in this manneron one sampling period (Table 3) and hence also alow similarity between profiles over the sampling in-tervals.

Of course, it does not follow that those specieswhich did demonstrate a significant effect of depthon vertical distribution were necessarily also passive-ly distributed; they could be showing a significantdepth effect produced in a different manner to thatfor Ankyra. This possibility was tested for each spe-cies which had shown a significant depth effect, bysumming the ranks at each depth over the four sam-pling intervals. These summed ranks were then cor-

81

--- -r'

31 ._ - ._

------ - Y

82

Table 3. Results from each sampling period for the most abundant phytoplankton species of tests for the effect of depth on verticaldistribution (Friedmans method, x2), the similarity between repeated vertical profiles (Kendall's coefficient of concordance, W) and thesimilarity between profiles for each species and those of the 'reference' species Ankyrajudayi (correlation coefficient, r). Significancelevels are shown as * (p<0.05), ** (p<0.01) and *** (p<0.001).

2 August 4 August

X2 W r x2 W r

Ankyra judayi 82.9*** 0.942 82.2*** 0.934

Chlamydomonas sp. 39.6* 0.450 0.048 60.5*** 0.688 0.800***Cryptomonas marssonii 51.4*** 0.584 -0.811*** 24.4 0.277Dinobryon divergens 14.3 0.162 40.0** 0.454 0.704***Mallomonas akrokomos 69.1*** 0.785 0.547* 26.0 0.295Mallomonas reginae 51.8*** 0.588 -0.171 34.7* 0.394 -0.710***Spiniferomonas bourrellyi 67.8*** 0.771 0.741*** 60.4*** 0.686 0.539**

related with those for Ankyra from the appropriatesampling period. For those species which showed asignificant positive correlation with Ankyra it is notpossible to reject the null hypothesis that their verti-cal distribution may most easily be described as apassive one.

Distribution of individual species

Ankyra judayi

The distribution of this small, non-motile specieswas used as a reference for assessing the distribu-tions of the other, flagellated species. Although thedistribution of Ankyra may be assumed passive, itwas certainly not random. No significant horizontalvariability was detectable on either of the scales test-ed (replicate samples at single sites and betweensites). The vertical distribution showed a highly sig-nificant effect of depth (Table 3) with the cells con-centrated in the upper 1 m. The maximum densityof cells was consistently found between 0.6 and0.8 m, where the temperature-associated metalim-netic density gradient was maximal (Fig. 2). Fewcells were ever detected below a depth of 1 m. Theseresults suggest that the sinking rate of this small algawas very low; slow sinking through the upper waterlayers was largely arrested at the metalimnion andthe resulting concentration of cells at that depth wasnot effectively redispersed by the limited (mainlyconvective) mixing generated in the shallow epilim-

nion. Hence the two series of successive depth pro-files both gave values for the coefficient of concor-dance which were only slightly below exactsimilarity. This distribution pattern for Ankyra inLake Nimet6n is a more extreme form of the patternreported for this species from large enclosures inBlelham Tarn by Reynolds & Wiseman (1982).

Spiniferomonas bourrellyi

This small flagellate consistently exhibited a verticaldistribution which differed little from that ofAnkyra (Fig. 4, Table 3). With a volume of onlyaround 80 /tm

3 and several long, slender spines, thespherical cells probably have a rate of sinking littledifferent from that of Ankyra. Certainly Spinifero-monas did not show the same tendency to accumu-late in the metalimnion (Fig. 4). On 2 August thehighest density of cells was found at, or 0.1 m below,the surface while on 4 August the cells were evenlydistributed within the shallow epilimnion. It appearsthat this species has limited swimming capacity, butsufficient to counter any tendency to sink andperhaps also to achieve small scale orientation inwater columns with only weak turbulence.

Chlamydomonas sp.

Vertical profiles of Chlamydomonas showed a sig-nificant depth effect (Table 3). However, on 2 Au-

83

gust the profiles showed no correlation to those ofAnkyra and also gave a low coefficient of concor-dance. At the start of the sequence the cells were con-centrated near the surface and thereafter becameprogressively more evenly distributed to 3 m or be-low (Fig. 4). No downward moving peak of cell den-sity was evident, suggesting that the movement ofcells was only loosely synchronised. On 4 August thesequence of profiles showed a high correlation tothat of Ankyra with a similar tendency towards ametalimnetic maximum. In view of the greatly in-creased total number of cells in the final profile(Fig. 4) the apparent upward redistribution must betreated with caution. Nevertheless, this species ap-pears to have been undertaking active, but weaklysynchronised, vertical migrations with downwardmovement in the evening and upward movementthrough the morning.

Cryptomonas marssonii

On 2 August the depth distribution of this speciesshowed a significant depth effect (Table 3) but thestrong negative correlation with the distribution ofAnkyra and the rather low coefficient of concor-dance show the observed pattern to be the result ofactive orientation. At 17.00 the bulk of the cells werealready between I and 2 m and continued downwardmovement is shown in the depth profiles (Fig. 4) andfrom the decline in total cell numbers in the 0- 3 mwater column. A large proportion of the populationevidently moved below 3 m during the night. A re-verse sequence was evident in the series of profilesfrom the morning of 4 August, with the ascent hav-ing apparently just started at 03.00 and being almostcomplete by 09.00. The migrations of this speciesshowed a greater degree of synchrony than was ap-parent for Chlamydomonas.

The magnitude of the migrations undertaken byC. marssonii was surprising. The movement of cellsfrom near the surface to below 3 m took them acrossa temperature gradient of 14 °C (Fig. 2). This is farin excess of most previous reports (see Kamykowski& Zentara, 1977), although migrations acrossequivalent temperature gradients have recently beenreported from another lake in the Evo district (Arvo-

la, 1984). Moreover, the cells clearly descended todepths at which no oxygen was present and presuma-bly remained there for several hours. This contrastswith the consistent avoidance of the anoxic hypolim-nion by Ceratium hirundinella observed by Heaney& Telling (1980).

There was some indication that Cryptomonasovata was following a similar migration pattern toC. marssonii, but cell counts were too low at this timeto permit further analysis. The pattern of migrationof Cryptomonas at this time in Lake Nimeton,descending around sunset and ascending aroundsunrise, is the same as that reported previously forseveral species of Cryptomonas from a variety oflakes (Soeder, 1967; Happey-Wood, 1976; Burns &Rosa, 1980; Frempong, 1981; Arvola, 1984) and maybe a widespread behavioural trait of this genus.

Dinobryon divergens

Like Ankyra, this species was effectively confined tothe upper I m water layer (Fig. 4). Despite this, nosignificant effect of depth was demonstrated on 2August (Table 3) due to the constantly changingshape of the profiles. The very low coefficient ofconcordance for this sequence of profiles reflects thediscrete peak of cells moving progressively downthrough the water column (Fig. 4). On 4 August asignificant depth effect was indicated as well as sig-nificant correlation with the distribution of Ankyra(Table 3), although the rather low coefficient of con-cordance is indicative of some changes in the depthdistribution. The observed sequences can be inter-preted as sinking during the evening and some activemovement in the morning to locate more favourableconditions for photosynthesis.

Mallomonas akrokomos

On 2 August the vertical distribution of this speciesshowed a significant effect of depth, significantpositive correlation with that of Ankyra and amoderately high coefficient of concordance (Ta-ble 3) and consequently may be considered primari-ly passive. However, on 4 August no significant

84

depth effect was detected and the very low coeffi-cient of concordance indicates a shifting vertical dis-tribution. During this early morning period, upwardmovement of cells was taking place (Fig. 4).

Mallomonas reginae

As noted earlier, this species was the only one toshow significant horizontal variation between thelake sites. The vertical profiles showed a significanteffect of depth on both occasions, but no, or nega-tive, correlation with the distribution of Ankyra (Ta-ble 3). On both occasions the coefficient of concor-dance was moderately low, indicating directional butloosely synchronised shifts in distribution. An even-ing descent was apparent (Fig. 4) but commencinglater than for the cryptomonads, and even by 09.00on 4 August only a partial ascent had occurred. Sincethe cells evidently descend well below 2 m and maynot return until late in the morning, the observedhorizontal variability between lake sites (Table 1)could be caused by failure of the 2 m tube samplerto capture the entire water column population atsome of the sites. This would occur if the verticalmigrations were out of phase at different sites, forexample in response to varying irradiance conditionsaround the lake.

Discussion

This work has investigated the vertical distributionsof some flagellated phytoplankton species by refer-ring them to that of a non-motile and close to neu-trally buoyant species. Use of a non-motile alga asa marker of possible effects of water turbulence onvertical distributions has been made previously bySoeder (1967) and Happey-Wood (1976). However,use of a reference species in this way does assumethat passive influences on distribution have an equaleffect on the reference species and those to be com-pared with it. This assumption may be violated in anumber of ways, for example by differences in sink-ing rates or susceptibility to grazing, as well as byvariations in horizontal heterogeneity between spe-cies. In this study horizontal heterogeneity was insig-

nificant except in the case of Mallomonas reginae,while differential loss rates would hardly have beenapparent over the short sampling intervals em-ployed. Therefore the use of a reference species wasvalid in this case.

All the flagellated species examined, with the ex-ception of Spiniferomonas bourrellyi, showed verti-cal distributions which both deviated significantlyfrom the reference distribution and shifted signifi-cantly during 6 hour periods around sunset or sun-rise. Since these shifts in vertical distribution ap-peared to involve most of the population of eachspecies and also produced no net directional move-ment over a 24 hour period, they may be consideredexamples of true vertical migration, as the term hasbeen traditionally used in limnology. This is equiva-lent to the 'homeostatic migration' category definedby Taylor (1986).

Although the migrations undertaken by differentspecies varied in their vertical extent and degree ofsynchrony, they all clearly conformed to a singletemporal pattern. This involved downward move-ment in the evening and a return, upward journey inthe early morning. This pattern is the one most fre-quently observed in migrating phytoplankton, bothfreshwater and marine (Taylor, 1980). Only rarelyhas the opposite pattern been reported (e.g. Tilzer,1973). The data presented here do not allow anyjudgement of the regularity of these diel migrations,but other studies in Lake Nimet6n and nearby lakesstrongly support the view that diel vertical migra-tions are a constant feature of healthy populationsof these species of flagellates (Arvola, 1984; Salonenet al., 1984b; Arvola et al., 1987).

The extent of vertical movements differed betweenspecies (Fig. 4). Dinobryon divergens was restrictedto the upper 1 m of the water column and may there-fore have been avoiding oxygen depleted water. Thephased downward movement of this species on 2 Au-gust (Fig. 4) indicates a rate of travel of 2 m d - 1,which is far greater than reported typical sinkingrates even for large diatoms (Burns & Rosa, 1980;Reynolds, 1984) and suggests the colonies were ac-tively swimming downwards rather than settling pas-sively. The more extensive distance (several metres)covered by other species, particularly the cryp-tomonads, involved crossing a temperature gradient

85

> 14 °C and penetrating far into the anoxic hypolim-nion. This must have entailed strong swimming inboth directions; cryptomonads typically have onlyvery low sinking rates (Burns & Rosa, 1980; Reynolds& Wiseman, 1982).

Some species (Dinobryon divergens, Cryptomo-nas marssonit) showed strongly synchronised move-ments which must reflect a uniform and sensitive re-sponse of individuals within the population to anappropriate stimulus. This might be internal (circa-dian) or it could be external, provided the stimulusaffected all individuals in a similar fashion (eg.changing irradiance). Both types of stimulus havebeen reported. The evening descent noted for mostspecies in this study could certainly have been adirect response to decreased irradiance, and the post-sunrise ascent of species such as Mallomonasakrokomos could equally have been a response to in-creasing irradiance. However, such a response seemsunlikely to explain the timing of ascent by speciessuch as Cryptomonas marssonii which commencedbefore any detectable increase in incident irradianceand was from depths at which light penetration wasinfinitesimal (Figs. 3, 4). These species may havebeen responding to endogenous rhythms. Apparentcircadian rhythms have been demonstrated withdinoflagellates in experimental tanks (Eppley et al.,1968; Heaney & Furnass, 1980; Cullen & Horrigan,1981). Phototactic responses have been reportedfrom laboratory and field studies, although the lat-ter will rarely allow firm identification of themechanisms underlying observed movements. Otherpossible stimuli for vertical migrations such as nutri-ent depletion are unlikely to act on all individuals inthe same way and would not produce the syn-chronous migrations observed here. However, nutri-ent depletion may modify underlying rhythms ofmigration in at least some species of phytoplankton(Heaney & Eppley, 1981).

The adaptive significance of flagellate migrationsremains uncertain. In the weakly turbulent watercolumns of forest lakes such as Lake Nimeton,phytoplankton must be able to effectively countertheir natural tendency to sink. Small cell size with alow sinking rate may be an adequate strategy (e.g.Ankyra) but diatoms are notably absent from theseenvironments in which the overwhelming bulk of the

phytoplankton is flagellates (Jones & Arvola, 1984).Motility allows these algae to remain in suspension,but that does not account for the high frequency ofspecies which undertake diel migrations.

Cost benefit analysis of diel migrations ofdinoflagellates (Raven & Richardson, 1984) revealedthe costs (primarily the energy and nutrients re-quired to produce and operate the flagella appara-tus) to be substantially outweighed by the benefits(increased acquisition of photons and nutrients rela-tive to non-migratory cells). In that analysis the as-sumed swimming speeds (up to 40 m d-l) were anorder of magnitude greater than the highest valueswhich can be deduced from the vertical profiles inLake Nimet6n (Fig. 4). The costs of the shortermigrations in Lake Nimeton may therefore be as-sumed to be less. Since some flagellate species in thelake do not undertake migrations but can sustain anadvantageous position in the euphotic zone (e.g.Spiniferomonas), the chief metabolic benefit tomigratory species is probably the access they gain tothe substantial reserves of nutrients present in thehypolimnion. Retrieval of phosphorus from thehypolimnion in Lake Nimet6n by diel vertical migra-tions of phytoplankton was experimentally verifiedby Salonen et al. (1984b). Comparison with arealloading of phosphorus to the lake via inflows sug-gests that migratory retrieval of phosphorus fromthe hypolimnion can be the largest source of freshphosphorus input to the epilimnion during drierperiods in the summer.

However, vertical migrations may confer other ad-vantages besides metabolic ones. It was already not-ed that Cryptomonas ovata was under-represented inthe outflow when, at the time of sampling, the popu-lation was still mainly well below the surface. Allthose species which move below the water layerwhich is feeding the outflow from the lake will simi-larly reduce population losses through washout. Themean flushing rate for the epilimnion of Lake Nime-ton during the ice-free period is around 0.5 d-' (L.Arvola, unpublished data). Spending 8 hours ofdarkness below the epilimnion would not thereforebe of great selective advantage to migratory specieswith respect to this population loss process, exceptduring storm events when flushing rates are muchhigher. However, small flagellates, and particularly

86

naked species, are favoured food for herbivorouszooplankton and may face severe grazing pressure.Reynolds etal. (1982) showed that > 96% of the loss-es from a population of Cryptomonas ovata in largeenclosures was due to grazing. In Lake Nimetonmost zooplankton are migratory, coming to the up-per waters to feed at night and spending the day indeeper strata (Arvola et al., 1986). Phytoplanktonmigrations on the reverse pattern, as were all thosefound in this study, would therefore substantially re-duce grazing pressure without imposing any loss ofgrowth potential.

Separating the relative importance of these advan-tages gained from vertical migrations is difficult.The evolution of phytoplankton migratory be-haviour, whether endogenous or exogenous, mayhave responded to a number of separate selective ad-vantages. The situation is further confused by thespectrum of behaviour displayed by even the smallnumber of flagellate species in this study. What isclear is that more rigorous investigation of verticaldistributions is needed. Laboratory experiments inwhich possible stimuli can be controlled must becoupled with field studies designed to permit objec-tive interpretation of distributions. This paper haspresented some approaches to the latter, but moresensitive statistical tests may be needed to evaluatevertical migration patterns in water columns withgreater physical mixing.

Acknowledgements

The field work was carried out during tenure of aRoyal Society Study Visit Grant. I thank Lauri Arvo-la, Kalevi Salonen and Phil Peters for their com-ments on the manuscript.

References

Arvola, L., 1983. Primary production and phytoplankton in twosmall, polyhumic forest lakes in southern Finland. Hydrobi-ologia 101: 105-110.

Arvola, L., 1984. Diel variation in primary production and thevertical distribution of phytoplankton in a polyhumic lake.Arch. Hydrobiol. 101: 503-519.

Arvola, L., K. Salonen, R. I. Jones, I. Bergstr6m &A. Heinanen,1987. A three day study of the diel behaviour of plankton ina highly humic and steeply stratified lake. Arch. Hydrobiol.109: 89-106.

Blakar, I. A., 1979. A close interval water sampler with minimaldisturbance properties. Limnol. Oceanogr. 24: 983-988.

Burns, N. M. & F. Rosa, 1980. In situ measurement of the settlingvelocity of organic particles and 10 species of phytoplankton.Limnol. Oceanogr. 25: 855-864.

Cullen, J. J. & S. G. Horrigan, 1981. Effects of nitrate on the diur-nal vertical migration, carbon to nitrogen ratio and the pho-tosynthetic capacity of the dinoflagellate Gymnodium splen-dens. Mar. Biol. 62: 81-89.

Elliott, J. M., 1977. Some Methods for the Statistical Analysis ofSamples of Benthic Invertebrates, 2nd Edn. Sci. Publ. Fresh-wat. Biol. Ass. U.K., 160 pp.

Eppley, R. W., O. Holm-Hansen & J. D. H. Strickland, 1968.Some observations on the vertical migration of dinoflagellates.J. Phycol. 4: 333-340.

Fee, E. J., 1976. The vertical and seasonal distribution of chlo-rophyll in lakes of the Experimental Lakes Area, north westernOntario: implications for primary production estimates. Lim-nol. Oceanogr. 21: 767-783.

Frempong, E., 1981. Diel variation in the abundance, vertical dis-tribution and species composition of phytoplankton in a eu-trophic English lake. J. Ecol. 69: 919-939.

Ganf, G. G. & R. L. Oliver, 1982. Vertical separation of light andavailable nutrients as a factor causing replacement of green al-gae by blue-green algae in the plankton of a stratified lake. J.Ecol. 70: 829-844.

Happey-Wood, C. M., 1976. Vertical migration patterns inphytoplankton of mixed species composition. Br. phycol. J. 11:355-369.

Heaney, S. I., 1976. Temporal and spatial distribution of thedinoflagellate Ceratium hirundinella 0. F. Muller within asmall productive lake. Freshwat. Biol. 6: 531- 542.

Heaney, S. I. & R. W. Eppley, 1981. Light, temperature and nitro-gen as interacting factors affecting diel vertical migrations ofdinoflagellates in culture. J. Plankton Res. 3: 331-344.

Heaney, S. I. &T. I. Furnass, 1980. Laboratory models of diel ver-tical migration in the dinoflagellate Ceratium hirundinella.Freshwat. Biol. 10: 163-170.

Heaney, S. I. & J. F. Talling, 1980. Dynamic aspects of dinoflagel-late distribution patterns in a small, productive lake. J. Ecol.68: 75-94.

Ilmavirta, V., 1974. Diel periodicity in the phytoplankton commu-nity of the oligotrophic lake Paajdrvi, southern Finland. I.Phytoplanktonic primary production and related factors. Ann.bot. fenn. 11: 136-177.

Irish, A. E. & R. T. Clarke, 1984. Sampling designs for the estima-tion of phytoplankton abundance in limnetic environments.Br. phycol. J. 19: 57-66.

Jones, R. I. & L. Arvola, 1984. Light penetration and some relat-ed characteristics in small forest lakes in southern Finland.Verh. int. Ver. Limnol. 22: 811-816.

Kamykowski, D. & S.-J. Zentara, 1977. The diurnal vertical migra-

87

tion of motile phytoplankton through temperature gradients.Limnol. Oceanogr. 22: 148-151.

Klemer, A. R., 1976. The vertical distribution of Oscillatoriaagardhii var. isothrix. Arch. Hydrobiol. 78: 343-362.

Nauwerck, A., 1963. Die Beziehungen zwischen Zooplanktonund Phytoplankton in See Erken. Symb. Bot. Upsal. 17(5):1-163.

Platt, T. & K. L. Denman, 1980. Patchiness in phytoplankton dis-tribution. In I. Morris (ed.), The Physiological Ecology ofPhytoplankton. Blackwell, Oxford: 413-431.

Raven, J. A. & K. Richardson, 1984. Dinophyte flagella: a cost-benefit analysis. New. Phytol. 98: 259-276.

Reynolds, C. S., 1980. Phytoplankton assemblages and their peri-odicity in stratifying lake systems. Holarct. Ecol. 3: 141-159.

Reynolds, C. S., 1984. The ecology of freshwater phytoplankton.Cambridge University Press, Cambridge, 384 pp.

Reynolds, C. S. & S. W. Wiseman, 1982. Sinking losses ofphytoplankton in closed limnetic systems. J. Plankton Res. 4:489-522.

Reynolds, C. S., J. M. Thompson, A. J. D. Ferguson & S. W.Wiseman, 1982. Loss processes in the population dynamics ofphytoplankton maintained in closed systems. J. Plankton Res.4: 561--600.

Salonen, K., L. Arvola & M. Rask, 1984a. Autumnal and vernalcirculation of small forest lakes in southern Finland. Verh. int.Ver. Limnol. 22: 103-107.

Salonen, K., R. I. Jones & L. Arvola, 1984b. Hypolimnetic phos-phorus retrieval by diel vertical migrations of lake phytoplank-ton. Freshwat. Biol. 14: 431-438.

Salonen, K., K. Kononen & L. Arvola, 1983. Respiration of plank-ton in two small, polyhumic lakes. Hydrobiologia 101: 65 -70.

Sandgren, C. D. & J. V. Robinson, 1984. A stratified sampling ap-proach to compensating for non-random sedimentation ofphytoplankton cells in inverted microscope settling chambers.Br. phycol. J. 19: 67-72.

Soeder, C. J., 1967. Tagesperiodische Vertikalwanderung bei be-geisselten Planktonalgen. Umschau 12: 388.

Sokal, R. R. & F. J. Rohlf, 1981. Biometry, 2nd Edn. Freeman,San Francisco, 859 pp.

Sournia, A., 1974. Circadian periodicities in natural populationsof marine phytoplankton. Adv. mar. Biol. 12: 325-389.

Taylor, R. J. R., 1980. Basic biological features of phytoplanktoncells. In I. Morris (ed.), The Physiological Ecology ofPhytoplankton. Blackwell, Oxford: 3-55.

Taylor, L. R., 1986. Synoptic dynamics, migration and theRothamstead Insect Survey. Presidential Address to the BritishEcological Society, December 1984. J. anim. Ecol. 55: 1-38.

Tilzer, M. M., 1973. Diurnal periodicity in the phytoplankton as-semblage of a high mountain lake. Limnol. Oceanogr. 18:15 -30.