Distribution of Macrophytes and Condition of the Physical

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Limnologica 37 (2007) 146–154 www.elsevier.de/limno

Distribution of macrophytes and condition of the physical environment of streams ﬂowing through agricultural landscape in north-eastern Slovenia Ursˇka KuharÃ, Tatjana Gregorc, Miran Rencˇelj, Nina Sˇraj-Krzˇicˇ, Alenka Gabersˇcˇik

Department of Biology, Biotechnical Faculty, University of Ljubljana, Vecˇna pot 111, SI-1000 Ljubljana, Slovenia

Received 9 August 2006; received in revised form 23 November 2006; accepted 23 November 2006

Abstract

The distribution and abundance of macrophytes, land-use beyond the riparian zone, characteristics of the riparian zone and selected channel properties have been studied in 9 streams flowing through an agricultural landscape in the north-eastern part of Slovenia. The streams studied supported a rich macrophyte community. Altogether, 53 plant taxa were observed on 93 km of the streams. Canonical correspondence analysis indicated that 10 out of the 11 parameters examined significantly influenced macrophyte distribution. Of these, substrate characteristics and riparian vegetation type were the most influential. Species composition analysis revealed that the majority of species indicated moderate nutrient load while, in the more strongly modified reaches, species indicating eutrophic conditions, i.e. Myriophyllum spicatum and different species of genus Potamogeton, were more abundant. r 2007 Elsevier GmbH. All rights reserved.

Keywords: Macrophytes; Lowland streams; Channel morphology; Riparian zone; Agricultural landscape

Introduction channel (Strayer et al., 2003). Intensive agriculture affects soil properties, reducing the soil’s capacity to Rivers and streams are complex and dynamic systems retain water and nutrients (Rabeni & Sowa, 2002). supporting high landscape biodiversity (Chovanec et al., Consequently, major water level fluctuations in the 2000). Despite this fact they are among the environ- stream and increased runoff of soil particles, fertilizers ments most disturbed and threatened by humans and biocides occur, affecting biocoenosis. With intensi- (Nilsson & Jansson, 1995; Nilsson & Svedmark, 2002). fication of agriculture, riparian vegetation, including In many parts of the world they have been modified riparian forests and wetlands that play a crucial role in intensively by urban and agricultural development, reducing input of matter from the terrestrial to the channelization, impoundment and pollution (Harding, aquatic environment, is more and more disturbed Benfield, Bolstad, Helfman, & Jones III, 1998; Naiman, (Anbumozhi, Radhakrishnan, & Yamaji, 2005; Brinson, Bilby, & Bisson, 2000; Naiman & De´ camps, 1997). 1993; Karr & Schlosser, 1978; Vought, Pinay, Fuglsang, Changes of land use are the most extensive changes & Ruffinoni, 1995). Changes in riparian vegetation, affecting rivers and streams (Brookes, 1994), altering the together with modification of stream morphology input of water, organic matter and other materials to the resulting from channelization, decrease habitat diversity, affect water quality and consequently alter the ÃCorresponding author. Tel.: +386 1 4233388; fax: +386 1 2573390. composition of stream communities, including macro- E-mail address: [email protected] (U. Kuhar). phytes. The latter are essential for the structure and

A H Drava Ljubljana I SLOVENIA HR Maribor

Hocki potok Drava Pavlovski potok Libanja D R A V S K O P O L J E Trnava Framski potok Ptuj SREDIŠKO POLJE Studencnica Polskava Reka Drava Struga N

Dravinja 5 km

Fig. 1. Location of the streams studied. Investigated parts are shown in bold. functioning of freshwater ecosystems (Baattrup-Peder- Methods sen & Riis, 1999). They provide habitat and shelter for numerous other organisms whose condition is Study area indicated indirectly by that of the macrophytes (Gabersˇcˇik, 1997). Deterioration of the physical stream The nine streams investigated are located in the north- environment and eutrophication have resulted in eastern part of Slovenia, where agriculture is most changes in macrophyte distribution, decline in macro- intensive. They belong to the watershed of the Drava phyte species richness and greater abundance of more (Fig. 1), a tributary of the Danube River. Struga and resistant species (Egertson, Kopaska, & Downing, 2004; Studencˇnica are short lowland streams (5.7 and 10.9 km Germ, Dolinsˇek, & Gabersˇcˇik, 2003; Preston, 1995; Riis long, respectively), originating under the lowest River & Sand-Jensen, 2001; Sand-Jensen, Riis, Vestergaard, & Drava terrace and flowing through the eastern part of Larsen, 2000). Dravsko polje. Hocˇki potok, Framski potok, Reka and In Slovenia, agriculture developed mainly in lowlands Polskava streams (22.9, 18.0, 14.8 and 42.3 km long, along the rivers. The agricultural landscape is rather respectively) originate in a hilly area west of Dravsko heterogeneous, with small patches of forest. Streams are polje and, in the lowland parts, flow through Dravsko only partly regulated. The heterogeneity presumably polje. Pavlovski potok, Libanja and Trnava streams increases habitat diversity in the streams and therefore (16.2, 12.0 and 13.6 km long, respectively) flow from the the potential for colonization by different macrophyte hilly area to the lowland Sredisˇko polje. Most of the species. Effective stream management requires an streams cut their beds into gravel deposits covered understanding of how the surrounding terrestrial land- with clay. Altitudinal gradients of the parts of the scape, including the riparian zone, influences the streams studied are low, altitudes ranging from 280 to structure and the function of a stream. This aspect is 180 m a.s.l. also included in the EU Water Framework Directive (Council of the European Communities, 2000) which quotes macrophytes as an important biological element Survey of macrophytes and environment needed to determine the ecological status of running waters. The present study was aimed at examining the The study was performed in the peak vegetation species composition and abundance in streams flowing period (August) in years 2002–2005. The streams were through an agricultural landscape and at establishing divided into 172 reaches of different lengths the relation between distribution and abundance of (100–1200 m), giving a total length of 93 km. A new macrophytes and the land-use type beyond the riparian reach started where environmental characteristics or zone, the characteristics of the riparian zone and the species composition or abundance changed. The co- channel morphology. ordinates of each reach were determined using the ARTICLE IN PRESS 148 U. Kuhar et al. / Limnologica 37 (2007) 146–154

Global Positioning System (GPS, Garmin International Results Inc.). We examined the whole length of each stream. The exceptions were Hocˇki potok and Framski potok, where All the streams examined flow through an agricultural we studied only the lowland part of the streams, landscape, but the degree to which they have been constituting about 50% of the stream length, and altered by agriculture varies. Our data show that the Polskava, where only selected reaches in the upper, most modified are the streams of the western part of middle and lower flows of the lowland part were Dravsko polje, where as much as 83% of the land surveyed, altogether comprising 17% of the stream beyond the riparian zone was used to cultivate row length. crops (Fig. 2). The riparian zone was poorly developed, The survey of macrophyte species was conducted consisting of grasses and sparse trees and shrubs, from the bank. For sampling we used a rake with hooks. maximum 5 m wide, and with frequent breaks. The Relative abundance was evaluated using a five-degree landscape of the eastern part of Dravsko polje and scale as follows: 1 ¼ very rare; 2 ¼ infrequent; Sredisˇko polje was more diverse (Fig. 2). Besides 3 ¼ common; 4 ¼ frequent; 5 ¼ abundant, predominant pastures and row crops, there were small patches of (Kohler & Janauer, 1995; Schneider & Melzer, 2003). forest along the streams. The riparian zone consisted Species were identified using the following keys: mainly of trees, especially in the middle flows of Martincˇicˇet al. (1999), Preston (1995) and Lauber & Sredisˇko polje streams and in the second part of Wagner (2001). Plants that were sampled in the Studencˇnica. The belt of riparian vegetation was up to phenological phase that prevents identification to the 30 m wide. Streams followed a natural course, with species level are presented as genus. Callitriche sp. from meanders and riffles and pools. Dravsko polje streams Sredisˇko polje and Callitriche palustris from Dravsko were similar with regard to bank and some channel polje are presented as Callitriche spp. Sparganium sp. characteristics. Sediment accumulation and a few loose from Sredisˇko polje and from the eastern part of retention structures (old logs) were present in these Dravsko polje, and Sparganium erectum from the streams. Bank undercutting was frequent. In Sredisˇko western part of Dravsko polje are presented as polje streams retention devices were more frequent, Sparganium spp. In cases where Polygonum hydropiper while sediment accumulation was sparse. Banks con- and Polygonum mite were growing together we were sisted of loose soil in half the length of these streams. not able to estimate the abundance of each species Stream bottoms were built of silt, gravel and sand and, separately and therefore listed them as Polygonum in the case of Sredisˇko polje and the western part of spp. Dravsko polje, also of stones. Silty bottom prevailed in Selected environmental properties were surveyed streams of the eastern part of Dravsko polje. Detritus using the modified Riparian, Channel, and Environ- consisted of leaves and wooden particles in the middle mental (RCE) inventory proposed by Petersen (1992). parts of Sredisˇko polje streams, elsewhere leaves and We assessed 11 parameters describing land-use type wood were scarce or absent (Fig. 2). beyond the riparian zone, characteristics of riparian The streams showed great variability in macrophyte zone (width, completeness and vegetation type) and distribution. Fifty-three plant taxa were observed on the morphology of the stream channel (bank structure total length of 93 km. Six taxa occurred in only one of and undercutting, occurrence of retention structures all streams studied: the stonewort Nitella mucronata and and sediment accumulation, type of stream bottom and pondweed Potamogeton perfoliatus were found in detritus and dynamics of the flow). Each parameter was Studencˇnica, Eleocharis palustris and Hydrodictyon sp. estimated by four categories (Fig. 2). in Hocˇki potok, Najas minor in Reka and Typha angustifolia in Trnava (Fig. 3). Dravsko polje streams were characterized by high Data analysis species numbers. We found a total of 49 macrophyte species; 31, 28, 31, 23, 28 and 34 in Hocˇki potok, Canonical correspondence analysis (CCA) was used Framski potok, Reka, Polskava, Struga and Studencˇni- to assess the relationship between plant species compo- ca, respectively. The streams were overgrown with sition and abundance and environmental parameters. macrophytes from source to outflow. In streams of the Environmental parameters were coded numerically from western part of Dravsko polje we found a maximum of 1 to 4. Reaches were weighted with regard to their 20 species per reach and in streams of the eastern part length. Forward selection was used to determine the a maximum 23 species per reach. Potamogeton was contribution of each parameter to the variance in species the most frequent genus in the western part, with composition. The statistical significance of environmen- four species occurring abundantly (Potamogeton berch- tal parameters was tested by the Monte Carlo permuta- toldii, Potamogeton crispus, Potamogeton nodosus tion test. Analyses were performed using Canoco for and Potamogeton pectinatus). Filamentous algae, Spar- Windows Version 4.5. ganium erectum, Phalaris arundinacea and Myriophyllum ARTICLE IN PRESS U. Kuhar et al. / Limnologica 37 (2007) 146–154 149

LAND USE: forest, wetland / pasture, wood, swamp, DW few crops / crops, pasture, settlement / DE crops, settlement S

WIDTH OF RIPARIAN ZONE: > 30 m / DW 5 - 30 m / 1 - 5 m / vegetation absent DE S

COMPLETENESS OF RIPARIAN ZONE: complete / DW breaks at > 50 m intervals / breaks at 50 m intervals / DE disturbances frequent or vegetation absent S

VEGETATION OF RIPARIAN ZONE: woody and marshy DW species / pioneer trees and shrubs / grass, few DE trees and shrubs / mainly grass, few shrubs S

RETENTION STRUCTURES: firmly set rocks and logs / DW rocks and logs, back filled with sediments / loose / DE loose sandy silt, few retention structures S

SEDIMENT ACCUMULATION: do not cause channel DW enlargement / some gravel bars of stones, little silt / DE bars of rocks, sand, silt / braided or channelized S

BANK STRUCTURE: stable, firmly held by roots / DW firm but loosely held by roots / loose / DE unstable, easily disturbed S

BANK UNDERCUTTING: little or none / DW on curves or at constrictions / frequent / DE on the whole length, banks falling in S

STREAM BOTTOM: stony, interstices obvious / DW stones, little silt / silt, gravel, sand / DE sand, silt S

RIFFLES AND POOLS, OR MEANDERS (Flow dynamics): DW on 5-7 x stream width / irregularly spaced / long DE pools following short riffles / absent S

DETRITUS: leaves and wood / coarse organic DW debris / coarse and fine organic DE matter / anaerobic sediment S 0 20 40 60 80 100 % of total length

Fig. 2. Environmental characteristics of the streams, assessed by the modified Riparian, Channel, and Environmental (RCE) inventory (Petersen, 1992), summarized for streams flowing through the western part of Dravsko polje (DW), streams flowing through the eastern part of Dravsko polje (DE) and streams flowing through Sredisˇko polje (S). White colour indicates the first category, bright grey the second, dark grey the third and black the fourth category of the environmental characteristic. spicatum were also abundant. The most abundant rather poor, and in about 30% of the stream length no species in the eastern part were Phalaris arundinacea, macrophytes were found. The total number of taxa and Nasturtium officinale, Berula erecta and Myriophyllum number of taxa per reach were higher in the upper than verticillatum, which occurred in almost the entire length in the lower parts of the streams. The most abundant of these streams. Potamogeton natans, Potamogeton species were Berula erecta, Phragmites australis and pectinatus and Ceratophyllum demersum only colonized Phalaris arundinacea, occurring mostly in the upper a few reaches, but in great abundance. parts. Some species, e.g. Potamogeton crispus, Potamo- In the streams of Sredisˇko polje we found 33 geton nodosus and Zannichellia palustris only colonized macrophyte species, which is fewer than in the Dravsko the last parts of the streams. polje streams. In Pavlovski potok, Libanja and Trnava The relationship between distribution of macrophyte streams we found 24, 20 and 25 taxa, respectively. In the species and environmental characteristics of the survey middle parts of these streams, plant colonization was units was identified using CCA. Twenty-five survey units ARTICLE IN PRESS 150 U. Kuhar et al. / Limnologica 37 (2007) 146–154

DW DE S ABC DEF G H I Agrostis stolonifera L. Agr sto Algae filamentosae Alg fil Alisma plantago-aquatica L. Ali pla Berula erecta (Huds.) Coville Ber ere Bryophyta Bryophy Callitriche spp. Cal spp Caltha palustris L. Cat pal Ceratophyllum demersum L. Cer dem Chara spp. Cha spp Eleocharis palustris (L.) Roem. et Schult. Ele pal Elodea canadensis L. C. Rich. Elo can Epilobium hirsutum L. Epi hir Equisetum palustre L. Equ pal Glyceria maxima (Hartm.) Holmb. Gly max Hydrodictyon sp. Hyd sp. Iris pseudacorus L. Iri pse Juncus effusus L. Jun eff Leersia oryzoides (L.) Sw. Lee ory Lemna minor L. Lem min Lemna trisulca L. Lem tri Lycopus europaeus L. Lyc eur Lythrum salicaria L. Lyt sal Mentha aquatica L. Men aqu Myosotis scorpioides L. Myo sco Myriophyllum spicatum L. Myr spi Myriophyllum verticillatum L. Myr ver Najas marina L. Naj mar Najas minor All. Naj min Nasturtium officinale R. Br. in Aiton Nas off Nitella mucronata A. Br. Nit muc Phalaris arundinacea L. Pha aru Phragmites australis (Cav.) Trin. ex Steud. Phr aus Polygonum spp. Pol spp Potamogeton berchtoldii Fieber Pot ber Potamogeton crispus L. Pot cri Potamogeton natans L. Pot nat Potamogeton nodosus Poir Pot nod Potamogeton pectinatus L. Pot pec Potamogeton perfoliatus L. Pot per Potamogeton trichoides Cham. et Schld Pot tri Ranunculus circinatus Sibth. Ran cir Rorippa amphibia (L.) Besser Ror amp Rumex hydrolapathum Hudson Rum hyd Scrophularia umbrosa Dumort. Scr umb Sparganium spp. Spa spp Spirodela polyrhiza (L.) Schleid. Spi pol Trapa natans L. Tra nat Typha angustifolia L. Typ ang Typha latifolia L. Typ lat Utricularia vulgaris L. Utr vul Veronica anagallis-aquatica L. Ver ana Veronica beccabunga L. Ver bec Zannichellia palustris L. Zan pal

Fig. 3. The spatial distribution and abundance of macrophytes in the streams ﬂowing through the western part of Dravsko polje (DW) (A – Hocˇki potok, B – Framski potok, C – Reka, D – Polskava), through the eastern part of Dravsko polje (DE) (E – Struga, F – Studencˇnica), and through Sredisˇko polje (S) (G – Pavlovski potok, H – Libanja, I – Trnava). Black bars indicate the abundance of macrophytes ( – very rare, – infrequent, – common, – frequent, – abundant, predominant).

were not taken into account because of the absence of inﬂuence of the width of the riparian zone was not macrophytes and only 10 out of the 11 environmental signiﬁcant after forward selection (P40.1). Sediment parameters were retained in the analysis, since the accumulation, riparian zone vegetation type, stream ARTICLE IN PRESS U. Kuhar et al. / Limnologica 37 (2007) 146–154 151

Table 1. Summary statistics for the canonical correspondence analysis performed on macrophyte species in lowland streams, and the percentage of variance explained by environmental parameters

Axis 1 Axis 2 Axis 3 Axis 4

Eigenvalues 0.347 0.178 0.097 0.072 Species-environment correlations 0.892 0.830 0.706 0.683 Cumulative percentage variance explained By species only 8.5 12.9 15.3 17.1 By species and environment 38.1 57.6 68.2 76.1

Environmental variables Explained variance (%) P-value F-ratio

Vegetation of riparian zone (R veget) 4.67 0.001 7.23 Sediment accumulation (Sed acc) 4.67 0.001 7.37 Stream bottom (Bottom) 2.95 0.001 4.59 Flow dynamics (Flo dyn) 2.70 0.001 4.65 Bank undercutting (B undc) 1.72 0.001 2.98 Bank structure (B struc) 1.47 0.001 2.38 Detritus (Detrit) 1.23 0.001 2.32 Retention structures (Ret str) 1.23 0.002 2.10 Land use (Lnd use) 0.98 0.035 1.66 Completeness of riparian zone (R compl) 0.74 0.087 1.34

Codes used in Fig. 4 are given in parentheses. bottom type and flow dynamic were shown to be the Discussion parameters exerting the greatest influence on macrophyte community, together explaining 15% of the Of all the parameters studied, substrate characteristics variance (Table 1). The latter three parameters were and riparian vegetation type are the most important correlated mainly with the first axis in the CCA environmental factors determining macrophyte distribu- ordination and the first one with the second axis tion in these small lowland streams. In an agricultural (Fig. 4). These two axes explained 12.9% variance of landscape the riparian zone constitutes a crucial link species data. The first axis represents a gradient from between the agricultural lands and the stream environ- reaches with trees and shrubs growing in the riparian ment (Anbumozhi et al., 2005). Width and type of zone, sandy and silty bottom and dynamic flow, to vegetation in the riparian zone define the effectiveness of reaches characterised by grassy riparian vegetation, removal of nutrients and sediments originating from coarser bottom and no flow dynamic (Fig. 4a). This agricultural runoff (Nakamura & Yamada, 2005; axis separates Sredisˇko polje and the eastern part of Syversen, 2005). Complex buffer zones consisting of Dravsko polje streams (right side of the ordination different plant communities have higher purification plot) from streams of the western part of Dravsko efficiency than those with a simple structure (Mander, polje (left side of the ordination plot). The second Kuusemets, & Hayakawa, 2005). Vegetation along the axis opposes reaches without sediment accumulation streams ranged from wide tree belts to narrow strips of to reaches where accumulation is common, thus grass. CCA analysis revealed no significant influence of separating Sredisˇko polje streams (upper part of vegetation width on macrophyte distribution, which is a ordination plot) from the eastern part of Dravsko polje consequence of the fact that 75% of the riparian zone streams (lower part of ordination plot). The relationship was in the same width category (1–5 m). In parts where between macrophyte species and environmental para- the riparian zone was colonized by trees, the channel meters is shown in Fig. 4b. Elodea canadensis, Myr- was rich with logs and limbs, and detritus consisted iophyllum verticillatum, Potamogeton perfoliatus, mainly of rough particles such as twigs and leaves. Potamogeton natans and Zannichellia palustris were However, the characteristics of the retention structures associated with sandy and silty bottom and sedi- and detritus had minor impact on plant distribution. ment accumulation, while Potamogeton nodosus, Pota- The same holds true for bank structure and under- mogeton crispus, Potamogeton pectinatus, Myriophyllum cutting and for land use pattern behind the riparian spicatum and Trapa natans were related to coarser zone. Research on areas with diverse land cover substrate and also to landscape heavily impacted by demonstrates that land use has an important influence agriculture. on stream vegetation (Riis, Sand-Jensen, & Vestergaard, ARTICLE IN PRESS 152 U. Kuhar et al. / Limnologica 37 (2007) 146–154

1.0 Our analysis shows the importance of substrate particle size and sediment accumulation for the distribution of stream vegetation, as have many other investigations (Ferreira & Moreira, 1999; Germ, Ga- bersˇcˇik, & Urbanc-Bercˇicˇ, 1999; Mackay, Arthington, FLO DYN Kennard, & Pusey, 2003; Riis & Biggs, 2003; Riis et al., R VEGET 2000). Stream bottoms were built mainly of sand, silt DETRIT B STRUC and gravel and, in some streams, sediment accumulation R COMPL BOTTOM occurred frequently. Our ﬁndings agree with observa- tions that ﬁne textured substrata are favourable for

LND USE aquatic plant development and growth (Abou-Hamdan, Haury, Hebrard, Dandelot, & Cazaubon, 2005; Baattr-

B UNDC up-Pedersen, & Riis, 1999; Hoyer, Frazer, Notestein, & RET STR Canfield, 2004). Myriophyllum verticillatum, Zannichel- lia palustris, Potamogeton natans, Potamogeton perfoliatus and Elodea canadensis preferred finer sediment, as observed elsewhere (Haslam, Sinker, & Wolseley, 1975). SED ACC The last three species grew abundantly in soft silty -1.0 sediment in the lower part of Studencˇnica. Potamogeton -1.0 1.0 natans and Elodea canadensis thrive in such substrates (a) (Bornette & Amoros, 1991; Trei & Pall, 2004) while 1.0 Potamogeton perfoliatus may also anchor between gravel Equ pal and stones (Baattrup-Pedersen & Riis, 1999). Potamo- geton nodosus, Potamogeton crispus, Potamogeton pecti- Gly max Ror amp natus and Myriophyllum spicatum were primarily Ver ana Men aqu associated with coarser substrate, as reported (Haslam Utr vul Naj min et al., 1975; Preston, 1995). FLO DYN Lyc eur R VEGET Phr aus Pot tri Cha spp In total we found 53 macrophyte species. Streams Jun eff Bryophy DETRIT Ali pla Agr sto Lyt sal B STRUC Epi hir flowing through Dravsko polje were overgrown from Typ ang Pol spp Iri pse Cat pal Ber ere Spa spp R COMPLTyp lat their source to the outflow, indicating environmental Spi polNaj mar Pha aru BOTTOM Tra nat Pot nodPot cri Lee ory conditions that favoured macrophyte growth. That was Alg fil Ele pal Pot pec Scr umb not the case in the middle parts of Sredisˇko polje LND USE Hyd sp. Myr spi Ver bec Ran cir streams, characterised by a woody riparian zone and Cal spp Pot ber Nit muc consequently heavy shade, where no macrophytes were B UNDC Lem min RET STR Cer dem Myo sco found. Hoyer et al. (2004) reported that little or no Zan palPot nat aquatic vegetation developed in locations where the Rum hyd Nas off Myr verElo can incident light reaching the substrate was less than 10%. Lem tri As the proportion of incident light depends on channel SED ACC Pot per dimensions (Davies-Colley & Rutherford, 2005), reaches -1.0 in the lower flow of Studencˇnica, also surrounded with -1.0 1.0 trees and shrubs but with a wider channel, were (b) characterized by rich macrophyte growth. Because reaches without macrophytes were excluded from the Fig. 4. CCA ordination diagrams showing the relationship between the vegetation of the streams and environmental CCA, the aspect of shading was less important in parameters. (a) Environmental parameters and stream reaches. explaining the relationship between macrophyte distri- K – Hocˇki potok, ’ – Framski potok, m – Reka, . – bution and riparian zone structure. Polskava, – Struga, – Studencˇnica, J – Pavlovski potok, Overall, the genus Potamogeton was the most frequent & – Libanja, } – Trnava. Codes for environmental and abundant. Its representatives prefer slow or parameters are given in Table 1. (b) Environmental parameters moderate flow, fine substrates and mesotrophic or and the abundance of macrophytes. Codes for macrophyte eutrophic water (Preston, 1995). Seven Potamogeton species are given in Fig. 3. species were found, among them Potamogeton berchtoldii, Potamogeton crispus, Potamogeton nodosus and Potamogeton pectinatus being dominant. These species 2000; Strayer et al., 2003). No evidence for this was appeared frequently and in great abundance in streams found in our study, since the majority of streams reaches of the western part of Dravsko polje, together with were influenced by agriculture. Myriophyllum spicatum and Sparganium erectum. All ARTICLE IN PRESS U. Kuhar et al. / Limnologica 37 (2007) 146–154 153 these species are characteristic of eutrophic waters regulated and unregulated Danish streams. Freshwater (Haslam, 1987; Preston, 1995; Schneider & Melzer, Biology, 42, 375–385. 2003). They occurred less frequently and less abundantly Bornette, G., & Amoros, C. (1991). Aquatic vegetation and in streams of the eastern parts of Dravsko polje and hydrology of a braided river floodplain. Journal of Sredisˇko polje, where agriculture was somewhat less Vegetation Science, 2, 497–512. developed and the riparian zone more preserved. There Brinson, M. M. (1993). Changes in the functioning of wetlands we found species that tend to occur most frequently in along environmental gradients. Wetlands, 13, 65–74. Brookes, A. (1994). River Channel Change. In P. Calow, & G. mesotrophic or meso-eutrophic waters, e.g. Berula E. Petts (Eds.), The rivers handbook. Hydrological and erecta Nasturtium officinale Mentha aquatica , , and ecological principles, Vol. 2 (pp. 55–75). Oxford, London, Myosotis scorpioides (Haslam, 1987; Schneider & Edinburgh, Boston, Melbourne, Paris, Berlin, Vienna: Melzer, 2003). Filamentous algae, which were found in Blackwell Scientific Publications. abundance in the western part of Dravsko polje, could Chovanec, A., Schiemer, F., Cabela, A., Gressler, S., Gro¨ tzer, temporarily appear elsewhere if the amount of nutrients C., Pascher, K., et al. (2000). Constructed inshore zones as in water suddenly increased (Smith, Tilman, & Nekola, river corridors through urban areas – the Danube in 1999). Vienna: Preliminary results. Regulated Rivers: Research & More detailed analysis, including some other para- Management, 16, 175–187. meters such as water chemistry, flow velocity and light Council of the European Communities. (2000). Directive 2000/ conditions might provide a better explanation of the 60/EC of the European Parliament and of the Council variance in macrophyte composition. However, the establishing a framework for the Community action in the present results on macrophytes, their geographical field of water policy. 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