effect of macrophyte community composition and nutrient enrichment on plant biomass and algal blooms
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Basic and Applied Ecology 11 (2010) 432–439
ffect of macrophyte community composition and nutrient enrichment onlant biomass and algal blooms
.S. Bakkera,∗, E. Van Donka,b, S.A.J. Declercka,c, N.R. Helmsinga, B. Hiddinga, B.A. Noletd
Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Rijksstraatweg 6, 3631 AC Nieuwersluis, The NetherlandsInstitute of Environmental Biology, Palaeoecology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The NetherlandsLaboratory of Aquatic Ecology and Evolutionary Biology, K.U. Leuven, Ch. De Beriotstraat 32, B-3000 Leuven, BelgiumDepartment of Animal Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands
eceived 2 November 2009; received in revised form 18 May 2010; accepted 19 June 2010
bstract
Submerged freshwater macrophytes decline with increasing eutrophication. This has consequences for ecosystem processesn shallow lakes and ponds as macrophytes can reduce algal blooms under eutrophic conditions. We hypothesize that theroductivity of submerged vegetation, biomass change under eutrophication and the suppression of algal blooms may beffected by macrophyte community composition. To test our hypothesis, we established three macrophyte community types in6 fishless experimental ponds: one dominated by the oligotrophic species Chara globularis, one dominated by the eutrophicpecies Potamogeton pectinatus and a diverse vegetation which became co-dominated by Elodea nuttallii and C. globularis,nd we fertilized half of the ponds.
The macrophyte communities produced different amounts of biomass and they responded differently to fertilization. Theommunity dominated by Potamogeton produced the lowest overall biomass, but was not affected by nutrient addition. Theommunities dominated by Chara and co-dominated by Elodea and Chara produced more than four-fold the amount of biomassroduced in Potamogeton communities under oligotrophic conditions, but were strongly negatively affected by nutrient addition.
Phytoplankton abundance did not differ significantly among the plant community types, but showed large variation withinommunity types. There was a significant negative relationship between spring macrophyte biomass and the probability ofummer algal blooms. The occurrence of algal blooms coincided with low daphnid densities and high pH (>10).
We conclude that the macrophyte community composition, characterized by the dominant species, strongly affected themount of biomass production as well as the short-term response of the vegetation to nutrient enrichment. Macrophyte communityomposition had no direct effect on algal blooms, but can affect the occurrence of algal blooms indirectly as these occurred onlyn ponds with low (<100 g/m2 DW) spring macrophyte biomass.
usammenfassung
Mit zunehmender Eutrophierung verschwinden die submersen Süßwassermakrophyten. Dies hat in flachen Seen und Teichen
onsequenzen für die Ökosystemprozesse, da Makrophyten die Algenblüten unter eutrophen Bedingungen reduzieren können.ir stellen die Hypothese auf, dass die Produktivität der submersen Vegetation, die Veränderung der Biomasse bei Eutrophierungnd die Unterdrückung von Algenblüten von der Zusammensetzung der Makrophyten-Gesellschaft beeinflusst wird. Um unsereypothese zu prüfen, etablierten wir drei Typen von Makrophyten-Gesellschaften in 36 fischfreien experimentellen Teichen:
∗Corresponding author. Tel.: +31 294 239 357; fax: +31 294 232224.E-mail address: [email protected] (E.S. Bakker).
439-1791/$ – see front matter © 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved.oi:10.1016/j.baae.2010.06.005
E.S. Bakker et al. / Basic and Applied Ecology 11 (2010) 432–439 433
einen Typ, der von der oligotrophen Art Chara globularis dominiert wird, einen, der von der eutrophen Art Potamogetonpectinatus dominiert wird und eine diverse Vegetation, in der sich eine Kodominanz von Elodea nuttallii und Chara globularisentwickelte, und wir düngten die Hälfte dieser Teiche.
Die Makrophyten-Gesellschaften produzierten unterschiedliche Mengen an Biomasse und reagierten unterschiedlich auf dieDüngung. Die Gesellschaft, die von Potamogeton dominiert wurde, produziert insgesamt die geringste Biomasse und wurde vonder Nährstoffzugabe nicht beeinflusst. Die Gesellschaften, die von Chara dominiert bzw. von Elodea und Chara kodominiertwurden, produzierten mehr als die vierfache Menge der Biomasse, die in den Potamogeton-Gesellschaften unter oligotrophenBedingungen produziert wurde, wurden jedoch sehr stark negativ von der Nährstoffzufügung beeinflusst. Die Abundanz desPhytoplanktons unterschied sich nicht signifikant zwischen den Typen von Pflanzengesellschaften, sondern zeigte eine großeVariation zwischen den Gesellschaftstypen. Es gab eine signifikante, negative Beziehung zwischen der Makrophyten-Biomasseim Frühling und der Wahrscheinlichkeit einer Algenblüte im Sommer. Das Auftreten von Algenblüten fiel mit geringen Dichtenvon Daphnien und einem hohen pH-Wert (>10) zusammen.
Wir schließen daraus, dass die Zusammensetzung der Makrophyten-Gemeinschaft, charakterisiert durch die dominantenArten, sowohl das Ausmaß der Biomassenproduktion in starkem Maße als auch die kurzfristige Reaktion der Vegetation aufeine Nährstoffanreicherung beeinflusste. Die Zusammensetzung der Makrophyten-Gesellschaften hatte keinen direkten Einflussauf die Algenblüten, kann jedoch das Auftreten von Algenblüten indirekt beeinflussen, da diese nur in Teichen mit geringerMakrophytenbiomasse (<100 g/m2 Trockengewicht) im Frühjahr auftraten.© 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved.
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eywords: Aquatic plant; Chara; Clear water; Ecosystem functiurbidity
. Introduction
Eutrophication causes a degradation of aquatic systemsorldwide (Carpenter et al. 1998; Tilman et al. 2001). In shal-
ow freshwater systems, increased nutrient loading resultsn a loss of aquatic vegetation followed by phytoplanktonominance (Jeppesen, Jensen, Sondergaard, & Landkildehus000). The loss of submerged macrophytes has consequencesoth for the diversity and ecosystem functioning of shal-ow freshwater bodies, such as lakes and ponds. Submergedacrophytes provide a habitat for other biota and they
ncrease the water transparency and nutrient retention inquatic ecosystems (Carpenter and Lodge 1986; Engelhardtnd Ritchie 2001).
We hypothesize that the response of the submergedegetation to nutrient enrichment may depend on the macro-hyte community composition. Macrophyte stands are oftentrongly dominated by a single species and are generallypecies poor at the local community level. Different macro-hyte species may have a different tolerance to increasedutrient availability (Portielje and Roijackers 1995; Sand-ensen et al. 2008) and subsequent reduced light levelsSand-Jensen and Madsen 1991). Therefore, the declinef submerged vegetation under nutrient enrichment mayepend on which species dominates the vegetation.We fur-her hypothesize that the identity of the dominant species inhe macrophyte vegetation may affect the ecosystem func-ions of macrophytes. We focus on the role of macrophytesn enhancing clear water conditions through the suppres-
ion of algal blooms, which is commonly characterizedy the mere presence or absence of submerged vegetationDeclerck, Vanderstukken, Pals, Muylaert, & De Meester007; Scheffer, Carpenter, Foley, Folke, & Walker 2001).3fwa
dea; Eutrophication; Phytoplankton; Potamogeton; Productivity;
owever, this positive feedback between macrophyte pres-nce and water transparency is in fact a density-dependentrocess (Schriver, Bogestrand, Jeppesen, & Sondergaard995) and differences in biomass production among macro-hyte communities may result in a different effect on theccurrence of algal blooms. Furthermore, macrophyte com-unity composition may have a direct effect on algal growth
hrough species-specific effects on water chemistry includ-ng nutrient availability, pH and alkalinity (Engelhardt anditchie 2001; Van den Berg, Coops, Simons, & Pilon 2002)r through allelopathic effects (Gross 2003; Hilt and Gross008).
In this study we test whether macrophyte community com-osition affects (i) the biomass production of submergedegetation, (ii) the change in biomass of submerged vegeta-ion in response to nutrient addition and (iii) the occurrencef algal blooms. We conducted a large outdoor experimentn 36 ponds planted with three macrophyte community typesith different dominant species which we subjected to two
evels of nutrient addition in a full factorial design.
. Materials and methods
.1. Experimental ponds
We established 36 outdoor experimental ponds (in a 6 × 6rid) at Loenderveen in winter 2005 (52◦12′N, 5◦02′E, Theetherlands). The ponds were 5 m × 5 m wide at the top and
m × 3 m at the bottom, 1.2 m deep and lined with waterproofoil. The ponds were filled with a 10:1 sand–clay mixtureith a depth of 0.3 m and conditioned with 10 L mud fromditch. The ponds were filled with water (11,800 L) from
434 E.S. Bakker et al. / Basic and Applied Ecology 11 (2010) 432–439
Fig. 1. Percent volume infestation (PVI) of macrophytes in May, July and August 2006 (A). Open symbols represent ponds with no extran ans ± Sc with (a nuttal
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utrients, closed symbols ponds with added nutrients. Data are meommunity in July 2006. Ponds with no extra nutrients are indicatedre: Ch: dominated by Chara globularis, El: co-dominated by Elodea
nearby lake which contained dephosphatized water (waterroperties: N-NH4 = 0.650 mg/L, N-NO3 = 0.650 mg/L and-PO4 = 0.007 mg/L, Waternet, unpubl. data) and water depthas maintained at 0.7 m through an overflow. Water from
he overflows was collected and redistributed during the firstummer after planting, but not during the second year. Inebruary 2006 we removed all aboveground dead material,lled the ponds with fresh lake water and installed wire torevent birds to enter the ponds. Fresh lake water was sup-lied through a tap in each pond when the water level droppedy more than 5 cm. The ponds remained fish-free during thexperiment.
.2. Macrophyte treatments
In April 2005 the ponds were planted with three differ-nt macrophyte communities, in a total density of 100 shootsr tubers (for Potamogeton pectinatus L.) per pond. First weelected two eutrophic species, P. pectinatus and Ceratophyl-um demersum L. for monocultures. We composed the thirdommunity of a mixture of species, including these two.s the Ceratophyllum plants grew very poorly, and Charalobularis Thuill. colonized the ponds during the summer of005, we used Chara as a replacement for Ceratophyllum asdominant species, which enabled us to compare a more
ligotrophic and eutrophic species dominated communitynd a diverse community containing both oligotrophic andutrophic species. The diverse community consisted of sevenacrophyte species, i.e. P. pectinatus, Potamogeton perfolia-
us L., Potamogeton lucens L., Myriophyllum spicatum L.,anunculus circinatus Sibth., Elodea nuttallii (Planchon) St.ohn and C. globularis. After the initial planting, we let theubmerged vegetation establish and allowed for spontaneousegetation development (see Fig. 1). We refer to the differ-
nt planted communities by their (co-)dominant species: thei) “Chara”, (ii) “Elodea” (which became co-dominant withhara in the diverse community), and (iii) “P. pectinatus”ommunity.catw
E. (B) Relative abundance (as % of PVI) of plant species in each−), ponds with added nutrients with (+). Macrophyte communitieslii and Chara globularis, Po: dominated by Potamogeton pectinatus.
.3. Nutrient addition
In 2006 we added nutrients to half of the ponds correspond-ng to 0.5 mg P/L (25 g KH2PO4) and 3.0 mg N/L (102 gH4NO3) per pond per week. We used these high values
o create a situation under which algal blooms would be pos-ible (Portielje and Roijackers 1995; Van de Bund and Vanonk 2004). Because the lake water which was used to fill
he ponds was dephosphatized we added only KH2PO4 forhe first 6 weeks starting on 19 May 2006. We started to addH4NO3 from late June and added both nutrients weekly assolution to the water column until the last week of August006.
.4. Sampling and sample analysis
Macrophyte biomass was sampled on 2 June 2006, whenhe canopy forming species had reached the water surface,nd on 17 August 2006. Macrophytes were collected by handrom a round metal sampler (0.5 m diameter, 0.5 m height),hich was pressed into the sediment. In June one sample was
aken at a random location per pond to minimize disturbancef the vegetation and in August two samples on different loca-ions, which were pooled. The plants were rinsed and dried at0 ◦C for 72 h. We visually estimated plant cover in each pondn 8 May, 5 July and 10 August 2006 and measured planteight of ten individuals per species or less, depending onvailability, from which we calculated percent volume infes-ation (PVI) (sum of plant cover × mean height per speciesivided by pond water volume). We could not estimate covern three ponds in July and nine ponds in August due to turbid-ty but have data for at least n = 3 for each community typend nutrient combination per date.
To determine phytoplankton density and water nutrient
oncentrations we collected water samples from each pondt 2-week intervals at 5 locations evenly distributed acrosshe pond surface using a tube sampler (integrating the entireater column), resulting in pooled samples of 13.5 L per
E.S. Bakker et al. / Basic and Applied Ecology 11 (2010) 432–439 435
Fig. 2. Macrophyte biomass (g DW/m2) in June (A) and August (B) 2006 for each plant community and nutrient treatment. Different lettersi s ± SEa by EP
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ndicate statistically different amounts of biomass. Data are meanre: Chara: dominated by Chara globularis, Elodea: co-dominatedotamogeton pectinatus.
ond. Chlorophyll-a concentrations were determined usinghe PHYTO-PAM fluorometer (Heinz Walz GmbH, Effel-rich, Germany) (Lürling and Verschoor 2003). We filteredhe water samples (0.8 �m mesh) and determined the PO4,O3 and NH4 concentrations in the water with continuousow analysis on an auto-analyzer (Skalar Sanplus Seg-ented Flow Analyser, Skalar Analytical BV Breda, Theetherlands). The pH was measured every 2–3 weeks in
itu with a portable probe (340i SET, 2E30-101B02, WTWissenschaftlich Technische Werkstätten GmbH, Weilheim,ermany). We sampled zooplankton in the ponds first on 18ay 2006 before the first nutrient addition and subsequently
n 14 June and 10 August. Zooplankton samples were col-ected similarly to the phytoplankton samples, filtered (60 �m
esh) and preserved in ethanol. All cladocerans were countednd identified to the genus or species level with a stereomi-roscope. At very high densities, a subsample (50% of theotal) was counted.
.5. Data analysis
We tested the effect of nutrient addition and plant commu-ity composition on macrophyte biomass, phytoplankton andooplankton densities, water nutrient concentrations and pHith a two-way repeated measures Anova, with plant com-unity composition and the nutrient addition treatment asxed factors and time as the repeated factor. Phytoplank-
on, zooplankton and nutrient data were log 10 transformedo obtain normality and meet homoscedasticity assumptions,his was not necessary for macrophyte biomass data. Forooplankton grazer density we used the summed density ofaphnia galeata and Daphnia pulicaria (the only Daph-ia species present) and Ceriodaphnia sp. as these were theost abundant large filter feeders. We further refer to these
roups collectively as daphnids. We used the mean summer
hlorophyll-a concentration (in July and August 2006; meanf four sampling dates in total) in the ponds to quantify sum-er algal blooms. All data were analyzed with Statistica 7.1StatSoft Inc. 2005).JA
. See Section 3 for statistical analyses. Macrophyte communitieslodea nuttallii and Chara globularis, Potamogeton: dominated by
. Results
.1. Macrophyte PVI
The canopy forming P. pectinatus reached the highest PVIn 2006 (Fig. 1A). C. globularis reached a mean height of5 ± 2 cm (mean ± SE), resulting in a PVI of almost 50%.ean height of Elodea was 28 ± 3 cm and PVI was about
0% in the ponds without nutrient addition, but only around0% in the ponds where nutrients were added, due to a lowerlant cover in these ponds (Fig. 1A). The species compositionn the ponds was similar within each community type acrossutrient treatments (Fig. 1B).
.2. Macrophyte biomass
Plant biomass differed strongly among plant communityypes (repeated measures Anova: F2,30 = 37.14, P < 0.001)nd the communities responded differently to nutri-nt addition (nutrient treatment: F1,30 = 17.48, P < 0.001;utrients × community type: F2,30 = 7.11, P = 0.003). Plantiomass was higher in the Chara dominated community com-ared to the Elodea co-dominated ponds and the communityominated by P. pectinatus in ponds where no nutrients weredded (Fig. 2). Biomass declined significantly with nutrientddition in both the Chara dominated and the Elodea co-ominated community whereas the vegetation dominated by. pectinatus was not affected by nutrient addition (Fig. 2).lant biomass was not significantly different between Junend August (time: F1,30 = 0.96, P = 0.34) and the effect ofommunity type and nutrient addition did not vary with timeall interactions with time P > 0.39).
.3. Phytoplankton
Chlorophyll-a concentrations were low from May throughuly, but increased in August (Fig. 3; repeated measuresnova, time: F7,196 = 22.59, P < 0.001). There was no signif-
436 E.S. Bakker et al. / Basic and Applied Ecology 11 (2010) 432–439
Fig. 3. Chlorophyll-a concentrations for each macrophyte commu-nity in the ponds with no nutrients added (A) and with nutrients(B) during the experiment. Open symbols represent ponds withnDc
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Fig. 4. Relationship between macrophyte biomass in June, beforephytoplankton blooms started, and mean summer (July–August)chlorophyll-a concentration. Open symbols represent ponds with noextra nutrients, closed symbols ponds with added nutrients. Eachsymbol represents an individual pond, n = 36. See Fig. 2 for thecomposition of the plant communities.
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o extra nutrients, closed symbols ponds with added nutrients.ata are means ± SE. See Fig. 2 for the composition of the plant
ommunities.
cant effect of macrophyte community type on chlorophyll-aoncentrations for either nutrient treatment (Fig. 3; com-unity type: F2,28 = 1.09, P = 0.35; community × nutrients:2,28 = 0.46, P = 0.64). Phytoplankton abundance tended
o be higher where nutrients were added (F1,28 = 3.85,= 0.06). An interaction between nutrient treatment and time
F7,196 = 2.30, P = 0.028) reflected the increased chlorophyll-concentrations in some of the nutrient rich ponds later in theummer (Fig. 3B). There were no significant differences inean summer chlorophyll-a concentrations among nutrient
reatments and macrophyte communities (Kruskall–Wallisest: H5,36 = 4.64, P = 0.46).
.4. Relationship between macrophyte biomass
nd algal bloomsMost ponds had a mean summer phytoplankton concentra-ion between 10 and 50 �g chlorophyll-a/L, but seven ponds
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ig. 5. Density of daphnid grazers (A and B) and grazing pressure (expresommunity in the ponds with no nutrients added and the ponds with nutrif the plant communities.
xperienced algal blooms (defined as containing on average55 �g Chl-a/L (Ibelings et al. 2007); Fig. 4). There was noignificant relationship between macrophyte biomass in earlyune and mean summer phytoplankton concentration (loga-ithmic regression: R2 = 0.035, P = 0.28), but ponds whichontained a low amount of macrophyte biomass in June hadhigher chance to develop a phytoplankton bloom over the
ummer (logistic regression: χ2 = 5.98, P = 0.014). All theonds that experienced phytoplankton blooms in our study
ontained less than 100 g/m2 (DW) macrophyte biomass inarly June (Fig. 4).sed as zooplankton:phytoplankton ratio, C and D) per macrophyteent addition. Data are means ± SE. See Fig. 2 for the composition
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.5. Zooplankton grazing pressure
The density of daphnids decreased over the summer inhe ponds without nutrient addition but increased in theonds where nutrients were added (Fig. 5AB; repeated mea-ures Anova, nutrient treatment: F1,30 = 5.75, P = 0.023; time:2,60 = 3.15, P = 0.05; time × nutrient treatment: F2,60 = 4.51,= 0.015). The effect of plant community type was not sig-
ificant (community type: F2,30 = 2.76, P = 0.08; communityype × nutrient interaction: F2,30 = 0.31, P = 0.74). Grazingressure, the ratio of daphnid density (individuals/L) to phy-oplankton concentration (chlorophyll a in �g/L), declineduring the summer in the ponds without nutrient addition,ut not in the ponds with nutrient addition (Fig. 5C and D;ime: F2,60 = 10.08, P < 0.001; nutrients × time F2,60 = 4.55,= 0.014; nutrient treatment: F1,30 = 7.70, P = 0.009). Graz-
ng pressure was similar in the nutrient treatments in Maynd June, but significantly higher in ponds where nutri-nts were added in August (Fig. 5C and D). There was noffect of plant community type on grazing pressure (data nothown).
The ponds that experienced algal blooms under eutrophiconditions (Fig. 4), contained significantly lower densi-ies of daphnids in August than the eutrophic ponds thatemained clear (daphnid individuals per L: 8.6 ± 8 in bloomonds (n = 6), 51.9 ± 11 in clear ponds (n = 12); indepen-ent t-test: t = −4.31, P < 0.001). This resulted in a lowerrazing pressure in the bloom ponds (0.02 ± 0.02) com-ared to the clear ponds (2.45 ± 0.52, t-test: t = −4.62,< 0.001). The density of daphnids in August was nega-
ively related to the pH (Ancova; pH: F1,33 = 28.48, P < 0.001;utrient treatment: F1,33 = 19.19, P < 0.001). This relation-hip was significant in fertilized ponds (Pearson correlation:= −0.81, P < 0.001), but not in ponds without nutrient addi-
ion (r = −0.26, P = 0.29).
.6. Nutrients and pH
The concentrations of NO3, NH4 and PO4 and the pH wereignificantly higher in the ponds where nutrients had beendded but did not differ among macrophyte communitiesAppendix A: Fig. 1 and Table 1).
. Discussion
Macrophyte community composition affected the biomassroduction and short-term response of the vegetation biomasso nutrient addition. The community dominated by Chararoduced most biomass, but plant biomass strongly declinednder nutrient enrichment. On the contrary, the community
hat was dominated by P. pectinatus produced little biomass,ut was not affected by nutrient addition. Due to its canopy-ike growth form P. pectinatus exposes most of its biomass toigh light availability and can tolerate turbid conditions ratherndta
Ecology 11 (2010) 432–439 437
ell, but often does not accumulate a lot of biomass (Hidding,olet, De Boer, De Vries, & Klaassen 2010; Van den Berg,cheffer, Van Nes, & Coops 1999), but see Van Wijk (1988).harophytes, which grow on the bottom of the water column,re known to be negatively affected by nutrient enrichmentBlindow, Andersson, Hargeby, & Johansson 1993; Van deund and Van Donk 2004).
.1. Macrophyte community composition andlgal blooms
Subsequently, we asked whether macrophyte communityomposition affects the occurrence of algal blooms. Differ-nces in the occurrence of algal blooms among macrophyteommunities could occur through direct species specificffects or through indirect effects where a higher macrophyteiomass results in general positive feedbacks regardless ofpecies composition (Scheffer et al. 2001). No communityype could completely prevent the occurrence of a summerlgal bloom and differences among plant communities wereot significant. We did find that high early season macrophyteiomass decreased the probability of algal blooms later in theummer in our experimental ponds. However, in our study,he amount of biomass differed with plant community type.herefore we cannot separate the effect of plant biomass andpecies specific effects. In our study only one pond from theommunity dominated by Chara experienced algal bloomshereas this happened to four ponds of the community domi-ated by P. pectinatus. This can be due to the generally largeriomass in the Chara ponds, but additionally Chara plantsay have had allelopathic effects on algal growth (Mulderij,an Donk, & Roelofs 2003) or changed the water chemistry
Kufel and Kufel 2002; Van den Berg et al. 2002).
.2. Relationship between zooplankton grazingressure and algal blooms
Despite the generally high amount of daphnids in the fer-ilized ponds, six of these ponds did experience algal bloomsn August. A comparison showed that the daphnid densityn the ponds with algal blooms in August was much lowerhan in the ponds without blooms and thus a reduction ofrazing pressure can have resulted in a switch towards algallooms. Daphnid densities may have been reduced due to theery high pH that was measured in the fertilized ponds inate July, just before the main algal blooms started. A pH of10 may cause reduced survival of eggs and neonates in sev-ral daphnid species, whereas a pH of >10.5 may reduce adulturvival (Hansen, Christensen, & Sortkjaer 1991; Vijverberg,alf, & Boersma 1996). Once the high pH reduces daph-
id densities, the effect is self-enhancing as phytoplanktonensities increase when grazing pressure decreases and pho-osynthesis of the algae increases the pH further (Jeppesen etl. 1998).
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38 E.S. Bakker et al. / Basic and
.3. Field relevance
The relationship between nutrient level and communityomposition of submerged macrophytes is not so strongVestergaard and Sand-Jensen 2000; Sand-Jensen et al.008), therefore different plant species may dominate atutrient levels at which clear and turbid conditions are bothossible. Even charophytes, which are generally found underligotrophic–mesotrophic conditions can be the dominantlant species when the system is about to switch from macro-hyte to phytoplankton dominated (Rip, Rawee & De Jong006; Ibelings et al. 2007). In our experiment, we measuredhe short-term response of the vegetation to nutrient enrich-
ent. Under field conditions, eutrophication is a slower pro-ess where nutrients accumulate over the years. This may leado longer persistence of macrophyte vegetation due to a shiftn species composition towards eutrophic, tolerant speciesBlindow et al. 1993; Van den Berg et al. 1999), but eventu-lly the macrophyte vegetation will disappear and a differentommunity composition of the vegetation cannot prevent this.owever, year-to-year fluctuations in nutrient concentrations
n the water are common and large fluctuations in plant abun-ance and clear-to-turbid transitions can occur in responseRip et al. 2006). In this case, a different community compo-ition of the vegetation may just have enough effect to preventhe loss of vegetation or the occurrence of algal blooms.
In systems which include fish, top-down control of theooplankton increases the likelihood of algal blooms (Van deund and Van Donk 2004). However, macrophytes may mod-late fish effects by acting as refugia for daphnids (Schrivert al. 1995). Due to this additional positive indirect effect ofacrophytes on water clarity, the effects of macrophyte den-
ity and community composition on the occurrence of algallooms may be stronger in water bodies with planktivoroussh then demonstrated in our fishless experimental ponds.
. Conclusions
We conclude that the macrophyte community composi-ion strongly affected the amount of biomass production asell as the short-term response of the vegetation to nutri-
nt enrichment. Macrophyte community composition had noirect effect on algal blooms, but can affect the occurrencef algal blooms indirectly as these occurred only in pondsith low (<100 g/m2 DW) spring macrophyte biomass. In
onclusion, the amount of spring macrophyte biomass seemso be a useful predictor of the occurrence of summer algallooms, but the potentially additional effects of species iden-ity should be further tested by manipulating plant biomassnd species composition in a full factorial design.
cknowledgements
Thanks to all NIOO colleagues who helped with the exper-mental pond facility and Koos Swart for pond maintenance.
J
Ecology 11 (2010) 432–439
e thank Peter de Vries and Thijs de Boer for data col-ection and processing of the samples, Arie Kersbergen forooplankton counting and Gerrit Rouwenhorst from Utrechtniversity for the chemical analysis of the water samples.e thank Waternet for permission to work at Loenderveen.
his is publication 4815 of the NIOO-KNAW Netherlandsnstitute of Ecology.
ppendix A. Supplementary data
Supplementary data associated with this arti-le can be found, in the online version, atoi:10.1016/j.baae.2010.06.005.
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