Hydrobiologia 134: 117-120 (1986).© Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands.

Rotifer sampling - a complete species list from one visit?

Linda MayInstitute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian, Scotland, U.K.

Keywords: rotifer, method, indicator, resting eggs, sediment

Abstract

This paper describes a possible method of compiling a rotifer species list for any site from a single visit.Sediment samples collected from Loch Leven on one occasion in winter were exposed to light and a rangeof temperature conditions in the laboratory. Resting eggs in the sediment were induced to hatch. The resul-tant species list was very similar to that obtained from regular plankton collections over the previous six yearperiod.

Introduction

Rotifers are well known to be discriminatinganimals in their responses to environment, sincemany species are commonly associated with partic-ular environmental conditions. In principle thissuggests that a study of changes in rotifer speciescould be used to biologically monitor the integrat-ed effects of environmental change. However, intemperate regions, many species occur seasonallyand the practical problems of obtaining representa-tive samples have restricted their use as biologicalmonitors. Using standard methods of collection, acomplete species list for each location can be ob-tained only by several visits during the year. Thispaper suggests a method of compiling a species listfor any given site from a single visit. The proceduretakes advantage of the fact that indigenous rotiferspecies which are not active in the plankton at anygiven time, can be found in the sediments as restingeggs.

Site

Loch Leven (56°10'N,30 30'W) is a shallow, eu-

trophic loch situated in the Scottish lowlands. Ithas mean and maximum depths of 3.9 m and25.5 m respectively and covers an area of13.3 km2 . The chemical, physical and geologicalfeatures of the loch are well documented (Holden& Caines, 1974; Smith, 1974; Kirby, 1974) and itshistory, fauna and flora were summarised by Mor-gan (1974). Loch Leven was chosen as the ex-perimental site because rotifer samples had beencollected there at weekly or fortnightly intervalssince 1977, and so detailed species lists were availa-ble for reference (May, unpubl. data).

Methods

Sediment samples were collected close to an openwater site which had been sampled regularly overthe previous six years. Six equi-distant sites along astraight line between Reed Bower and the southshore were sampled (Fig. 1). The sampling date, 16February 1984, was chosen because rotifer densitiesin the open water are low in winter and most spe-cies are over-wintering as resting eggs in the sedi-ments at this time of year.

An undisturbed sediment core, 7 cm in diameter,

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Even

South Queic

Water

Fig. 1. A map of Loch Leven showing sediment sampling sites ( v), open water sampling site (A) and depth contours. Inset shows thelocation of the loch in Scotland.

was collected at each site with a Jenkin surface-mud sampler (Ohnstad & Jones, 1982). The coredepth varied according to the substrate, but alwaysexceeded 10 cm. A 250 ml surface water samplewas also taken at each sampling point to provide in-formation on the open-water rotifer community atthat time.

On return to the laboratory, the water above eachsediment core was discarded and the cores werepushed to the top of the sample tube with a plungeroperated from below. A purpose made guillotinewas used to remove the top 5 cm of mud from eachcore. These sub-samples of sediment were placed inseparate wide-necked plastic jars and mixed well.Three 20 ml aliquots of the mud suspension wereremoved from each sample at random, and placedseparately into graduated, 250 ml Erlenmeyerflasks. The sample in each flask was diluted withLoch Leven water to give a final volume of 200 ml.This water had been previously filtered throughWhatman GF/C grade glass microfibre filters

(Whatman Ltd, England) to remove zooplanktonand prevent contamination of the sediment samplewith active animals. The conical flasks were co-vered with self-cling plastic film and one samplefrom each site was incubated at 5C, another at10 °C, and the third at 15 C for the duration of theexperiment. All samples were kept under artificiallight, LD = 12:12. These conditions were chosenbecause exposure to light and changes in incuba-tion temperature have been shown to induce hatch-ing of resting eggs in some rotifer species (Clementet al., 1981; Pourriot et al., 1981; Pourriot et al.,1982).

At weekly intervals, the clear surface water ineach conical flask was carefully decanted andreplaced with fresh, filtered, Loch Leven water. Thedecanted liquid was treated with procaine hydroch-loride to anaesthetise soft-bodied rotifers (May,1985). Formaldehyde was added as a preservative,giving a final concentration of 4 per cent. Therotifers in each sample were concentrated by

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sedimentation and the species present were identi-fied under an inverted microscope. The experimentwas continued until no further animals were col-lected from the samples on two consecutive occa-sions (approximately 12 weeks). A species list ofanimals hatching from the sediments was com-piled. This was compared to the species list ob-tained from open-water samples collected at weeklyor fortnightly intervals from 1977 until 1982.

Results

Table 1 shows a species list of planktonic rotiferscollected in open-water samples from Loch Levenbetween 1977 and 1982. The monthly occurrence ofeach species over this six year period is indicated.The table shows that some species, in particularKeratella cochlearis (Gosse) and Synchaeta kitinaRousselet, were generally present throughout theyear and would be found in almost every samplecollected. In contrast, the winter species, Notholcasquamula (Muller) and Polyarthra dolichoptera(Idelson), were usually present from December toApril and November to June, respectively. Theserotifers would only occur in samples collected dur-ing the winter and early spring. Similarly, the warmstenotherms, such as Pompholyx sulcata Hudson

and Trichocerca pusilla (Lauterborn), would berecorded only from summer samples. Table 1 showsthat there is no time of year when a single open-water sample would yield a complete species list forthe loch. Indeed, the single water sample collectedon 16 February 1984 (in parallel to the sedimentsampling program) contained very few species, atvery low levels of abundance (Table 2).

In comparison, a single sample of sediment col-lected in February 1984, when the majority ofrotifer species were over-wintering as resting eggs,and even the cold water forms had not emergedcompletely, provided a full species list from a singlevisit (Table 1).

Table 2. Rotifer species diversity and abundance in open watersamples from Loch Leven collected on 16 February 1984. Thesamples were collected in parallel to the sediment sampling pro-gram.

Species Individuals per litre

Northolca squamula 18Keratella cochlearis 3Synchaeta kitina 2Colurella adriatica 1Brachionus angularis 1Keratella quadrata 1

Table 1. A species list of planktonic rotifers from Loch Leven, compiled from a six year series of weekly and fortnightly samplingvisits (1977 - 1982). The figures 1 - 6 show the relative frequency with which a given species was found in each month. For comparison,those species which were hatched from sediment samples collected on a single sampling occasion in February 1984 are indicated bya cross (+).

Species In open water samples Hatchedfrom

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec sediment

Keratella cochlearis 6 6 6 6 6 6 6 6 6 6 6 6 +Synchaeta kitina 5 6 6 6 5 5 6 6 6 4 4 4 +Notholca squamula 6 6 6 6 1 1 2 +Polyarthra dolichoptera 6 6 6 6 6 6 1 1 1 1 2 +Keratella quadrata 4 4 5 5 5 5 5 4 4 2 3 3 +Asplanchna priodonta 4 4 1 1 +Conochilus unicornis 1 5 5 3 1 1 +Polyarthra major 1 4 4 +Pompholyx sulcata 2 4 5 6 6 5 1 +Trichocerca pusilla 1 6 6 6 3 +Lecane lunaris 1 2 3 1 2 1 +Colurella adriatica 1 2 1 2 3 1 2 1 1 +Synchaeta grandis 1 I 1 2 3 1 1 2 1 1 +Filinia longiseta I I 1 2 4 1 1 +Brachionus angularis 2 +

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Discussion

Many rotifer species prefer a well-defined rangeof environmental conditions and their use as en-vironmental indicators is probably limited only byour lack of knowledge of the factors controllingtheir distribution (Pejler, 1981). Surveys over a widerange of lake types have suggested a close correla-tion between distribution and environmental condi-tions in many species (Pejler, 1965). Nevertheless,the use of these animals as environmental indica-tors is complicated by the practical problems of ob-taining representative samples for any given site.Many species occur seasonally and frequent visitsover the whole year are needed to compile a com-plete list of species. Pejler (1965) discusses thisproblem and argues that, for distribution studies, asuperficial survey of many lakes is better than adetailed study of a few. The author chooses to col-lect one or two plankton samples from a large num-ber of sites between June and September and ac-cepts the consequent loss of information on coldwater species.

The present study was stimulated by the need foran efficient method of obtaining a complete rotiferspecies list for any lake from a single visit. Theresults suggest that, in early spring, most rotiferspecies are 'overwintering' in the surface mud andcan be induced to break their dormancy by incuba-tion under controlled laboratory conditions. In thisseries of experiments exposure to light and a varietyof temperature conditions seemed to induce restingeggs to hatch. A complete species list was producedin less than 6 weeks.

Although these results were not produced instan-taneously, the technique has two important advan-tages over previous survey methods. Firstly, itrepresents a significant decrease in effort comparedto several return visits to a remote sampling siteduring the year. Secondly, the method overcomesthe inherent problems of identifying preserved,soft-bodied rotifers because the animals can beidentified live as they hatch from the sediments.For general use, however, the method may requirefurther development to ensure the hatching of rest-ing eggs whose dormancy is not broken by light andtemperature changes.

In addition to providing information on presentday rotifer communities, this method may also have

potential in tracing the past histories of lake en-vironments. Rotifer resting eggs, being resistant todecomposition, are preserved in lake sediments(Gilbert & Wurdak, 1978) and may still remain via-ble after many years. Resting eggs taken from deep-er sediments could hatch to reveal environmentalindicator species which would provide useful infor-mation on past conditions.

Acknowledgements

I am grateful to Mr. A. Kirika and Mr. D. H.Jones who helped to collect the samples. I alsothank Dr. A. E. Bailey-Watts and Mr. D. H. Jonesfor reviewing the draft of this paper.

References

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