Diurnal dynamics of the vertical distribution of zooplankton in an oligotrophic lake

162

ISSN 1995-0829, Inland Water Biology, 2009, Vol. 2, No. 2, pp. 162–170. © Pleiades Publishing, Ltd., 2009.Original Russian Text © A.E. Dobrynin, published in Biologiya Vnutrennikh Vod, No. 2, 2009, pp. 62–71.

INTRODUCTION

A high water transparency and low concentration ofphytoplankton are characteristic of oligrotrophic water-bodies. Low phytoplankton concentrations are tradi-tionally considered (within the framework of the “bot-tom-up and top-down” [21] theory) one of the main fac-tors influencing the vertical distribution (VD) ofzooplankton. At the same time, a clear dependence ofdiurnal VD dynamics on the extent of water columnillumination was revealed in fresh waterbodies [1]. It isknown that, on the one hand, light serves as a signaltriggering the migration mechanism; on the other hand,it directly negatively affects the zooplankton [17].

In mesotrophic and, especially, in eutrophic waterbod-ies, the short wave part of the solar radiation is absorbed inthe near-surface water layers. In the oligrotrophic water-bodies, it penetrates to depths of up to

≥

7

m [23]. How-ever, the maximal absorption of ultraviolet light by thewater falls upon the shortest (230–280 nm) waves (UV–S)[12]. These waves do not penetrate deeper than the near-surface layers. The UV-S particularly are the mostimportant in terms of having photo-destructive effects onzooplankters by affecting the DNA molecules [10]. Therecovery of DNA occurs with the participation of theenzyme photoliase at the long-wave (350–400 nm) partof UV radiations (UV-A) and visible light [22]. Thelong-wave part of UV radiation may also result in photo-destructive reactions in DNA. However, this is only pos-sible at high intensities and doses of radiation: one tothree orders higher in intensity and five to six ordershigher in doses of radiation than the respective parame-ters of short-wave UV radiation [10]. In addition, the

effect of waves of the middle part of UV radiation(UV–B) leads to the formation of hydroxyl radicalsdamaging DNA and oxidizing fatty acids and proteins[15]. The experiments on exposing daphniae [13] andcopepods to UV–B [16] revealed that the survival ofthese animals does not depend on the oxygen concentra-tion but increases upon a drop in temperature. This mayexplain why, during the day, zooplanktons leave warmnear-surface water layers that are saturated with UV–Sand move deeper to an area there there is still UV–A,UV–B, and visible light. These parts of radiation are nec-essary for photoliase functioning, while a decreased tem-perature facilitates the animal’s survival after irradiationby UV–B.

Studies on waterbodies with high water transpar-ency have not yet yielded an unequivocal answer con-cerning the relation between the extent of water columnillumination and zooplankton distribution. In some oli-gotrophic waterbodies, the organisms prefer to avoidthe upper five meters of the water layer [14], while inothers such phenomenon was not recorded [5].

The goals of this study are to reveal the peculiaritiesof the vertical distribution of zooplankton and its diur-nal dynamics in the oligotrophic Lake Koskovskoyeand to asses the influence of water transparency uponthese parameters.

MATERIALS AND METHODS

Koskovskoye Lake is a small but deep waterbody(the surface area is about 0.13 km

2

and the maximaldepth is 36 m) located in Vologod raion (Vologod

Diurnal Dynamics of the Vertical Distribution of Zooplankton in an Oligotrophic Lake

A. E. Dobrynin

Institute for Biology of Inland Waters, Russian Academy of Sciences, Borok, Yaroslavl oblast, 152742 Russiae-mail: [email protected]

Received January 28, 2008

Abstract

—The diurnal dynamics of the vertical distribution of zooplankton was studied in an oligotrophic lakewith a high water transparency. Those sedentary species which occupy the upper water layers in waterbodieswith low water transparency shift to a border between epy– and metalimnion and stay there during most of theday in Koskovskoye Lake, which has a high water transparency. Active diurnal vertical migrations are charac-teristic of large organisms. Increased water transparency weakly affects large zooplankters that occupy metal-imnial layers in mesotrophic waterbodies. The relationship between the zooplankton’s vertical distribution andthe distribution of phytoplankton and fish is analyzed.

Key words

: zooplankton, vertical distribution, oligrotrophic waterbody, transparency.

DOI:

10.1134/S1995082909020096

ZOOPLANKTON, ZOOBENTHOS AND ZOOPERIPHYTON

INLAND WATER BIOLOGY

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No. 2

2009

DIURNAL DYNAMICS OF THE VERTICAL DISTRIBUTION OF ZOOPLANKTON 163

oblast). The sampling station was located in the pela-gial at a depth of 14 m (

59°16

′

06.9

″

North;

39°03

′

45.0

″

East). During the period of diurnal sampling (July 23–24, 2000), the sun set at 21:12 and rose at 03:47.

The zooplankton was sampled using a 5 l PB–5bathometer (two replicate samples from each waterlayer). The samples were collected from depths of 0, 1,3, 4, 5, 7, 9, 11, and 13 m. The time interval betweensamplings was 2 h. Before sampling the water temper-ature was measured from the surface to the bottom witha 1 m interval. The water transparency was determinedusing a Secchi disk. After the daily sampling was over,the phytoplankton was sampled from epy-, meta-, andhypolimnion. The spatial distribution of fish was stud-ied using an echo sounder. The hydroacoustic surveyswere carried out along the lake’s transverse section at03:00, 07:00, and 18:00.

The zooplankton samples were processed accordingto commonly accepted methods [7]. A total of108 quantitative samples were collected and processed.The chlorophyll

‡

(Chl

a

) content in the phytoplanktonsamples was determined by N.M. Mineeva.

RESULTS

During the observation period, a stratification oflake water was revealed: the well-pronounced ther-mocline was situated at depths of 3–7 m (Fig. 1). Thelakes has signs of oligotrophy. The Chl

‡

content in theepylimnion is 1.67

µ

g/l; in metalimnion, 1.49

µ

g/l; inhypolimnion, 2.91

µ

g/l. The increase in the Chl

a

con-tent with depth is likely related to high (>5 m) watertransparency, and hence with the excessive insolationof the upper water horizons. In addition, as a rule, theconcentration of nutrients in hypolimnion is higherthan in the water layers above it.

The lake’s fish fauna consists of six species: roach(

Rutilus

rutilus

(L.), bleak (

Alburnus alburnus

(L.),perch (

Perca fluviatilis

L.), ruffe (

Gymnocephalus cer-nuus

L.), pike (

Esox lucius

(L.), and burbot (

Lotalota

(L.) [11]. Data on the vertical and horizontal distri-bution of fish during certain hours obtained by echo-sounding surveys did not reveal any considerable modi-fications in the distribution of fish in the pelagial (Fig. 2).Throughout the whole period of surveys, the fish occu-pied depths of 10–15 m, with a slight dispersiontowards deeper water in the morning (07:00). At thesewater horizons, roach, perch, and pike were found [11].The diurnal horizontal migrations of fish were morepronounced.

In the zooplankton composition, 15 species wererecorded: six species of Rotatoria, six species of Cla-docera, and three species of Copepoda (see the table).

The Calanoida nauplii and copepodits dominated innumber. The densities of

Asplanchna priodonta, Kera-tella cochlearis, Bosmina coregoni, Daphnia cucullata,Diaphanosoma brachyurum, Eudiaptomus graciloides,Mesocyclops leuckarti

, and

Cyclops strenuous

, as well

of Cyclopoida copepodites, were lower.

Keratellaquadrata

,

Filinia

sp.,

Polyarthra

sp.,

Trichocerca

sp.,

Leptodora kindtii

,

Alonella nana

, and

Oxyurella teni-caudis

also had low densities. The zooplankton biom-ass was rather low, averaging 0.95 g/m

3

. Calanoidacopepodites and sexually mature

Eudiaptomus gracil-oides

dominated.The vertical transformations of zooplankton com-

munity were weakly pronounced. During the most ofthe day, the maximal zooplankton biomass was noted ata depth of 5 m, i.e., in the middle of the metalimnion(Fig. 3). In the evening (17:00 to 23:00), zooplanktonmoved to the upper layers of the metalimnion (depthsof 3–4 m). At night, a slight descending of zooplankterswas observed, which intensified to the initial depths inthe morning and daytime.

An analysis of data on the vertical transformations ofseparate species revealed that those zooplankters withmaximal numbers at near-surface layers in waterbodieswith a lower water transparency descend to depths of 3to 4 m (

Daphnia cucullata

,

Bosmina coregoni

, and

Kera-tella cochlearis

) (Figs. 4a–4c) or to 5–7 m (Calanoidacopepodits (Fig. 4d) in Lake Koskovskoye. Species suchas

Daphnia cucullata

,

Bosmina coregoni

, and

Keratellacochlearis

concentrated near the surface for a shortperiod of time in the midnight and left this water horizonat the beginning of dawn. Later, during the day, thenuclei of the populations occupied the upper parts ofmetalimnion. In Calanoida copepodites, the pattern ofdiurnal vertical migrations was similar, but these animalsdescended deeper, to 2–3 m, and occupied the samewater horizons as

Cyclops strenuous

(Fig. 4e). The pres-ence in the lower water horizons, mainly in hypolimnionand at the border between meta– and hypolimnion, ischaracteristic of the latter species. This is presumablyrelated to the larger size of this crustacean and, as a con-sequence, to its tendency to avoid the pressure of the

8

6

12

24

Ï

°

C

Fig. 1.

The water temperature (

°

C) at different depths (m) atthe sampling station on July 23, 2000.

164


Vol. 2

No. 2

2009

DOBRYNIN

3 h

10

30

7 h

18 h

10

30

10

30

m

Fig. 2.

The echogram of fish distribution in Koskovskoye Lake. The wavy line is the bottom profile, the horizontal line is the depth,the dots are the position of fish (the size of a dot depends on the size of separate specimens), the arrow indicates the location of thesampling station.

m

4

8

23 1 3 5 7 9

11 13 15 17 19 21h

1 g/m

3

4

8

Fig. 3.

The diurnal vertical distribution of zooplankton biomass (g/m

3

) in Kosovskoye Lake on July 23–24, 2000. The figures underthe symbols are the time of day and the

Y

axis is the depth.


Vol. 2

No. 2

2009


visual planktophages, along with its eury–oxibionticproperties and preference for cold waters, although thelatter statement is debatable [9].

The nauplii of Copepoda (usually found in massamounts and serving as a food for many planktivorousfish) have a weak mobility and, as a rule, occupy met-alimnion throughout the whole day. The vertical migra-tions of these animals are very inconsiderable. Similarresults were obtained for the Koskovskoye Lake(Fig. 4f). The highest number of nauplii was noted at adepth of 5 m. There, the maximal decrease in watertemperature was registered, causing maximal drop inwater density and viscosity and an accumulation ofdetritus and bacterioplankton at this depth.

The “twilight” migrations are characteristic of

Asplanchna priodonta

(Fig. 4g). During most of theday, the nucleus of the population was presented inmetalimnion, while after sunrise and before sunset theascent of some specimens to epilimnion was observed;it was more pronounced in the second case.

Considerable transformations of vertical distribu-tion were registered in

Mesocyclops leuckarti

,

Eudiap-tomus graciloides

, and

Diaphanosoma brachyurum

(Figs. 4h–4j). In other waterbodies these species areactive migrants [6], although sometimes a lack of ver-tical redistribution of these species densities werenoted [2].

A complicated interdependence was revealed in thedistribution of grabber-zoophage

Mesocyclops leuckarti

and its prey. This reflects the ability of this predator tofeed on varius animal food. A maxima of abundance wasnoted in

M. leuckarti

at 23:00. The first maximum (nearsurface) coincided with the highest concentrations of


,

Bosmina coregoni

, and

Daphnia cucullata

; the second (at a depth of 3 m) coin-cided with

Eudiaptomus graciloides

. At 01:00 and03:00, the pattern of vertical distribution of

Mesocyclopsleuckarti

was similar to that in

Diaphanosoma brachyu-rum

and


(by the sum numberof these species) and the maxima of

M. leuckarti

and


densities coincided. During themorning hours, the vertical distribution of

M. leuckarti

was in fact similar to that in


.Unfortunately, the small amount of cyclopids in the sam-ples collected during the day (from 11:00 to 17:00) didnot allow for a full analysis for this part of the day. At19:00 the distribution of

M. leuckarti

was related to thesum distribution of its preys listed above. At 21:00 therelation was the same and the coincidence of this speciesdistribution with


and

Eud-iaptomus graciloides

was maximal.

DISCUSSION

An analysis of data presented here revealed that, interms of the diurnal dynamics of vertical distribution, theclosest relations were observed between

Mesocyclopsleuckarti

and the highly abundant

Diaphanosoma

brachyurum

. The high efficiency of the latter species wasdescribed earlier [8]. The maxima of the numbers ofthese species coincided in six out of twelve surveys. Thecoincidences of distributions of cyclop populations andthe whole set of its potential prey was more pronouncedduring dark and twilight hours, when the limitationsrelated to the insolation of upper water horizons wereminimal and

M. leuckarti

was able to being exploitingmore available food. The fact that the predator popula-tion follows


and

Eudiapto-mus graciloides

is presumably related to two factors: thehigh feeding value of the former species and the migra-tion of both species from the upper water layers to thehorizons lying below the level of transparency. This fea-ture of the above species differs from

Bosmina coregoni

and

Daphnia cucullata

; these species remained at theborder between epy– and metalimnion.

Species composition, number, and biomass of zooplanktonin Kosovskoye Lake (July 23–24, 2000)

Species Number,specs./m

3

Biomass, mg/m

3

Rotatoria

Asplanchna priodonta

Gosse 5753 115

Keratella cochlearis

(Gosse) 4549 0.9

K. quadrata

(O.F. Müller) 15 0.003

Filinia

sp. 138 0.04

Polyarthra

sp. 279 0.1

Trichocerca

sp. 497 0.15

Total for the group 11231 116

Cladocera

Bosmina coregoni

Baird 5521 72

Daphnia cucullata

Sars 6084 91


(Lievin)

7703 46

Leptodora kindtii

(Focke) 74 15

Alonella nana

(Baird) 3 0.007

Oxyurella tenicaudis

(Sars) 3 0.05

Total for the group 19 390 224

Copepoda


Lill.

3287 159

Mesocyclops leuckarti (Claus)

1809 54

Cyclops strenuus (Fisch.) 1424 100

Nauplii 13422 54

Calanoida copepodites 11319 192

Cyclopoida copepodites 4862 49

Total for the group 36123 608

Total for zooplankton 66744 949

Note: Average daily values are given.

166

INLAND WATER BIOLOGY Vol. 2 No. 2 2009

DOBRYNIN

The dynamics of the distribution of Diaphanosomabrachyurum and Eudiaptomus graciloides had a classi-cal pattern of direct migrations: nighttime ascendingand daytime descending. The range of the migrations ofthese species throughout the day reached 5 m, which, in

case of the clear presence of populations nuclei, is quitesignificant for freshwater zooplankters (Figs. 4i, j).

In Koskovskoye Lake, in several species similar interms of feeding spectra, the spatial–temporal diver-

m

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

(a)

(b)

(c)

Fig. 4. Diurnal vertical distribution of zooplankton. (a) Daphnia cucullata; (b) Bosmina coregoni; (c) Keratella cochlearis; (d) Calanoidacopepodites; (e) Cyclops strenuus; (f) Copepoda nauplii; (g) Asplanchna priodonta; (h) Mesocyclops leuckarti; (i) Eudiaptomusgraciloides; (j) Diaphanosoma brachyurum; (k) Cyclopoida copepodites. Figures depict specific number (% of the total number for thewhole water column) at various horizons. Other desidnations are the same as in Fig. 3.



gence noted earlier in other waterbodies (particularly inSiverskoye Lake, which also situated in Vologda oblast[4]) [18] were revealed. This was noted both in activemigrants and in species that were did not, in fact,migrate. For instance, the transformation patterns ofDiaphanosoma brachyurum and Eudiaptomus gracil-oides vertical densities were similar, but the maximum of

the former species density throughout the day was abovethat of the latter species. Such phenomenon wasobserved despite the fact that Diaphanosoma brachyu-rum is a microfiltrator, while Eudiaptomus graciloides isa macrofiltrator [19, 20]. However, the diapasons offeeding particles consumed by these species overlap.Weakly migrating Cyclopoida and Calanoida copep-

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

(d)

(e)

(f)

Fig. 4. Contd.

168


DOBRYNIN

odites were also disconnected in time and space. Theformer species spent most of the day in metalimnion; thelatter spent most of the day in hypolimnion. Each descentof Cyclopoida copepodites (at 03:00, 11:00, and 17:00)to the border between meta– and hypolimnion corre-

sponded to Calanoida copepodites either moving out ordispersing at these horizons (Fig. 4d, 4k).

The results of the diurnal survey have shown thatwater transparency influences, first of all, the speciestending to the near-surface water horizons. The popula-

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

(g)

(h)

(i)

Fig. 4. Contd.



tions of the small zooplankters with low mobilitydescend to the horizons in the range multiple of 0.5 to1 of the transparency level. The larger and more mobileorganisms actively migrate throughout the whole day.The zooplankters occupying meta– and hypolimnionspread out in the waterbodies with less transparentwater in a similar fashion. Thus, high water transpar-ency leads to a descent of the “near-surface” speciespopulations and weakly affects the zooplankters dwell-ing in the lower layers of the water column. The factthat the “near-surface” species at high water transpar-ency leave the upper water horizons may be explained by(i) the higher irradiation of the these layers by UV–S,which does not penetrate deeper than 1 to 1.5 m even invery transparent water and (ii) the fact that theincreased illumination of the upper water horizonsleads to the stronger pressure of planktivorous fish. Theanalysis of the vertical distribution of fish and zoop-lankters did not reveal any direct relations betweenthese variables. Presumably, their interactions are of amore complicated character than invertebrates simply

avoiding the horizons occupied by fish. In addition, justthe presence of fish kayromones in water may triggerdiurnal vertical migrations of planktic animals [21, etc.].It is necessary to note that in Koskosvkoye Lake,despite the maximal Chl a concentration in thehypolimnial zone, the zooplankters preserve theirmigrations to the upper water layers at night, i.e., thepattern of these migrations are similar to those inmeso– and eutrophic waterbodies in which the maxi-mal Chl a concentration is found in the epylimnial lay-ers. The Chl a concentration reflects the productionactivity of phytoplankton, but not its biomass (the valueof which is the most important for zooplankters).Unfortunately, there are no data on the phytoplanktonbiomass at different horizons in the lake water. How-ever, as was noted in waterbodies similar to Kosk-ovskoye Lake upon the maximal Chl a concentration inhypolimnion, the maximal phytoplankton biomass maybe found in the epylimnion [3]. As a result, the biolog-ical role of diurnal vertical migrations stays important.

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

4

8

23 1 3 5 7 9

11 13 15 17 19 21

4

8

20 %

(j)

(k)

Fig. 4. Contd.

170


DOBRYNIN

CONCLUSIONS

The species that have low mobility and occupy thenear-surface water horizons in the weakly transparentwaterbodies move to the border between epy– and met-alimnion and stay there for most of day in KosovskoyeLake. The increase in water transparency has littleeffect on the zooplankters tending to the metalimnion.The active diurnal vertical migrations are characteristicof large organisms under the conditions of high watertransparency, No relation was revealed between thedynamics of vertical distributions of fish and zooplank-ton. Despite the maximal Chl a concentration inhypolimnion, zooplankters continued to migrate to theupper water horizons at night.

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Diurnal dynamics of the vertical distribution of zooplankton in an oligotrophic lake