Hydrobiologia 161: 99-123 (1988)R. 1. Jones and V. Ilmavirta (eds.). Flagellates in Freshwater Ecosystems 99Q, Kluwer Academic Publishers

Vertical-migration patterns of flagellates in a community of freshwaterbenthic algae

Christine M. Happey-WoodSchool of Plant Biology, University College of North Wales, Bangor, LL57 2UW, Gwynedd, UK.

Key words: Chlamydomonads, chrysomonads, cryptomonads, endogenous rhythms, epipelon, euglenoids,movement patterns, survival strategies, trachelomonads

Abstract

The population densities of sediment-inhabiting flagellates sampled from a shallow eutrophic lake in April1986 were investigated at intervals of 1 or 1.5 h over a twenty hour period in the laboratory under natural irradi-ance and in controlled conditions. In natural irradiance the flagellates exhibited a vertical migration rhythmup onto the sediment surface after dawn and down into the sediment during the afternoon. Details of the timingof the migration movements and period of time flagellates were present on the sediment surface differed be-tween species and five flagellate groups: trachelomonads, green euglenoids, chlamydomonads, chrysophytesand cryptomonads. During daylight, twenty-seven species maintained population maxima at the surface ofthe sediment on eight occasions. The species composition of these maxima differed and eight species werefound to have two maxima on the surface of the sediment at different times of day. Numbers of three speciesof chrysophycean flagellates and Rhodomonas minuta increased again on the sediment surface after dark.Under continuous irradiance at 10 °C, the migration cycle of all five groups of flagellates was affected. Someevidence for an endogenous nature of this rhythm was found for green euglenoids and chlamydomonads. Num-bers of chrysophytes, particularly Synura spp. increased in constant light in the surface layers of sediment.

Thus a mosaic of vertical migration patterns was described in an epipelic community of algae dominatedby five groups of flagellates. The importance of this in the survival strategy and ecology of these sediment-inhabiting algae is discussed.

Introduction

Patterns of vertical-migration have been describedfor a variety of algal populations both in benthiccommunities (e.g. Bracher, 1919; Faur6-Fremiet,1950, 1951; Palmer & Round, 1965, 1967; Round &Palmer, 1966) and planktonic situations (e g. Hasle,1950; Eppley et al., 1968; Berman & Rodhe, 1971).Descriptions of vertical-migrations in phytoplank-ton have, in the main, been given for phytoplanktondominated by one species of large alga such asdinoflagellate species (Berman & Rodhe, 1971; Tail-ing, 1971; Heaney, 1976; Heaney & Talling, 1980;

Frempong, 1984), although smaller organisms suchas Chrysococcus diaphanus Skuja may move in ver-tical rhythms under calm water conditions (Happey& Moss, 1967). Associations of mixed flagellates anda sulphur bacterium have been shown to exhibit suchdaily vertical-movements in the water column of ashallow eutrophic pool (Happey-Wood, 1976), andthese varied between species. In sediments verticalmovements of different algae are distinct. In a ma-rine estuarine community with turbid waters, move-ment patterns of Euglena contrasted with those ofdiatom species (Palmer & Round, 1965; Round &Palmer, 1966) and were related to tidal cycles, where-

100

as in clearer coastal waters Perkins (1960) found thatmovements of diatom populations were diurnal. Infreshwaters, migration movements of sediment-inhabiting algae are diurnal (Round & Eaton, 1966)and Round & Happey (1965) gave evidence to showthat different diatom species are present in maxi-mum numbers at the sediment surface at differenttimes of day.

This study investigated the vertical movements ofepipelic algae in a flagellate-dominated freshwatercommunity. Movement of sediment-inhabiting al-gae may be studied readily in controlled laboratoryconditions (P. Jones, in preparation). Thus the aimswere to study the vertical movement patterns of arange of flagellates and to distinguish possibledifferences between migration characteristics of par-ticular algal phyla or species. Also, it was hoped todifferentiate the endogenous or exogenous nature ofvertical migration of flagellates in and out of surfacesediments in samples from a shallow fresh waterlake.

Methods

Sediment samples for the detailed study of verticalmigration of epipelic algae were collected on 29thApril 1986 from Rhos Ddu, Newborough Warren,Anglesey, North Wales (National Grid Reference ST428649), a shallow eutrophic lake managed as a birdreserve. This site was selected as it was known to sup-port high standing crops of epipelic algae dominatedby flagellate species, since the epipelic algal popula-tions had been studied previously in a survey of lakesin North Wales using the methods given in Happey-Wood & Priddle (1984).

For the investigations of vertical migration, eightreplicate samples of surface sediment, algae andoverlying water were taken at 1100 h into plastic bot-tles, by suction from within areas of sediment de-fined by a perspex cylinder (Eaton & Moss, 1966).After returning to the laboratory the samples wereallowed to settle for six hours in the dark, the super-natant water was decanted carefully and the repli-cates were then bulked and thoroughly mixed. Equalquantities of sediment were poured into eight 9 cmdiameter plastic Petri dishes, each with small regu-

larly placed holes in their bases, to allow for drain-age of excess water. The Petri dishes were lined withfilter paper (Whatmann No. 9). After an hour, thesediment surfaces were covered with triple layers oflens tissue (Greens, 105), each square of area2 x 2 cm. Three replicate dishes were maintained inthe laboratory at a temperature 15-20C, undernatural vertical irradiance (designated treatmentL:D) and three replicate samples were kept in a Fis-on's Controlled Growth Cabinet (Model600G3/TTL) at a temperatur eof 10 °C and constantirradiance of 13 uE cm-' s -1 (treatment L:L). Thetwo remaining replicate samples of sediment were re-tained for statistical analysis of counting precisionby video-photo-microscopy.

The following day, from 05.00 h until midnight,counts were made of the algae contained within thelens tissue squares from both the L:D and L:L treat-ments. Two treatments were used, firstly to inves-tigate algal movement under natural conditions ofirradiance, secondly, to test the possible endogenousnature of any vertical migration rhythm under con-stant conditions. The algae were studied at intervalsof either 1 h or 1.5 h and a total of fifteen countswere made during the 20 h time-period. For eachcount a triple layered square of lens tissue was re-moved from the sediment onto a glass slide on whichthree drops (0.02 cm3) of 1% glutaraldehyde hadbeen placd as fixative. The use of glutaraldehyderesults in less distortion of delicate flagellates thanother preservatives (Happey-Wood, 1978; Happey-Wood & Priddle, 1984). The three layers of lens tissuewere separated into top, middle and bottom layers toenable determination of any vertical segregation ofdifferent algae on the sediment surface, representedby the lens tissue. Thus counts of approximately0-50, 50-100 and 100-150 m surface zones ofsediment were obtained for each of the fifteen sam-ples. Algae were identified using standard works ofEttl (1970, 1976, 1979), Huber-Pestalozzi (1941,1950, 1955) and Hustedt (1930), and counted undera Leitz Laborlux photo-microscope at x400 mag-nification. A minimum of 40 fields of view werestudied or a minimum of 100 individuals of the pre-dominant organisms recorded.

Statistical analyses on replicate counts made byvideo-photography were carried out to define the

101

counting precision. Algae from the two remainingfield replicate samples, one of which had been keptovernight in natural irradiance (L:D) and the secondof which had been under constant conditions over-night as previously described (L:L), were removed inlens tissue squares and transferred from the sedi-ment surfaces at 10.00 h on to a substrate of talcumcontained within 4.5 cm diameter glass rings in aPetri dish base and surrounded by distilled water.The samples were put in constant conditions in theFison's Growth Cabinet. Observation, which wastwo days after collection, was at x400 magnifica-tion via a Leitz Labourlux microscope retained in thegrowth cabinet. Records were made of forty randomfields on videotape using a Panasonic WVP-GIEColour Video-Camera at thirteen intervals of be-tween 0.5 and 5 h during the twenty four hour period09.00 1 May to 09.00 2 May 1986. Counts were madesubsequently of total numbers of bothtrachelomonads and green euglenoids viewing thefilm on a Bell & Howell 1202M 26" colour monitor,using a Panasonic AG-6200 Video Cassette Record-er. Values for standard deviation and coefficient of

variance were calculated. Solar radiation for the L:Dtreatment was measured using a LintronicSolarimeter in the laboratory and irradiance underL:L conditions was measured using a Crump radi-ometer.

Results

Species composition of the epipelic community

The sediment-inhabiting algal community at RhosDdu had a seasonal periodicity of algae during 1980.This was dominated in Spring by a variety of flagel-lates (Fig. 1). The chlamydomonads were the mostnumerous organisms together with euglenoids inMarch, April and early May, and at this time diatomnumbers were low. However, during the Summer,Amphora ovalis and three other species of diatombecame dominant; the trachelomonads reachedmaximum population densities in June. Numbers ofeuglenoids declined during May with a second popu-lation maximum in August-September. Diatom

log10 cells. cm- 2

4Chla. gloeognma 3

24

Chlor. rap+govei 3

2Chlam. coninis

2Chlor. guan 3

2Crpto 3

E. granulata [

23

E clavata 2

3Trachs 2 l

log 10 celts. cm -2

LLI~A

F A A

Amphra3

N angfica 4

2 _4FN. falsiensis 3

2 i

N. gastrum 3 [2 -

Os.ill tenuis [D F A J A 0 D

M M J S N

1980

Fig. 1. The standing crop, expressed as log10 cells cm-2, of the predominant algae on sediment from Rhos Ddu lake during 1980.

M M J S N

1980

-

102

Table 1. Predominant epipelic algae and their maximum stand-ing crops sampled from Rhos Ddu on 30th April 1986. Data aretaken from L:D treatment*.

Species Maximum standing crop(individuals cm-2)X

Trachelomonads

Trachelomonas hispida (Perty)Stein emend. Deflandre

T. hispida var. coronata Lemm.T. pulcherrima Playf.T. urniga SkujaT. varians DeflandreT. volvocina Ehrenb.

100003300

800 +

180011006300

Green euglenoids

Euglena acus Ehrb.E. charkowiensis Swir.E. clavata SkujaE. ehrenbergii KlebsE. granulata (Klebs) Lemm.E. viridis Ehrb.Phacus pleuronectes (O.F.M.)

DujardinP. rudicula (Playf.) Pochm.

18003200 (8000)*7800 (8600)*4600

700800

15002000

Chlamydomonas

Chlamydomonas gloeogamaKorsch.

C. minutissima Korsch.C. sphaeroides GerloffC. spinifera EttlChloromonas grove G.S. West

(Gerloff)

6400600

43001400

10300

Chrysophytes

Chrysococcus rufescens KlebsMallomonas caudata Iwan.M. globosa SchillerSynura pertersenii Stein emend

Korsch.S. ulvella Korsch.

390012002700

200 ( 800)*2000 (2200)*

standing crop decreased from July through the re-

mainder of the year. Thus, for a eutrophic situation,

Rhos Ddu appears unusual in the lack of vernal and

autumnal diatom maxima. It was selected as a site

to study flagellate movement, due to the variety of

algal groups present and the dominance of flagel-

lates in the algal community.

During the 1986 investigation of vertical migra-

tion, four groups of flagellate algae predominated;

Euglenophyta, which have been separated into

trachelomonads and green euglenoids for ease of

presentation, Chlorophyta, Chrysophyta and Cryp-

tophyta. Although present, diatoms were less abun-

dant than the majority of these flagellate groups.

Numbers of Protozoa were low, one colourless Am-

phidinium sp. was present occasionally and Oscil-

latoria tenuis was rare.

The maximum standing crops recorded on any

one occasion during the twenty hour study, given as

individuals cm-2, were trachelomonads

12000 +± 1320, green euglenoids 17 800 ± 3560,

chlamydomonads 23500 + 3050, chrysophytes

12 500 + 2 500, cryptophytes 10 500 ± 2100, and dia-

toms 14200 ± 2130 cells cm- 2 (95% confidence

limits).

A total of 50 taxa of microbes were distinguished.

Apart from the predominant species (Table 1), other

organisms included species of Carteria, Chlamydo-

monas, Chloromonas, Chromulina, Golenkinia,

Ochromonas, Phacotas, Pyramimonas, Synecoc-

cocus. Diatoms included species of Cymbella, Gom-

phonema, Navicula anglica, N. falsiensis and Pin-

nularia. Colourless and 'green' species of both

Amoeba and Paramecium, together with Menodium

sp., comprised the Protozoa.

Statistical analysis

Chroomonas coerulea (Geitl.) SkujaCryptomonas obovata SkujaC. playuris SkujaRhodomonas minuta Skuja

4900680019001400 (2600)*

* Data in parentheses are for L:L treatment when the maxi-mum standing crop recorded was greater than in the L:Dtreatment.

x Sum of counts for the three layers of lens tissue.+ Recorded only at 09.00 h; this species is not presented in the

figures.

Numbers of Trachelomonas spp. and green Euglena

spp. (Fig. 2) counted per field by video-photography

were low. However, changes in numbers during the

twenty four hour period were significant. Values for

coefficients of variance are given in Table 2. The pre-

cision of enumerating Trachelomonas spp. was

greater than that for Euglena spp., but there was less

difference between values of the coefficients of vari-

Cryptophytes

103

B)

-ia

X)

-_ I L

c 1.0,a)

0)L. 08

Zz

06

0'4

02

fl

L:L

I \

I \

I

1~ /111, ,III

090012O o oo oco ooo

usI)

.800

.600

-200

0

1May TIME (h) 2 May 1May 1986 2 May1986

Fig. 2. Total numbers of A) Trachelomonas spp. and B) green euglenoid spp. expresed as numbers of cells field - l and numbers cm- 2

under L:D ( - ) and L:L (o --- o) treatments. Each point represents the mean value of 40 observations by video-photomicroscopy;error bars represent 2 x SD.

Table 2. Summary of changes in standing crop of total Trachelomonas spp. and green Euglena spp. and values of coefficient of vari-ance calculated from replicate observations by video-photography during twenty-four hours.

Standing crop (cells cm-2) C.V. (o%)

Treatment Maximum Minimum

Trachelomonas spp. L:D 2400 750 11.2L:L 3000 1600 7.6

Euglena spp. L:D 800 120 20.6L:L 1020 60 22.2

A)

'a.!Z:

:,0-2

C.R _ A,~~~~~~~~~~~~~~~~.E .5-

wi . - E~~~1]

ance between L:D and L:L pre-treatments than be-tween values for the two flagellate types.

Variations in the algal populations present on thesediment surface

During the twenty hour experiment, the natural pho-toperiod involved dawn, between 06.30-07.00 h(sunrise 06.38) and dusk, between 21.30-22.00 h(sunset 19.41); all times given relating to the experi-ment are British Summer Time (B.S.T.). Changes in

200-

1 200

200]

400t

>

0-.

2001a

Protoza 10]

PDARK LIGI

laboratory temperature were small, from 15 °C at5 a.m. to a maximum of 20 °C from 09.00 h decreas-ing at 20.00 h to 17 °C at midnight. Levels of irradi-ance were low due to continuous cloud cover, and to-talled 45 W m - 2 for the day. Results of theexperiment will be treated firstly in terms of groupsof microbes, i.e. trachelomonads, green euglenoids,chlamydomonads, chrysophytes, cryptophytes, dia-toms and Protozoa; secondly, each 'flagellate group'will be considered on a species basis, and thirdly, thedepth distribution of the flagellates at the surface ofthe sediment will be discussed.

HT DARK. . . 1 . . .

12 66 24 - I

u

L

:' - *_-

6 68 lb 1'2 1 4 16TIME (h)

18 20 22 24

Fig. 3. Total numbers of six algal groups and Protozoa counted in lens tissues by microscopy: samples in L:D ( - e ) and L:L ( o --- o )conditions during a twenty hour period. Natural light conditions are shown along the top of the figure.

104

NPOW

105

Flagellate groupsAt the start of the experiment numbers of all thegroups of algae counted in the lens tissues were ex-tremely low (Fig. 3). This applies to both L:D andL:L treatments. Maximum numbers oftrachelomonads in the L:D treatment were recordedat 08.00 h, remained high for three hours, decreasedin the mid-part of the day, reached a second maxi-mum at 17.00 h and then declined rapidly by 18.00 hprior to dusk. Under L:L conditions numbers wereconsiderably less, the maximum population densitywas only ca. a third of that in L:D conditions andoccurred at 12.30 h, but, as under L:D conditions,numbers were minimal again after 18.00 h. Numbersof green euglenoids increased later thantrachelomonads, under both treatments. Maximumpopulation density, which was greater than for thetrachelomonads, was recorded at 11.00 h; cell num-bers then declined gradually throughout the ex-perimental period. Behaviour of green euglenoidsunder constant conditions was similar in periodicity

C'4

o

Total 5I

Total OTrachs

T. hispida var 251coronata 0 J

5

T. hispida 2'5

0

T. volvocina 25J

T. varians 2

T. urniga 25]2·O

10

/t

__--~ t EO-'

-- -

.. 0. 0- - -

L ' -

but the initial increase in numbers early in the daywas delayed by two hours in comparison with L:Dconditions.

Under L:D conditions, chlamydomonads in-creased slightly in numbers at dawn, but the majorappearance of these smaller flagellates on the sedi-ment surface was noted between 14.00-15.30 h. Atthis time chalmydomonads were more numerousthan any other flagellate group throughout the ex-perimental period. In constant conditions, no in-crease in chlamydomonad numbers occurred untilafter 11.00 h. Maximum numbers in both ex-perimental treatments were coincidental.

Chrysophytes appeared less numerous than theother contributing organisms. However, it should beemphasised that colonial Synura spp. were majorcontributing organisms to algal groups. Some in-crease in numbers was found at the sediment surfaceat 08.00 h and a second peak, the population maxi-mum, was counted at 15.30 h. Numbers of thesegolden brown flagellates showed some increase in

TRACHELOMONADS

o-0 -0 0..~~0 _ _ -0 -. - J- - -U

6 8 10 1 ' 14TIME (h)

16 18 20 22

Fig. 4. Population densities of total Trachelomonas spp. and of T hispida var. coronata, T hispida, T volvocina, T varians, T urniga.

The star marks the only record for T pulcherrima.

____ -1-- --P-h-O---_h__o-�-- -s----n---

106

numbers under constant conditions during the firstten hours of the experiment and again between20.00- 24.00 h, the period corresponding to dusk inthe natural environment.

Cryptomonads increased gradually in numbersfrom 06.00 h until 09.30 h, then again more marked-ly to reach their maximum numbers at 15.30 h. Afterdeclining in numbers for three-four hours a thirdpeak in numbers was recorded just prior to dusk.Under L:L conditions, numbers at the sediment sur-face increased after 11.00 h, similar to thechlamydomonads under these conditions. Thehighest count of cryptophytes under constant condi-tions coincided with that under L:D conditions and,

(itc

so 20-

TotalEuglenoids 10-

(green)

5 oE. 2'5

granulata

E. 25-ehrenbergii 0

75-E.

clavata 5251

075

charkowiensis 5

2.50-

E. acus 25]

P. rudicula 2.5

P. pleuronectes 2 52.5

like the chrysophytes under constant conditions,large numbers of cryptophytes were at the sedimentsurface during the dusk-time period.

Diatoms were rare at the sediment surface for thefirst few hours of the study. Numbers increased be-tween 09.30-10.00 h and declined from 15.30 hthroughout the rest of the study. Populations of dia-toms recorded under constant conditions were lessand more variable than under natural irradiance.Protozoa, although few in number, were mostnumerous at 09.30 h; however, data for theseanimals should be treated with care as counts werevery low and probably not statistically significant.Under constant irradiance, Protozoans were noted

GREEN EUGLENOIDSLightDark Dark

A 10 10 -1

p0,

0' "N0~. - 0

- --0--.. _~-;~--·~-lt ~ 0- -

6 8 10 12 14TIME (h

16 18 20 22 24

Fig. 5. Population densities of total green euglenoid spp. and of E. granulata, E. ehrenbergii, E. clavata, E. charkowiensis, E. acus, Phacusrudicula and P. pleuronectes.

-

107

extremely rarely between 07.00-09.00 h, and werenot found again during the experiment.

Thus the different flagellate groups increased innumbers and achieved maximum population densi-ties at the sediment surface at different times of day.Under natural irradiance, this sequence wastrachelomonads, followed by green euglenoids, withchlamydomonads, chrysophytes and cryptophytestogether at 15.30 h. Comparing the two experimen-tal treatments, maximum numbers recorded at anyone time at the sediment surface were smaller underconstant conditions than under natural daylight,apart from the green euglenoids. The diurnal patternof increase and decrease in cell numbers, except forthe green euglenoids and chlamydomonads, alsodiffered under constant temperature and irradiance.The most pronounced effect appeared to be a delayin the increase in cell numbers of green euglenoids,chlamydomonads, chrysophytes, cryptophytes, dia-toms and Protozoa.

Thachelomonads (Fig. 4)Increases in population density of the five species ofThachelomonas were recorded at different times dur-ing the period of the experiment under L:D condi-tions. Maximum numbers of T hispida var. coronatawere observed at 08.00 h as were the maxima for Tvolvocina and T varians. Greatest population densi-ty for any species of trachelomonad was found forT7: hispida at 17.00 h. Under constant light and tem-perature (L:L) the numbers of all trachelomonadspecies were less than in natural irradiance, and thetiming of the presence of cells at the sediment sur-face differed. The peak in total trachelomonads inthe morning resulted from increases in populationdensity of all five species, whereas the afternoonpeak was composed primarily of T hispida and toa lesser extent T hispida var. coronata.

Green euglenoidsThe most common green euglenoid was Euglena

E

20-

Total 15-Chlmys. 10

5-

10-

Chlor.grovel 5-

c. 25]gloeogama

C. 251sphaeroides 0 J

c. 11spinifera 0

C.minutissima 0

A 8 ib 1'2 14TIME (h)

16 18 20 22 24

Fig. 6. Population densities of total chlamydomonads and of Chloromonas grovel, Chlamydomonas gloeogama, C. sphaeroides, C.spinifera and C. minutissima.

:__�__ __ _ __. I N W I IWNNI I 4

108

clavata. Increases in the population densities for all5 Euglena species and Phacus rudicula occurredfrom 07.00 h onwards. Numbers of P pleuronectesdid not increase until after 09.00 h. High populationdensities ofE. ehrenbergii were noted from 09.30 hto 14.00 h, whereas after increasing from08.00-11.30 h, numbers of E. clavata declinedsteadily until 17.00 h. Standing crops, even at maxi-mum population densities, of E. granulata, E. char-kowiensis, E. acus, P. rudicula and R pleuronecteswere all low in comparison with E. ehrenbergii andE. clavata. The effect of constant conditions (L:L)was least marked for E. clavata, where the maximumpopulation density under L:L conditions was slight-ly greater (see Tabl 1) than under L:D conditions.This maximum corresponded in time with that un-der L:D conditions, but the commencement of theincrease was one hour later and numbers increasedmore rapidly. Under constant conditions, the pres-ence and increase in numbers of E. charkowiensiswas five hours later than in the L:D treatment. How-ever, the maximum population density, at 14.00 hwas more than twice that recorded in natural irradi-

gil

ance. When subjected to L:L conditions numbers ofall other green euglenoids were lower than in the L:Dtreatment.

Chlamydomonads (Fig. 6)Chloromonas grovei and Chlamydomonas spiniferawere the first chlamydomonads to increase in num-bers on the sediment surface between 06.00 and07.00 h. Numbers of the other three small chloro-phytes increased later and more gradually from08.00 h until mid-afternoon. Maximum populationdensities of all these green flagellates except C.spinifera were recorded at 15.30 h. Numbers thengradually declined until 22.00 h. Throughout the ex-periment, under constant conditions, numbers of allchlamydomonads were less than in natural irradi-ance. The pattern of population increase and de-crease at the sediment surface was similar in bothL:L and L:D conditions for Chloromonasgrovei andChlamydomonas sphaeroides.

Chrysophytes (Fig. 7)The migration patterns of the two species of Synura

CHRYSOPHYTES

....... I11_ 15A

Total 5Chrysos 2.5

0

S. ulvella 0]

S. petersenii 0

2M. gobosa 0

M. caudata 05

Chr. rufescens 2]

6 8 10 12 14 16 18 i20 22 24TIME (h)

Fig. 7. Population densities of total chrysophytes and of Synura ulvella, S. petersenii, Mallomonas globosa, M. caudata and Chrysococ-

cus rufescens.

v

19)

109

differed. The maximum occurrence of S. ulvella wasat 08.00 h, in both experimental treatments, and asecond, smaller population maximum was noted at15.30 h in L:D conditions. Very few colonies of S.petersenii were found on the sediment surface until15.30 h. The greatest population density in L:D con-ditions was recorded at 22.00 h. Under constant con-ditions colony numbers of this large Synura wereconsiderably greater than under L:D conditions andthe maximum colony density was found at 11.00 h.Thus the migration patterns of the two Synura spe-cies differ with respect to both time of day and ex-perimental conditions. Two species of Mallomonaswere amongst the algae quantified, and like the twoSynura species, their migration patterns contrasted.M. globosa exhibited two populaton maxima on thesediment surface, at 09.30 and 15.30 h, whereas onlythe mid-afternoon peak was recorded for M. cauda-ta. The vertical migration patterns during the experi-ment different for these two Mallomonas species un-der the two experimental conditions. Chrysococcus

!101

TotalCryptos 5]

Cry oplatyuris 05

5'Cry.

obovataO

R. minuta 1

Chro. 5coerulea 25-

rufescens was recorded at greatest population num-bers in mid-afternoon in the L:D treatment. UnderL:L conditions, numbers of this alga were muchreduced on the sediment surface. The highest stand-ing crop of Chrysophytes on the sediment surfaceoccurred under L:D conditions at 15.30 h and is ac-counted for by the maximum numbers of M. globo-sa, M. caudata and Chrysococcus rufescens. Therewas some increase in numbers of cells at the sedi-ment surface under darkness, i.e. at 22.00 h in theL:D treatment; this applied to S. petersenii and bothMallomonas spp.

Cryptomonads (Fig. 8)Numbers of both Cryptomonas species increased onthe surface of the sediment before either Rhodomo-nas minuta or Chroomonas coerulea. All four cryp-tomonads reached their maximum population den-sities at 15.30 h, but a smaller increase in cellnumbers under L:D conditions was found at20.00 h; this was pronounced for C. platyuris and

CRYPTOMONADSLightDark Dark

M---_*--_0 0

6 8 10 12 14TIME (h)

16 18 20 22

Fig. 8. Population densities of total cryptophytes and of Cryptomonas platyuris, C. obovata, Rhodomonas minuta and Chroomonascoerulea.

24

.. . . . . . . . .-

-

110

less so for C. obovata, Rhodomonas and Chroomo-nas. Under constant conditions, numbers of C.platyuris were very much less than under L:D condi-tions, apart from a parallel of the evening peak at20.00 h. Under constant conditions C obovata andChroomonas were present in lower numbers andRhodomonas in greater numbers.

Diatoms (Fig. 9)Cymbella sp. was present on the sediment in themorning only under L:D conditions. The most fre-quent diatoms were two species of Navicula, N. an-glica and N. falsiensis, with maximum numberscounted at 15.30 and 14.00 h respectively. The num-bers of Pinnularia spp. increased most rapidly be-tween 12.00 and 14.00 h. Population densities of alldiatoms were lower in the L:L treatment.

The depth distribution of flagellates on thesediment surface

TrachelomonadsDuring the first three hours of the experiment, thefew trachelomonads that were recorded were evenly

. n,.

distributed in all three layers of lens tissue (Fig. 10a,i.e. depths 1, 2 and 3) in both experimental treat-ments. By 08.00 h the majority were present in thesurface 50 Am depth, i.e. the top layer of lens tissue,and more were found in the 100-150 Am depth cate-gory than in the middle layer in the L:D treatment.Such an increase in numbers of these brown eugle-noids was not recorded at this or any other time dur-ing the experimental period under L:L conditions. InL:D conditions numbers of trachelomonadsdeclined at the surface of the sediment between09.00-12.00 h and then reached a seond populationmaximum at 17.00 h, with the entire assemblage oftrachelomonads in the surface layer of lens tissue(Fig. 10). These algae were recorded below a depthof 50 Am only in the L:L treatment. By 18.00 h, thetotal numbers of trachelomonads counted haddecreased by 58% from ca. 11000 to 4600 cellscm - 2 and, unlike 17.00 h, these were more or lessevenly distributed between the three depth zones. Asmall number of trachelomonads remained in thelens tissue for the rest of the experiment, there wereslightly more than present at the start of the experi-ment, 1000 compared with 850 cells cm- 2 respec-tively. Under L:L conditions, there were much lower

DIATOMS

Total 10Diatoms

0 -Cymbella 021

N.anglica 0,5j

0 1

N. falsiensis 05

Pinnularia 2]01

i 12 14TIME (h)

i6 18 20 22 2

Fig. 9. Population densities of total diatoms and of Cymbella sp., Navicula anglica, N. falsiensis and Pinnularia spp.

L I I i I I I I i I

_

, , .· . . . . . .-

111

numbers of trachelomonads recorded in all the lenstissue layers. Maximum numbers in constant condi-tions were in the deepest layer, 100-150 im, at12.30 h and only a quarter of the maximum recordedunder natural illumination.

The populations of Trachelomonas spp. present atthe sediment surface and in the depth zones

Fig. 10. The distribution of a) total trachelomonads and b) fivespecies of trachelomonads in three depth layers at the sedimentsurface during the twenty hour experimental period. Depths: 1represents 0-50, 2 represents 50-100 and 3 represents100-150 m from sediment surface. Black: L:D treatment, white:L:L treatment.

50-100 Am and 100-150 tm changed in speciescomposition under L:D conditions during the ex-periment (Fig. IOb). Trachelomonas hispida var.coronata was one of the first species to increase innumbers and achieved a population maximum at thesurface of 2000 cells cm-2 at 08.00 h. The numbersdecreased and at 12.00 h a total of only 250 cellscm-2 were found from all three depths. Cell num-bers of this alga increased in the surface 150 Am ofsediment with second a maximum at 15.30 h andthen the population density declined to almost zeroby 21.30 h.

T hispida increased gradually in numbers, from06.00 to 08.00 h (Fig. lob), similar to T hispida var.coronata. Total numbers remained between1500-2500 cells cm- 2, until 15.30 h, when a largeincrease in numbes of this trachelomonad wererecorded up to a total of 10000 cells cm-2, 98% ofwhich were present in the surface 50 um. Populationdensity decreased between 17.30 and 20.00 h, withan inverse gradient of cells from the surface to thedeepest layer of lens tissue. The depth distributionof T hispida at times of population maxima differedfrom that of T hispida var. coronata. The cells of Thispida were found exclusively in the surface 50 mdepth, whereas cells of T hispida var. coronata weredistributed along a depth gradient with 50, 31, and19% of the total population in the 0-50, 50-100and 100-150 Mm depth-layers respectively.

The population density of T volvocina increasedslightly between 05.00 h and the subsequent twosampling times. Between 07.00 and 08.00 h, totalnumbers rose rapidly from 1050 to 6300 cellscm-2, with 67% in the surface 50 Mm, 14% at50-100 Am and 19% between 100-150 Mm. In thefollowing eight hours, T volvocina was a majorcomponent of the trachelomonad component of thealgae on the sediment surface, although less numer-ous than at 08.00 h. However, from 16.00 h onwards,numbers of T volvocina declied, and very few werefound on the sediment surface after 17.00 h.

T varians was less common than the previousthree trachelomonads. None were recorded at05.00 h, and the population density increased from06.00 h to reach a maximum at 08.00 h when 55%of the total population were in the depth100-150 /Am, 18% were at 100-50 Am and 27% in

t-

112

the surface 50 pm. Total numbers then decreasedand none were observed at 14.00 h. A second popu-lation increase had occurred by 16.00 h, with similardepth distribution to the earlier peak in cell numbersat 08.00 h.

T urniga was not recorded until 10.00 h. Maxi-mum numbers of 1800 cells cm- 2 were noted at12.30 h. Similar to T varians the greatest cell num-bers were found in the deepest layer of lens tissue,44% of total cell numbers at this time. Overall, num-bers then declined, but a small population of T urni-ga was found in the surface regions of the sedimentthroughout the remaining experimental period.

Green euglenoids (Fig. 11)For the first two hours of the experiment, numbersof green euglenoids were very low under both treat-ments. By 08.00 h, 4500 cells cm -2 were counted inthe upper two layers of lens tissue under L:D condi-tions. No such increase in the number of green eugle-noid cells was found at this time under constant con-ditions. The numbers of green euglenoids increasedprogressively in natural irradiances, maintaining agradient of population density with depth throughthe three layers of lens tissue, until 11.00 h. The max-imum number of green euglenoids recorded at thistime were 17800 cells cm-2 , of which 78% of thetotal were counted in the surface 50 Arm depth. Thetotal green euglenoid population density declinedfrom midday on, moreover the gradient of decreas-ing cell numbers with depth, within the surface150 pm, was retained throughout the period of ob-servation in L:D conditions. In constant conditions,numbers of green euglenoids recorded were consis-tently lower than in natural irradiance, and apartfrom observations at 15.30 and 17.00 h, a largerproportion of the total population was found in thetwo deeper layers of lens tissue and a smaller propor-tion in the surface layers than with L:D treatment.

The large, conspicuously warted, Euglenagranulata, was recorded in the lens tissues only from08.00 h until 14.00 h (Fig. llb). The highest num-bers of E. granulata were noted at 11.00 h, when theentire population recorded, 700 cells cm-2, was inthe surface 50 Am of the depth profile.

E. ehrenbergii was the second most abundantgreen euglenoid under L:D conditions (see also

rcells.cn

.0

~10

.0_1

:E

0)r'

03 cell cn2

.0, .

.0

5

%0

2-5

5

0

X,-5

5

,0

2-5

Fig. 11. The distribution of a) total green euglenoids and b) eightspecies of green euglenoids in three depth layers at the sedimentsurface as presented in Fig. 10.

Fig. 5). It was present in the lens tissues from 07.00 hthroughout the experimental period. The mostmarked increase in cell numbers occurred between08.00 and 09.00 h, a change in total population den-

-3 -2

113

sity from 1600 to 4600 cells cm-2 . On all samplingoccasions, E. ehrenbergii was found at all threedepths investigated and generally the distributionexhibited a gradient of decreasing cell numbers withdepth.

The most frequent green euglenoid, E. clavata,was observed on all occasions during the twentyhour period, but only occasional cells were observedat 05.00-07.00 h and 22.30-midnight. Populationdensity started to rise at 07.00 h and increased stead-ily to the maximum number recorded, 7800 cellscm-2 at 11.00 h. Depth distribution involved 68°7of this total in the surface 50 um, 14% between50-100 pm and 18% in the deepest 150 pm. Like E.ehrenbergii, E. clavata occupied the surface 150 pmof sediment, represented by the three layers of lenstissue.

E. charkowiensis was less common than the previ-ous two species of euglena. The first record of E.charkowiensis was at 07.00 h and the populationdensity increased until 09.30 h, when 70% of the to-tal number of cells recorded was in the surface50 pm. During the mid-part of the day, numbers ofE. charkowiensis decreased, but the maximum num-bers of 3200 cells cm-2 were present at 18.00 h. Atthis time, 69% of the total were in the surface layer,19% occupied the 50-100 tm depth and 12% wererecorded in the bottom layer of lens tissue.

The elongate E. acus was noted on the sedimentat 08.00 h and maximum numbers, only 1800 cellscm- 2 , were achieved at 09.00 h. Half of these wereat the surface, 39% in the mid-layer and 11% in thedeepest layer of lens tissue. E. acus was not observedat either 12.30 or 14.00 h and recorded on only oneother occasion, 17.00 h, in very small numbers in theuppermost 50 pm of the surface of the sediment.

Phacus rudicula was present at 07.00 and 08.00 h,numbers increased by 09.30 h and the counts re-mained between 1600 and 2000 cells cm- 2 for thesubsequent 4.5 hours. During this time the propor-tion of the population within the surface 50 Am in-creased until at 14.00 h it occupied only the 50 pmsurface layer.

The highest count for E. viridis occurred at11.00 h, and all 800 cells cm- 2 were found in thesurface layer of lens tissue. This euglenoid was

recorded on only 4 other occasions in the twentyhour experiment.

Phacus pleuronectes was not found in the countsuntil 12.30 h. Then the majority of the total popula-tion (1000 cells cm-2) were in the surface 50 pm.Maximum cell numbers were noted at 15.30 h,1500 cells cm- 2, with 60% in the top layer, noneapparently in the middle layer and 40% of the popu-lation in the bottom layer of lens tissue. The popula-tion density of P. pleuronectes declined in the sur-face 50 pm by 19.00 h and a small populationremained near the surface of the sediment, in the100-150 m deep layer, for the duration of the ex-periment. The major surface population of eugle-noids between 09.00-12.00 h was due to large num-bers of E. ehrenbergii, E. clavata, E. charkowiensisand E. acus, with Phacus rudicula having more im-portance after 10.00 h.

Chlamydomonads (Fig. 12)The population density of chlamydomonads, underL:D conditions, increased predominantly in the sur-face to 50 m layer from 07.00-14.00 h. At 14.00 h,84% of these were in the surface 50 m depth, 12%in the 50-100 Mm region and only 4% in the layerat a depth of 100-150 m. The maximum popula-tion density of chlamydomonads in these conditionsoccurred at 15.30 h when some depth redistributionappeared to have taken place, 75% of the total of23 500 cells cm - 2 occupied the upper-most layer oflens tissue, 24% were found in the middle layer andonly 1% of the total population were below a depthof 100 pm. This distribution of chlamydomonadswith a marked concentration of the population inthe top 50 m layer of the sediment remained until19.00 h, although the total number of cells in thethree layers had declined. The population density ofthese small green flagellates decreased greatly by20.30 h, and on this occasion and at 22.00 h thedepth stratification of cell numbers was no longerrecorded. Under constant conditions (L:L) numbersof chlamydomonads were consistently lower thanunder natural irradiance. Both increase in numbersof cells was later and depth stratification of cellnumbers in the three layers of lens tissue ws lesmarked than in L:D conditions.

114

Chloromonas grove, the most commonchlamydomonad, was the major numerical compo-nent of the algal population at 11.00 h (Fig. 12b).This minute flagellate made up 59% of the totalchlamydomonads, of which 92% of C. grove werein the surface 50 m. The maximum numbers of thisflagellate, 17400 cells cm-2, were recorded at15.30 h, and again this population was concentratedin the surface layer (71% of total C. grovel). Themost striking feature was the rapid increase in num-

6

Fig. 12. The distribution of a) total chlamydomonads and b) fivespecies of chlamydomonads in three depth layers at the sedimentsurface as presented in Fig. 10.

bers of C. grovel between 09.30 and 11.00 h. The de-cline in population density was gradual from 17.00until 20.30, with cells recorded mostly in the surface50 Mm layer, which indicates that movement throughthe 150 MAm depth of three layer of lens tissue by thissmall flagellate is rapid and/or by the shortest directroute, i.e. vertically down into the sediment. Some ofthe C. grove population were noted at 150 um depthat 22.30 h, i.e. in darkness.

Numbers of Chlamydomonas gloeogama in-creased gradually from 08.00 h retaining a gradientin all numbers decreasing with depth from the sur-face from 09.00 h until 14.00 h. The maximumpopulation density coincided with that of Chloro-monas grovel at 15.30 h, with 75%/o of the cells in thesurface layer of lens tissue. Numbers of Chlamydo-monas gloeogama decreased but the remainingpopulation was in the surface layer of lens tissue.

Chlamydomonas sphaeroides was the third mem-ber of this group to reach maximum numbers in mid-afternoon. At 15.30 h, 77°7o of the total 4300 cellscm- 2 were in the surface 50 Am, the rest were in themiddle layer of lens tissue. For the following threeobservations C. sphaeroides was found only in thesurface 50 Am. The numbers of cells in the surfacelayers had decreased to very few by 22.30 h.

Numbers of C. spinifera and C. minutissima wereboth low. Results (Fig. 12b) suggest that there mayhave been upward movement of C. spinifera throughthe layers of lens tissue and an increase in populationdensity at the surface between 07.00-09.00 h. Themaximum count was at 09.00 h, with all 1400 cellsin the uppermost 50 pm. This maximum decreasedgradually during the rest of the experimental period,but small numbers of C spinifera were recordedthroughout this time. C. minutissima was found ononly seven occasions. It was always in the surface50 m, and recorded below this depth only at15.30 h when the maximum number (600 cellscm- 2 ) was noted.

Chrysophytes (Fig. 13)During the first population increase of chrysophyteson the sediment surface, between 07.00-09.30 h,the colonies and cells were more evenly distributedin the three depth zones, than the three flagellategroups previously discussed. At 09.30, the chryso-

115

phytes were distributed with 28% of the total in thesurface tissue, 40% in the middle and 32% in thebottom layer. Similarly the population maximum,was distributed evenly through the surface 150 Amof the sediment at 15.30 h. Of this total 12500 in-dividuals cm-2, 36% were in the surface 50 Am,38% were between 50-100 Am and 26% were in the

Fig. 13. The distribution of a) total chrysophytes and b) six spe-cies of chrysophytes in three depth layers at the sediment surfaceas presented in Fig. 10.

100-150 Am depth layer. The second maximum at15.30 h declined very rapidly at all sampling depths.The population recorded at dusk (20.30 h) was situ-ated almost entirely in the deepest layer at100-150 s4m from the surface. These distributionswere found under L:D conditions. In constant irradi-ance and temperature, numbers of chrysophyteswere lower, and apart from at 06.00 h and 08.00 h,the majority of the population was below the surface50 iAm of the sediment. The major components inL:L conditions were the two species of Synura.

The individual species of Chrysophytes (Fig. 13b)in the surface 150 Am of sediment were present incounts for shorter periods than most of the previousthree groups of flagellates (Figs. 10, 11, 12). Coloniesof Synura ulvella were recorded from06.00-17.00 h, apart from at 14.00 h. The highestnumbers of this colonial alga occurred at 08.00 h at100-150 Mm depth. S. petersenii was infrequent inoccurrence, and was not recorded in the surface50 Mtm. Mallomonas globosa increased in numbersgradually from 06.00- 09.30 h, the maximum num-bers at any one depth level were between100-150 Mm depth at 09.30 h. Numbers increased asecond time during the afternoon, and the greatestnumber of cells was found in the middle layer of lenstissue. M. caudata was less common, but again maxi-mum numbers were mid-afternoon. Occasional cellsof Chrysococcus rufescens were recorded between06.00 and 11.00 h, but the majority were present inthe surface region between 14.00- 18.00 h. Numbersincreased in the surface two layers of lens tissue,0-100 m, and the majority of the populationwere concentrated in the upper 50 Mm, particularlyat 15.30 h. Chromulina sp. was not recorded until10.00 h. Very few of these small brown flagellateswere found in the surface layer of lens tissue, num-bers increased, particularly in the 50- 100 Am layer,from 10.00-14.00 h, but the population maximumwas recorded in the deepest layer, 100- 150 Am fromthe surface, at 15.30 h. The morning chrysophytepeak may be ascribed to S. ulvella and Mallomonasglobosa, whereas the population maximum record-ed in the afternoon was dominated by Chrysococcusrufescens and Chromulina sp. together with all otherfour chrysophytes.

116

Cryptophytes (Fig. 14)The number of cryptophyte cells increased sequen-tially with time and proximity to the sediment sur-face from 08.00-11.00 h in L:D conditions(Fig. 14a). At 08.00 h the greatest proportion, 48%of the total numbers between 100-150 m deep, at09.00 h 62% was in the depth interval 50-100 /Amand by 11.00 h the majority (85%) were in the top50/ m. However, the total recorded down thesediment-surface depth profile had decreased by11.00 h compared with 09.30 h. The maximumpopulation was recorded at 15.30 h, when 46% were

Fig. 14. The distribution of a) total cryptomonads and b) fourspecies of cryptomonads in three depth layers at the sediment sur-face as presented in Fig. 10.

in the deepest layer, 28% in the middle layer and26% in the surface 50 um. After declining in num-bers, there was an increase in population density at20.00 h in the lowest level, 100-150 ,tm depth. Inconstant conditions the numbers of cryptophyteswere lower throughout the three depth intervals thanin L:D conditions. The increase in numbers was ca.four hours later and less pronounced than in L:Dconditions. The concentration of cells at100-150 ,tm at 20.30 h was also evident in L:L con-ditions.

Cryptomonasplatyuris was found predominantlyin the deeper 2 layers of lens tissue (50-150 ltmdepth) at 07.00 and 08.00 h (Fig. 14b). Total num-bers of this large flagellate then increased after mid-day, and again the majority of the population wasin the deeper two layers of lens tissue. At 20.30 h, C.platyuris occupied only the 100-150 tum depth fromthe surface. C. obovata was recorded throughout thestudy and was the major component of the cryp-tomonad group; increases in numbers of this flagel-late were mainly responsible for the population-depth maxima at 09.30 and 15.30 h. At these times,62% were present in the mid-depth zone and 50%in the deepest layer, respectively. C. obovata was not-ed at 20.30 and 22.00 h in the deepest layer of lenstissue. The two peaks in population density ofRhodomonas minuta at 08.00 and 15.30 h bothshowed a direct relationship between populationdensity and distance from the sediment surface;2000 cells cm- 2 were found at 100-150 /tm at08.00 h and 2000 cells cm -2 were recorded at thisdepth at 15.30 h. Similar numbers of cells were pres-ent in the 20.30 and 22.00 h samples in this, thedeepest layer. Chroomonas coerulea reached maxi-mum population density in the morning at 09.30 h,with an inverse gradient of population with depth atthe sediment surface. Numbers declined to almostzero at midday. This blue-green coloured flagellatereached a second smaller peak in the afternoon, andpersisted from 19.00 h to the end of observations inthe top 50 um of sediment only.

Discussion

The precision of counting the groups of flagellate al-gae studied was lower than values given in the litera-

117

ture for estimations of standing crops of epipelic al-gae such as diatoms (Eaton & Moss, 1966), andsimilar to those for nanno-Chlorophyta (Happey,1970) and flagellate Volvocales (Happey-Wood &Priddle, 1984). Differences in population densitiesof any flagellates during the twenty hour period dis-cussed here involved changes in numbers of in-dividuals in the counts by one or two orders of mag-nitude, and in population densities from zero toseveral or tens of thousands cm -2. Values for count-ing precision (Table 2) have been calculated fromdata collected two days after collection of the sam-ples from the field, with lower standing crops of al-gae than found on the previous day. This might beascribed partly to longer time from sampling in thefield and also to the substrate on which they wereplaced. Further investigations of different substratesfor epipelic flagellates are indicated, although dia-toms have been found to migrate through talcum ina manner comparable to that on the sediment fromthe natural environment (P. Jones, in preparation).Nevertheless, despite the low standing crops oftrachelomonads and green euglenoids, changes innumbers were shown to exist, differences in popula-tion densities far exceeded the mean values+ 2 x SD (Fig. 2). Investigations of the statisticalvariables for sampling and counting an epipelicchlamydomonad (Happey-Wood, 1984) demonstrat-ed differences in counts arising in replicates from thePetri Dish sample in the laboratory and in countsfrom different field samples. In this study, an at-tempt to minimise such patchiness in algal distribu-tion was made by mixing all eight field samples to-gether before counting was attempted. The actualcounts for the majority of algal species during thetwenty hour period spanned the range zero to greaterthan two orders of magnitude, and taking the preci-sion of counting estimated in this investigation andvalues already in the literature, the changes in popu-lation densities of flagellates at the sediment surfacewere real. Thus vertical migration of these epipelicflagellates, onto the surface and back into the sedi-ment, was taking place during the period of the ex-periment.

Some patchiness of organisms on and within thesediment is likely, due to their ability to move, andthis will be in a state of flux, since the individuals

are capable of movement within a three-dimensionalenvironment, involving the sediment, interstitialwater and overlying water. Such movement may havebeen limited in the experimental design used here,since the film of water overlying the sediment deter-mined the extent of vertical movement. In the natu-ral environment this distance moved might be great-er, involving transfer of some of the algae, eitherparticular species or very actively motile cells, intothe plankton above the sediment-water interface.Transfer of epipelic chlamydomonads from sedi-ment and subsequent planktonic growth has beendocumented (Happey-Wood, 1978).

Some of the fluctuation in cell numbers (ca. 20%)during the period has been associated with errors inquantitative estimations of the organisms, andsome, yet undetermined, might be associated withrandom movement or movement mainly in ahorizontal plane, both of which are worthy of fur-ther investigation. However, changes in numbers be-tween minimum and maximum of 1000- 12200 fortrachelomonads, 100-17800 for green euglenoids,550- 23 500 for chlamydomonads, 550-12 500 forchrysophytes and 500-12500 for cryptomonads (allin individuals cm- 2) between the start and thepopulation maxima, and all followed later by similardecreases in numbers confirm the presence of a verti-cal migration rhythm under conditions of natural ir-radiance in the laboratory.

The diurnal migration rhythms of flagellatesdescribed by Round & Eaton (1966) involved changesin total counts of the genus Phacus and to a lesserextent Euglena and Trachelomonas of between ca.100 and just greater than 200 cells during a similar20 hour period. Changes in numbers of flagellatesin this investigation were greater and occurred morerapidly for all algal groups. However, the work ofRound & Eaton (1966) described the vertical-migration rhythm of an epipelic community domi-nated by diatoms rather than mostly flagellates ashere for Rhos Ddu. The flagellates from this epipeliccommunity increased in population density on thesediment surface prior to the diatoms; this order wasreversed in the results of Round & Eaton (1966).

Vertical movement of Euglena obtusa Schmitz.from the River Avon, England, was found to be tidalin the natural environment, but diurnal in simulated

118

day-night conditions at 15 C in the laboratory(Palmer & Round, 1965). Their results for E. obtusaindicated the retention of a surface population ofsimilar standing crop of 400-500 cells mm-2 foran eight hour period with little fluctuation in num-bers compared with the data presented here wheretotal numbers of Euglena spp. increased for a fourhour period to reach a maximum at the sedimentsurface and then declined gradually in numbers overa ten hour period.

An interesting feature of the vertical-migrationmovements of the Rhos Ddu epipelic flagellatepopulation was the differences between the move-ment cycle of major algal groups and species withinthese groups. The trachelomonads were present onthe sediment surface earliest and increased in num-bers rapidly from 07.00 h, with greatest populationdensities attributable to five species. Numbers ofthese flagellates remained high for nine hours anddeclined rapidly after 17.00 h. Increase in numbersof green euglenoids was more gradual over fourhours to a single population maximum and thendeclined steadily during the subsequent ten hours.Seven species of green euglenoid achieved popula-tion maxima at three of the sampling times; four ofthese were at 11.00 h. The chlamydomonads reachedmaximum total numbers on the sediment surface at15.30 h, and of the five major species three were pre-dominant at this time. Similarly, at 15.30 h, bothchrysophytes and cryptomonads were maximal innumbers at 15.30 h. Four species of chrysophyte andthree cryptomonads were the major components ofthese two maximum population densities.

Migration upwards of the twenty-seven mostnumerous flagellates resulted in algal populationmaxima at the sediment surface of different speciescomposition at eight sampling times (Table 3). Sevenof these species were found to accumulate in twopopulation maxima on the sediment surface duringthe period. Apart from 17.00 and 18.00 h when sin-gle species, T hispida and E. charkowiensis respec-tively, were dominant, the algal population maximawere of mixed species composition (Thble 3).

Within the confines of sampling intervals, 1 or1.5 h and the three depth intervals of lens tissue usedon the sediment surface, it was possible to identifyprogressive upward movement of algal cells through

the lens tissue over more than two consecutive sam-pling intervals and similar downward movement bydecreasing numbers of individuals, for only sixflagellate species. Such upward migration was notedfor E. charkowiensis by 09.30 h, E. ehrenbergii, E.clavata, Chlor. grovei (Fig. 12) and Chroom.coerulea (Fig. 14) by 11.00 h and T hispida (Fig. 11)by 16.00 h. Progressive downward movement wasrecorded for E. ehrenbergii and E. clavata through-out the afternoon, Chroom. coerulea from15.30-17.00 h and T. hispida from 16.00-18.00 h.Surface or sub-surface population maxima of theother predominant flagellates developed morerapidly than could be identified by the sampling in-tervals, both in time and depth intervals, or involvedthe aggregation at one depth interval of a populationpreviously abundant at all three depth intervals sam-pled (e.g. T volvocina at the surface at 14.00 h,Fig. 11; Crypt. obovata between 50-100 m at09.30 h, Fig. 14) or extreme horizontal patchiness indistribution. More detailed examination of the dis-tribution and movement of these flagellates is re-quired at shorter time intervals and smaller depth in-tervals.

Studies in vertical migration patterns inphytoplankton in the field have demonstrated move-ment of different algal species at different times dur-ing the day resulting in populations concentrated atparticular depth zones (Happey-Wood, 1976). Epi-pelic flagellates demonstrated a similar phenome-non with particular species dominant on the surfaceof the sediment at specific times of day, althoughthere is no information concerning their depth dis-tribution in the sediment below a depth of 150 m.Distances moved by flagellates in sediments, partic-ularly in a vertical direction, are likely to be muchless than by planktonic species in calm water condi-tions. This may be manifested by the presence of epi-pelic flagellates for longer periods of time on thesediment surface (e.g. Trachelomonas spp. for9 hours, Fig. 4; Chlor. grove for 14 hours, Fig. 6)than for planktonic population maxima within aparticular depth zone. Water movements will begreater in open water in the natural environmentthrough wind stress and disturb the inherent plank-ton distribution whereas in the laboratory studyhere, water movements and sediment disturbance

119

Table 3. Summary of timing of major populations of flagellates on the surface of sediment under natural irradiance. Depth of surfacemaxima, T = 0 - 50 tim, M = 50 - 100 am and B = 100 - 150 1tm depth and percentage of the population at that time at that depth aregiven in brackets.

Time (h) 08.00 09.30

Species: T. hispida var. coronata (T 55%)* E. charkowiensis (T 70%)*T. volvocina (T 67%) E. acus (T 50%)T. varians (B 22%)* Chi. spinifera (T 100%)Rh. minuta (B 52%)* Mall. globosa (B 54%o)*S. ulvella (B 47%) Cry. obovata (M 74%)*

Chroom. coerulea (T 47%)*

Time (h) 11.00 12.30

Species: E. granulata (T 100%) T. urniga (B 44%)E. ehrenbergii (T 76%)E. clavata (T 68%)E. viridis (T 100%)Chlor. grove (T 92%)*

Time (h) 4.00 15.30

Species: Ph. rudicula (T 100%) T. hispida var. coronata (T 50%)*T. varians (T 70%)*Ph. pleuronectes (T 60%)Chlor. grovei (T 75%)*Chl. sphaeroides (T 77%)Chl. minutissima (T 66%)Mall. globosa (M 67%)*Mall. caudata (T + M 50%)Chrys. rufescens (T 69%)Chromulina sp. (B 82%)Crypt. platyuris (M 33%)Crypt. obovata (B 50%)*Rhod. minuta (B 71%)*Chroom. coerulea (T 68%)*

Time (h) 17.00 18.00

Species: T. hispida (T 100%) E. charkowiensis (T 69%)*

*denotes species which exhibited two population maxima during the observations.

was minimised. Thus populations may have persist-ed longer at the sediment surface than in the fieldsituation.

P. Jones (pers. commun.) has demonstrated a ver-tical migration rhythm in estuarine diatoms, wherethe speed of movement of Pleurosigma angulatumand net distance moved correlated closely with theactive upward and downward phases of movementof the diatom out of and into the sediment. Differentflagellates move at different rates and in different

characteristic pathways. This aspect of their biologyneeds further investigation and may help to accountfor the range of migration rhythms recorded in thisinvestigation. However, in addition to the charac-teristic features of motility, other factors, both auto-genic and allogenic, may affect the movement of theorganism and thus directly the vertical migration cy-cle. Irradiance, particularly at high intensities, hasbeen found to affect the depth interval at which thesubsurfaces maximum population density of a

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plankton alga may be found in natural phytoplank-ton (e.g. Berman & Rodhe, 1971; Tailing, 1971;Heaney & Tailing, 1980).

During periods of thermal stratification, migra-tion movements of phytoplankton involve migrationof the population down the water column, usuallyin the dark phase, towards regions of increased es-sential nutrients such as ammonium-nitrogen andphosphorus (e.g. Happey-Wood, 1976; Harris et al.,1979; Heaney & Tailing, 1980). Phenomena ofstratification such as changes in concentrations ofdissolved gas and essential nutrients will be similarat the sediment-water interface to the metalimnionof eutrophic lakes but reside within a much narrowerdepth-interval involving steeper concentration gra-dients than in open water. Migration movements ofCeratiumfurca (Ehr.) Clap. et Lachm. and Gonyan-lax polyedra Stein have been shown to be affectedby the interaction of light, temperature and nitrogenin laboratory studies (Heaney & Eppley, 1981). Theretrieval of phosphorus from subsurface depths of120-160 cm and transfer to surface regions of ex-perimental tubes has been ascribed to the active ver-tical migration of motile algae, particularly Crypto-monas marssonii Mars. in natural planktonpopulations (Salonen et al., 1984). Thus it is likelythat similar factors at the sediment-water interfaceand in the surface regions of the sediment may in-fluence or be involved in migration movements ofepipelic algae and these require experimental investi-gation. Diatoms have been found to grow in benthicsediments when absent from overlying plankton andwhen no detectable silica was present in the overlyingwater (Happey-Wood & Priddle, 1984) indicating thelikelihood of nutrient uptake from the sedimentboundary layers and interstitial water. Such uptakeprocesses and nutrient accumulation would be aidedby downward migration of algae into the surface lay-ers of sediment.

Timing of the migration movements, the persist-ence of algae on the sediment surface and the num-ber of population maxima on the surface during theday varies between the algal groups and within thealgal species found in samples from Rhos Ddu. Themost consistent group were euglenoids, where fourspecies of green euglenoid were coincident on thesediment surface at 11.00 h and three species of

trachelomonads were on the surface at 08.00 h, butof these three species, two manifested second after-noon maxima. Experimental manipulation of en-vironmental conditions such as irradiance, bothquantity and quality, temperature and nutrientsboth inorganic and organic may help to explain themosaic of migration patterns recorded for thesetwenty-seven flagellates. Such features may affectthe rate or direction of movement both of which mayvary at particular times of day.

Some diurnal rhythms of movement of epipelic al-gae have been shown to be endogenous and main-tained when the algae are retained under conditionsof constant light and temperature (Palmer & Round,1965, 1967; Round & Happey, 1965; Round & Eaton,1966). In this investigation the expression and phas-ing of the vertical migration rhythm in constant ir-radiance at 10 °C was affected least for the green eu-glenoids and chlamydomonads (Fig. 3), particularlyE. clavata, E. acus and possibly R rudicula (Fig. 5)and Chlor. grovei (Fig. 6). Thus the rhythm in thesealgae may be endogenous, but further confirmationis required. For all other flagellates, the verticalmigration rhythm was suppressed or altered, similarto that described for freshwater diatoms from streamsites (Round & Happey, 1965). Of interest is the in-creased population densities of Synura spp. on thesediment surface under continuous light. This wasthe only algal genus with more individuals presentin constant irradiance. Further explanation is re-quired for the increase in numbers of S. petersenii,M. globosa, M. caudata and Rhod. minuta on thesediment surface after darkness.

The presence of vertical migration rhythms inflagellates may confer some selective advantage tothe survival and growth of flagellate species in themixed algal population assemblage of freshwaterepipelic floras. Flagellates as a group of micro-organism have several unique physiological charac-teristics in addition to their ability to move. Thus thepossible selective advantage or disadvantage ofmovement in relation to these strategies will be con-sidered in the context of the complex and variableenvironment of the surface sediments.

Movement itself will require some utilisation ofcellular energy. This is thought to be relatively littlein comparison to the energetics of the cell as a whole,

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except perhaps in the case of very small flagellatesof diameter less than 2 Am due to the relatively great-er effect of surface tension in an organism with alarge surface area to volume ratio (Raven, 1987). Thecost of movement to cells in a benthic environmentis likely to be less than in planktonic communitiessince the forces of water turbulence are likely to beless and distances moved shorter than forphytoplankters in open water. Sediment distur-bance, either by animals or wind action may reducethe effective movement of epipelic algal inhabitantsand thus increase cellular cost in energy of theirmigration movements. The movement of flagellatesmay enable them to adopt a depth position or timeperiod during the day on the sediment surface sub-ject to optimum photosynthetic active radiation.Similarly, movement may enable the uptake of es-sential nutrients in the sediments as described forcertain phytoplankton species (Salonen et al., 1984).

Flagellates as a biological group exhibit variedmetabolism and nutrition, some photosynthetic spe-cies utilise dissolved inorganic carbon, some requireother organic substrates and others are capable ofheterotrophic nutrition or may be able to ingest or-ganic material (Klaveness, 1985). Flagellates areoften dominant in acidic conditions such as Finnishlakes (e.g. Salonen et al., 1984) and Happey-Wood(1980) showed a relationship by ordination betweenpopulations of epipelic chlamydomonads and thebicarbonate-alkalinity in the overlying water of anacidic pool. Acidity and the inorganic carbonequilibria in freshwater have an important role in de-termimng the growth of oligotrophic and eutrophicspecies (Moss, 1973) or the change in pH and availa-bility of different forms of inorganic carbon duringthe day at different times of year (Talling, 1976). In

anaerobic sediments or under stratified water condi-tions levels of dissolved carbon dioxide are high(Hutchinson, 1957; Mortimer, 1941, 1942), thusmovement might be important for the acquisition ofdissolved inorganic carbon in addition to essentialnutrients such as nitrogen and phosphorus. The in-organic form or forms of carbon utilized by flagel-lates is thus worthy of further study. Movement islikely to be of great importance to flagellates requir-ing organic substrates for growth, since the levels ofdissolved organic substrates will be much higher inthe sediment and interstitial water than in the overly-ing water. The ability to move is likely to be of advan-tage to the survival of flagellates in terms of obtain-ing different sources of inorganic carbon, essentialnutrients and organic substrates.

Flagellates exhibit a wide range of photosyntheticpigments and also ornamentation such as warts andtheca which may be coloured, as in the case oftrachelomonads. The absorption of irradiance forphotosynthesis will vary between species dependingon their complement of major and accessory pho-tosynthetic pigments but also may be affected by cellornamentation. Thus an assemblage of mixedflagellate species such as described here will containan array of irradiance absorptions, and this togetherwith migration of different species of flagellatesonto the sediment surface or to a sub-surface depthof optimum irradiance at different times of day, mayenable the coexistence at any one time of a popula-tion of mixed species composition.

The strategies of movement, varied nutrition andrange of pigmentation in flagellates all confirmselective advantages to flagellates for their survivalat the sediment-water interface (Table 4). This, to-gether with the possession of vertical-migration

Table 4. Summary of flagellate characteristics and their possible selective advantage or disadvantage in the epipelic environment.

Characteristic (Strategy) Selective advantage Effect on flagellatesor disadvantage

Movement + Some use of energy. Wider range of niches exploited.

Varied nutrition inorganic C + Wider range of niches exploited, thus survival inorganic C + competition

Different photosynthetic pigmentation + Optimum irradiances attained.

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movements on to the sediment and re-burial at alater point in time, with patterns of movementcharacteristic of different species, aids the coexis-tence of a diverse flora of flagellates in the complexenvironment of shallow-water sediments. Evolutionto the present complex mosaic of movement patternsmay have involved the gradual directionalisation ofrandom movements by flagellates through externalstimulii such as the 'flickering light' characteristic ofunderwater situations (O. Lundqvist, pers. comm.),resulting in an exogenous rhythm of physiologicalsignificance aiding survival which, over time in somesituations, may have become endogenous.

Acknowledgements

The author is particularly grateful to Peter Jones forassistance with the twenty hour experiment, thestudy of the sediment by video-photography andmaking the data in Fig. 2 available for publication.Thanks are also due to the Natural ConservancyCouncil for permission to sample Rhos Ddu.

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