Hydrobiologia 306: 21-32, 1995. 21( 1995 Kluwer Academic Publishers. Printed in Belgium.

The influence of macrophytes on a phytoplankton community inexperimental conditions

Iwona JasserDepartment of Hydrobiology, Institute of Zoology and Ecology, University of Warsaw, ul. Nowy Swiat 67, 00-046Warszawa, Poland

Received 5 May 1993; in revised form 3 May 1994; accepted 8 June 1994

Key words: macrophytes, phytoplankton, cyanobacteria, allelopathy, dialysis bags, extracts

Abstract

The impact of submerged macrophytes or their extracts on planktonic algae was studied under experimental condi-tions. Live Ceratophyllum demersum L., its extract, and extracts of four other plant species induced modificationsin the phytoplankton dominance structure. These modifications were: a decline in the number of Oscillatorialimnetica Lemm., which was the most numerous cyanobacterian species, and a decline in biomass and percentagecontribution of all cyanobacteria to total algal biomass. This was accompanied by an increase in biomass and per-centage contribution of green algae, especially Chlorella sp. and Chlamydomonas sp. Also, there was an increasein biomass and percentage contribution of nanoplankton (under 50 /pm) to total phytoplankton biomass.

The isolation of planktonic algae from direct influence of C. demersum by means of dialysis membranes caused anincrease in number, biomass and percentage contribution of cyanobacteria. Release of organic compounds of over3000 daltons by macrophytes apparently contributed to a decline of cyanobacteria by changing the phytoplanktondominance structure.

Introduction

The dominance of phytoplankton and the retreat ofsubmerged macrophytes from the littoral are wide-ly known symptoms of eutrophication (Unni, 1977;Phillips et al., 1978; Ozimek & Kowalczewski, 1984).Whereas there is no doubt that both these communitiesuse the same source of energy, identifying the sourcesof their nutrition is by far more complicated. Carig-nan & Kalff (1980) suggest that macrophytes utilizenutrients from interstitial water, even in hypertrophicconditions. However, there are indications that macro-phytes are able to assimilate nutrients from a watercolumn when nutrient concentration in it is higher thanin interstitial water (Carignan, 1982), or when anatom-ical features of the plants give a greater share of theabove-bottom parts in assimilating nutrients (Denny,1972). There are several theories explaining the retreatof macrophytes and dominance of phytoplankton dur-ing eutrophication. Wetzel & Hough (1973) suggestthat the retreat of plants from eutrophic lakes is caused

by phytoplankton shading of macrophytes. An alterna-tive theory by Phillips et al. (1978) suggests that therapid growth of algae is the result rather than the rea-son for this retreat conditioned, especially in shallowlakes, by rapidly growing periphyton and filamentousalgae. Other theories by Balls et al. (1989) point toeither sediment transformations, which are responsi-ble for preventing growth of submerged plants, or tocompetition for nitrogen which results in an intensivegrowth of small algae cells.

Field observations carried out over several years(Schreiter, 1928; Moss et al., 1986; Rorslett, 1986) ineutrophic waters indicated that, with the nutrient sup-ply stable, in some periods phytoplankton abundancewas accompanied by smaller macrophyte biomasswhile in other periods it was the other way round.Other data showed that the abundance of phytoplank-ton in the immediate vicinity of a plant community wasdifferent than in open waters free from plants (Poko-rny et al., 1984; Jones, 1990; Moss, 1990). All theseexplanations seem to suggest that there are antagonistic

22

relations between macrophytes and phytoplankton andthat macrophytes are able to inhibit phytoplankton.

Several authors (Hogetsu et al., 1960; Dokulil,1973; Brammer, 1979; Moss, 1990) indicate that theinfluence of plants on algae could take form of com-petition for light and/or nutrients or could be causedby allelopathic mechanisms. Most studies on rela-tions between macrophytes and phytoplankton are usu-ally concerned with algae biomass (measured onlythrough chlorophyll concentrations). Some laboratorystudies have concentrated on the influence of macro-phytes on selected algal species. But general relationsbetween these two communities remain hypothetical.This study presents a quantitative and qualitative anal-ysis of changes taking place in the phytoplankton underthe influence of macrophytes.

Materials and methods

Macrophytes and water with its natural communityof phytoplankton used in the experiments came fromLake RoS, in the Mazurian Lake District, northeast-ern Poland. Lake RoS has an area of 1888 ha, a meandepth 8.1 m and a maximum depth 31.8 m. At thetime of the experiments, soluble reactive phosphorus(SRP) was not detectable in the water and phytoplank-ton was dominated by cyanobacteria, mainly Oscil-latoria, Aphanizomenon and Microcystis, which con-stituted 75% of the total phytoplankton biomass. Theremaining 25% was composed of diatoms (9%) - withMelosira, Fragilaria, Nitzchia, Synedra and Cyclotel-la; green algae (8%) - Chlorella, Chlamydomonas; andother algae (8%) dominated by dinoflagellates. Totalphytoplankton biomass in the lake ranged from 9 to12 mg of wet biomass 1 - '.

The phytoplankton was incubated in the presenceof macrophytes or their extracts. Five species ofmacrophytes were used: Ceratophyllum demersum L.,Myriophyllum spicatum L., Potamogeton lucens L.,Stratiotes aloides L. and Chara fragilis Desv. A fieldexperiment analysed the influence of live macrophyteson the phytoplankton community inside transparentplastic bags submerged in the littoral of Lake Ros. Thefirst of laboratory experiments probed the influence ofmacrophyte extracts on the algal community while thesecond investigated the influence of macrophytes onalgae separated from the plants by dialysis membranes.In all these experiments, two identical proportions otmacrophyte biomass to water volume were maintained,that is 7 and 28 g of wet biomass 1- l . Biomasses were

characteristic of shallow littoral of eutrophic lakes inthe Mazurian District.

Before the experiments, plants were cleaned ofperiphyton under running water and dried on filterpaper. All living macrophytes remained in good condi-tion throughout the experiments, which was confirmedby the presence of new green shoots following theexposure. The initial and final biomass of the plantsvaried only slightly. After a 10-day exposure, the com-position, number and biomass of phytoplankton werecompared with controls (free from either macrophytesor extracts). At the end of the experiments, 100 mlsamples of water were collected from all containersand preserved with formaldehyde and Lugol solution.The algae were concentrated by sedimentation to 1 ml,counted and identified according to Siemitiska (1964),Starmach (1966, 1989) and Hindak (1978).

The biomass of each taxon and relative total phy-toplankton biomass (mg of wet biomass 1- ) were cal-culated on the basis of the number of taxa, size of theircells and the shape ratio established by Simm (1985).Species with cells smaller than 50 m were defined asnanoplankton. The results were compared with con-trols using the Mann-Whitney non-parametrial sta-tistical test. Statistical relevance was established atc= 0.05.

Field experiment

Samples of 70 (small biomass) and 280 g (largebiomass) C. demersum were placed in double-layeredtransparent plastic bags containing 10 I of lake waterwith its natural algae community. Three identical setsof bags were placed at 0.5 m depth in the littoral, sowere three sets of macrophyte free control bags.

Laboratory experiment with macrophyte extracts

Two series of this experiment were carried out inAugust, each lasting 10 days, using lake phytoplanktonand extracts from five macrophyte species. Portions of1, 4 and 8 g of each species were homogenized in 30 mlof distilled water and centrifuged for 10 minutes (1000r sec- l). The supernatant was filtered through What-man GF/C filters. This method of preparing extractswas based on Kleiven & Szczepaiska (1988). Allextracts were placed in experimental bottles containing0.5 1 of lake water. In this way, three concentrations ofextracts were used: I - 2 g 1- 1 -small, II - 8 g 1- ' -medium, III - 6 g 1- ' - large. Three batches of eachconcentration was prepared. Three bottles with extract-

23

free lake water were used as a control. All bottles werecovered with a net and left outdoors in a shaded place.The macrophyte extracts provided phytoplankton withnutrients and organic compounds. By the same token,the phytoplankton was able to utilize resources whosequantity might otherwise be diminished by the macro-phytes.

Laboratory experiment with dialysis membranes

In aquaria, the phytoplankton was enclosed in 0.1 1bags made of dialysis membranes, so that it was iso-lated from freely floating C. demersum. The bagswere permeated by mineral nutrients and organic com-pounds under of 3000 daltons but blocked the flow ofbigger organic compounds. Plant samples of 70 (smallbiomass) and 280 g (large biomass) were put in twoaquaria, each with 101 of lake water. A third aquarium,without macrophytes, was set up as a control. Threedialysis bags were placed in each aquarium. Bags inthe control aquarium constituted a control for phyto-plankton in bags exposed to macrophytes. All bagswere placed in aquaria on a coarse net just below thewater's surface to protect them from possible shadingby macrophytes. The aquaria were left outdoors in ashaded place for 10 days; the water temperature inthem was within the range of the littoral waters.

Results

Macrophyte impact on phytoplankton in thefieldexperiment

In the control, cyanobacterial communities were dom-inated by: Oscillatoria limnetica, Lyngbya vacuo-lifera Skuja and Aphanizomenon flos-aquae L. Theoccurrence of 0. limnetica, the most numerous of allcyanobacteria, was more than 100 times lower whenexposed to C. demersum than in the control (Fig. la).L. vacuolifera occurred only in the presence of a smallbiomass of macrophytes, where it was 25 times lessnumerous than in the control. A.filos-aquae was discov-ered in only one bag with a large macrophyte biomass.Cyanobacterian biomass was 0.07 and 0.05 mg 1-1 inthe presence of plants, much lower than the controlwhich was 4.8 mg 1- ' (Fig. Ib). Percentage contribu-tion of cyanobacteria to total phytoplankton biomasswas thus 20 and 70 times lower than in the control. Thelower biomass and contribution of cyanobacteria wasaccompanied by much higher biomass of green algae,

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Fig. 1. The number and s.d. of dominant cyanobacterian species(a), the biomass of phytoplankton taxa (b) and the percentage contri-bution and s.d. of nanoplankton to total phytoplankton biomass (c),in the control (C) and in the presence of small (Ms) and large (MI)macrophyte biomasses in the field experiment. Gaps in (a) indicateabsence of particular species; s.d. in (b) is for total phytoplanktonbiomass.

up to 10 times, in the presence of large plant biomass.Percentage contribution of green algae was thus 76 and86% of total phytoplankton biomass; much higher thanin the control. Green algae were dominated by small(under 20 m) unicellular genera: Chlorella, Chlamy-domonas, Treubaria, quadricellular Scenedesmus, and

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24

filamentous (bigger than 50 m) Oedogonium andMougeotia. The biomass and contribution of diatomsin the presence of plants and in the control did not dif-fer, while the biomass and contribution of other algae,mainly dinoflagellates, in the presence of plants waslower than in the control.

Total algal biomass in the presence of a smallmacrophyte biomass was about half of that in the con-trol. However, in the presence of large macrophytebiomass it did not differ significantly from the control,even though the average value was lower. Percentagecontribution of nanoplankton to total phytoplanktonbiomass was about 5 times higher in the presence ofC. demersum than in the control (Fig. Ic). Differencesbetween percentage of nanoplankton in the presenceof small and large macrophyte biomasses were not sig-nificant.

Impact of macrophyte extracts on phytoplankton

Green algae were represented in both series mainly bysmall (under 20 gtm) Chlorella sp., Chlamydomonassp. and Monoraphidium irregular G. M. Smith.Cyanobacteria were represented by 0. limnetica,L. vacuolifera, and A. flos-aquae, bigger than 50 im.Nitzschia sp., Synedra sp. and Fragilaria sp., also over50 tim, were the most numerous among the diatoms.Other algae were represented by Peridinium sp. andGymnodinium sp. (under 50 ttm).

The occurrence of 0. limnetica, L. vacuolifera andA. flos-aquae in the presence of macrophyte extractswas in most cases significantly lower than in the con-trol (Fig. 2a, b). The biggest cyanobacterian responseoccurred in the series I in the presence of C. demersumand S. aloides. In both cases, L. vacuolifera and A. flos-quae were not found, while the number of 0. limneticawas many times lower than in the control. A. flos-aquae was most sensitive to macrophyte extracts. Itwas either very scarce or totally absent in the pres-ence of macrophyte extracts. L. vacuolifera showedmoderate sensitivity to extracts. 0. limnetica, the mostnumerous cyanobacterian species, showed the smallestsensitivity. There was no case of 0. limnetica disap-pearing completely in the presence of extracts but itsnumber was sometimes only one per cent of the con-trol.

In the presence of macrophyte extracts, cyanobac-terian biomass was in all cases in series I and in 9cases out of 12 in series II lower than in the control(Fig. 3a, b). It ranged from being 2 times lower in thecase of extracts of P. lucens to being 70 times lower

in the case of extracts of C. demersum. Cyanobacteri-an biomass accounted for between I and 26% of totalphytoplankton biomass, compared to 47-53% in thecontrol. Lower cyanobacterian biomass in the pres-ence of extracts was accompamied by higher biomassof green algae (in 14 cases out of 15 in series I and in6 out of 12 cases in series II). The biggest green algaebiomass occurred in the presence of medium and largeconcentrations of extracts of Ch. fragilis (Fig. 3a). In25 cases out of 27, the contribution of green algaeto total phytoplankton biomass was greater than in thecontrol, reaching up to 90%. However, no clear relationwas established between biomass of cyanobacteria andgreen algae and increasing concentrations of extracts.In both series, no regularity in occurrence of diatomsand other algae was noted.

Total phytoplankton biomass in series I did notshow a clear relation to the presence and concentra-tions of extracts (Fig. 3a). However, total phytoplank-ton biomass higher than in the control was observedin the presence of medium and large concentrationsof Ch. fragilis extracts (3 and 4 times). In series II,total biomass was in 9 cases out of 12 about half ofthat in the control (Fig. 3b). Percentage contributionof nanoplankton in the presence of extracts was, in20 cases out of 27, bigger compared to the control(Fig. 4). In 10 cases, nanoplankton accounted for morethan 80% of total algal biomass.

Macrophyte impact on phytoplankton isolated bydialysis membranes

Aquaria with macrophytesPhytoplankton in the control aquarium was dominat-ed by dinoflagellates (Peridinium sp., under 50 am).This taxon accounted for 45% of total phytoplank-ton biomass while diatoms (Fragilaria, Synedra,Nitzschia, all over 50 tm) accounted for 31% (Fig. 5).Green algae mainly Chlamydomonas sp., Chlorellasp. and Treubaria sp. (under 20 glm) made up 23%.Cyanobacteria accounted for only one percent. Onthe contrary, green algae dominated phytoplankton inthe aquaria containing small and large C. demersumbiomass, where they accounted correspondingly for50 and 70% of total phytoplankton biomass. The con-tribution of dinoflagellates and diatoms in those aquar-ia was small. Cyanobacteria accounted for only about3% of total biomass but still their contribution to phyto-plankton biomass in the presence of large macrophyte

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Fig. 2 a. The number and s.d. of dominant cyanobacterian species in the control (C) and in the presence of macrophyte extracts. I -small, 1 -medium, III - large concentration of extract. Gaps indicate absence of particular species. Series I - early August.

biomass was significantly lower than in the presenceof small plant biomass.

Total phytoplankton biomass reached about 0.4 mg1-1. Percentage contribution of nanoplankton was notsignificantly different in the control from that in thepresence of small plants biomass (54 and 63%). But inthe presence of large macrophyte biomass it was higher(70%).

Dialysis bags0. limnetica was the most numerous of the cyanobac-terian species both in the control and in the presence ofC. demersum. It was followed by A. fios-aquae. Boththese species had filaments longer than 50 pm. Greenalgae were dominated by small, unicellular Chlorellasp., Chlamydomonas sp., Treubaria sp. and quadricel-lular Scenedesmus sp., all under 20 /tm. Diatoms wererepresented by Synedra sp., Nitzschia sp. and Fragi-

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Fig. 2 b. The number and s.d. of dominant cyanobacterian species in the control (C) and in the presence of macrophyte extracts. I -small, II -medium, III - large concentration of extract. Gaps indicate absence of particular species. Series II - late August.

laria crotonensis Kitton., all over 50 pm. Other algaewere dominated by Peridinium sp. and Gymnodiniumsp. (under 50 am). In bags placed in the aquariumwith small macrophyte biomass, the number of O. lim-netica was 10 times higher than in the control, whilein the aquarium with large plant biomass it was 100times higher (Fig. 6a). The number of A. flos-aquaedid not differ. Cyanobacterian biomass was higher inthe presence of plants (9 and 26 times) than in the con-trol (Fig. 6b). The contribution of cyanobacteria was33 and 44% of total phytoplankton biomass, while itwas about 1% in the control. In the presence of macro-phytes, diatom biomass was lower than in the con-trol. The biomass of green algae and other algae didnot indicate that they were influenced by macrophytes.The isolation of phytoplankton from direct macrophyteexposure led to differences in the composition of thephytoplankton community in dialysis bags and aquaria.In the presence of plants the contribution of cyanobac-teria in bags was 10 times higher than in the aquariacontaining these bags, while green algal contribution

(dominating in the aquaria with plants) was lower inbags than in aquaria. Total phytoplankton biomass inbags was significantly lower in the presence of smallplant biomass than in the control, while in the presenceof large plant biomass it did not differ (Fig. 6b).

Total algal biomass was in all cases significantlyhigher in bags than in aquaria. The percentage contri-bution of nanoplankton in bags was higher in the pres-ence of plants then in the control (Fig. 6c). But the con-tribution of nanoplankton in bags placed in the aquariawith macrophytes was lower than in these aquaria.

Discussion

Numerous studies carried out since the 1920s implythat there are antagonistic relations between macro-phytes and phytoplankton. On the basis of fouryearenvironmental observations, Schreiter (1928) as wellas Hasler & Jones (1949) on the basis of their exper-iments, stated that the presence and intensive growth

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Fig. 3 a. The biomass of phytoplankton taxa in the control (C) and in the presence of macrophyte extracts. I - small, 11 - medium, I11I - largeconcentration of extracts; s.d. is for total phytoplankton biomass. Series I - early August.

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Fig. 3 b. The biomass of phytoplankton taxa in the control (C) and in the presence of macrophyte extracts. I - small, II - medium, 11I - largeconcentration of extracts; s.d. is for total phytoplankton biomass. Series II - late August.

of Elodea canadensis L. had an inhibiting effect onthe abundance and composition of phytoplankton inponds. Other studies and hypotheses by Hogetsu et al.(1960), Kogan et al. (1972), Dokulil (1973), Bram-mer (1979) and Wium-Andersen et al. (1982) impliedthat macrophytes had a restricting influence on phy-toplankton development because of the following fac-tors: competition for nutrients, secretion of inhibitoryorganic compounds, and shading.

The present study showed that whenever anyphytoplankton community had a direct contact withmacrophytes, it would become significantly differ-ent from the phytoplankton community found in themacrophyte-free control. Differences in the composi-tion of the phytoplankton community in each of theexperiments showed in a patterned replacement ofdominant taxa. And so, cyanobacteria (mainly 0. lim-netica, A. fios-aquae and L. vacuolifera) dominatedin the macrophyte-free control but their biomass and

28

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Fig. 4. The percentage contribution with s.d. of nanoplankton to total phytoplankton biomass in the control (C) and in the presence ofmacrophyte extracts. I - small, II - medium, III - large concentration of extracts.

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Fig. 5. The biomass of phytoplankton taxa in the control (C) andin the presence of small (Ms) and large (MI) macrophyte biomassesin aquaria; s.d. for total phytoplankton biomass.

percentage contribution in the presence of macrophyteswere in almost all cases significantly lower (Fig. 7).

This regularity is consistent with the results ofHasler & Jones (1949) who, however, did not carryout a detailed analysis of that particular phytoplank-ton group. But Kogan et al. (1972), analysing theinfluence of C. demersum and Myriophyllum spica-tum on pure cultures of Anabaena sp. and Microcys-tis aeruginosa Kutz, recorded an almost 90% dropin their quantity. In the present study, no proportion-al relationship between the degree of cyanobacterianlimitation and the biomass of macrophytes or concen-tration of extracts was detected. But in the field exper-iment and experiments with extracts, a lower quantityof cyanobacteria in the presence of macrophytes wasaccompanied by a considerably higher quantity andcontribution of green algae -mainly nanoplankton gen-era: Chlamydomonas, Chlorella, and Scenedesmus.Such results correspond to those of Kogan et al. (1972)which were obtained on pure cultures of cyanobacteriaand green algae.

According to the present study, green algae(Chlorella and Scenedesmus) did not change in quanti-ty under the influence of C. demersum and M. spicatum,while cyanobacteria showed considerable sensitivityto them. As was the case with cyanobacteria, no pro-portional relation between biomass of green algae andbiomass of macrophytes was noticed. What is more sig-nificant, even a small biomass or extract concentrationproduced changes in the structure of the phytoplanktoncommunity. In the presence of macrophytes or macro-phyte extracts neither diatoms nor other algae groupsshowed any particular reaction.

There was a higher percentage contribution ofnanoplankton to total phytoplankton biomass in thepresence of macrophytes or their extracts than in the

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Fig. 6. The number and s.d. of dominant cyanobacterian species(a), the biomass of phytoplankton taxa (b) and the percentage con-tribution of nanoplankton to total phytoplankton biomass (c), in thedialysis bags placed in: the control (C) and in the presence of small(Ms) and large (MI) macrophyte biomasses. Gaps in (a) indicateabsence of particular species; s.d. in figure (b) is for total phyto-plankton biomass.

control (Fig. 7). As was the case with cyanobacte-ria and green algae, there was no distinct relation-ship between nanoplankton contribution and small orlarge macrophyte biomass or an increasing concentra-tion of extract. It seems that macrophytes can have a

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31

Fieldexperiment

EXTRACTSOF

C.demersumM.spicatumPlucensS.aloidesCh.fragilis

EXTRACTS OF:

C.demersurrM.spicatumP.lucensS.aloides

CONTRIBUTION TO TOTALPHYTOPLANYTON BIOMASS

GREEN AILAECYANO- NANO- TOTAL

nochanges

SERIES I

~ changes

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-C 6 6 changes

SERIES II

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___ noIn s aquoria changes L_ _ changesIn ~ ~ chann°g es

Fig. 7. Schematic diagram of phytoplankton modifications causedby presence of macrophytes or extracts of them. Arrows directionindicate the rise or decline of particular value in relation to thecontrol. The width of arrows indicate the magnitude of modifications.

restricting influence on the amount and composition ofthe phytoplankton community, in line with the 'all ornothing' principle. This means that, even the smallestamount of macrophytes would not allow a large phy-toplankton biomass to develop and curb growth of cer-tain phytoplankton groups. It is possible that even thesmall biomass of macrophytes and the small concen-tration of extracts used in the experiments were higherthan the threshold biomass of macrophytes sufficientto cause changes in the phytoplankton community'sstructure. Contrary to some earlier papers (Dokulil,1973; Gibbs, 1973; Brammer, 1979; RorSlett, 1986)this work did not establish a clear relation between totalphytoplankton biomass and the presence of macro-phytes or their extracts. However, it showed that therewas a tendency for phytoplankton biomass to be limit-ed by macrophytes. Small differences in phytoplanktonbiomass were caused by the replacement of dominantphytoplankton groups under the influence of macro-phytes.

One hypothesis explaining how macrophytes affectphytoplankton suggests that macrophytes have a com-

petitive edge in nutrient utilization (Brammer, 1979).This study does not rule out such a possibility, butimplies another mechanism is possible. An inhibitinginfluence of macrophyte extract on cyanobacteria andthe lack of this influence in the case of green algae sup-ports this suggestion. The use of macrophyte extracts(also used by Kleiven & Szczepafiska, 1980 to examineallelopathic relations between macrophytes) prevent-ed utilization of nutrients by macrophytes. It is pos-sible that the presence of macrophyte extracts couldhave fertilized the water with suspended organic mat-ter which would quickly decompose (Saunders, 1980).Therefore, extracts could cause a development of phy-toplankton biomass proportiomal to the fertilizationby decomposing organic matter. With the exception ofthe series containing Ch. fragilis, such a phenomenonwas not observed. This suggests that some other sub-stances, e.g. allelopathic, restricting cyanobacteriagrowth, were present in the extracts. Obviously theinfluence of these substances was stronger than troph-ic stimulation.

Such allelopathic substances inhibiting phyto-plankton growth in laboratory conditions were isolatedfrom Charales by Wium-Andersen et al. (1982). Inthis study, in the presence of the small concentrationof Chara extracts there was no significant difference inphytoplankton biomass as compared with the control,and there was only a small growth of cyanobacteria.Higher phytoplankton biomass in the presence of twoother concentrations of Chara extracts indicated thatfertilization in this case was stronger than allelopath-ic influence on phytoplankton. On the other hand, itshould be also noticed that Forsberg et al. (1990) intheir comparable field studies did not find any allelo-pathic effect of Chara on phytoplankton biomass.

A completely different situation occurred in exper-iments with dialysis bags where phytoplankton didnot have direct contact with macrophytes. In bagsplaced in aquaria containing macrophytes, there wasa significantly higher number of the dominant species(0. limnetica) and a higher percentage contributionof cyanobacteria (Fig. 7) than in the bags placed inthe macrophyte-free aquarium. It was also stated thatthere was a lower contribution of green algae andnanoplankton to total phytoplankon biomass in bagsfrom aquaria with macrophytes than in the water ofthe aquaria themselves. The phytoplankton communi-ty's structure in dialysis bags from aquaria containingmacrophytes confirmed the existence of macrophyte-produced allelopathic substances which restrict growthof phytoplankton, especially cyanobacteria.

_12)�T�I�PLYI�

_ _ r

32

Other papers (Hasler & Jones, 1949; Brammer,1979; Wium-Andersen, 1982) indicate that the growthof the whole phytoplankton community in the pres-ence of macrophytes was restricted. It is possible thatmore extensive experiments would confirm this state-ment. However, the present results clearly indicatethat cyanobacteria are more sensitive to macrophyte-produced allelopathic substances than green algae.

Acknowledgments

Materials presented here, are a part of a M. Sc. thesisprepared at the Department of Hydrobiology, Univer-sity of Warsaw, Poland. Supervision, help and criti-cal remarks of Prof. Ewa Pieczyfiska and Dr AndrzejKowalczewski are kindly acknowledged. Two anony-mous referees are acknowledged for their remarks andlanguage improvements.

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