Hydrobiologia 200/201: 399-407, 1990.
R.D. Gulati, E.H.R.R. Lammens, M.-L. Meijer & E. van Donk (eds), Biomaniputation - Toot for Water Management.
© 1990 Kluwer Academic Publishers. Printed in Belgium.
Can macrophytes be useful in biomanipulation of lakes? The Lake
Teresa Ozimek, Ramesh D. Gulati I & Ellen van Donk 2
Department of Hydrobiology, Institute of Zoology, University of Warsaw, Nowy Swiat 67, 00-046
Warsaw, Poland; l Correspondence addres: Limnological Institute, 'Vijverhof' Laboratory, Rijksstraatweg
6, 3631 A C Nieuwersluis, The Netherlands; 2Provincial Waterboard of Utrecht, Postbox 80300, 3508 TH
Utrecht, The Netherlands
Key words: lake biomanipulation, submerged macrophytes, filamentous green algae, phosphorus and
Lake Zwemlust (area 1.5 ha, Z m 1.5 m) has been the object of an extensive limnological study since its
biomanipulation involving removal of planktivorous fish (bream) in March 1987 and emptying of the lake.
In the subsequent summer period of 1987 the Secchi depth increased to the lake bottom (2.5 m), compared
with ca 30 cm in the earlier summers. The reaction of submerged macrophytes to improving under-water
light climate was rapid. In summer 1987, besides the introduced Chara globularis, 5 species of submerged
macrophytes occurred and colonized 10~o of the lake area. In 1988 and 1989 only quantitative changes
were observed; new species did not appear, but the area colonized by macrophytes increased by 7 and
10 times, respectively. Elodea nuttallii was dominant among the macrophytes and Mougeotia sp. among
the filamentous green algae. Their abundance, contributed to transient N-limination of phytoplankton
causing a persistent clear water phase in 1988 and 1989, unlike in 1987 when zooplankton grazing
contributed chiefly to the water clarity. Laboratory bioassays on macrophytes confirmed nitrogen
With increase of lake trophy, the area occupied by
and biomass of submerged macrophytes de-
creases (Phillips et al.,
Kowalczewski, 1984; Lachavanne, 1985); so in
hypertrophic lakes plant vegetation is scarce or
absent. Phillips etal. (1978) suggested that in
shallow lakes the main cause of decline of macro-
phytes is shading by epiphytes, loosely connected
filamentous algae and subsequent development of
high phytoplankton concentrations. The multiple
role of macrophytes in functioning of shallow lake
ecosystems is well documented (e.g. Boyd, 1971;
1978; Ozimek &
Pieczynska & Ozimek, 1976; Carpenter & Lodge,
1986; Engel, 1988). Disappearance of macro-
phytes causes, directly or indirectly, changes in
other communities in la.kes. So, restoration of
shallow, hypertrophic lakes should be very closely
connected with restoration of water plants (Moss,
We studied changes in submerged macrophytes
in a shallow hypertrophic Lake Zwemlust (The
Netherlands) in a two-year period (1988-'89)
after its restoration by biomanipulation in spring
1987. The lake has been the object of extensive
limnological studies since its biomanipulation
involving removal of planktivorous fish (bream)
in March 1987 (Van Donk et al., 1989). During
this operation the lake was emptied by pumping
out water and its fish was simultaneously
removed. The lake got refilled within a week
chiefly by nutrient seepage water from the River
Vecht. Before biomanipulation macrophytes were
absent. During the biomanipulation operation
Nuphar lutea L. and Clara globularis Thuill. were
introduced to the lake as refuges for pike.
In this paper we examine the changes after
biomanipulation in composition and in relative
abundance of the total biomass of macrophytes,
and present data on growth and nutrient uptake
rates ofEIodea nuttalli, the most important among
the macrophyte species present in the lake. Also,
bioassay experiments were carried out on Elodea
sp. in the sunamer of 1988 when N-NO3 and
N-NH 4 concentrations had declined to near
detection levels but P-PO 4 level was high and still
close to the pre-biomanipulation level of ca: 1 nag
P 1 ~ (Van Donk et al., 1989).
Material and methods
Lake Zwemlust (area, 1.5 ha; mean depth 1.5 m,
maximum depth 2.5 m) is located in the Province
of Utrecht, The Netherlands. It is used for recrea-
tive purposes with 100-300 visitors daily in sun>
mer, but there may be a tenfold increase in num-
bers on warm days.
The lake was sampled three times during
August-November in 1988 and once in July 1989.
Samples were collected from 5 transects (Fig. 1);
in total 50 samples were collected on each sam-
pling date. A modified Bernatowicz type sampler
(sampling area, 0.1 m 2) was used to collect
samples to study species composition and
biomass of macrophytes
Macrophytes were segregated into species and
periphyton and calcium deposits were removed
by washing; the dry weight, nitrogen and phos-
phorus contents were measured. The nitrogen
content of the plants was determined using a
CHN analyzer (Perkin-EImer 240). Phosphorus
was determined with a molybdate method accord-
ing to Golterman (1969). Filamentous green algae
+, SUBMERGED) MACROPHYTES
~-.: FILAMENTOUS GREEN ALGAE
Fig. I. Schematic distribution of submerged macrophytes and filamentous green algae in Lake Zwemlust in August 1988 and
floating on water surface, and farming a scum-like
material, were collected from a quadrat of
100 cm2; 20 samples were collected each time.
The growth rate and N and P uptake rates were
determined in water enriched with N and P levels
comparable with those measured in Lake Zwem-
lust directly after biomanipulation in 1987. The
biomass per unit area of Elodea nuttallii used in
such experiments was similar to the mean
biomass m - 2 noted in Lake Zwemlust in summer
Plants and water for the experiments were col-
lected from the lake in September 1988. The
plants were washed under running tap water, tak-
ing care not to damage the plant tissues. Before
the experiment the apical portion (ca 20 cm) of the
mother shoot was cut off and acclimatized 3 days
in the laboratory at 19 °C, 16 : 8 light : dark cycle,
and at light intensity (PhAR) of about 30 W m - 2.
Several samples of plants were weighed fresh after
blotting out the surface moisture, dried at 105 °C
for 24 h and then weighed again, to get the fresh
weight and dry weight, respectively.
The biomass increase ofE. nuttallii was studied
at different nitrogen levels. Five shoots were
cultivated in four sets (I-IV) of two-litre aquaria
using: one-litre filtered (Whatman GF/F) water
from Lake Zwemlust (I), filtered water + 2 mg
N-NO3 (II), filtered water + 2 mg N-NH 4 (III),
and filtered water + 2 mg N-NO 3 + 2 mg N-NO4
(IV). The experiment lasted 14 days. At the
beginning and end of experiment total dry weight
of the plant material and concentrations of
N-NH4, N-NO3 and P-PO4 in each aquarium
were measured. Each treatment was replicated
five times. Uptake rates of N-NH4, N-NO 3, and
P-PO4 were also measured at different levels of
To determine temporal changes in nutrient con-
centration due to uptake, Elodea nuttallii (0.5 g dry
weight) was cultivated in 1 1 filtered lake water
with 2 mg 1 - 1 each of N-NH 4 and N-NO3. Phos-
phorus and nitrogen concentrations were meas-
ured after 4, 8, 16, 32 and 64 h. For each exposure
period 3 aquaria with plants and 3 blanks were
used. The experiment was carried out in labora-
tory conditions described above. P-PO 4 was
determined according to Murphy & Riley (1962),
N-NO) according to Stainton et al. (1974) and
N-NH 4 following Verdouw et al. (1977). For all
determinations a Cerco automated analyzer was
Distribution. biomass, nitrogen and phosphorus
In the summer of 1988 macrophytes and fila-
mentous green algae occupied ca one ha, i.e.
about 70~o of lake bottom; in summer of 1989
almost 100~o of the lake bottom was covered. The
distribution of macrophytes and filamentous
green algae is shown schematically in Fig. 1. Six
species of aquatic vascular plants, Chara globu-
taris and 9 taxa of filamentous algae were present
in both 1988 and 1989 (Table I). Among the
macrophytes E. nuttallii attained the highest fre-
quency (Table 1) and among the algae, Mougeotia
(August-November 1988) and Cladophora (July
1989). C. globularis was introduced into the lake
during biomanipulation operation, the rest of sub-
merged macrophytes occurred naturally in the
lake. The highest macrophyte biomass observed
during the study namely, ca 120 g dry weight m - 2
for the whole lake area, was in September-October
1988 (Fig. 2) and algal maximum, ca 13 g dry
weight m - 2, occurred in August-September 1988
(Fig. 3). The standing cl:op maxima of macro-
phytes and algae during the investigation period
were: 1255.8 kg DW and 144.0 kg DW, respec-
tively. In July 1989, macrophyte and algal stand-
ing crops on areal basis of 320 and ca 25 g DW
m - 2, respectively, were three and two times higher
than in the summer months of 1988 (Figs. 2
E. nuttallii was dominant in both the years: it
contributed ca 70~ to the total macrophytes
biomass in 1988 and 82~o in 1989. In 1988 it
Table I. Frequency of occurrence and contribution to biomass of the submerged macrophytes in Lake Zwemlust, in August 1988
and July 1989,
Species Frequency (~g)
Contribution to biomass (~o)
1988 1989 1988 1989
Elodea nuttallii (Planch.) St. John
Ceratophylhon demersum L.
Chara globularis Thuill,
Elodea canadensis Michx.
Potantogeton bertholdii L.
Potamogeton crispus L.
Potamogeton acutifolius L.
Fig. 2. Average dry weight (95"/0 confidence limits) of sub-
merged macrophytes in Lake Zwemlust.
Fig. 3. Average dry weight (95~% confidence limits) of fila-
mentous green algae in Lake Zwemlust.
occurred in clumps growing from the bottom to
water surface with shoots ca 2m long; also
smaller shoots (20-40 cm) grew on the bottom
and non-rooted, floating shoots were present. The
biomass ofE. nuttallii ranged from 0.1 g DW m - 2
for the floating shoots to ca 500 g DW m - 2 for the
clumps. The clumps occupied < 5 ~o of lake sur-
face in 1988, but > 30~o in 1989. E. nuttallii is
evergreen, it spends winter as dormant species. In
1988 the plant started to produce many dormant
apices already in October and the old shoots
started decaying. In early November the dormant
apices contributed ca 23~o to the biomass.
In December the old shoots were noted only
sporadically (R. Kornijow, pers. communica-
The macrophytes and algae accumulated sub-
stantial amounts of N and P (Table 2). The dif-
ferences in the accumulated contents of N and P
by macrophytes and algae resulted from large dif-
ferences observed in their biomasses, since N and
P contents per unit dry weight in macrophytes and
algae differed only slightly (unpublished data). In
winter, about one-third of the total P and N in the
plants was stored in dormant apices (Table 3).
The biomass increments of E. nuttallii were from
2 to 4 times greater than N-enriched (2 mg 1-1
each of N-NH 4 and N-NO3) water than in the
controls (Fig. 4); the differences between treat-
ments II, III and IV and the control if compared
as final dry weights were significant (Mann-
Table 2. Accumulation of nitrogen and phosphorus in macrophytes and in filamentous green
August-2 November 1988.
algae in Lake Zwemlust, 31
Plant Period Nitrogen
g m- 2 total kg g m- 2 total kg
Table 3. Accumulation of nitrogen and phosphorus in old summer shoots and in new winter shoots of E. nuttallii in Lake
Zwemlust, November 1988.
Shoots Dry weight
~o D.W. gm -2
11 111 IV
 Final i
Fig. 4. Changes of dry weight f E. nuttallii under different
nitrogen enrichment levels: I, blank; II, water from Lake
Zwemlust + 2 mg N-NO3; III, +2 mg N-NH4; and IV,
+ 2 mg N-NO3 + 2 mg N-NH4.
Whitney test, P < 0.01). In the 14-days bioassay
the plants absorbed about 50~ and 75-90~o of
initial P and N content, respectively (Fig. 5).
Depending on N concentrations in the ambient
medium E. nuttallii can utilize 1.3-1.6 mg P g- 1
DW and 0.1-9.0 mg N g- I DW.
Elodea nuttallii depleted N in the water very
rapidly. It preferred N-NH 4 to N-NO 3 if both
ions were available in water in similar concen-
tration (Fig. 6). A reduction of 50~ of initial
N-NH4 content was noted after 8 h. The plants
absorbed almost all N-NH 4 in about 32 h. The
high uptake of N-NO3 by plants began from 32 h
when N-NH 4 was almost exhausted. After 64 h
75 ~o of the N-NO3 was taken up by plants. The
plants assimilated only about 10~o of P during the
experiment lasting 64 h (Fig. 6).
An important aim of the biomanipulation strategy
to restore lakes has been to improve the under-
water light climate in the lakes, with zooplankton
reducing the seston, including algae, while
xx xxx Iv Experiments no.
~p_P04 ~N_NH4 m~N_N03 Initial
F(~. 5. Changes in the concentrations of N-NH 4, N-NO3 and P-PO 4 in the medium after 14 days of cultivation E. mma//ii; codes
as Figure 4.
- __N_NH 4
I ~ - - P- PO 4
s ie 3z 64
h o u r s
• With E. nuttallii
FL~. 6. Uptake rates of nitrogen and phosphorus ofE. m~uallii in water enriched with 2 mg N-NH 4 and 2 mg N-NO3; solid circles,
blanks: and solid squares, with E. matallii.
nutrient levels stay still high (Shapiro & Wright,
1984; Lampert, 1988; Gulati, 1989). Light is con-
sidered to be a key factor regulating the growth
and distribution of submerged macrophytes
(Spence, 1972; Barko & Smart, 1981; Barko &
Filbon, 1983). The biomanipulation approach
should create conditions that stimulate growth of
submerged macrophytes, particularly in spring
when plants start to grow from the bottom.
In Lake Zwemlust, the response of submerged
macrophytes to improving light climate was rapid.
The course of changes in vegetation, namely from
macrophytes and filamentous algae in 1986 (i.e.
year before biomanipulation) to 1989, is shown
schematically in Fig. 7. In 1987, in the first sum-
mer after biomanipulation, besides the species
introduced, 5 species of submerged macrophytes
occurred and had colonized 10 ~/0 of the lake area
(Van Donk et al., 1989). In 1988 only quantitative
changes were observed; no more new species
appeared, but the area colonized by the macro-
phytes increased 7 times compared with 1987.
The areal biomass increased to a level similar to
that in eutrophic lakes, e.g, in many Polish
eutrophic lakes (Pieczynska & Ozimek, 1976). In
1989 the area occupied by macrophytes increased
further such that virtually the entire lake bottom
was covered, and the macrophyte biomass
increased to a level, prevalent in fertile ponds
(Pokorny & Ondok, 1982) and lake habitats
fertilized by municipal sewage (Ozimek, 1978).
The importance of submerged macrophytes in
ecosystem functioning is reported to be propor-
tional to their biomass and productivity
• Phytoplankton predominant
• Water transparency low
• Macrophytes absent
Chronology of limnological events
in Lake Zwemlust (1987-1989)
• Situation comparable with
summer 1988 - a further 50%
increase in macrophytes
Fig. 7. Scheme illustrating changes in macrophytes in Lake Zwemlust in 1986-1989.
4 ,,, ,,,,,,
• High zooplankton grazing
• Increased water clarity
• Appearance of macrophytes
and fil. green algae
• "Clear water" - resulting from
• Predominance of macrophytes
• Subdominance fil. green algae
• Zooplankton grazing pressure
• Improved underwater light
• High P, low N
Suppression of phytoplankton
due to N-limitation and shading
and allelopathy (?) caused by
• High water clarity, high P
• Grazing pressure moderate
(Carpenter & Lodge, 1986). In this respect
E. nuttallii has apparently played a central role in
Lake Zwemlust, especially in view of the aim of
the biomanipulation measures. E. nuttallii started
growing actively very early in the year when water
temperature is about 4 °C, even though it is
known to grow more intensively at temperatures
between 10 and 17 °C (Kuni, 1982). Possibly, an
early canopy growth (Barko & Smart, 1981;
Moss, 1990) of E. nuttallii, the dominant species
in the lake, enables this species to successfully
compete with other macrophy~es, but also with
filamentous algae and phytoplankton. Similarly, a
positive effect of biomaniputation on submerged
macrophytes was noted also in other Dutch lakes,
e.g. in Lake Bleiswijkse Zoom (Meijer etal.,
In Lake Zwemlust, together with macrophytes,
filamentous green algae occurred in large
amounts. Some workers have reported fila-
mentous green algae to negatively affect the
growth of some species of submerged macro-
phytes, e.g.E, canadensis (Simpson & Eaton,
1986) and to cause their decline (Phillips et al.,
1978). It remains to be seen if further develop-
ments in Lake Zwemlust will be commensurate
with the hypothesis proposed by Phillips et al.
Macrophytes often accumulate large quantities
of inorganic nutrients early in the growing season
(Boyd, 1971). Nutrients stored during early spring
growth are utilized for growth later. So, macro-
phytes which start to grow early in the season
have a competitive advantage over other macro-
phyte species and phytoplankton. Dense stands
of macrophytes can cause deficiencies of nutrient
in water (Boyd, 1971). Such a situation was
observed in Lake Zwemlust. Rapid growth and
high biomass of plants caused limiation of N in
summers of 1988 and 1989, but not of P, the level
of which remained high (Van Donk et al., 1989;
Van Donk et al., 1990). An important question
about the role of macrophytes in lakes is the
extent to which macrophytes beds act as source
or sink for a nutrient. Generally, macrophyte
stands always act as sink for dissolved N (Mickle
& Wetzel, 1978; Howard-Williams, 1981); as
regards dissolved P, the macrophytes may act as
a sink usually in spring but also sometimes as a
source usually in summer (Prentki et al., 1978;
Landers, 1982). In Lake Zwemlust in summers of
1988 and 1989 dense stands ofmacrophytes acted
as sink for both N and P. Our bioassay experi-
ments supported N limitation by plants in the
lake; this may explain inhibition of phytoplankton
and periphyton growth (see e.g. Fitzgerald, 1968)
both of which had very tow biomass in 1988 and
1989. So, the persistence of clear water in 1988
and 1989 was probably caused by macrophytes,
unlike in 1987 when zooplankton grazing con-
tributed chiefly to water clarity (Gulati, 1989).
Macrophytes can affect phytoplankton not
only by competing for nutrients but also by shad-
ing (Goulder, 1969) and, possibly, by allelopathy
(Wium-Andersen etal., 1982). Role of macro-
phytes in biomanipulation of lakes should be
based not only on their negative effects on phyto-
plankton but also on their positive effects on
zooplankton and fish. In deep lakes zooplankton
has refuges against fish predation in deeper layers
of lakes, but in shallow lakes plants may take over
this role of 'sheltering' zooplankton. Species and
size composition of fish in shallow lakes is asso-
ciated with type and abundance of vegetation
(Grimm, 1989; Engel, 1988). Engel (1988)
reported that plant beds denser than 300 g DW
m-z are difficult for fish to penetrate. In Lake
Zwemlust macrophytes attained such high levels
of biomass in many parts, despite the macrophyte
removal by harvesting in early July 1989. Water
within the dense beds of macorphytes may
become deoxygenated to a level deleterious to fish
(Davis, 1975). Therefore, such situations should
be prevented by control measures. Macrophytes
create not only refuges but also foraging environ-
ment for fish. For example, macrophytes comprise
a significant portion of diet of rudd (Prejs, 1984)
which was common in Lake Zwemlust in 1988
In evaluating the role of aquatic plants in lake
restoration their important features are: early
active growth at low temperature, temperate pro-
ductivity levels, high capacity for absorption of
minerals and nutrients, mainly directly from
water, storage of accumulated nutrients for long
periods (overwintering plants), low P release rates
and a likely release of allelopathic substances
which negatively affect phytoplankton growth.
Besides, the macrophyte standing crop can be
regulated by repeated harvesting during their ex-
tended growth period.
The first author feels indebted to the Limnological
Institute, for providing funds for travel to The
Netherlands and stay at the Vijverhof Laboratory
of the Institute. Special thanks are due to Klaas
Siewertsen, who did the chemical analyses of
plants and Cecilia Janssen-Kroon for typing the
Barko, J. W. & G.J. Filbin, 1983. Influences of light and
temperature on chlorophyll composition in submersed
freshwater macrophytes. Aquat. Bot. 15: 249-255.
Barko, J. W. & R. M. Smart, 1981. Comparative influences
of light and temperature on the growht and metabolism of
selected submersed freshwater
Monogr. 51: 219-235.
Boyd, C. E., 1971. The limnological role of aquatic macro-
phytes and their relationship to reservoir management.
Res. Fish Limnol. 8: 153-166.
Carpenter, S. R. & D. M. Lodge, 1986. Effects of submerged
macrophytes on ecosystem processes. Aquat. Bot. 26:
Davis, J. C., 1975. Minimal dissolved oxygen requirements of
aquatic life with emphasis on Canadian species: a review.
J. Fish Res. Bd Can. 32: 2295-2332..
Engel, S., 1988. The role and interactions of submerged
macrophytes in a shallow Wisconsin Lake. J. Freshwat.
Ecol. 4: 229-341.
Fitzgerald, G. P., 1969. Some factors in the competition or
antagonism among bacteria, algae, and aquatic weeds. J.
Phycol. 5: 351-359.
Golterman, H. L., 1969. Methods for chemical analysis of
freshwaters. IBP Handbook 8, Blackwell Scientific Publi-
cations, Oxford, 166 pp.
Goulder, R., 1969. Interactions between the rates of pro-
ductions of a freshwater macrophyte and phytoplankton in
a pond. Oikos 20: 300-309.
Grimm, M.P., 1989. Northern pike (Esox lucius L.) and
aquatic vegetation, tools in the management of fisheries
and water quality in shallow waters. Hydrobiol. Bull. 23:
Gulati, R.D., 1989. Structure and feeding activities of
zooplankton community in Lake Zwemlust, in the two
years aftcr biomanipulation. Hydrobiol. Bull. 23: 35-48.
Howard-Williams, C., 1981. Studies on the ability of a
Potamogeton pectinatus community to remove dissolved
nitrogen and phosphorus compounds from water. J. appl.
Ecol. 18: 619-637.
Kuni, H., 1982. The critical water temperature for the active
growth ofElodea nuttalli(Planch.) St. John. Jap. J. Ecol. 32:
Lachavanne, J.B., 1985. The influence of accelerated
eutrophication on the macrophytes of Swiss lakes:
abundance and distribution. Verb. int. Ver, Limnol. 22:
Landers, D. H., 1982. Effects of naturally senescing aquatic
macrophytes on nutrient chemistry and chlorophyll a of
surrounding waters. Limnol. Oceanogr. 27: 428-439.
Lampert, W., 1988. The relation between zooplankton
biomass and grazing. A review. Limnologica 19:11-20.
Meijer, M. L., A. J. P. Raat & E. W. Doef, 1989. Restoration
by biomanipulation of Lake Bleiswijkse Zoom (The
Netherlands): first results. Hydrobiot. Bull. 23: 49-57.
Mickle, A. M. & R. G. Wetzel, 1978. Effectiveness of sub-
mersed angiosperm-epiphyte complexes on exchange of
nutrients and organic carbon in littoral systems. Aquat.
Bot. 4: 303-329.
Moss, B., (1990). Engineering and biological approaches to
the restoration from eutrophication of shallow lakes in
which aquatic plant communities are important compo-
nents. Hydrobiologia 200/201: 367-377.
Murphy, J. & J. P. Riley, 1962. A modified single solution
method for determination of phosphate in natural waters.
Analyt. chim. Acta 26:31-36.
Ozimek, T., 1978. Effects of municipal sewage on the sub-
merged macrophytes of lake littoral. Ekol. pol. 26: 1-39.
Ozimek, T. & A. Kowalczewski, 1984. Long-term changes of
the submerged macrophytes in eutrophic Lake Miko-
lajskie (North Poland). Aquat. Bot. 19:1-11.
Phillips, G. L., D. Eminson & B. Moss, 1978. A mechanism
to account for macrophyte decline in progressively
eutrophicated freshwaters. Aquat. Bot. 4: 103-126.
Pieczynska, E. & T. Ozimek, 1976. Ecological significance of
lake macrophytes. Int. J. Ecol. Envir. Sci. 2: 115-128.
Pokorny & Ondok, 1982. Photosynthesis and primary pro-
duction in submerged macrophyte stands. In: B. Gopal,
R.E. Turner, R.G. Wetzel & D.E. Whigham (eds).
Wetlands: Ecology and management, Internat. Sci. Publ.
Jaipur, India: 206-214.
Prejs, A., 1984. Herbivory by temperature freshwater fishes
and its consequences. Envir. Biol. Fishes 10: 281-296.
Prenki, R.T., T.D. Gustafson & M.S. Adams, 1978.
Nutrient movements in lakeshore marshes. In: R.E.
Good, D. F. Zhigham & R. L. Simpson (eds). Freshwater
Wetlands. Academic Press, New York, London: 169-194.
Shapiro, J. & D. I. Wright, 1984. Lake restoration by bio-
manipulation: Round lake Minnesota, the first two years.
Freshwat. Biol. 14: 371-383.
Simpson, P. S. & J. W. Eaton, 1986. Comparative studies of
photosynthesis of the submerged macrophyte Elodea
canadensis and filamentous algae Cladophora glomerata
and Spirogyra sp. Aquat. Bot. 14: 1-22.
Spence, D. H. N., 1972. Light on freshwater macrophytes.
Botanical Society of Edinburgh Transactions, 41:
Stainton, M. P., M. J. Capel & P.J. Armstrong, 1974. The
chemical analysis of freshwater. Miscellaneous special
publications no. 25. Research and Development Direc-
torate Freshwater Institute, Winnipeg, Manitoba.
Van Donk, E., in press. Changes in community structure and
growth limitation of phytopla~akton due to top-down food-
web manipulation. Verh. int. Ver. Limnol. 24.
Van Donk, E, R. D. Gulati & M. P. Grimm, 1989. Food-web
manipulation in Lake Zwemlust: positive and negative
effects during the first two years. Hydrobiol. Bull. 23:
Van Donk, E., M.P. Grimm, R.D. Gulati & J.P.G.
Klein-Breteler (1990). Whole-lake food-web manipulation
as a means to study community interactions in a small
ecosystem. Hydrobiologia 200/201: 275-289.
Verdouw, H., C. J. A. Echteld & E. M.J. Dekkers, 1977.
Ammonia determination based on indophenol formation
with sodium salicylate. Wat. Res. 12: 399-402.
Wium-Andersen, S. U., U. Anthoni, C. Cristophersen & G.
Houen, 1982. Allepathic effects on phytoplankton by sub-
stances isolated from aquatic macrophytes (Charales).
Oikos 39: 187-190.