Journal of Plankton Research Vol.20 n o i l pp.2071-2087, 1998
Plankton development in a rapidlyflushedlake in the River Spree
system (Neuendorfer See, Northeast Germany)
Norbert Walz and Martin Welker
Institutfur Gewdsserokologie und Binnenfischerei, Muggelseedamm 260,
D-12562 Berlin, Germany
Abstract. The Neuendorfer See is a shallow lake through which the River Spree flows. In the lake,
abundances of zooplankton (mainly rotifers) and phytoplankton increased exponentially over the
flowing stretch between the inflow and the outflow of the river. Increase in space is enhanced by
increased residence time in the lake. Sensitivity of plankton development on residence times of <8
days weakened at longer residence times. This pattern holds for all zooplankton species. Rates of
change in chlorophyll and rotifers (after correction for temperature) are first enhanced, then are
decreased at longer residence times. Concentrations of dissolved inorganic nutrients [Si, soluble
reactive phosphorus (SRP) and NO3"] and the N/P ratio diminished with increasing residence time.
The plankton dynamics in the lake were dependent on discharge and the lake behaved as a 'plug flow
tubular reactor' with minor longitudinal mixing. This was not different from the transportation in
large rivers.
Introduction
From the beginning of plankton research (Hensen, 1887), the main focus of interest has been on lentic waters, especially large lakes. At the turn of the 19th to the
20th century, however, plankton of lotic waters were also studied intensively.
Lauterborn (1893) published his 'Contributions to the rotifer fauna of the River
Rhine and its old branches' and Schroder (1897) 'On the plankton of River Oder'.
Zacharias (1898) established the term 'potamoplankton' to point out the
peculiarities of this community. Woltereck (1908) approached this subject with
his paper 'Plankton and lake outflow'. The paradigm, however, ended abruptly
with Brehm's (1911) insistence on a lentic origin of the plankton and his opinion
that they could survive only for short periods in the water current. In contrast to
the study of benthos in brooks and rivers, there was little research on potamoplankton for more than three-quarters of a century. With the exception of taxonomic reports and a few ecological studies, e.g. in the Nile (Tailing and Rzoska,
1967), it is only in the last decade that the study of plankton ecology in lotic water
bodies has been taken up again (Reynolds, 1995). This is especially true for larger
rivers such as the lower Rhine (De Ruyter van Steveninck et a/., 1992), the River
Meuse (Descy and Gosselain, 1994), the lower Orinoco (Saunders and Lewis,
1989) or the Rideau River, Canada (Basu and Pick, 1997).
For this reason, the question of whether the potamoplankton originated in the
river channel or are derived from lentic sections has not been fully answered.
Recent investigations show that both mechanisms contribute (Saunders and
Lewis, 1989; Reynolds and Glaister, 1993). Moreover, a meroplanktonic origin is
increasingly supported (Reynolds and Descy, 1996). The time for the advective
transport along the river stretch is a critical factor in the development of potamoplankton populations (Margalef, 1960). This term interacts with the generation
© Oxford University Press
2071
N.Watz and M.Welker
times of the individual species. For the formation of zooplankton populations in
temperate regions, this condition will be met only in the lower reaches of larger
and longer rivers (De Ruyter van Steveninck et al., 1992). Additionally, there
must be a steady upstream inoculation into the main channel. The source of
plankton inoculation must be situated in zones with long biotic residence times.
The origin of fluvial populations is served by lotic in-stream refugia (Lancaster
and Hildrew, 1993) and 'dead-zones', e.g. lateral sites with dense macrophyte
stocks, or lentic riverine lakes, backwaters or connected oxbow lakes. Such
natural zones are not much studied in this respect. Reynolds (1995) demonstrated
that lateral 'storage-zones', as he called them, in the main channel can account
sufficiently for the recruitment of plankton. This is in addition to the effect of artificial impoundments and riverine reservoirs (e.g. S0balle and Kimmel, 1987) on
the downstream plankton.
The present investigation was carried out on the River Spree, a cascade of
natural riverine lakes and lotic sections (see Kohler, 1994). The average time for
advective transport of the plankton is prolonged by these lakes, which are potentially 'plankton reactors'. The question is: will densities of phytoplankton and
zooplankton increase in such 'reactors' and what is the relationship of this
production to the flushing rate? The qualitative and quantitative outcome will
depend on whether the reactor is mixed (i.e. a chemostat) or a tubular flow
reactor (piston flow, plug flow reactor) (Uhlmann, 1972).
Study site
The Neuendorfer See is a lentic dilation along a sixth-order section (Strahler,
1957) of the River Spree, situated -60 km southeast of Berlin, Germany. The
River Spree rises in the Lusatian Mountains at 580 m a.s.l. near Neugersdorf
(Czech Republic) and flows northwards through Saxony and Brandenburg. Prior
to reaching the Neuendorfer See (241 km), the Spree flows through the Spremberg Reservoir (153 km), where the residence time is -17 days (Kohler, 1994).
The river then anastomoses through the 'Spreewald', an inland delta, before
finally converging -4 km above the Neuendorfer See (Figure 1, Table I). The
output from the lake is situated on the east side. This downstream river reach,
Table L Morphometric data of the Neuendorfer See
Geographic coordinates
Catchment area at inflow
Direct catchment area
Area"
Volume*
Maximum length
Maximum width
Maximum depth
Mean depth1
Mean residence time1
Height above sea level1
•According to Ripl and Schubert (1993).
2072
52o07"N, \y56~E
4S30 km2
34 km2
3.33 km2
8.5 x 10* m3
5.4 km
2.0 km
4.5 m
2.5 m
3.9 days
45 m
Plankton development in a rapidly flushed lake
Station 4>
Neuendorfer See
Fig. 1. Bathymetric map of the Neuendorfer See (from Ripl and Schubert, 1993).
the so-called 'Krumme Spree', flows eastwards to the next lake, the Schwielochsee. The fate of the plankton in this river stretch is the subject of another
paper (Welker and Walz, 1998).
The Spree is the only large river entering the Neuendorfer See. The shoreline
of the lake is characterized by a 10- to 30-m-wide reed zone and by many small
bights. The main flow of the river skirts a somewhat isolated northern bight, but
its volume was not deducted in the calculation of average residence time of the
Neuendorfer See. The surrounding plain provides no shelter from the prevailing
south-western winds. Secchi depth varied between 0.40 and 1.20 m, depending
mainly on resuspension of fine sediments.
In the future, the Neuendorfer See will be subject to substantially lower
discharges from the River Spree. With the decline of the Lusatia opencast coal
mining in the upper reaches of the river, not only will the input of sump water
into the River Spree decrease, but also Spree water will be used to fill up large
opencast mining pits.
The four sampling stations were arranged along the length axis of the lake.
Station 1 was in the inflow, Station 2 at the transition of the channel-like part of
the inlet into the main basin, Station 3 in the outflow, and Station 4 was situated
in the isolated northern bight. The distance between Stations 1, 2 and 3 was
-3 km, respectively.
2073
N.Wab and M.Welker
Method
Two samples were taken by a 3.41 Ruttner sampler (Limnos™) from 0.5,1.0 and
1.5 m depth, respectively, and combined for a 20.4 1 sample which was filtered
through 30 um plankton gauze. The screened zooplankton were refilled to 0.7 1,
narcotized with carbonated water and fixed in 4% formalin. In the laboratory, the
sample volumes were reduced to 100 ml and stained with 1 ml erythrosine solution (0.8 g erythrosine/100 ml water) which coloured rotifers and crustaceans
brightly red, but detritus only slightly.
Zooplankton species were determined according to the following literature:
protists (Patterson and Hedley, 1992), rotifers (Koste, 1978), cladocerans
(Flossner, 1972) and copepods (Einsle, 1993). Rotifers were determined from
living material. Polyarthra dolichoptera and Polyarthra vulgaris were not separated in the routine counts. Nauplius larvae and copepodite stages were differentiated into 'cyclopoids' and 'calanoids'. Subsamples of 10 ml (Hensen stempel
pipette) were counted (100-fold magnification) under a compound microscope
(Ergaval, Carl Zeiss, Jena). At least 100 individuals of one species were counted,
normally several hundreds.
For the calculation of rotifer carbon (C), 0.05 ug C per rotifer was assumed as
a mean value from Rothhaupt (1993). Chlorophyll was converted to dry weight
(DW) using 9.22 ug Chi a mg"1 DW (Behrendt and Opitz, 1996). As diatoms
dominated in the lake, 24.3% carbon content of DW was assumed (Behrendt,
1990).
Chemical analyses of the water samples were carried out after filtration
through Sartorius cellulose nitrate membrane filters (0.45 mm). The following
analytical methods were used.
Soluble reactive phosphorus (SRP): ammonium molybdate method according
to DIN 38.405.
Total phosphorus (TP): as SRP, after disintegration with 10 N sulphuric acid.
Ammonia: indolphenol blue method according to 'Ausgewahlte Methoden der
Wasseruntersuchung' (1986).
Nitrate: ion chromatographic method according to DIN 38405-D19.
Temperature, oxygen: WTW Oximeter EOT 196 in 0.5 m intervals.
Chlorophyll: samples were filtered in the field through Whatman GF/C fibre
glass, using a hand vacuum pump (Nalgene). The niters were wrapped in
aluminium foil, transported in dried silica gel and frozen at -17°C. The filters were
extracted in 10 ml of boiling ethanol (90%, 78°C) and homogenized (Polytron,
Kinematica, 45 s at 8000 r.p.m.). The determination of chlorophyll and phaeophytin followed DIN 38412-L12.
The water discharge volumes were obtained from the regional water authorities (Landesumweltamt Brandenburg) based on daily water gauge measurements at Leibsch.
The growth rates (i.e. rates of change) of rotifers (ind. I"1) and of phytoplankton
(chlorophyll, ug I"1) could be calculated with a tubular plug flow reactor model
(see below) by an exponential increase, as in a batch reactor (Bailey and Ollis,
1986):
2074
Plankton development in a rapidly flushed lake
r = (In A U p u t - l n Winpu,)/r
(1)
r = (ln Chi aomput - In Chi ainput)/f
(2)
and
For t, the calculated residence time was taken as this represents the average time
available for growth. With Qio = 2.7, these growth rates were related to 20°C in
order to remove temperature effects:
log r(20=C) = log r(Temp) + (log Qio X DT)I1O
(3)
where DT is the temperature difference between measured temperature and
20°C.
Qi0 = 2.7 was introduced from the temperature relationship of respiration and
of growth rates in Brachionus spp. (Rotifera) (Galkovskaya, 1987; Walz et al.,
1989).
Results
Dominant zooplankton species
Fast-growing species dominated in the plankton of the Neuendorfer See. During
this study, eight species of protozoa were found, 62 species of rotifers, 17 species
of cladocerans, nine species of copepods and the veligers of the bivalve Dreissena
polymorpha. Of the rotifers, Synchaeta oblonga, Keratella cochlearis (f.
cochlearis, f. tecta, f. robusta), Trichocerca pusilla, Brachionus calyciflorus,
Anuraeopsis fissa and Polyarthra dolichoptera/vulgaris were most numerous.
During the study period, a few species contributed high percentages of the total
numbers of rotifers: Kxochlearis (Nmax = 2401 ind. I"1) and S.oblonga (Nmax =
1549 ind. I"1) accounted for >50% of rotifers in nearly all samples. The highest
numbers of rotifers were found at Station 4 in the isolated northern bight.
Abundances of Crustacea were lower, except for cyclopoid nauplius larvae
(Wmax = 1993 ind. H) and cyclopoid copepodids (Nmax = 401 ind. h 1 ). Adult
cyclopoid copepods reached a maximum abundance of 89 ind. I"1 (Acanthocyclops robustus), but were present in densities of <5 ind. I"1 in all other samples.
Calanoid copepods were absent during the whole study period. Of the cladocera,
Bosmina longirostris (Nmax = 19 ind. I"1), Daphnia cucculata (Nmax = 35 ind. I"1)
and Diaphanosoma brachyurum (NmiX = 101 ind. I"1) were the most abundant and
most continuous species.
Zooplankton abundance along the flowing stretch within the lake
In 1993, the abundance of zooplankton was determined only along the course of
the flowing stretch in the lake at three stations. Results from Station 4 were not
considered further. Rotifer densities at the inflow were always low, ranging from
12 to 131 ind. I"1. The abundances of all species increased along this course, the
2075
N.Wah and M.Welker
highest numbers always being found at the outflow station (Figure 2). There, the
densities ranged from 92 to 1477 rotifers H. This pattern of increase was found
in all major species, in rotifers as well as in crustaceans, and is consistent with a
tubular flow reactor rather than with a mixed one. After mid-September 1993,
water temperatures were <13.5°C and populations did not increase substantially
within the lake. At this temperature, the generation time of K.cochlearis was 9
days (Walz, 1983), which was too long at the available residence time.
The species composition of the plankton community did not change dramatically during its passage through the lake, although the abundances increased by
one or two orders of magnitude. Species that dominated at Station 1 dominated
at the other stations, too. The Shannon-Weaver diversity indices for the
zooplankton populations were 1.32, 1.20 and 1.15 for the three stations, but an
ANOVA revealed no significant difference [F= 3.19 < F(2,9r P = o.os) 4.26].
Assuming a given water parcel moving linearly through the lake in dependence
on the total water residence time, one is able to estimate the flow velocity of that
parcel. In the Neuendorfer See, this ranged from 3 to 9 mm S"1, depending on the
discharge, and was much lower than in lotic sections (50-60 cm sr1; Kohler, 1994).
This linear relationship also allowed the calculation of partial residence times for
the point of the different stations, i.e. 'partial residence time' means the time the
water travelled in the lake up to these stations. In Figure 3, zooplankton abundances from Stations 1-3 and from all dates of the year 1993 are plotted versus the
partial residence times, not taking into account other factors (e.g. temperatures).
The slopes of these semi-logarithmic plots represent the exponential growth rates
of the populations (Table II). The growth rates differed between species, with
100
217.1933
100
10
10
1
'ioo
(0
3
10
100 18.8.1983
30.4-C
10
1
100
10a 13.10.1993
117-0
10
10
Sample points
Fig. 2. Increase in individual densities of zooplankton species along theflowingstretch (Stations 1-3)
within the Neuendorfer See in 1993. Synch, Synchaeta oblonga; Kcoch, Keratella cochlearis; Kquad,
Keratella quadrata; Poly, Polyarthra dolichoptera/vulgaris; Bcaly, Brachionus calyciflorus; Crust, all
crustaceans.
2076
Plankton development in a rapidlyflushedlake
highest rates for S.oblonga, followed by Keratella quadrate, K.cochlearis and
B.calyciflorus. Growth rates of crustaceans were the lowest. Growth rates of
P.dolichoptera/vulgaris were not significantly dependent on residence time.
Rotifer abundance and chlorophyll concentrations depending on total
residence times
In 1993, mainly short residence times were found. Lower discharges during 1994
gave longer average residence times and it should be noted that only Stations 1
Table D. Relationship between zooplankton abundance (ZN) and partial residence time (r) in the
Neuendorfer See according to ZN, = ZN0 X e " " . Exponential growth rate (r, ± SE), coefficient of
determination (r2), number of samples (N), error of probability (P) for Ho:r2 = 0
Species
r(day-')
±SE
Synchaeta oblonga
Keratella quadrata
Keratella cochlearis
Brachionus calyciflorus
Polyarthra vulgaris/dolichoptera
All rotifers
All crustaceans
0.35
0.27
0.24
0.22
0.11
0.24
0.21
0.06
0.06
0.04
0.11
0.06
0.04
0.05
N
0.53
0.51
0.53
0.20
0.10
0.56
0.35
29
23
29
18
29
29
29
<0.0001
<0.0001
<0.0001
0.05
0.09"
<0.0001
<0.0006
a
Not significantly different from zero.
Brachionus calyciflorus
Keratella cochlearis
100
10
o.i
100 Keratella quadrata
Synchaeta oblonga
10O0O
1000
JO
100
CO
10
1
All Crustaceans
All Rotifers
10000
Partial residence time (d)
Fig. 3. Zooplankton abundances as a function of partial residence times along the river stretch
(Stations 1-3) within the Neuendorfer See in 1993. Slopes (representing the speafic growth rates, see
Table II) are given ± 95% confidence limits.
2077
N.Wah and M.Welker
and 3 were sampled, so the difference between lake input and output was taken
as a measure of the downstream plankton development. The input concentration
of chlorophyll was much more variable than that of zooplankton, which was
always low. The zooplankton abundance increased proportionally to the residence time of the water in the lake. As shown in a semi-logarithmic plot (Figure
4), rotifers increased with an exponential growth rate of 0.32 (day-1) (r2 = 0.49, N
= 12, P = 0.01). The increase in chlorophyll, however, did not follow this model
(r2 = 0.1, N = 9, P = 0.4), but increased even more strongly. When the residence
time was >8 days, the rotifer growth reached a plateau of -4000 ind. H. This level
remained constant at even much higher residence times of up to 53 days. In
contrast, chlorophyll seemed to decrease from maximum concentrations of -100
ug I"1 (dashed line in Figure 4). However, this decrease was not significant (exponential rate = -0.01 day-1, r2 = 0.20, N = 8,P = 0.26).
Individual rotifer species also conformed to this pattern (Figure 5). The abundance of some populations, however, even decreased at high residence times.
This pattern was also consistent for different stages of cyclopoids and for Daphnia
cucculata. The increase in the crustaceans, however, was much slower and only
low numbers were ever reached.
Among individual species, several types of residence time-dependent increase
can be distinguished. In one type, population densities increased sharply with
increasing residence times up to -8-10 days; when the residence time is prolonged
further, a decline in densities occurs which might lead eventually to the disappearance of the species; S.oblonga and K.quadrata are examples of this type.
Another type of increase was found in A.ftssa and D.cucculata, for example: their
5
10
20 30 40 50
Residence time (d)
60
Fig. 4. Chlorophyll concentrations and rotifer densities in the outflow of the Neuendorfer See as a
function of residence time in 1993 and 1994.
2078
Plankton development in a rapidly flushed lake
Polyattva dolichJvuig.
Anuraaopsis flssa
1000
1000
10
10
0.1
{•
Diaphanosoma brachkirurr
1000
10•,
1
0.1 •
m*
i
1
cydopoid nauplii
1
Synchaeta obtonga
Brachionus calydflorus
(I/N)
1000
1000
10
10
1000
r
>*
10
0.1
(0
idu
(0
0.1 J-•»
1000
1*
0.1
0.1
l^**>
Keratella cochlearis
1000
1000
1
0.1
1
I
0.1
1
fV
IN.
20
10
4
.
0
6
0.1
0 0
\
.
cydopoid adults
1000 -
1000
1000
0.1
• >
Daphnia cucullata
Keratella quadrata
* / *
10
10 -
10
^ — • "
*
V"*
10
cydopoid copepodids
Trichocerca pusila
>
0.1
r
10
t
-i*
01
21 0
4 10
6
0
0
• / •
-••—i—i—
60
40
20
Residence time (d)
Fig. 5. Abundance of individual zooplankton species in the outflow of the Neuendorfer See in
relationship to residence time in 1993 and 1994. Lines were fitted by eye.
densities increased with residence times up to -10 days. Still longer residence
times resulted neither in increases nor in decreases in these species. This type of
population growth lies between the first type and a third one. This is the steady
increase with maximum densities reached at maximum residence times. Such a
population growth is realized in cydopoid copepods (A.robustus), for example,
and may be seen as population growth still below the maximum. This maximum
was not reached within 53 days of residence in the lake.
The growth rates in relation to residence time showed different patterns
(Figure 6). When the residence times were low, growth rates increased with residence time. When maximum chlorophyll concentrations or maximum rotifer
numbers were reached (see Figure 5), between ~6 and 8 days, average growth
rates became lower. This showed that intensive growth took place only in the first
days of residence in the lake.
2079
N.Wab and M.We!ker
0.5
Chlorophyll a
AD Rotifers
10
20
30
40
50
60
Residence time (d)
Fig. 6. Growth rates of chlorophyll and rotifers of the Neuendorfer See as a function of residence
time in 1993 and 1994. Lines were fitted by eye.
Nutrient concentrations in relation to residence time
Simultaneously with the supposed growth of the phytoplankton in the lake,
concentrations of soluble nutrients showed a sharp decline (Figure 7). The SRP
first decreased by -80%, but to only -50% of the inflow concentration at
prolonged residence times. In comparison, the decrease in dissolved silicate (DSi)
and nitrate was slower, but definitely lower levels were reached for nitrate at
longer residence times. This resulted in a continuous decrease in the N/P ratio
(nitrate-N + ammonia-N/SRP). In spite of this substantial decrease, nutrient limitations did not develop as the absolute concentrations remained high. Mean SRP
decreased from 18.1 ug H at the lake input to 13.2 ug I"1 at the output, and mean
NO3~ from 1.64 to 1.10 mg I"1 (values from 1993 and 1994, N = 16).
Statistical analysis of factors determining rotifer abundance and chlorophyll
concentration
Both zooplankton abundance and chlorophyll increased during passage through
the lake, and more so at longer residence times. Therefore, a positive correlation
was found between rotifer biomass and phytoplankton concentration. Rotifer
carbon depended on phytoplankton carbon by the following power function:
rotif-C = 0.066 X phyto-C° 74, r2 = 0.55, N = 9,P = 0.022 for HQ: r2 = 0
2080
Plankton development in a rapidly Boshed lake
40
0
-40
-80
40
SRP
-40
-80
20
0
-40-
3? -80300
N/P
0
5
10
15
20
25
30
Residence time (d)
Fig. 7. Relative changes in concentrations of dissolved silicate (DSi), SRP (soluble reactive phosphorus) and NOj" between inflow and outflow and N/P ratio in the outflow in relationship to residence time of the Neuendorfer See in 1993 and 1994. Lines were fitted by eye.
A slope (b) of <1 indicated a slower growth of zooplankton in relation to phytoplankton, but it could not be significantly distinguished from 1 (P > 0.1 for Ho:
b = \).
The development of plankton in the Neuendorfer See was negatively correlated to the discharge, i.e. positively correlated with residence time (see Figure
4). However, as discharge was negatively correlated to temperature (Table III),
both parameters could have influenced population dynamics. A closer analysis
with simple Pearson coefficients showed significant positive correlations for the
rotifers versus temperature and inverse correlations to discharge. However,
chlorophyll was only correlated with temperature, not with discharge (Table III).
To exclude the effect of the second respective parameter, partial correlations
were calculated (Sachs, 1992). This measure showed that discharge and temperature alone had about the same (positive or negative) influence on rotifers (Table
III). The positive relationship between chlorophyll and rotifers was only evident
when discharge was held constant. When only the effect of temperature was eliminated, no significant correlation arose. This means that discharge played the
major role in controlling rotifers. Larger parts of the variability of the rotifer
numbers were explained by the combined influence factors when discharge was
included (multiple r2 > 0.9; see Table HI). However, this multiple coefficient of
determination was much lower for chlorophyll (multiple r2 = 0.49) where only
half of the variability could be explained by the two factors. Other factors, such
as nutrients or light, were not included in these correlations.
2081
N.Wab and IVLWelker
Table IIL Correlation coefficients between log rotifer abundance (N I"1), log chlorophyll
concentration (ng 1-') and temperature (°C) and discharge (m-1 s~')- (a) Simple Pearson correlations,
(b) partial correlations with constant discharge or temperature, respectively, (c) multiple correlations
dependent on two parameters respectively
(a) Pearson correlations
Discharge
Chlorophyll
Rotifers
Temperature
Discharge
Chlorophyll
-0.72,/> = 0.001
0.70, P = 0.01
0.87, P < 0.0001
-0.48, ns
-0.89, P < 0.0001
0.74, P = 0.02
(b) Partial correlations
Chlorophyll
Rotifers
Discharge
Temperature
constant discharge constant temp.
Chorophyll
Chlorophyll
constant discharge constant temp.
0.59, P = 0.05
0.73, P = 0.01
0.79,/' = 0.05
0.05, ns
-0.77, P = 0.01
0.38, ns
(c) Multiple correlations
Chlorophyll
Rotifers
Temperature + discharge
Chlorophyll +• temperature
0.70, P = 0.05
0.95, P = 0.001
0.89, P = 0.01
Chlorophyll + discharge
0.96, P = 0.001
ns, not significant, P > 0.1.
Discussion
The origin of the phytoplankton of the River Spree was localized in the Spremberg impoundment, 82 km above the Neuendorfer See, with a residence time of
17 days (Kohler, 1994). In this reservoir, mainly centric diatoms were dominant.
The additional time for the flow from this reservoir through the Spreewald is ~6
days. As in the Seine River (France) (Gamier et al., 1995) and the Lower Rhine
(Admiraal et al., 1994), in the river stretch mainly diatoms grew in spring and both
diatoms and chlorophytes in summer (Kohler, 1994).
The results show that in flushed lakes, plankton growth is dependent on residence time. Dickman (1969) observed an inverse relationship between discharge
and primary productivity in a rapidly flushed British Columbia lake. In Scottish
lochs and Newfoundland ponds, phytoplankton and zooplankton only increased
in periods of greater residence times (Brook and Woodward, 1956; O'Connell and
Andrews, 1987). When these trade-offs are quantified, however, good correlations were only- found between zooplankton and residence time or discharge in
rivers (Van Dijk and Van Zanten, 1995). Basu and Pick (1996) determined the
same retention-dependent slope (after transformation to natural logarithms) for
total zooplankton in eastern Canadian rivers exactly as in our Figure 4. Concerning phytoplankton only, Threlkeld and Choinsky (1985) in a flushed impoundment and Van Dijk and Van Zanten (1995) in the Lower Rhine found a positive
relationship to residence time and a negative correlation to discharge.
2082
Plankton development in a rapidly Sashed lake
There is a probability that plankton development is not initiated at shorter residence times less than those found in the Neuendorfer See. A residence time of
-3 days may be critical. The shortest onset of growth was found after 3-4 days
(Perry et al., 1990; Basu and Pick, 1996). This threshold is in agreement with the
generation times of rotifers (Walz, 1987). In any case, the phytoplankton were
already present when the zooplankton started to grow in the Neuendorfer See.
For this reason, a relationship between phytoplankton and rotifer biomass has a
slope of <1. Interestingly, an inter-lake regression between mean phytoplankton
and zooplankton fresh weights (McCauley and Kalff, 1981) gave a similar slope
(0.72). In the rivers Rhine and Meuse, the time delay of zooplankton development in lakes was shown in a spatial development along the flowing stretch with
a downstream rotifer and crustacean peak behind an upstream chlorophyll peak
(De Ruyter van Steveninck et al., 1990). The spatial phytoplankton growth was
not studied in the Neuendorfer See.
When only the general pattern of the plankton development in the Neuendorfer See in relationship to residence time (Figure 4) is examined, this lake could
also be classified as a mixed reactor (chemostat). This view is compounded by the
fact that also in a mixed reactor, concentrations of organisms first increase with
residence time and then decrease slowly at longer ones (Herbert, 1958; Bailey
and Ollis, 1986; Walz, 1993a). However, due to the increase in plankton organisms along the flowing stretch within the lake (Figure 2), it was possible to characterize the Neuendorfer See clearly as a plug flow tubular reactor (Uhlmann,
1972). Such a plankton increase was also found along the longitudinal axis of
reservoirs (Pourriot et al., 1994) and was taken as characteristic in a conceptual
reservoir model (Marzolf, 1990).
The fact that the plankton in the Neuendorfer See started to grow without a
lag phase is further evidence that the conditions in the lake are not fundamentally different from those in the River Spree. This is due to a general correspondence between tubular flow and riverine transport. The model of tubular flow
implies full lateral mixing (Bailey and Ollis, 1986), as in a dilated river channel
with reduced time of travel (Reynolds and Descy, 1996). Rivers and flow-through
lakes, therefore, are in a first approach to be classified in a continuum (Vannote
et al., 1980). Reynolds et al. (1994) emphasized that the selection of species in
rivers and shallow lakes is less different than that between deep and shallow
lakes. But now the question arises as to why maximum chlorophyll concentrations were not reached in the River Spree before the lake entrance. Residence
times were long enough in the upper course. Indeed, high phytoplankton biomass
was found in the Spremberg impoundment (Kohler, 1994), which even increased
during its passage through the Spreewald in spring, but decreased in summer.
Welker and Walz (1998) could show that the summer plankton in the outflow of
the Neuendorfer See, the Krumme Spree, was diminished exponentially along the
passage of this 21 km stretch. To a high degree, according to experimental
measurements (Siefert, 1996), this decrease could be explained by the clearance
activity of large unionid mussels due to their high abundance in the river bed. In
the lake, benthic filter feeders were present as well (personal benthic observations, occurrence of Dreissena larvae), but apparently did not influence the
2083
N.Walz and M.Welker
plankton community as they did in the river downstream. Lake plankton had less
chance of coming into contact with exposed surfaces [see Butman et al. (1994) on
effects of concentration boundary layer]. This and other discontinuities between
rivers and lakes, and between different river sections, gave rise to a discontinuum
concept (Ward and Stanford, 1995).
In many flushed lakes and large rivers, rotifers are dominant and, in particular,
those species also found in the Neuendorfer See (De Ruyter,van Steveninck,
1992; Pace et al., 1992; Van Dijk and Van Zanten, 1995). The development of the
plankton in this flushed lake is time limited. For that reason, organisms with high
potential growth rates (rmax) are selected (Walz, 1995): rotifers (lowest generation
times 3 days) against crustaceans (lowest generation times 8-14 days; Porter et
al, 1983); within the rotifer group Synchaeta spp. and Brachionus spp. against
Polyarthra spp. and Kellicottia longispina (Walz, 1995). In such 'time-limited'
environments, the intra-community regulation recedes into the background,
favouring the importance of physical factors (Walz, 1993b). Only if residence
times were >8-10 days did a kind of succession take place that is reminiscent of
plankton dynamics in lentic waters. Crustacea occurred in higher numbers and
rotifer numbers were declining, at least in several species. This may represent the
transition from mainly physically determined plankton dynamics to biologically
determined succession (Reynolds and Descy, 1996).
At high residence times, chlorophyll concentrations in the lake tended to
decline (Figure 4). With a clearance rate of -2.5 ul rotifer"1 h"1 (a value reached
at high food concentrations for B.calyciflorus; Rothhaupt, 1990), 4000 rotifers I"1
clear -240 ml I"1 day"1, consistent with an instantaneous mortality rate for phytoplankton of 0.24 day 1 (Sterner, 1989). With this grazing activity, rotifers could
have cleared - 2 1 % of the water column per day and lowered phytoplankton to
50% in 2.9 days if there was no growth. In the river impoundment Baldeneysee
(Essen, Germany), the rotifer population (up to 3400 Brachionus spp. I"1) was
responsible for sharp chlorophyll crashes (Koppe et al., 1985). However, grazing
may not be the only process responsible for algal decline. Algal losses increased
with residence times >10 days in impoundments of Des Moines River due to sedimentation and light limitation (S0balle and Bachmann, 1984).
Acknowledgements
This study was part of a thesis by M.W. and he wants to thank his parents for
financial support. We thank the Chemistry Laboratory of the IGB for nutrient
analyses, Marianne Graupe and Ingrid Hoffmann for chlorophyll analyses, and
Achim Schonborn and Bernd Schiitze for help on sampling excursions. The
Landesumweltamt Brandenburg-Wasserwirschftsamt Cottbus kindly made the
hydrolological data of the Spree available.
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Received on November 14, 1997; accepted on June 22, 1998
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