Journal of Plankton Research Vol.19 no.ll pp.1743-1762, 1997
Zooplankton development in two large lowland rivers, the Moselle
(France) and the Meuse (Belgium), in 1993
L.Viroux
Facultis Universitaires Notre-Dame de la Paix, Unit of Freshwater Ecology, 61
Rue de Bruxelles, B-5000 Namur, Belgium
Abstract. Zooplankton population dynamics were studied along a 120 km section of the River
Moselle, in France, and at one location along the Belgian course of the River Meuse. Community
composition and population densities were assessed on fresh material. Rotifers overwhelmingly
dominated the zooplankton of both rivers, especially in the Meuse where microcrustaceans were
abundant only seasonally. The maximal densities recorded lie in the range of values commonly
reported for large European nvers. Zooplankton dynamics in these tworiversare discussed in relation
to their hydrological characteristics, differing morphologies and degree of regulation.
Introduction
Important studies on both the temporal and spatial components of zooplankton
dynamics in rivers worldwide include those by Enaceanu (1967), Tailing and
Rz6ska (1967), Rai (1974), Jos£ de Paggi (1978), Dzyuban (1979), Shiel et al,
(1982), Saunders and Lewis (1989) or de Ruyter van Steveninck et al. (1992).
Other studies, e.g. de Ruyter van Steveninck et al. (1989), Admiraal et al. (1990,
1994), Thorp et al. (1994), have limited their scope to relatively short sections of
large rivers, comprising only a few sampling points. In other cases, the studies
have chiefly focused on the dynamics of certain limited categories of organisms,
like rotifers (Guisande and Toja, 1988; Ferrari et al, 1989; Papinska, 1990) or
microcrustaceans (Bother, 1986,1987,1988; Bother and Kiss, 1990).
Perhaps the most complete among the recent studies on large river zooplankton are those carried out by Shiel et al. (1982) on the Murray, Saunders and Lewis
(1989) on the Orinoco and de Ruyter van Steveninck et al. (1992) on the Rhine,
who not only considered the dynamics of the principal mesozooplankton groups
that may be encountered in freshwaters, but also extended their work to cover
wide geographical areas. Most recently, Basu and Pick (1997) have presented an
overview of plankton development in a North American river.
The zooplankton of both the Meuse and the Moselle rivers are not well known.
Most studies on the zooplankton of the Meuse along the Belgian stretch have
been limited to short sections of the river, or were dedicated to the assessment of
the ecological impact of various sources of pollution (Jeuniaux et al, 1984;
Sanderson, 1992; Marneffe et al, 1996), whereas other data sets also exist for the
Dutch part of the river (Peelen, 1975). On the other hand, to the author's knowledge, no published data on the zooplankton of the Moselle are available. The
Moselle, like the Neckar, is one of the most important tributaries of the Rhine,
at least in terms of the contribution to the total plankton load (de Ruyter van
Steveninck et al, 1992). In the Moselle, sharp declines in phytoplankton populations at the onset of summer have been recorded since 1990, followed by
© Oxford University Press
1743
UViroux
summer periods when low biomasses have been maintained. This is one
phenomenon in which zooplankton grazing might play an important role. The
main aim of the present study was to describe both the temporal and longitudinal development of zooplankton in the Moselle during a complete growth season
(1993), and to compare its seasonal features with the development recorded on
one location on the River Meuse during the same period, as these two rivers
exhibit similar hydrological characteristics. Such data may, eventually, contribute
to an improved understanding of river plankton dynamics and add to the relatively limited body of data available on large river zooplankton.
General description of the two rivers and geographical framework of the study
Both the Moselle and the Meuse rise in the north-east of France, and partly lie
on either side of a common watershed. The course of the Moselle (Figure 1) flows
for some 535 km. Its catchment area totals -28 000 km2, which makes it the most
important tributary of the Rhine in terms of the drainage area. The river runs
through France in a more or less north-eastward direction, constitutes a short
section of the border between Luxemburg and Germany, then continues towards
the Rhine. The average slope is 1.2%o. The river traverses, from source to mouth,
a sequence of formations: Vosgian sandstone, calcareous soils of the Plateau de
Lorraine and schists of the Rhenan Massif. The water in the reach studied is
eutrophic in character, with a salinity deeply influenced by the inflows of its two
main French tributaries: the Meurthe and the Seille (Gigleux, 1992). In the
section considered, the river is regulated, but, due to its uneven depth (1-6 m), it
retains some seminatural features with large shallow sections unsuitable for navigation. As a result, in many sections the natural sinuosity is barely altered, and
the slope and vegetation of the banks have been preserved.
Five stations were chosen along the river (Figure 1): Frouard (river km 195),
the reference upstream station north of the city of Nancy; Millery (km 200),
downstream of the confluence with the Meurthe; Hauconcourt (Argancy Dam,
km 269), downstream of the city of Metz, the confluence with the Seille, and the
outlet of the cooling basin of La Maxe (EDF power plant); Koenigsmacker
(km 297), at the level of the Cattenom EDF nuclear power plant; and finally
Sierck (km 310, just before the border). The stretch covered by this study therefore extended to some 120 km. These sampling stations are indicated on Figure
1 by their initials.
The Meuse, another eutrophic river, flows through France, Belgium and the
Netherlands where it enters the Rhine delta (Figure 1). It flows for 885 km, and
its catchment area just exceeds 36 000 km2. The average slope is ~0.45%o. In contrast to the Moselle, regulation along the entire Belgian stretch is intense. The
channel is modified to accommodate large boats and >90% of the banks on both
sides are artificial. The general features of the river itself, as well as the sampling
location, have been described elsewhere (Descy, 1987; Descy and Gosselain,
1994; Gosselain et al., 1994).
Both rivers support an important phytoplankton population, as the high levels
of nitrogen and phosphorus are not limiting (Descy, 1987; Descy and Willems,
1744
Zooplankton in the Moselle and Meuse
North Sea
Fig. L Map of the drainage area of the rivers Meuse and Moselle, showing the location of all sampling stations (identified by their initials) and major tributaries.
1991; Gosselain et al., 1994). In the Meuse, diatoms (mainly belonging to the
Stephanodiscus hantzschii Grunow group) dominate the spring phytoplankton,
whereas chlorophytes contribute substantially to summer communities (Descy,
1987; Gosselain etai, 1994). In the Moselle, more diversified diatom communities
1745
LViroax
can be observed throughout the season, with contributions from halophilic taxa
originating from the Meurthe and Seille (Descy and Willems, 1991). The phytoplankton are often dominated by Skeletonema potamos (Weber) Hasle, which
barely develops in the Meuse, and can account for >50% of the total algal
numbers at times.
Method
Collection of samples
Fifteen samples were collected each fortnight between April and October 1993.
In Koenigsmacker, zooplankton were sampled from a pier at the entrance of the
weir, using a 3 1 Van Dorn bottle. Five samples were taken and their contents
sieved through a 63 urn nylon net, so that a pooled 15 I sample was obtained. The
plankton collected on the net were then resuspended in -200 ml of filtered river
water, and placed in a cooler. The samples from the four other stations (Frouard,
Millery, Hauconcourt and Sierck) were collected by simply lowering a 101 bucket
from a bridge, at the mid-course of the river. Though simple, this technique has
been used in past studies on the Rhine (de Ruyter van Steveninck et al., 1989,
1992; Tubbing et al, 1994; van Zanten and van Dijk, 1994), and is the standard
method used by the institute in charge of the sampling. These samples were then
forwarded to Koenigsmacker in a truck equipped with a cooling system, and
delivered for subsequent treatment. Between 6 and 10 1 were processed in the
fashion described above.
Samples from the Meuse were collected weekly or bi-weekly, between March
and September. The procedure followed for sampling zooplankton was identical
to that described for Koenigsmacker.
Processing of samples
Qualitative and quantitative analyses of the samples were always performed on
fresh material. In the laboratory, samples were stored in the refrigerator at 4°C
and processed either immediately (Meuse) or the following day (Moselle). The
analysis of all samples was completed within the 48 h following the time of collection. The 200 ml samples were further concentrated by filtering through a 63
urn nylon net, and the seston was resuspended in a few millilitres of filtered water,
giving a final concentration factor between 250 and 1000 (depending on the sestonic load). Severe clogging of the filters, both in the field and in the laboratory,
was encountered on several occasions, and greatly influenced the final volume
adopted.
Enumeration of the organisms was performed in a two-step protocol. Slowmoving, generally abundant animals, such as rotifers, nauplii and veligers of
Dreissena polymorpha Pallas, were counted first. Subsamples of 500 ul-1 ml were
drawn from the samples, after careful mixing, using a micropipette with a widebore tip, following the recommendations of MacCallum (1979). The subsample
was then divided into 10-12 drops on a multidepression microscope slide. In
general, the animals were sufficiently torpid, following their stay in the
1746
Zooplankton in the Moselle and Mease
refrigerator, so as not to be excessively active, although it was sometimes
necessary to prevent them from moving by adding one droplet of very dilute
(<5%) glycerol to each dish, which inhibited swimming without causing any
noticeable damage to the animals. Four successive subsamples were examined
under a dissecting microscope at a magnification of 35 x. Rotifers were determined to the genus or species level using Ruttner-Kolisko (1974) and Pontin
(1978). Copepod nauplii were enumerated regardless of their developmental
stage, and Dreissena veligers regardless of their size. Protozoa were not taken into
account.
The overall volume that was eventually examined during this first step was
always a small fraction of the volume of the original sample before subsampling.
The necessity to operate within narrow time intervals, and the time-consuming
processing methods involved, made it difficult to count larger volumes, or more
subsamples, without jeopardizing the full analysis by allowing enough time for
zooplankton to reproduce. However, the volume of either the sample or the subsample was often altered in order to achieve a minimal density of 60 individuals
of the main species in the subsample (Bottrell et al., 1976; MacCauley, 1984).
During the second step, faster swimmers like copepod? and cladocerans, which
are also less abundant, were enumerated. When rotifer counting was completed,
the sample was first fixed with 4% formaldehyde and the entire sample scanned
under a dissecting microscope for every microcrustacean. These were counted as
cladocerans or copepods, but not identified further.
Measurement of phytoplankton biomass
Phytoplankton abundance was assessed as chlorophyll a concentration. River
water was filtered through Whatman GF/C glass fibre filters, and the pigments
extracted according to Pechar (1987). Chlorophyll a concentration was then
expressed following Lorenzen (1967). Data for the other stations were kindly
provided by the Institut de Recherches Hydrologiques of Nancy, France.
Results
In the Moselle, rotifers dominated the communities on most occasions (Figure 2).
At the beginning of the growth season, rotifer densities increased as the water
body moved downstream, by roughly an order of magnitude from Frouard to
Sierck. The respective maxima for each station were reached at different times.
In the downstream part of the studied stretch, maximum densities were recorded
in spring, while most upstream stations had a summer maximum. Hauconcourt
was intermediate, with an exceptional peak of almost 6000 individuals H in June.
Microcrustaceans (Figure 3) were generally more abundant in the upstream
section, especially in Hauconcourt, where they reached maximal densities of -225
individuals I"1. Cladocerans often dominated the microcrustacean zooplankton in
Frouard, whereas their abundance was more or less comparable to the other
classes elsewhere. In contrast, the abundance of Dreissena veligers (Figure 4) was
higher in the downstream section, where they also appeared earlier. Neverthe1747
LViroiK
2500
2000
1500
1000 500 0
160
120
80
40
_D
s ™
sampling date
Fig. 2. Temporal variations of total rotifer densities (bars) in the River Moselle in 1993, combined
with phytoplankton biomass (line) expressed as chlorophyll a concentration. Sampling stations are
ordered starting from upstream (top) towards downstream (bottom), and are indicated by their initials. To ensure clarity, the outstanding maximum recorded in (H)auconcourt on 7 June has been cut
short to permit standardization of the scales.
less, with the exception of intense seasonal 'pulses', their presence in the water
column was often unimportant.
Perhaps the most striking feature of the overall plankton dynamics over the
year was the significant summer decline of both phyto- and zooplankton observed
in the downstream section (Koenigsmacker and Sierck), a phenomenon that
began as early as the end of June and that was particularly intense at Sierck,
1748
Zooplanktou in the Moselle and Meuse
200
150
100
50
0
• •
•
I,
"
^
nauphi
Copepoda
'Cladocera
en
^
sampling date
Fig. 3. Temporal evolution of copepod nauplii, copepodids and cladocerans in the River Moselle in
1993. Sampling sites are ordered as in Figure 2.
whereas phytoplankton densities remained fairly high at the same period at the
upstream stations (Figure 2).
The spatiotemporal variations in the composition of the total rotifer community (Figure 5) showed various interesting features, the most remarkable being
perhaps the steadily decreasing importance of Keratella cochlearis Gosse when
going downstream. With the exception of the spring period, when Keratella
quadrata Mtiller bloomed everywhere, and in the downstream section where
K.cochlearis populations were depressed, Brachionus calyciflorus Pallas and
K.cochlearis were the two most abundant species, often contributing at least 50%
1749
L.Viroux
•
300 200 100 n -
n 400
300
200
100
F
l.
I-
1
..
•
-
H
•
l
•H
H
warn
mm
1
500 400 H
3
o
M
K
1_
1 1
300
200 100 -
• 1
III!..
A
finn -r
400
300
200
100
s
•
500
-
I 1
I 1
1 1
Hi
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o
o
o
o
o
o
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o
)
0
o
)
0
»-
f ~ * - l 0 0 1 C V J t O » -
sampling date
Fig. 4. Occurrence of Dreissetw polymorpha veligers in the water column of the River Moselle in
1993. Sampling sites are ordered as in Figures 2 and 3.
of the total rotiferan zooplankton. Synchaetidae {Synchaeta Ehrb. and Polyarthra
Ehrb.) were seldom an important component of zooplankton, except at the very
end of the growth season, while a miscellaneous assemblage of various subdominant species was observed throughout the year. Table I gives the list of the
species recorded, with details on their seasonality.
Overall, the timing of planktonic events along the Moselle, as deduced from
Figure 2 alone, did not, in any station, mimic the 'classical' inverse relationship
between phyto- and zooplankton densities commonly observed and described in
lakes. Episodes of phytoplankton blooms and intense zooplankton growth were
often completely asynchronous. On more than one occasion, their respective
maxima were recorded at the same time.
1750
Zooplankton in the Moselle and Meuse
100
Sampling date
LJ
WA
B calyciflonjs
K cochleans
K. quadrata
LJ
Ulllll
Synchaetidae
Various
Fig. 5. Relative composition of rotifer assemblages in the River Moselle in 1993. Species of secondary importance in terms of abundance have been combined and referred to as 'Synchaetidae'
(Polyarthra spp. and Synchaeia spp.) or 'various' (all other non-dominant species).
The dynamics observed in the Meuse in 1993 differed from the situation
described for the Moselle in more than one aspect (Figure 6). Zooplankton densities were maximal in the summer, following a short and discrete early spring
maximum, and alternation of phyto- and zooplankton blooms was much more
apparent. Rotifer dominance was conspicuous, while microcrustaceans occurred
at low densities for most of the year before rising to a very short period of high
1751
to
Table I. Seasonal occurrence of rotifer species in the River Moselle and the River Meuse in 1993. All species are classified into four categones. depending on
their abundance and regularity in the samples. Whenever feasible, the period of maximal abundance is noted
Asplanchna spp. Gosse
Brachionus calyciflonis
B.angularis Gosse
B.urceolaris MUller
B.diversicornis Daday
B.leydigi Cohn
B.quadridentatus Hermann
Cephalodella sp. Bory de St. Vincent
Epiphanes sp. Ehrb.
Euchlanis dilatata Ehrb,
Filinia longiseia Ehrb.
Keralella cochlearis
K.quadrata
K. valga Ehrb.
Kellicottia longispina Kellicot
Lecane sp. Nitzsch.
Notholca acuminata Ehrb.
Polyarthra spp.
Rhinoglenafronlalis Ehrb.
Squatinella sp. Bory de St. Vincent
Synchaeta spp.
Tesludinella sp. Bory de St. Vincent
Trichocerca sp. Lamarck
Bdelloidea
Frouard
Millery
Hauconcourt
*•
*
•*•
* (June-July)
• ••
*
• *
*
*
Koenigsmacker
Sierck
La Plante
•**
•++
•*•
•*
* (until July)
**
* (July)
• (April)
# (April)
• (June-July)
*
*
* (July)
*
*
*
• (April)
• *•
•• (July)
* (until May)
#
***
+*•
***
***
* (August-September)
** (from July on)
# (July)
• (until May)
#
*
**•
***
• *•
# (April)
*
• (May-June)
* (until May) • (April only)
* (until June)
# (April)
**
* (until May)
**
# (April)
*
* (April)
# (July)
• (April)
# (April)
• (Augusl)
***
***
•* (from July on)
# (May)
* (May-June)
# (April)
• (April only)
• (April)
** (August)
# (April)
# (Apnl)
**
# (June)
# (May)
# (October)
* (until June)
# (September)
# (April)
# (June)
*•
# (August)
# (August)
# (September)
#, encountered once; *, erratic; •*, common bul seldom abundant; ***, very common and often abundant.
# (May)
• (April-June)
Zooplankton in the Moselle and Meuse
Rotifers
2000
1500
1000
r —
\
\
500
ilfTTTX
160
120
so
40
0
fI
s:
I
i
microcnistaceans
nauplii
Copepods
nCladocerans
Dreissena
100%
•g 50%
sampling date
B. calyciflorus
K cochlearis
K. quadrat a
Synchaetidae
various
Fig. 6. Seasonal variations of plankton abundance in the upper Belgian River Meuse in 1993 and relative composition of the rotifer assemblages (bottom). The way these data are presented conforms to
the lay-outs adopted in Figures 2-5.
numbers in late August-early September, when they dominated the zooplankton,
a bloom which coincided with the lowest discharge values, as shown in Figure 7.
Dreissena veligers were an order of magnitude less abundant than in the Moselle
and were found only in summer (June-August). This may indicate differences in
benthic community composition between the two rivers. The seasonal variations
in the composition of the rotifer community resembled that at Frouard (Figure
5) in the Moselle, with B.calyciflorus and K.cochlearis always contributing
a minimum 30% of rotiferan zooplankton, but with a less intense summer
1753
LViroiii
4S0
on
350
300
250
200
ISO
100
50
0
1 e
o
—
1 S
g
= a
date
3
Fig. 7. Seasonal variation of discharge (in m s~', thin line) and water temperature (in °C, bold line)
recorded in Tailfer (River Meuse, top) and Cattenom (River Moselle, bottom) in 1993. Data are from
the CIBE (Meuse) and from EDF (Moselle).
dominance of K.cochlearis. Brachionus calyciflorus was mainly present during
spring, and again in late summer. Polyarthra sp. bloomed in mid-August.
Discussion
The maximal densities of zooplankton reported here from both the Moselle and
the Meuse rivers are well within the range of values reported for other large European eutrophic rivers such as the Guadalquivir (Guisande and Toja, 1988), and
the Meuse itself (Sanderson, 1992; Maraeffe et al, 1996), the Po (Ferrari et al,
1988) or the Vistula (Papinska, 1990), but are somewhat higher than the highest
values recently recorded for the Rhine, into which the Moselle discharges (de
Ruyter van Steveninck et al, 1989,1992; Admiraal et al, 1990, 1994; van Zanten
and van Dijk, 1994). This would tend to confirm the importance of the Moselle
with respect to its contribution to the plankton load of the Rhine.
Hydrodynamics probably play a key role in determining plankton dynamics in
a body of flowing water. In particular, the mean water residence time, and dilution effects resulting from lateral inflows from the tributaries, determine the
opportunities for organisms to multiply in the water column (e.g. Descy et al,
1987).
1754
Zooplankton in the Moselle and Mease
June 1993
»o
«rT
CT
o"
o*
oo
V
*"*
distance from source (km)
Fig. 8. Longitudinal variation of mean water velocity (in m s~', thin line) and water transfer time (in
days, bold line) mathematically calculated for the French section of the River Moselle in 1993 for two
distinct hydrological conditions: June (top) and August (bottom). Data were obtained using the
PEG ASE mathematical model. Sampling stations are outlined by vertical dotted lines with the exception of Sierck, due to its closeness to the border. Station Hauconcourt is identified by a bold arrow,
and water travelling time up to that point is projected on the right vertical axis.
There seem to be marked differences between the residence time in the Rhine
and that in its tributary the Moselle. According to de Ruyter van Steveninck et
al. (1992), the average travelling time of Rhine water from the outlet of Bodensee
to the North Sea is -11 days. A calculation of water velocity profiles in the
Moselle, based on simulation of flow using PEGASE, a mathematical model
developed at the University of Liege (E.Everbecq, personal communication),
shows that maximal velocities in the stretch between Frouard and Sierck, under
two characteristic hydrological situations (June and August 1993), seldom exceed
0.4 m s"1 and are often as low as 0.1-0.2 m s"1 (Figure 8). The model calculates
that, in typical summer conditions of low discharge (-60 m3 s"1), the travelling
time along the 120-km-long studied stretch is -11 (June) to 15 days (August),
1755
L-Vironx
which is comparable to the average residence time of the water along the entire
length of the Rhine. The explanation for the low densities reported in the Rhine,
and for the scarcity of microcrustacean zooplankton in this river when compared
to the Moselle, might thus lie in physical conditions. This would be particularly
critical for slow-growing organisms such as microcrustaceans, which may not have
enough time to build up their populations, and are only abundant in the vicinity
of the sea, where water velocity is reduced (de Ruyter van Steveninck etal., 1989,
1992; Admiraal et al., 1990,1994).
However, water velocity is by no means homogeneous across a transverse
section of any body of flowing water, and sinuosity and the presence of natural
vegetation along the banks might provide potential shelters for fast-swimming
organisms and prevent them from being washed away rapidly. The importance of
river retention and heterogeneities of flow for phytoplankton development was
demonstrated convincingly in the case of the River Severn (Reynolds etal., 1991;
Reynolds and Glaister, 1993). One may hypothesize that similar retentive
mechanisms are also relevant to a still greater extent, if not essential, to slowdeveloping riverine zooplankton.
Another central aspect of river zooplankton ecology concerns the origins of the
plankton. In contrast with the recent argument (Reynolds and Descy, 1996) that
the algal component of the potamoplankton may be completely autochthonous
to the river, rather than originating from backwaters or any limnetic habitat connected to the main channel, little is known about the origins of zooplankton.
Whatever the source of these animals, one may postulate that there is not a
unique source located upstream, but a series of sources spread along the course,
each contributing a fraction of the 'inoculum'. Headwater lakes (Tailing and
Rz6ska, 1967), backwaters (OSmera, 1973) and channel habitats (Saunders and
Lewis, 1989) have all been proposed as potential zooplankton sources, but any
clear evidence of truly autochthonous recruitment, solely based on egg banks
deposited in the sediment for instance, has yet to be presented.
To assess the 'necessity' of intervention of retentive mechanisms or allochthonous plankton sources on zooplankton development (i.e. to see whether a point
source located at the level of headwaters can at all times provide a sufficient
inoculum to account for the densities observed downstream), one must first try
to evaluate the demographic potential exhibited by zooplankters, then confront
it to water retention times. Table II gives four simple examples of such calculations, based on the densities recorded in Hauconcourt on 7 June and on a selection of exponential growth rates observed from laboratory or field experiments
for major components of riverine zooplankton. Considering the two hydrological
regimes described above, with mathematically deduced water travelling times
from the source to the sampling point of 14 (June) and 18 days (August) (Figure
8), and assuming that growth is actually exponential, food limitation does not
impede population growth, and predation is always negligible, initial densities
required as inocula can be calculated using the Malthusian exponential growth
equation (Edmondson, 1977):
N, = N
1756
Zooplankton in the Moselle and Meuse
Table n. Attempts to calculate the inocula required at the level of the headwaters, to render possible
the development of various important categories of zooplankton as collected in Hauconcourt on 7
June 1993, on the sole basis of exponential growth rates gathered from the literature, and assuming
that growth goes unimpeded by biotic factors. Two situations are considered, June and August,
corresponding to both hydrological conditions referred to in the text as well as in Figure 8
Taxa
N,
(ind. I"1)
u
(day 1 )
No.—June No.—August Reference for u values
situation
situation
(ind. 1-')
(ind. h 1 )
Brachionus calyciflorus
Bosmina longirostris
O.F.Muller
Rotifers
5720
1.416
1.4 x 1CH
4.8 x 10-8
Bennett and Boraas, 1989
95
6000
0.55
0.48
4 x 10-2
7.23
4g x 10-3
1.061
Bothar, 1987
de Ruyter van Steveninck
0.23
3.795
1.512
de Ruyter van Steveninck
el al., 1992
etai, 1992
Microcrustaceans
95
with N, the observed density, No the initial inoculum required, T the time elapsed
between the source and the sampling point, and u the exponential growth rate.
The required inocula at the level of the headwaters for the first two taxonomic
categories considered (Table II) are reasonably low, well below one individual
per litre, and are in good agreement with the densities reported from headwater
streams for 'euplanktonic' species (i.e. Sandlund, 1982; Shiozawa, 1986; Richardson, 1990). Such organisms, if properly inoculated into the system, may simply
rely on their specific dynamic capacities to develop the apparently huge densities
observed in Hauconcourt on 7 June. Furthermore, hydrological conditions are
even more suitable for comparable developments in August (Table II). However,
when computing the values presented by de Ruyter van Steveninck et al. (1992)
for rotifers and microcrustaceans in the Rhine, the initial densities required rise
quite sharply, and may prove unrealistic if the inoculation from supplementary
sources along the way is not taken into account. These simple, rough calculations
do not indicate effects of river retention, or the existence or absence of allochthonous plankton sources, but they do suggest a role for them. Both Bothar
(1987) and de Ruyter van Steveninck et al. (1992) obtained their demographic
estimates by simply examining the longitudinal variation of zooplankton numbers
in samples, and these empirical deductions might prove more realistic for field
studies than idealized laboratory situations like the one described in Bennett and
Boraas (1989).
Now, considering only the growth rates of the populations versus the mean time
of downstream transfer is clearly too simple to provide a fully comprehensive
approach. Whether or not riverine zooplankters always find the suitable food
items in adequate amounts, to be thus able to feed and grow optimally at all times,
is simply unclear.
One of the most striking contrasts between the Meuse and the Moselle is the
difference in microcrustacean densities. Their constant presence in the Moselle
may also be attributed to the possible persistence along the river of shelters
(meandering sections, macrophyte patches where water velocity is reduced)
where cladocerans and copepods could thrive and be inoculated randomly in the
1757
L-Viroux
main channel, at the mercy of hydrological events, therefore constituting a truly
'autochthonous' plankton source. This would be made possible by the relatively
intact sinuosity of the river, and by the natural state of its banks in many sections.
Indeed, the development of microcrustaceans in the Moselle does not seem to
follow any precise spatial or temporal trend (Figure 3), whereas in the Meuse it
is clear (Figures 6 and 7) that the increase in abundance of microcrustacean zooplankton is strongly linked to minimal summer discharges. The obvious seasonal
occurrence of microcrustaceans in the upper Meuse, and their more intense
development and earlier presence downstream (see Marneffe et al, 1996), are
indications that if retentive mechanisms are weak, these organisms can only be
abundant when favourable hydrological conditions prevail, i.e. when flow is
reduced and retention time is prolonged. In the Moselle, the link between discharge and microcrustacean development is much less pronounced (Figures 3 and
7). However, one must also bear in mind that sampling on one location, using
low-volume devices like the Van Dorn bottle or a bucket, could have led to an
overall underestimation of microcrustacean densities, as these organisms often
exhibit strong avoidance reactions and rely on their swimming abilities to avoid
capture (Smyly, 1969; De Bernardi, 1984). Furthermore, the reduction in current
velocities near the banks might enable good swimmers to distribute themselves
in a heterogeneous fashion both transversally and with depth, a phenomenon that
was considered but shown to be insignificant by Bottrell (in Bottrell et al, 1976)
in the Thames, albeit strongly suggested by Shiel et al. (1982) in the Murray,
Thorp et al. (1994) in the Ohio, and by Marneffe et al (1996) in the Meuse.
In their extensive studies on the Murray-Darling River System, Shiel et al.
(1982), Shiel and Walker (1984) and Shiel (1985) drew a clear distinction between
regulated and unregulated rivers when describing their plankton. Unregulated
(or, rather, moderately regulated) rivers like the Darling seem to support a characteristically rotiferan zooplankton, whereas the Murray, a typically regulated
river, appears much richer in microcrustaceans. The upper Belgian Meuse, where
regulation is heavy, hosts a rotiferan zooplankton, while the Moselle, the 'least'
regulated of the two rivers, seems a more suitable environment for microcrustaceans. These apparently contradictory observations no longer hold when reference is made to the comparative morphologies of the various river channels. The
Murray, with its numerous side-habitats and its remarkably intricate floodplain
(Shiel etal, 1982), bears little resemblance to the Meuse despite the fact that they
are both intensely regulated. The Belgian stretch of the Meuse is, by contrast,
almost completely devoid of side-habitats and this only emphasizes their possible
importance as sources of plankton. In moderately regulated rivers like the
Darling and the French Moselle, similar observations can be made. The Darling
channel is deeply incised and its course is homogeneous (Shiel, 1985), while the
French Moselle is much more heterogeneous not only in terms of depth, but also
in the richness of its side-habitats. These two rivers thus have very little in
common, due to their morphological dissimilarities, and cannot be fully compared. The connection of plankton development with hydrology may thus be
strongly modulated by the overall complexity of the fluvial network. Finally, as
neither the Moselle nor the Meuse are impounded with true reservoir-like bodies
1758
Zooplankton in the Moselle and Meuse
of standing water where typically lacustrine zooplankton communities could
develop, I have not included those peculiar habitats in this discussion, though
their importance as plankton 'reactors' is a well-known phenomenon and adds to
the already long list of potential plankton sources (Tailing and Rz6ska, 1967;
Dzyuban, 1979; Shiel et al., 1982; Kiss, 1994).
I have already pointed out that the 'classical' alternation of phyto- and zooplankton blooms could only be observed in the Meuse (Figure 6; see also Marneffe et al, 1996), whereas in the Moselle no such clear illustration could be
discerned (Figure 2). However, as Tailing and Rz6ska (1967) emphasized, the
description of the plankton community on a given point of a body of flowing water
is barely a 'snapshot' of a dynamic situation. One can only draw assumptions on
what has happened on the way to the sampling point, and make careful predictions about the fate of that biomass as it is swept away downstream. As a consequence, one can state that the link between a sample taken on one end of the
studied stretch of the Moselle in summer and another taken 15 days later at the
other end is actually stronger than the link between two consecutive samples
taken on the same spot, keeping in mind the water travelling times deduced from
Figure 8, and that overall plankton dynamics must therefore be examined from
a totally different angle than simply analysing a collection of temporal profiles.
The convergence of spatial and temporal components of plankton development
in rivers must be carefully considered when trying to interpret the observed
dynamics. Clearly, one must not necessarily expect to observe the clear sequences
of planktonic events that one encounters in lakes. One may, however, observe
that in the Moselle, the more intense zooplankton development recorded downstream in spring, and the gradual displacement of the bloom upstream towards
the summer period (Figure 2), provide an illustration of the longitudinal sequence
of planktonic blooms as described by Gamier et al. (1995; their Figure 10).
The differences between the Moselle and the Meuse in 1993 may also be
related to the contribution of associated riverine habitats in shaping the zooplankton of a river. By providing potentially substantial plankton inocula, shelters for slow-developing species, or protection from rapid wash-out, their
influence may partly counteract the effects of purely physical conditions. It is
clear that the constraints of unidirectional flow are among the most relevant
driving forces in such systems, as is indicated by the striking differences underlined between the Rhine and the Moselle. However, river morphology must also
be considered carefully. An understanding of zooplankton dynamics in advective
environments, taking into account the inseparable temporal and longitudinal
components of development, seems difficult to acquire without the help of mathematical models able to describe accurately, with sufficient detail, the hydrodynamic behaviour of the whole fluvial system. In the absence of this type of
model, I hereby referred to simple calculations which should be improved substantially by the acquisition of new data and by implementing the models themselves. Including zooplankton population dynamics in those models primarily
implies an improved knowledge of sources and standing stocks based on accurate
estimates of population densities taking into account the microdistribution of
zooplankters, an aspect that has seldom been investigated. Secondly, these
1759
L-VTroui
observations also call for more large-scale studies aimed at following water
parcels during their transport downstream, as was done by de Ruyter van
Steveninck et al. (1992), using techniques yet to be defined and tested. An
extremely interesting method, modified from the classical limnocorrals, was
recently developed by Thorp et al. (1996) and may prove more efficient for the
study of longitudinal dynamics than rough mathematical estimates of downstream transport.
Finally, it is equally clear that trophic relationships, which were not considered
in the present study, also play a major role in the onset, decline and succession of
planktonic events. Future perspectives should, therefore, necessarily include the
study of phyto-zooplankton interactions in rivers.
Acknowledgements
I am indebted to Veronique Gosselain for her most valuable collaboration during
the sampling campaigns, to Etienne Everbecq (Centre Environment, University
of Liege) for providing the simulated hydrological data used in this paper, and to
the crew of the Institut de Recherches Hydrologiques in Nancy for their faithful
presence at the regular rendezvous, and their efficiency in collecting the samples.
Data for discharge were kindly provided by Electricite de France (Moselle) and
the Compagnie Intercommunale Bruxelloise des Eaux (Meuse). This paper
greatly benefited from the critical advice of Dr Jean-Pierre Descy. The present
study was part of the author's MSc Thesis. He is now a PhD researcher under a
grant from the Belgian FNRS.
References
Admiraal,W., van Zanten.B. and de Ruyter van Steveninck,E.D. (1990) Biological and chemical
processes in communities of bacteria, phytoplankton and zooplanklon in the lower river Rhine.
Limnol. Aktuel., 1, 151-160.
Admiraal.W., Breebart.L., Tubbing.G.M.J., van Zanten.B-, de Ruyter van Steveninck,E.D. and
Bijkerk,R. (1994) Seasonal variation in composition and production of planktonic communities in
the lower River Rhine. Freshwater Biol., 32, 519-531.
Basu,B.K. and Pick,F.R. (1997) Phytoplankton and zooplankton development in a lowland, temperate
river. / Plankton Res., 19, 237-253.
Bennett.W.N. and Boraas,M.E. (1989) A demographic profile of the fastest growing metazoan: a
strain of Brachionus calyciflorus (Rotifera). Oikos, 55, 365-369.
Botha>,A. (1986) Population dynamics and estimation of production in Bosmina longirostris
(O.F.MUller) in the River Danube (Danubialia Hungarica, CVIII). Hydrobiologia, 140, 97-104.
Bothar,A. (1987) The estimation of production and mortality of Bosmina longirostris (O.F.MUller) in
the River Danube (Danubialia Hungarica, CIX). Hydrobiologia, 145, 285-291.
Bothar,A. (1988) Results of long term plankton investigation in the River Danube, Hungary. Verfu
Int. Ver. Limnol., 23, 1340-1343.
Bothar^A. and Kiss.K.T. (1990) Phytoplankton and zooplankton (Cladocera, Copepoda) relationships
in the eutrophicated River Danube (Danubialia Hungarica, CXI). Hydrobiologia, 191, 165-171.
Bottrell,H.H., DuncanA, Gliwicz^Z.M., Grygierek.E., Herzig,A., Hillbricht-Ilkowska.A.,
Kurasawa.H., Larsson,P. and Weglenska,T. (1976) A review of some problems in zooplankton production studies. Norw. J. ZooL, 24, 419-456.
De Bernardi,R. (1984) Methods for the estimation of zooplankton abundance. In Downing J. A. and
Rigler.F.H. (eds), A Manual on Methods for the Assessment of Secondary Productivity in Fresh
Waters. Blackwell Scientific, Oxford, pp. 59-86.
de Ruyter van Steveninck,E.D., Admiraal.W. and van Zanten,B. (1989) Changes in plankton communities in regulated reaches of the lower river Rhine. ReguL Rivers: Res. Manage., 5, 67-75.
1760
Zooplankton in the Moselle and Meuse
de Ruyter van Steveninck.E.D., Admiraal.W., Breebart.L., Tubbing.G.M J. and van Zanten3. (1992)
Plankton in the River Rhine: structural and functional changes observed during downstream
transport. /. Plankton Res., 14, 1351-1368.
DescyJ.-P. (1987) Phytoplankton composition and dynamics in the river Meuse (Belgium). Arch,
Hydrobiol. Suppl. 78 Algol. Stud., 47, 225-245.
DescyJ.-P. and Gosselain,V. (1994) Development and ecological importance of phytoplankton in a
large lowland river (River Meuse, Belgium). Hydrobiologia, 289, 139-155.
DescyJ.-P. and Willems.C. (1991) Contribution a la connaissance du phytoplancton de la Moselle
(France). Cryptogam. Algol., 12, 87-100.
DescyJ.-P., Servais,P., SmitzJ.S., Billen.G. and Everbecq^E. (1987) Phytoplankton biomass and production in the River Meuse. Water Res., 21, 1557-1566.
Dzyuban.N.A. (1979) The zooplankton (metazoic) of the Volga. In Mordukhai-Boltovskoi (ed.), The
River Volga and its Life. Junk, The Hague, pp. 195-231.
Edmondson.W.T. (1977) Population dynamics and secondary production. Arch. Hydrobiol. Beih.
Ergebn. Limnoi, 8, 56-64.
Enaceanu,V. (1967) Das Zooplankton der Donau. In Liepolt,R. (ed.), Limnologie der Donau. E.
Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Section V, pp. 180-197.
Ferrari,!., Farabegoli.A. and Mazzoni,R. (1989) Abundance and diversity of planktonic rotifers in the
Po river. Hydrobiologia, 186/187, 201-208.
GarnierJ., Billen.G. and Coste,M. (1995) Seasonal succession of diatoms and Chlorophyceae in
the drainage network of the Seine River: Observations and modeling. Limnoi. Oceanogr, 40,
750-765.
Gigleux,M. (1992) Le deVeloppement phytoplanctonique dans la Moselle en aval de Metz et dans la
Seille. Mesure de la production primaire. PhD Thesis, Laboratoire d'Ecologie, University de Metz,
107 pp.
Gosselain.V., DescyJ.-P. and Everbecq,E. (1994) The phytoplankton community of the River Meuse,
Belgium: seasonal dynamics (year 1992) and the possible incidence of zooplankton grazing. Hydrobiologia, 289, 179-191.
Guisande.C. and TojaJ. (1988) The dynamics of various species of the genus Brachionus (Rotatoria)
in the Guadalquivir River. Arch. Hydrobiol., 112, 579-595.
Jeuniaux.Ch., Billen.G., DescyJ.-P., Everbecq.E., Servais,P., SmitzJ. and ThomeJ.-P. (1984) Surveillance dcologique de la Meuse en aval du site de Tihange. Rapport de Synthese en trois Volumes,
357 pp.
Jos6 de Paggi.S. (1978) First observations on longitudinal succession of zooplankton in the main
course of the Parana^ River between Santa Fe and Buenos Aires harbour. Stud. Neotrop. Fauna
Environ., 13, 143-156.
Kiss.K.T. (1994) Trophic level and eutrophication of the River Danube in Hungary. Verh. Int. Ver.
Limnoi, 25, 1688-1691.
Lorenzen.CJ. (1967) Determination of chlorophyll and phaeopigments: spectrophotometric
equations. Limnoi. Oceanogr, 12, 343-346.
MacCallum,I.D. (1979) A simple method of taking a subsample of zooplankton. N.Z. J. Mar. Freshwater Res., 13, 559-560.
MacCauleyJE. (1984) The estimation of the abundance and biomass of zooplankton in samples. In
DowningJ.A. and Rigler,F.H. (eds), A Manual on Methods for the Assessment of Secondary
Productivity in Fresh Waters. Blackwell Scientific, Oxford, pp. 228-265.
Mameffe,Y., DescyJ.-P. and Thome J.-P. (19%) The zooplankton of the lower River Meuse, Belgium:
Seasonal changes and impact of industrial and municipal discharges. Hydrobiologia, 317, 1-16.
OSmera.S. (1973) Annual cycle of zooplankton in backwaters of thefloodarea of the Dyje. Hydrobiol.
Stud., 3,219-256.
Papinska.K. (1990) Abundance and composition of rotifers in Vistula River. Pol. Arch. Hydrobiol.,
37, 449-459.
Pechar,T.T. (1987) Use of acetone:methanol mixture for extraction and spectrophotometric determination of chlorophyll a in phytoplankton. Arch. Hydrobiol. Suppl. 78 Algol. Stud., 46, 99-117.
Peelen.R. (1975) Changes in the composition of the plankton of the Rivers Rhine and Meuse in the
Netherlands during the last fifty-five years. Verh. Int. Ver. Limnoi, 19,1997-2009.
Pontin.R.M. (1978) A Key to British Freshwater Planktonic Rotifera. Scientific Publication 38. Freshwater Biological Association, 178 pp.
Rai.H. (1974) Limnological studies on the River Yamuna at Delhi, India Part II The dynamics of
potamoplankton populations in the River Yamuna. Arch. Hydrobiol., 73,492-517.
Reynolds.C.S. and DescyJ.-P. (1996) The production, biomass and structure of phytoplankton in large
rivers. Arch. Hydrobiol Suppl 113 Large Rivers, 10,161-187.
1761
UVlroux
Reynold.C.S. and Glaister,M.S. (1993) Spatial and temporal changes in phytoplankton abundance in
the upper and middle reaches of the River Severn. Arch. Hydrobiol. Suppi, 101, 1-22.
Reynolds,C.S., Carling,P.A. and Beven.KJ. (1991) Flow in river channels: new insights into hydraulic
retention. Arch. Hydrobiol, 121, 171-179.
Richardson.W.B. (1990) Seasonal dynamics, benthic habitat use, and drift of zooplankton in a small
stream in southern Oklahoma, U.S.A. Can. J. Zooi, 69, 748-756.
Ruttner-Kolisko,A. (1974) Plankton Rotifers: Biology and Taxomony. Die BinnengewSsser, Supplementary edn, English translation of Vol. 26(1). E.Schweizerbart'sche Verlagsbuchhandlung,
Stuttgart, 146 pp.
Sanderson.R. (1992) A preliminary study of the zooplankton of the River Meuse (Belgium). MSc in
Natural Resource Management, University of Leicester, 90 pp.
Sandlund.O.T. (1982) The drift of zooplankton and microzoobenthos in the river Strandaelva, western
Norway. Hydrobiologia, 94, 33-48.
SaundersJ.F.,111 and Lewis.W.M.Jr (1989) Zooplankton abundance in the lower Orinoco River.
Limnol. Oceanogr., 34, 397^*09.
Shiel.RJ. (1985) Zooplankton of the Darling River System, Australia. Verh. Int. Ver. Limnol., 22,
2136-2140.
Shiel.RJ. and Walker.K.F. (1984) Zooplankton of regulated and unregulated rivers: The MurrayDarling River System, Australia. In Lillehammer.A. and Saltveit.SJ. (eds), Regulated Rivers. University of Oslo Press, Oslo, pp. 263-270.
Shiel.RJ., Walker.K.F. and Williams.W.D. (1982) Plankton of the Lower River Murray, South
Australia. Aust. J. Mar. Freshwater Res., 33, 301-327.
Shiozawa.D.K. (1986) The seasonal community structure and drift of microcrustaceans in Valley
Creek, Minnesota. Can. J. Zool, 64, 1655-1664.
Smyly,WJ.P. (1969) Some observations on the effect of sampling technique under different conditions
on numbers of some freshwater planktonic Entomostraca and Rotifera caught by a water bottle. /
Nat Hisi., 2, 569-575.
TallingJ.F. and RzoskaJ. (1967) The development of plankton in relation to hydrological regime in
the Blue Nile. J. Ecol, 55, 637-662.
ThorpJ.H., Black,A.R., Haag.K.H. and WehrJ.D. (1994) Zooplankton assemblages in the Ohio
River: seasonal, tributary and navigation dam effects. Can. J. Fish. Aquat. 5c/., 51, 1634-1643.
ThorpJ.H., Black,A.R., JackJ.D. and Casper,A.F. (19%) Pelagic enclosures-modification and use for
experimental study of riverine plankton. Arch. Hydrobiol. Suppl. 113 Large Rivers, 10, 583-589.
TubbingJJ.G.MJ., Admiraal,W., Backhaus.D., Friedrich.G., de Ruyter van Steveninck.E.D.,
MullerJ). and Keller,I. (1994) Results of an international plankton investigation on the River
Rhine. Water Sci. Tech., 29, 9-19.
van Zanten,B. and van Dijk,G. (1994) Seasonal development of zooplankton in the lower River Rhine
during the period 1987-1991. Water Sci. Tech., 29, 49-51.
Received on April 15, 1997; accepted on July 9, 1997
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