Interactions between A splanchna and Keratella cochlearis in the Pluisee (north Germany)

Wolfgang HofmannMax- Planck-Institut fur Limnologie, Abt . Allgemeine Limnologie, Postfach 165, D-2320 Plon, F.R.G .

Keywords: rotifers, population dynamics, predation, competition, species diversity, Asplanchna, Keratella

Abstract

Extremely high abundance of Asplanchna priodonta led to a decline in the population of the preferred foodspecies, Keratella cochlearis and subsequently, to the extinction of the predator population . Kellicottialongispina was obviously favoured by the predatory losses of the Keratella. Thus, the interactions betweenAsplanchna and Keratella influenced the zooplankton community structure .

Introduction

Variation in the abundance of Asplanchna fromyear to year in the PluBsee, provided the opportuni-ty to examine the influence of this predator on thedynamics of individual rotifer populations andfurthermore, on the zooplankton community .

Samples were collected with a 5-1 vessel in 1969,1970, and 1972 (weekly vertical profiles, 1 m sam-pling interval) (Hofmann 1975) .

Results

In the PluBsee (Hofmann 1975) two Asplanchnaspecies occur : A. priodonta Gosse and A . girodi(De Guerne). The peak abundance for both speciesgenerally occurred in April or May. However, therewere great changes in species composition andabundance from year to year . In 1969, A . priodontapredominated, in 1970 only A . girodi was found,and in 1972 the genus was represented only by A.priodonta . Furthermore, in 1972, abundance wasexceptionally high compared to 1969 or 1970 (Fig .1). On three sampling dates abundance exceeded1 000 ind . 1 -1 with maximum abundance about2 240 ind . 1 -1 . The extremely high abundance of this

Hydrobiologia 104, 363-365 (1983) .© Dr W . Junk Publishers, The Hague . Printed in The Netherlands .

predatory rotifer probably influenced the dynamicsof its prey species .

According to Pourriot (1977), Asplanchna prio-donta preferentially feeds on Keratella cochlearis(Gosse). In the Plul3see this species usually reachesmaximum abundance in May, with the exception of1972 . In that year population growth stopped at theend of April followed by a rapid decline . If theabundance curves of Asplanchna and K. cochlearisare plotted together, they produce a typical pair ofpredator-prey curves (Krebs 1972) showing a nu-merical response of Asplanchna to the populationof Keratella . This response led to a decline of theprey population and, subsequently, to the declineand disappearance of the predator. In June thepopulation of Keratella started to grow again (Fig .2). Examination of gut contents of Asplanchnaproved that K. cochlearis was the exclusive animalfood .

During April and May, K. cochlearis was dis-tributed throughout the water column, showing asecond peak of abundance in the lower hypolim-nion. As only the epilimnic part of the populationcoincided with Asplanchna, only this subset of thepopulation has been included in Fig . 2 .

In Figure 3 the two vertically separated subunitsof the population are examined separately . If

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abundance dynamics and egg ratios are compared itbecomes evident that the decline in Keratella at theend of April (which occurred only in the epilim-nion) was not due to a decreasing birth rate . Duringthis period, the birth rate would have led to growthof the population by the factor 1 .1 day -1 , or almosta doubling in seven days (calculation according toEdmondson (1977)). However, mean abundance inthe upper 4 m decreased from 840 ind . 1 -1 (24 April)to just over 80 ind . 1 -1 (2 May) and to 13 ind . 1 -1(8 May) within 14 days . In the beginning of Maythere were more animals in the hypolimnion than inthe epilimnion .

Birth rate is another indication that the decline inK. cochlearis was not brought about by diminishedfood supply . Co-occurring Kellicottia longispina(Kellicott) with almost identical food preferences(Pourriot 1977; Dumont 1977) and which is notinjured by Asplanchna (Edmondson 1965) may beused as an indicator of the food conditions forKeratella . In the period under discussion Kellicot-tia exhibited maximum birth rates and a rapid in-crease in abundance (Fig . 4) indicative of favour-able food conditions for both species .

Furthermore, plotting the abundance curves ofKeratella and Kellicottia together results in a pairof curves which suggests the dynamics of two com-peting species (Fig . 5) . Kellicottia was much moreabundant in 1972 (when K. cochlearis was decimat-ed by Asplanchna) than in 1969 and 1970 whenabundance ofAsplanchna was low . Finally, growthof the Kellicottia population occurred just whenKeratella was declining. As concluded by Dumont(1977), the data presented by Edmondson (1965)indicate competition for food between these spe-cies .

Hence, high Asplanchna abundance not only hada direct negative effect on K. cochlearis, but wasprobably also of importance for species competingwith Keratella and on other components of thezooplankton community .

In 1970 and 1972 there were significant differen-ces in the structure of the epilimnic zooplanktoncommunity . In 1970 there were only two dominantspecies : Filinia terminalis (Plate) in early springand, subsequently, Keratella cochlearis . In 1972,under the influence of Asplanchna, there was agreat variety of important species in spring andearly summer, K. cochlearis being the most abund-ant species in only three samples . Diversity was

4

10 6 N/m 2

s

M

I

A

M

J

J

Fig. 1 . Pluilsee, abundance of Asplanchna in 1969, 1970, and1972 .

10 6 N/m2

M

I A I

M

I

J

I

J

Fig. 2. Plul3see, abundance of Asplanchna priodonta andKeratella cochlearis (0-10 m) in 1972 .

max . N/I

0-01969 priodonta 55-• 1970 girodi 15*-+ 1972 priodonta

2 240

3 .

2-

+-+ K. cochlearis ( 0-10m)

o-o A . priodonta

M

I

A

I

M

J

K. cochlearis0-10m

I-~ i11-22m

0 .2

I J

Fig. 3 . Plul3see, abundance and egg ratios (columns) of Keratellacochlearis in the epilimnion and the hypolimnion in 1972 .

E

0.6

0 .4

0 .2

10 6N/m2

3

2

Fig. 4. Plul3see, abundance and egg ratios (columns) of Kellicottialongispina in 1972 .

0 5 N/m2

s

i

M

I

A

I

M

•

1972 Keratella cochlearis•

1972 Kelticoftia longispino

1970

Kellicottia longispina

IIIi i

Fig. 5. Plullsee, abundance of Kellicottia longispina in 1969,1970, and 1972 ; abundance of Keratella cochlearis in 1972 .

clearly higher . In 1972 the mean relative abundanceof the dominant species was 38% and in 1970 it wasas much as 61% . Consequently, mean diversity wassignificantly higher in 1972 (2 .6) than in 1970 (1 .6)(Hofmann 1975). These differences were obviouslydue to the fact that in 1972 Asplanchna was feedingon the dominant species .

Discussion

The results show that evaluation of populationparameters such as abundance, egg ratio, and birthrate derived from field studies can spotlight some

0 .6

0.c

0 .2

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mechanisms operating in the zooplankton com-munity .

Abundance of planktonic rotifers is sometimesmainly a function of birth rate (and food) (Ed-mondson 1965), for example in Kellicottia longis-pina in 1972 and K. cochlearis in 1970, when theinfluence of Asplanchna was insignificant . Howev-er, high birth rates combined with decreasingabundance denotes high mortality (Edmondson1965) which could be attributed to the predatorAsplanchna priodonta in the case under discussion .

The numerical response of the predator to theprey population led to the extinction of the preda-tor, showing again that stable predator-prey oscil-lations obviously do not exist in field populations(Krebs 1972) .

A third component determining the abundanceofK. cochlearis is apparent from Fig . 3 : the hypo-limnetic part of the population disappeared in spiteof high egg ratios due to an abiotic factor, in thiscase the total depletion of oxygen in the hypolim-nion .

In specific situations the factors determiningpopulation dynamics can be separated and theirroles can be estimated on the basis of data derivedfrom field studies . In such a case it is also possible toreveal responses of the community to changingpredominant ecological factors .

References

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1969

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Dumont, H . J ., 1977 . Biotic factors in the population dynamicsof rotifers . Arch . Hydrobiol . Beih . 8 : 98-122 .

Edmondson, W. T ., 1965 . Reproductive rate of planktonic rotif-ers as related to food and temperature in nature . Ecol . Mo-nogr . 35 : 61-111 .

Edmondson, W. T ., 1977 . Population dynamics and secondaryproduction. Arch. Hydrobiol . Beih . 8 : 56-64 .

Hofmann, W., 1975 . The influence of spring circulation onzooplankton dynamics in the Plul3see . Verb . int . Ver . Lim-nol . 19 : 1241-1250 .

Krebs, C . J ., 1972 . Ecology. Harper & Row, N.Y ., Evanston,San Francisco, Lond . 694 pp .

Pourriot, R ., 1977 . Food and feeding habits of Rotifera . Arch .HydrobioJ. Beih . 8 : 243-260 .