Zooplankton Flowing Waters Sampling 1999 Belgium

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Zooplankton distribution in flowing waters and its implications for

sampling: case studies in the River Meuse (Belgium) and the River

Moselle (France, Luxembourg)

Laurent Viroux

Facults Universitaires Notre-Dame de la Paix, URBO Unit of Freshwater

Ecology, rue de Bruxelles 61, B-5000 Namur, Belgium

Abstract. Zooplankton spatial distribution was studied on four occasions by transect sampling on therivers Moselle and Meuse in July 1996. Sampling sites were selected for their variety in morphology.A pseudoreplicate-based sampling scheme was adopted that allowed small-scale longitudinal vari-ations in density to be tested along with transversal position and depth. In the Moselle, zooplanktonwere unevenly distributed transversally, an observation tentatively linked with the complexity of theriver channel and the influence of tributaries. In the Meuse, spatial heterogeneity was stronger, anddepth also played a key role. Its effect was different for rotifers and microcrustaceans. The occurrenceof zooplankton patches, similar to those commonly reported from lakes, was noted. The factorsleading to the establishment of such distribution patterns, and their relevance to routine sampling,are briefly discussed; a short list of recommendations for sampling in large rivers is proposed.

Introduction

The distribution of zooplankton in a body of water, as it is transferred throughadvection from the source of a river to its mouth, has very seldom been investi-

gated. Most studies concerning potamozooplankton describe its dynamics on thesole basis of point samples typically taken from the middle of the channel (Rzskaet al., 1955; Rai, 1974; Jos de Paggi, 1978; Basu and Pick, 1996, 1997; Viroux,1997), the bank areas being largely neglected for instance. In some cases, severaltransversal positions are sampled, but pooled so that spatial information is lost(Saunders and Lewis, 1989; de Ruyter van Steveninck et al., 1992; Pillard andAnderson, 1993; Tubbing et al., 1994), or sampling is carried out from varyingpositions depending on the station (Talling and Rzska, 1967; de Ruyter vanSteveninck et al., 1989, 1992; Admiraal et al., 1994). Even less frequent are indi-

cations on the depth at which sampling is carried out (see Reinhard, 1931; Rzskaet al., 1955; Bothr and Kiss, 1990; Pace et al., 1992; Meister, 1994, as illustrations).In the majority of cases, no details whatsoever are given concerning the exactpositioning for routine sampling (Shiel et al., 1982; Guisande and Toja, 1988;Ferrari et al., 1989; Papinska, 1990; van Dijk and van Zanten, 1995; Marneffe etal., 1996). As a general rule, few authors have addressed the question as towhether sampling from a particular position would significantly affect the esti-mate of densities, and to what extent it would do so. In an earlier paper, I alludedto this problem (Viroux, 1997).

The rare studies that have devoted some effort to the analysis of the possibleheterogeneity in zooplankton distribution either transversally or depthwise haveall indicated the existence of some spatial variations in one or both dimensions,although no constant pattern seems to emerge. Shiel et al. (1982) provided thefirst insight into this problem by examining cross-river variability in a downstream

Journal of Plankton Research Vol.21 no.7 pp.12311248, 1999

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stretch of the River Murray, Australia, where they found plankton densities to behigher in the centre than alongside the banks, and greater at 5 m depth than insubsurface samples. More recently, Thorp et al. (1994) carried out similar experi-ments in the River Ohio, USA. They found average annual densities of totalzooplankton to be greater in littoral areas, an observation they linked to highabundance of some taxa in the shallow nearshore habitats in summer, whereaszooplankton distribution was even during the rest of the year. Finally, the mostrecent account of such observations is documented by Marneffe et al. (1996) whosampled zooplankton in a very lentic section of the lower River Meuse, Belgium,and found depth to be the factor affecting distribution, densities being higher indeeper layers during the low-flow period.

From this census, one sees that the paucity of information regarding zooplank-ton distribution in large rivers makes it very difficult to attempt any preliminarygeneralization. Furthermore, as the reproducibility of these results is not clearly

evoked, if at all considered, by their authors, and as none of them has discussedtheir practical implications, their value as such may be limited. These obser-vations lack the necessary replication.

The aims of the present study were to analyse zooplankton distribution in avariety of hydro- and biological conditions (densities, discharge, channelcomplexity, local impact of tributaries), keeping in mind the necessity to examinethe consistency of the observations made, in an attempt to draw a set ofconclusions that could extend beyond their strict scope and help build more reli-able sampling programmes for the future.

Method

Selection of stations

In the River Moselle, all sampling was carried out from a ship during a 9 daycruise down the river, between 2 and 10 July 1996. Advantage was taken of alarge-scale limnological study of the river, set up by the Commissions Inter-nationales pour la Protection de la Moselle et de la Sarre, and carried out byresearch teams from various German institutes, to fit in the sampling programme.

Three sites were selected where differing hydrographic conditions prevailed. Thegeneral profile of the stations is depicted schematically in Figure 1.

On 3 July, the sampling station (river km 277.8 from the mouth in Koblenz) layright downstream at the point where two major branches of the river meet. Theleft-hand side of the river corresponds to the outlet of the deep, artificial naviga-tion channel, whereas the right-hand side receives the waters from the much shal-lower, natural section of the river known as the Moselle sauvage, which isunsuitable for commercial navigation.

On 4 July, with the boat travelling according to the discharge pattern of the river

as calculated using a model, the second sampling location lay downstream of thecity of Thionville (river km 266.6). This time, the channel is simple in its configur-ation, but receives the waters from a small tributary at the right-hand side, whilethe left-hand side is influenced by the discharge of a nearby wastewater treatmentplant that happened to be defective at the time. Bank areas are shallow and

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covered with macrophyte patches. Finally, on 9 July, the third sampling station wasselected along the border between Luxembourg and Germany (river km 221.9).

At this site, the channel is once again simple, and no major collateral inputs arepresent. Bank areas are occupied by large macrophyte patches.The depth of the River Moselle is very irregular, ranging from a few centi-

metres to 6 m (Viroux, 1997), and could not be ascertained on the selectedlocations before sampling was initiated.

Zooplankton distribution in flowing waters

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Fig. 1. Map of the Meuse and Moselle catchments, showing the location and schematic drawings ofthe selected sampling sites. Major tributaries on both rivers are numbered upstream to downstream,and listed. Single arrows on the small diagrams indicate the direction of flow and double arrows thelocation where transect sampling was carried out.


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In the River Meuse, the station of La Plante upstream of Namur (river km 537from the source, schematically laid out on Figure 1) was selected. At this location,the river is ~100 m wide, 4 m deep, and the banks are fully artificial with no appar-ent macrophyte colonization, thus contrasting with the semi-natural conditionsexperienced along the Moselle. The experiment was carried out on 29 July. Allsampling was carried out from the footbridge at the level of the weir. As this sitelay in the vicinity of a dam and flow was near its minimal, semi-lentic conditionswere expected, which was found worth investigating and motivated the selectionof the sampling date.

All sampling was thus limited to July 1996. Water velocities, calculated fromreference discharges and estimates of the section, were ~0.115 m s1 in theMoselle and ~0.075 m s1 in the Meuse.

Sampling schemeThe sequence of events, i.e. the order in which successive samples were taken,was different from river to river, as logistic conditions were themselvescompletely different.

In each of the three sites chosen along the Moselle, three transversal positionswere selected. Whenever possible, when depth was sufficient, two depths weresampled (subsurface and 2 m). Each coordinate was sampled three times.Although the channel might have been deeper than 2 m, no attempt was made totake samples nearer the bottom strata. Because time was limiting, all sampling

had to be completed within an hour, which imposed the use of plastic jerrycansfor storing samples before processing. At each transversal position, samples weredrawn alternatively from the surface and the depth, so that 34 min elapsedbetween two consecutive samples taken from any coordinate. The timing ofsampling can be visualized schematically in Figure 2 (top drawing). All samplingwas carried out using a 3 l opaque Van Dorn bottle, and bank areas were sampledfirst, before the boat resumed a central position to remain parked there for therest of the day. Each raw sample had a final volume of ~9 l.

In the Meuse, because the sampling site was easily accessible, the sampling

strategy differed. Six transversal positions were selected, and samples were takenat subsurface level and at a depth of ~2 m to conform to the scheme adopted inthe Moselle.

Because time was not limiting, sampling began close to the left bank andproceeded towards the right bank, before crossing the river to start anothertransect (Figure 2, bottom drawing). As a consequence, the time lag betweentwo consecutive samples taken from the same coordinate was longer than in theMoselle (a visual representation of these time lags is given in Figure 2). Onceagain, all sampling was carried out using the Van Dorn bottle and sample

size was ~3 l. Samples were initially kept in plastic jerrycans as well, beforeprocessing.Once all samples had been collected, the exact content of the jerrycans was

measured and sieved through a 35 m nylon filter. The seston was then resus-pended in a few millilitres of filtered river water and immediately fixed with acid

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Lugol iodine solution (Leakey et al., 1994). Samples were kept in 250 ml vials fortheir subsequent treatment.

Laboratory processing of samples

Samples were further concentrated before counting. This was achieved by settlingin plastic sedimentation columns; it is a time-consuming procedure, although ithas been recommended for rotifers to avoid the loss of small forms that mightresult from another filtration (James, 1991). Once settling was complete, samples

were resuspended in a known volume of 4% formaldehyde solution and storedin 25 ml glass vials for stocking.Examination followed the same two-step procedure as described in Viroux

(1997), with some refinements. Potentially abundant zooplankters like rotiferswere first counted by taking replicate subsamples from the suspension. However,


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Fig. 2. Schematic drawing of the sequence used for taking samples in the Moselle (top) and Meuse(bottom). Sampling is initiated in the bottom right corner (large blank dot) in each case. Each dotrepresents the location of the water parcels sampled and they are spread across the Z-axis to visual-ize the effect of flow. Arrows follow the succession of samples. The spatial, and hence temporal, delaybetween two consecutive samples taken on the same coordinate is represented by a thick line on theright-hand side of each drawing. (U) is for upstream, (D) for downstream.


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when densities were too low, the entire sample was scanned. Enumeration wasperformed under a Leica DMIL inverted microscope, at a magnification of100.Taxonomic identification was achieved using the keys of Pontin (1978) andRuttner-Kolisko (1974). When this step was completed, samples were entirelyscanned under a dissecting microscope and microcrustaceans were counted.Cladocerans were determined to genus or species level using Amoros (1984) andScourfield and Harding (1966). The percentage of egg-bearing females and thenumber of eggs per ovigerous female were also calculated from this examination.Copepods were not identified, but simply counted as copepodites or nauplii; theoccurrence of adults was noted.

Statistical analysis of data

The choice of multiple coordinates, and the taking of three samples for each,

made it possible to perform one-way or two-way ANOVAs on the resulting data.The preliminary condition that all samples be taken from a unique populationwas assumed to be fulfilled, although the definition of a population in suchenvironments (unity in time and space) is rendered difficult by the downstreamwash-out of the organisms. The implications of replication under these experi-mental conditions were extensively discussed by Hurlbert (1984). It was assumedthat the time lag between two samples taken at the same coordinate was shortenough to ensure that a true population was repeatedly sampled. Normality ofthe data distributions obtained was tested, and some sets were log transformed

whenever needed. When only one deep coordinate was sampled, like on 4 and 9July in the Moselle, one-way ANOVAs were performed and orthogonal contrastswere used to test the effects of depth and/or transversal position. Two-wayANOVAs were used in other cases.

Results

Table I lists the various species (rotifers and cladocerans) identified in thesamples from both rivers. Some of these were not observed by Viroux (1997),

largely owing to the use of different microscopic techniques.

General remark on Figures 38

The graphs in Figures 38 are arranged so as to show the water body seen fromthe point of view of someone standing upstream of the sampling point, hencewatching the water body flow away. Broken lines follow isodensity lines drawnby the graphical software (Microsoft Excel). Significance of statistical tests is indi-cated by either one (P< 0.05) or two (P< 0.01) asterisks. A summary of these

tests can be found in Table II.These diagrams are not meant to provide a precise picture of distributionthenumber of coordinates is clearly too small to achieve a higher level of precisionand all strata were not investigatedbut to help visualize the spatial changes indensity. I decided to use this simple graphical expression of the results rather than

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Table I. Checklist of the various species and categories observed in all samples from the rivers Moselleand Meuse in July 1996. Abundance and regularity in samples is indicated by asterisks

Taxa Moselle Meuse

RotatoriaAnuraeopsis fissa Gosse * *Asplanchna spp. Gosse * ***Bdelloidea * (July 4, left bank)Brachionus calyciflorus Pallas * ***B.angularis Gosse * ***B.budapestinensis Daday **B.urceolaris s.l. (O.F.Mller) * ***B.diversicornis (Daday) *Colurella sp. Bory de St. Vincent ** *Euchlanis dilatata Ehrb. **Filinia sp. Bory de St. Vincent ***Keratella cochlearis Gosse *** ***K.quadrata (O.F.Mller) * *

K.valga Ehrb. *Lecane sp. Nitzsch ** **Lepadella ovalis (O.F.Mller) *Mytilina sp. Bory de St. Vincent * (July 4, right bank)Platyias sp. Harring * (July 4, right bank)Polyarthra spp. Ehrb. * ***Synchaeta spp. ** ***Testudinella sp. Bory de St. Vincent *Trichocerca sp. Lamarck * *

CladoceraBosminidae

Bosmina sp. *** ***

ChydoridaeAlona sp. * **Chydorus sp. *Disparalona rostrata Koch ** **Eurycercus lamellatus (O.F.Mller) *Graptoleberis testudinaria (Fischer) *Pleuroxus sp. *Pleuroxus truncatus (O.F.Mller) *

DaphniidaeDaphnia cucullata Sars * **

LeptodoridaeLeptodora kindtii Focke *

MacrothricidaeDrepanothrix dentata Eurn *Ilyocryptus sordidus Livin *

SididaeDiaphanosoma brachyurum Livin * **

CopepodaCalanoida *Cyclopoida * ***

Adult males * *Adult females *

Harpacticoida *

*Found in only a few samples.

**Found in most samples.***Found in all or nearly all samples.


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TableII.Sum

maryofthestatisticaltestsperformedoneitherraworlog-transformeddata(indicatedbyanas

terisk).Themagnitudeofvariab

ilityisindicated

bytheranges

incoefficientsofvariation(first

threecolumnsforresidual,inter

-depthandinter-transversaldim

ensions).Fvaluesarethengiven,alongwiththe

associatedpro

babilities.Weaklysignificanttes

ts(P100

46.7>100

68.286.2

15.69

0.0

1

1.68

0.18

Otherspecies

16.3>100

44.196

56.290

45.79

0.0

1

3.03

0.029

Copepods

8.799.3

42.765.4

12.968.9

0.96

0.33

3.62

0.014

Nauplii

1282.2

34.746.8

19.571.6

7.78

0.0

1

1.31

0.29

Cladocerandynamicparameters

%o

fovigerousfemales(*)

587.8

54.956.4

52.886.8

Meanclutc

hsize(*)

132.6

12.726.3

9.129.4


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plotting the numbers on each coordinate, as in Marneffe et al. (1996). The numberof density classes is always limited to the minimum, and their choice is arbitrary.The reader will see that very seldom is the number of categories higher than thenumber of transversal coordinates.

Moselle

The densities measured for various categories of zooplankton in the RiverMoselle on the three sampling dates are depicted in Figures 35. As rotifer dens-ities were very low, only totals were used; representing the various species wouldbe of little interest.

On 3 July, an important influence of transversal position can be observed(Figure 3). The waters originating from the navigation channel (left-hand side)were richer in zooplankton, and this affected most groups. On the contrary

(bottom drawing), Chydoridae and Macrothricidae, which are primarily benthic,appeared more abundant in the samples deriving from the shallows (right-handside), although this was not found to be significant.

Depth was of much less relevance overall. An observation worth mentioningis the abundance of Bosmina sp. in the deep layer of the navigation channel, afact attributable to the presence of a dense cluster of small individuals in only oneof these samples, suggesting the existence of patches similar to those commonlyreported in lakes and ponds.

On 4 July, although channel morphology is simpler at that site, some influence

of transversal position could still be outlined (Figure 4). Both the tributary (right-hand side) and the wastewater treatment plant (left-hand side) contributed aninput of rotifers, although the species involved were not those observable in mid-channel (not shown). More surprising is the apparent vertical distribution ofcopepod nauplii. Overall, the densities observed were lower than at the firststation the day before.

On 9 July, finally, in a section where no collateral inputs are present, zooplank-ton densities still decreased (Figure 5). This time, no significant influence wasfound as plankton organisms were distributed evenly across the section.

Meuse

In the Meuse, very high rotifer densities were observed, far exceeding the valuespreviously recorded at the selected sampling location [see Viroux (1997) andGosselain et al. (1998); see also Marneffe et al. (1996) for comparison with down-stream stations]. Figure 6 depicts the distribution of the main rotifer groups in thesame fashion as Figures 35. Overall, surface layers hosted the highest densitiesand a strong, both horizontal and vertical, heterogeneity could be observed,

chiefly attributable to Keratella cochlearis and Polyarthra spp. (top threediagrams). Filinia differed from that general picture by being more abundant inthe deeper layer sampled. It is worth mentioning the occasional observation ofsmall clusters of a given species in some samples (not visible on the diagrams),such genera as Lepadella orAsplanchna being concerned.


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The spatial distribution patterns for various microcrustaceans on the same daycan be described from Figure 7. Although the most abundant cladoceran,Bosmina sp., was evenly distributed across the water column (top drawing), largeror more efficient swimmers exhibited strong spatial preferences. Diaphanosoma

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Fig. 3. Isodensity diagrams for the most relevant taxa observed in the Moselle on 3 July. The densitygradient is depicted by a grey scale. Densities are expressed in individuals l1. Arrows on top of thefigure indicate the positions where samples were taken. The significant effect of either transversalposition or depth is indicated (see the text).


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Fig. 4. Isodensity diagrams for the most relevant taxa observed in the Moselle on 4 July. Symbols areidentical to Figure 3.

Fig. 5. Isodensity diagrams for the most relevant taxa observed in the Moselle on 9 July. Symbols areidentical to Figures 3 and 4.


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brachyurum was far more abundant in the bottom layer, as were copepod naupliiand other large cladocerans, which included Daphnia cucullata and, sporadically,some predatory Leptodora kindtii. Copepods differed from that depth-oriented

spatial structure by adopting a transversal pattern.Although both the proportion of egg-bearing cladoceran females in thesamples and mean clutch size seemed to be higher in bottom layers (Figure 8),the absence of ovigerous females in some samples made it impossible to carry outtwo-way ANOVAs to test the significance of these observations.

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Fig. 6. Isodensity diagrams for the principal categories of rotifers observed in the Meuse on 29 July.


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Discussion

Very contrasted hydro- and biological conditions were experienced between thetwo rivers during the course of the present study. While the Moselle supported aweakly developed, rapidly flushed plankton, the Meuse provided much more

favourable conditions for an abundant zooplankton to develop (low discharges,hence low water velocity). Furthermore, the selection of sites with very dissimi-lar layouts (artificialized versus semi-natural, split versus single channel, presenceor absence of lateral inputs or vegetation) made it possible to analyse the varioussituations described here. The primary requirements of the study were met in


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Fig. 7. Isodensity diagrams for the various categories of microcrustaceans observed in the Meuse on29 July.


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accordance with the expectations, partly intentionally, partly owing to meteoro-logical conditions.

The factors governing zooplankton spatial distribution appear very distinctthanks to the contrasting situations. It seems clear that, in the Moselle, the abun-

dance of plankton alongside the bank areas was a direct consequence of the inputof different water masses. On 3 July (Figure 3), the deep, slow-flowing navigationchannel carried significantly more plankton than the parallel-running, much shal-lower Moselle sauvage. On 4 July (Figure 4), rotifer density and communitycomposition were different just downstream of the mouth of a small tributarythan in the main channel.

We could overlook these variations in zooplankton load as resulting from theparticular changes in population density within different water masses flowingindependently along morphologically distinct sections and for a variable distance,

the key factor being the differences in longitudinal transfer time. Assuming thatan identical inoculum can be provided at the point where the channel is split intoa series of collateral branches, the particular morphological features of eachbranch (length, sinuosity, water velocity) will allow time for a given number ofgenerations to appear, different from arm to arm. This concept is identical to theone stated for phytoplankton (see Reynolds and Descy, 1996), but for organismswith a longer generation time. Sampling exactly where these branches all meetwill provide a picture of zooplankton development in all of them. The situationobserved on 3 July provides a simple, yet spectacular illustration (Figure 3).

The influence of tributaries is often only seen as bringing about a dilution effect(Descy et al., 1987). More rarely are they seen as significant plankton sources. Thepresent study shows that even a small lateral input (Figure 4) can have an effecton zooplankton density in the main channel, bringing in species that might other-wise be absent there, like benthic forms for instance. The influence of lateral

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Fig. 8. Isodensity diagrams for a selection of dynamic parameters (egg ratio and clutch size) measuredon the cladoceran communities observed in the Meuse on 29 July.


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inputs may, however, be limited in space, and may no longer be observable atsome distance downstream.

One distinctive feature of two stations on the Moselle was the presence ofmacrophyte patches alongside the banks. These habitats may act as filters, partlyresponsible for the impoverishment of plankton being transferred from standingwaters to streams (Hamilton et al., 1990), but they may also act as refuges for largeplanktonic cladocerans against fish predation (Stansfield et al., 1995; Lauridsen etal., 1996). Their retentive capacity is largely based on the reduction of watervelocity within their stems (Sand-Jensen and Mebus, 1996). Unfortunately,because the densities measured in the Moselle were so low, and sampling wascarried out outside these patches, their influence could not be tested.

In the Meuse, depth and transversal positions were equally relevant as factorsdetermining zooplankton distribution. In the case of rotifers, the generaltendency is to find more organisms in the subsurface layer, and in mid-channel.

It looks as though a concentration takes place, from the peripheral (slower)towards the central (faster flowing) waters, as if driven by a purely physicalphenomenon, although low discharges were prominent at that time. It is likelythat the poor swimming abilities of rotifers do not enable them to determine theirposition in the water column actively, the magnitude of vertical and transversalmixing being too powerful for them to counteract even when flow is reduced. Thesituation of Filinia sp. does not follow that rule, however (Figure 6), and might

just shatter the latter hypothesis, or indicate the existence of slow-flowingpockets within the stream channel, exploited by some species for population

increase. The observation of clusters in some samples supports this.As a consequence, sampling in mid-channel, a commonplace practice, as we

have seen earlier on, might result in an overestimate of global rotifer densities,but it seems an adequate strategy if a maximum number of animals is to be caught.

Among cladocerans, only the strongest swimmers (Diaphanosoma brachy-urum, Daphnia cucullata, L.kindtii) seem able to migrate actively towards a givenlocation despite the flow. Richardson (1992) showed experimentally that largeDaphnia could not swim against water in motion at a velocity of 0.05 m s 1. Atthe time of sampling, given a section of ~400 m2 and a discharge of ~30 m3 s1,

water velocity in mid-channel was ~0.075 m s1

, greater than the critical speed formigration against the flow. However, this does not rule out the possibilities foreither lateral or vertical displacement, or even upstream swimming in the vicin-ity of the banks where flow is slower. The tendency large cladocerans exhibit forconcentrating in the deeper layers raises two questions: will sampling the subsur-face layer always lead to an underestimate of actual densities, and is that behav-iour similar to vertical migrations commonly reported in lakes? Sampling at nightfollowing a similar protocol should provide an answer to the latter. It is hard totell whether that vertical distribution will be observed in every situation, or if it

is the result of extremely low summer flows combined with the closeness of a dam.As none of the dynamic parameters measured here (egg ratio and clutch size;Figure 8) could be tested for the influence of either depth or transversal position,we cannot say whether collecting a suboptimal number of females will hamperanalysis of the state of their populations. Although true population dynamics


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cannot be derived from point samples in rivers because of the impossibility ofdefining a population as such (Hurlbert, 1984), it can be proposed that thedynamic state of metapopulations of transient individuals can be qualified usingthese two parameters and their variation in time and space.

To tackle the problems of sampling microcrustaceans, other sampling devicesmight be used. The choice of the Van Dorn bottle was motivated by its handiness,given the necessity to operate as quickly as possible to ensure reproducibility, andmost of all because it seems to be the only device that can be lowered at a givendepth with acceptable accuracy. Schindler traps of even the smallest volumes areswept by the flow, and if weighted they lose the handiness for which they weredesigned in the first place. Such devices are seldom used in rivers [see Shiel et al.(1982) and Ferrari et al. (1989) for examples]. Nets would be clogged withinseconds and sample volume would not be determined reliably. Finally, the use ofpumps and operational rapidity appear quite incompatible. The fact that an

opaque bottle was used might also cause some concern about the presence oflarge-bodied crustaceans in the subsurface samples. Although transparent bottlesare preferable (Smyly, 1968), it is likely that the high turbidity in lowland riversrenders an opaque device much less detectable than it would be in clear lake water.Finally, sample size was always small, but once again the need to collect all samplesin the shortest possible time was the criterion. The fact that the bottle was gentlylowered and rapidly retrieved probably resulted in limiting escape reactions.

Conclusions and recommendations for future sampling

We have shown distinct situations when the distribution of zooplankton in thewater column of large lowland rivers was not homogeneous. Both distance to thebanks and depth may account for heterogeneity in the distribution of organisms.If hydrological conditions (channel complexity, flow, current velocity) are themajor factor in shaping spatial patterns, the capacity of organisms to control theirposition when flow is reduced cannot be completely denied. These variations indensities across the section should be taken into account when designing samplingschemes in flowing waters, as in most studies sampling effort is to be minimized.

The following guidelines can be suggested.

1. The choice of stations should be restricted to areas where the channel is simpleand does not receive significant lateral inputs. Should tributary influence bemonitored, it is better to select a sampling site at some distance downstream,where the waters have presumably mixed.

2. Deeper layers are difficult to reach precisely using most samplers, but trans-versal heterogeneity in subsurface layers can and should be dealt with. Poolingwill result in the loss of spatial information, but might prove necessary to facili-

tate sample processing.3. Large sample size should be used for the evaluation of microcrustacean dens-ities, and the deduction of the dynamic parameters from the samples. Large-volume samplers are preferable for those organisms, even if their use canprove tricky in flowing waters.

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Acknowledgements

The author wishes to thank the CIPMS (Commissions Internationales pour laProtection de la Moselle et de la Sarre) for providing the opportunity to join thecrew on the boat along the Moselle, and the staff from the BfG (Bundesanstalt

fr Gewsserkunde, Koblenz, Germany) whose precious help was appreciatedwhen collecting the samples in the Moselle. The present study is part of theauthors PhD research under a grant from the Belgian FNRS.

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Received on February 3, 1998; accepted on February 26, 1999

L.Viroux

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Zooplankton Flowing Waters Sampling 1999 Belgium

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