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Plant communities in lowland Danish streams: species composition and environmental factors
Aquatic Botany 66 (2000) 255–272
Plant communities in lowland Danish streams: species composition and environmental factors Tenna Riis ∗ , Kaj Sand-Jensen, Ole Vestergaard Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød, Denmark Received 5 February 1999; received in revised form 16 July 1999; accepted 23 August 1999
Abstract We studied the quantitative composition of the vegetation at 208 stream sites distributed throughout Denmark with the purpose of identifying distinct plant communities, using cluster analysis, and their relationship to environmental conditions using indirect gradient analysis. Six plant communities were defined in the analysis, although the differences in species composition among them were small. The plant communities were mainly related to differences in water alkalinity with a Potamogeton-community being associated with high alkalinities, Sparganium-, Callitriche- and Batrachium-communities with medium alkalinities and a Myriophyllum alterniflorum-community associated with low alkalinity. Also stream size turned out to be an important variable to the separation of plant communities, with the Callitriche-community mainly found in small streams and the Potamogeton-community and a community of amphibious helophytes mainly found in larger streams. The most common species were Sparganium emersum and Elodea canadensis forming the widespread Sparganium-community at half of the stream sites throughout Denmark, a pattern most likely stimulated by the common practice of weed cutting. Though communities of stream plants could be defined, the overlap of species among them was very substantial. Communities are less distinct in Danish streams compared to streams in other regions including wider environmental gradients of temperature, current velocity, substratum, alkalinity and nutrient richness. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Stream plant communities; Alkalinity; Stream size; Weed cutting; Denmark
1. Introduction Distribution of plant species in an ecosystem can be viewed as a continuum, because plant species respond individually to variation in environmental factors and these vary con∗ Corresponding author. Present adress: National Environmental Research Institute, Vejlsøvej 25, P.O. Box 314, DK-8600 Silkeborg, Denmark. Tel.: +45-8920-1499; fax: +45-8920-1414. E-mail address: [email protected] (T. Riis)
0304-3770/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 9 ) 0 0 0 7 9 - 0
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tinuously in time and space (Gleason, 1926). Nonetheless, an operational way to describe the vegetation is to define plant communities as the collection of plant species growing together in a particular location and showing a definite association with each other (Kent and Coker, 1992). The reason why a certain plant community is present in a given environment is usually explained by the fact that the plant species in the community have similar environmental requirements (Kent and Coker, 1992). The focus of this study was a nationwide quantitative investigation of plant distribution in Danish streams. In streams different plant communities may arise as a result of differences in environmental factors such as stream size, water chemistry, flow velocity and substratum composition (e.g. Robach et al., 1996; Holmes et al., 1998). The assemblages of plants also are influenced by dispersal of the species by shoot fragments and turions drifting with the water, or by lateral ingrowth from the permanent amphibious populations on the banks (Johansson and Nielsson, 1993; Henry and Amoros, 1996). Therefore, the dispersal regime and environmental conditions in the riparian zone (e.g. bank steepness, soil type and land use) might influence species composition in the streams. The purpose of this work was to determine the composition of plant communities in Danish streams in relation to the environmental conditions. To ensure a proper evaluation of the differences in plant assemblages and their relationship to environmental variables, we included a large number of separate stream systems (29) and individual stream reaches (208) in the analysis and studied the vegetation and the environmental variables quantitatively. Numerical classification methods were used to group stream reaches into classes on the basis of their floristic composition, while indirect gradient analyses were used to evaluate the variation in plant composition in relation to environmental conditions. Our analysis was confined to Danish waters. However, as the species pool and physico-chemical conditions of Danish streams are very similar to those of other European lowland regions, we expect the patterns to reflect general mechanisms and regulations. The first specific objective was to determine if distinct plant communities in lowland streams exist. The second objective was to evaluate the relationships between the composition of stream plant communities and environmental conditions. 2. Methods Vegetation and environmental variables were studied in the summers of 1996 and 1997 at 208 unshaded stream sites each of 100 m length. Stream sites were located in 29 stream systems chosen so they were distributed throughout Denmark and representing different environmental and morphometric characteristics. The sites were selected on a map beforehand because they had to be close to a road for practical reasons. However, a site was excluded if it was shaded or if there was no vegetation at all. Mean width of the reaches varied from 0.6 m at upper reaches to 30 m at lower reaches. Danish stream systems are generally small, and 82% of the sites were between 0.6 and 10.0 m wide. 2.1. Sampling of vegetation data On each 100 m long stream site 10 transects were placed equally (one for each 10 m) across the stream. Along each transect, quadrats (25 × 25 cm) were placed side by side from
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one bank to the other. In each quadrat, the presence of vascular plant species and bryophytes was recorded. Observations were made by means of a viewing tube. Plants were identified to species when possible (Hansen, 1981; Moeslund et al., 1990). Species were not identified within the genera Callitriche and Epilobium, because identification is only certain when the individuals have flowers or seeds. Flowering is also required to identify some of the Batrachium species, but flowers were common. When flowers were absent, Batrachium aquatile, B. baudotii and B. peltatum could not be distinguished, and they were registered as Batrachium sp. Relative frequencies of species, or species groups, on each stream site were calculated from the number of quadrats with the species present and total number of quadrats with plants. A minimum of 100 quadrats with plants per site were analysed to ensure accurate calculations of frequencies. If 10 transects did not provide 100 quadrats with plants, additional transects were included. 2.2. Environmental data and their selection Environmental variables for each stream site were: mean depth and width, frequency of different bottom substrata, homogeneity of the spatial distribution of substrata, water velocity, light attenuation, alkalinity, bank steepness, soil type, land use and geographical region. Mean depth and width were chosen to characterise the size of the streams. Depth was measured in each quadrat included in the vegetation analysis, and mean depth of a stream site was calculated from all the quadrats examined. Mean stream width on a site was calculated as the mean of the widths of the 10, or more, transects. Proportions of different substrata were quantified because of their proposed importance for the development of plant species (Butcher, 1933, Carpenter and Lodge, 1986). The substrata were divided into stone (>30 mm diameter), gravel (3–30 mm diameter), sand (0.1–3 mm diameter), mud (<0.1 mm, black), hard clay and peat. Coverage of each substratum type on a reach was calculated as the relative frequency of all the quadrats examined. Homogeneity of the spatial distribution of substrata was quantified, because high homogeneity of an area is likely to restrict the number of niches and species richness (Harper, 1961). Substratum homogeneity was quantified from the spatial distribution of substratum types on the stream sites, as: P10 Pni 1, if x = y i=1 j =2 I (xj , xj −1 ) ; (x, y) = SH = P10 Pni 0, otherwise 1 i=1
j =2
where SH is substratum homogeneity, i are transects and j are quadrats. The value expresses how often two neighbouring quadrats differ from each other with respect to the substratum. The value can take any number between 0 and 1. The closer the value is to 1, the more often are two neighbouring quadrats alike, and the higher is substratum homogeneity. High water velocity and poor light penetration can restrict the growth of some submerged stream plants (Westlake, 1975). Therefore, these parameters were measured at each stream site. Mean water velocity on the stream site was measured by an integration method of dilution gauging (White, 1978). Mean water velocity was calculated from the time elapsed for a 5 l salt water solution (12% w/w) to pass through the 100 m reach. The salt water solution was added upstream of the reach at time 0, while conductivity was measured continuously
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at the 100 m point downstream of the reach. As the solution passed the downstream point, conductivity first increased and later decreased until the initial level was reestablished. By means of integration of conductivity over time, it was possible to determine the time at which half of the salt water solution had passed the downstream point. The mean water velocity on the stream site was assumed to represent the mean water velocity in the growing season. The annual variation in water velocity is not considered. Light attenuation of photosynthetically available radiation (PAR, 400–700 nm) on each stream site was determined by measurements of the photon flux at 10 cm depth intervals with a flat cosine-corrected sensor (Li-Cor 190). The vertical, diffusive attenuation coefficient (Kd ) was found by linear regression of the exponential decrease of photon flux density on logarithmic scale plotted against depth (Kirk, 1983). Alkalinity is often considered to be the water chemical variable which is most closely related to the distribution of submerged plants in lakes (Jackson and Charles, 1988; Vestergaard and Sand-Jensen, 1999), and may also be of importance for the distribution of stream plants (e.g. Holmes and Whitton, 1977; Haslam, 1978). Alkalinity has a close relationship to pH and conductivity, and therefore, these variables are not included. Alkalinity was determined by potentiometric end-point titration with HCL on 100.0 ml samples of stream water (Hutchinson, 1957). The following four variables: bank steepness, soil type, land use and geographical region are qualitative variables, which were described by different categories in the field. Each category was given a number, which made it possible to evaluate these variables together with the above-listed quantitative variables. The vertical profile of the stream bank was determined, because the steepness may influence the ability of amphibious species on the bank to colonise into the stream. Bank steepness in a distance of 0–0.5 m from the stream was measured and given a value from 1 to 3, with 3 representing a steep bank and 1 representing a less steep bank. If the two stream banks along the reach differed, the value selected was from the side of the stream holding the highest species richness. Several researchers have suggested that catchment soil type in the stream and the adjacent land can be a suitable predictor of the distribution of stream plants (e.g. Grasmück et al., 1995; Holmes et al., 1998). Soil type in the vicinity of stream reaches was found on soil maps. Four different categories were used: coarse sandy soil (category 1), fine sandy soil (category 2), clay-mixed-sand soil (category 3), and sand-mixed-clay soil (category 4). Land use in the area surrounding the stream reach was included in the analysis because many species, especially in small streams, derive from the banks, which in turn are colonised from the surrounding areas (Henry and Amoros, 1996; Henry et al., 1996). Modes of land use were divided into 10 groups representing increasing diversity of terrestrial plant species (Table 4). The relative differences in diversity was estimated from Nørrevang and Meyer (1969). The lowest plant diversity was observed in arable land and reed swamps, and the highest diversity was found in low and high meadow vegetations. We distinguished between five geographical regions in Denmark (Fig. 1) to reflect differences in alkalinity due to changes in clay and calcium carbonate content of the subsoils. The regions may also differ in discharge pattern and in summer temperature of the streams, and the geographical location may influence the immigration history of the species. Region
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Fig. 1. Map of Denmark showing the five geographical regions and the boundary of the ice cap during the last Ice Age ending about 11,000 years ago.
1 has sandy fluvial deposits from the last Ice Age (ending about 11,000 years ago) and predominantly old moraine deposits from the previous Ice Age (ending about 130,000 years ago). Region 2 has clay moraine from the last Ice Age and raised sea beds about 7000–8000 years old. Region 3 has clay moraine from the last Ice Age. Region 4 (island of Funen), and region 5 (island of Zealand) have mainly clay moraine soils. Nitrogen and phosphorus were not included in the analysis because they typically display high concentrations in lowland Danish streams (>3 mg total-N l−1 and >0.1 mg total-P l−1 ; Sand-Jensen and Lindegaard, 1996). Previous studies in Danish streams showed no correlation between species distribution of plants, tissue content of nitrogen and phosphorus in the plants and nutrient concentration in the stream water (Kern-Hansen and Dawson, 1978). Moreover, tissue content of nitrogen and phosphorus are generally above those levels likely to constrain plant growth rate (Jacobsen, 1993). 2.3. Data analysis First, a hierarchical agglomerative cluster analysis based on Bray–Curtis similarity measures was used to describe the percentage similarity in species distribution between localities
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(Clarke and Warwick, 1994). Relative frequencies of each species on each locality were not transformed or normalised. Rare species were downweighted by omitting those occurring at less than five localities. This operation was done because rare species are unlikely to be adequately described and may act as outliers in a multivariate treatment of the data (Økland, 1990). Moreover, species of Lemna were omitted because of their free-floating growth form. A final 75 taxa remained for further analysis. Groups of localities with a more or less similar floristic composition were defined based on the percentage similarity in species distribution between localities. These species present in the different groups of localities were defined as different plant communities. Second, environmental gradients related to the sample groups defined by the cluster analysis were looked for. Detrended correspondence analysis (DCA) was carried out by CANOCO (ter Braak, 1992) followed by a Spearman rank correlation analysis between environmental parameters and sample scores in the DCA analysis. Significant differences between the groups in the DCA ordination diagram were tested by multi-response permutation procedures in PC-Ord statistical package (McCune and Mefford, 1995). The quantitative environmental parameters were not normally distributed, and differences between the medians of the groups of sample sites were tested by Kruskal–Wallis test. For each group of sample sites differences in number of observations of the categories of qualitative variables (bank steepness, soil type, land use and region) were tested by χ 2 -tests. Differences between frequencies of the qualitative environmental variables and between groups of localities were also tested by χ 2 -tests. Correlations among the environmental variables were tested with a Spearman rank correlation test. 3. Results 3.1. Numerical classification The cluster analysis separated six groups of sample sites including 205 of the 208 sites. Similarities in species composition within the groups were small as the mean Bray–Curtis similarity in the groups was 25% (±5% SE; range 16–51%). However, the groups were significantly different from each other (MRPP-test, p < 0.01). Vegetation distribution and abundance in each group of sample sites were analysed. The 30 most abundant taxa in each group of sample sites were used to describe different communities (Table 1 ). Only 23 taxa were available in Group 3. The overall list of abundant taxa contained 56 taxa, implying a high overlap of species among communities. Six taxa were present in all six communities (Callitriche sp., Epilobium sp., Myosotis palustris/Myosotis laxa ssp. caespitosa, Phalaris arundinacea, Sparganium emersum and Sparganium erectum), and five taxa were present in five communities (Berula erecta, Elodea canadensis, Glyceria maxima, Potamogeton crispus and Ranunculus repens). Each community was characterised by dominant and exclusive species. A Sparganiumcommunity (Group 1) was dominated by S. emersum and E. canadensis, and only Sagittaria sagittifolia was restricted to this community. A Callitriche-community (Group 2) was dominated by species of Callitriche and B. peltatum, and three taxa were exclusively present in this community (Equisetum palustre, Montia fontana and Polygonum amphibium). A
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Table 1 The relative frequency of plant species in the six communities based on the presence in 208 localities. The values of the dominating. Species in the groups are in bold. Group 1: S. emersum-community. Group 2: Callitriche-community. Group 3: M. alterniflorum-community. Group 4: Potamogeton-community. Group 5: G. maxima-community. Group 6: B. aquatile-community, n = number of stream sites Species
Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 (n = 114) (n = 37) (n = 4) (n = 31) (n = 13) (n = 6)
Saggittaria sagittifolia L. 0.28 Agrostis stolonifera L. 0.46 Batrachium circinatum (Sibth.) Spach 0.65 Potamogeton praelongus Wulf. 0.60 Potamogeton pectinatus L. 1.29 Potamogeton natans L. 2.02 Butomus umbellatus L. 2.21 Nuphar lutea (L.) Sm. 0.59 Phragmites australis Trin. 0.48 Batrachium baudotii (Godron) F. Schultz 1.96 Glyceria fluitans (L.) R. Br. 1.95 Batrachium peltatum (Schrank) Presl. 1.02 Batrachium sp. 2.23 Potamogeton perfoliatus L. 0.55 Solanum dulcamara L. 0.63 Veronica anagallis-aquatica L. 0.95 Equisetum fluviatile L. 0.30 Scirpus lacustris L. 0.47 Alopecurus geniculatus L. 0.40 Elodea canadensis L. C. Rich 17.56 Berula erecta Hudson (Coville) 6.50 Ranunculus repens L. 0.28 Glyceria maxima (Hartman) Holmberg 5.96 Potamogeton crispus L. 1.16 Callitriche sp. 7.38 Epilobium sp. 0.88 Myosotis palustris L. / laxa Lehm ssp. caespitosa (C.F. Schultz) Hyl. 0.82 Phalaris arundinacea L. 4.81 Sparganium emersum Rehman 29.23 Sparganium erectum L. 2.48 Equisetum palustre L. Montia fontana Cham. ssp. Fontana Polygonum amphibium L. Agrostis sp. Myriophyllum alterniflorum D. C. Stellaria alsine Grimm Zannichellia repens Boenn. Cardamine amara L. Nasturtinum sp. Carex rostrata Stokes Mentha aquatica L. Poa sp. Deschampsia caespitosa (L.) Beauv. Potentilla palustre (L.) Scop.
0.63
0.36 17.75
2.39 19.89 4.75 1.54 0.51 1.13
15.74 6.18
0.33 1.86 16.39 0.79 4.02 1.13 0.36 0.46 0.28 1.28 20.84
2.14 0.42
2.21 0.84 2.95 2.34 1.38 1.46 7.13 2.36 1.51 4.17 0.55 2.32
4.62 2.17 2.08 0.32 0.26 0.39
3.57 4.56
0.43 8.00 2.22 0.64 3.50 18.32 2.30 0.28
25.78 0.50 1.10 1.14
4.86 0.27 8.48 0.27 6.30 0.86
0.15 0.08 7.34 0.55
0.39 3.14 4.91 2.69
0.72 5.39 1.36 10.48
1.99 1.04 5.25 5.39
3.46 3.55 0.54 5.29 2.17 20.79 0.98
6.30 0.30 0.26
4.07 2.55 5.48 2.80 0.37 1.15 2.29 0.19 0.39 0.53 0.71 1.76 2.58 0.66 1.04 1.11
0.45 22.05 0.76 0.24 0.13 0.07 4.24 4.06 0.64 0.09 1.92
1.21 1.05 1.57
1.29
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Table 1 (Continued). Species Carex sp. Juncus effusus L. Potamogeton lucens L. Iris pseudacorus L. Batrachium aquatile (L.) Wimmer Alisma plantago-aquatica L. Myriophyllum spicatum L. Typha latifolia L. Carex acutiformis Ehrh. Veronica beccabunga L. Catabrosa aquatica (L.) Beauv. Fontinalis antipyretica Hedw.
Group 1 (n = 114)
Group 2 (n = 37)
Group 3 (n = 4) 0.45 0.90
Group 4 (n = 31) 0.25 0.45 0.65 0.39
Group 5 (n = 13)
Group 6 (n = 6)
1.10 1.57 2.92 50.44 0.55 3.71 9.38 0.75 0.69
0.63 0.07 0.21 0.27
Myriophyllum alterniflorum-community (Group 3) was dominated by M. alterniflorum, Potamogeton natans and G. fluitans and two taxa (Deschampsia caespitosa and Potentilla palustre) were restricted to this community. A Potamogeton-community (Group 4) was dominated by three species of Potamogeton (P. perfoliatus, P. crispus and P. pectinatus) and Potamogeton lucens was restricted to this community. A G. maxima-community
Fig. 2. Ordination diagram by DCA showing the scores of 205 localities based on the relative frequencies of 75 plant species. The groups of stream sites representing the plant communities defined by Cluster analysis are marked by different symbols and framed. Eigenvalue of Axis 1 was 0.53 and of Axis 2 was 0.48, and the gradient lengths were 4.54 and 3.65 SD units for Axis 1 and 2, respectively.
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(Group 5) was dominated by G. maxima and had three exclusive species (Alisma plantagoaquatica, Myriophyllum spicatum, and Typha latifolia). A B. aquatile-community (Group 6) was dominated by B. aquatile, while Catabrosa aquatica and Fontinalis antipyretica were restricted to this community. The DCA-analysis of 205 samples with 75 taxa revealed that the groups of sample sites separated along two axes (Fig. 2). Along Axis 1, Group 2 and 3 were located to the left in the diagram, Group 1, 5 and 6 in the center and Group 4 farthest to the right. This pattern indicates the existence of an underlying environmental gradient between the plant communities at different stream sites. The eigenvalue of Axis 1 was 0.53, and the gradient length was 4.54 standard deviation (SD) units, which indicate a full turn over of species on the localities farthest away from each other along Axis 1 (Økland, 1990). Spearman rank correlation analysis between sample scores from the DCA analysis and the environmental variables revealed that alkalinity had the highest correlation with the sample sites. Besides alkalinity, also stream width, stream depth, light attenuation coefficient, proportions of mud in the substratum, soil type and region correlated positively to Axis 1, while the proportions of peat and sand, and the land use correlated negetatively to this axis (Spearman rank correlation, p < 0.05; Table 2). Thus, the analysis shows that alkalinity is higher in sample sites from Group 4 than Group 1, 5 and 6, which in turn have a higher alkalinity than sites in Group 2 and 3. Along Axis 2, Group 3 was located at the bottom of the ordination diagram, Group 1, 2 and 4 in the middle, and Group 5 and 6 at the top. The Axis-2 eigenvalue was 0.48 and the gradient length was 3.65 SD units. The proportion of stone and gravel and geographical region were positively correlated to Axis 2, while substratum homogeneity and bank steepness were correlated negetatively to this axis (Table 2).
Table 2 Spearman rank correlations coefficients between the sample scores on the DCA axis 1, 2 and 3 and environmental variables (p < 0.05) Variable Alkalinity Attenuation coefficient Water velocity Depth Width Stone Gravel Sand Mud Clay Peat Substratum homogeneity Bank steepness Soil type Land use Region
Axis 1
Axis 2
0.49 0.38
0.35 −0.26 −0.14 −0.15
0.35 0.42
−0.35 0.34 0.20 −0.36 0.25 0.40 −0.24 0.44
Axis 3
0.23 0.33 −0.14 −0.20 −0.17 −0.32 −0.21
0.34
0.20
0.19 0.21
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3.2. Environmental properties The indirect gradient analysis suggested the existence of differences in environmental variables among groups of stream sites containing the different communities. To test this pattern, medians of the quantitative variables were compared between groups of stream sites (Kruskal–Wallis test; Table 3). Differences in the number of observations of the categories of Table 3 Environmental parameters of the six groups of stream sites representing the six plant communities. The total number of sample sites is 205a Group 1 (n = 114) Alkalinity (meq l−1 ) Median 2.36 c Range 0.59–5.78 Light att. coef. (m−1 ) Median 1.00 ab Range 0.20–2.80 Water velocity (m s−1 ) Median 0.24 Range 0.01–0.72 Mean depth (cm) Median 57 b Range 15–157 Mean width (cm) Median 583 b Range 124–2500 Stone (%) Median 0.5 Range 0–35.9 Gravel (%) Median 4.6 b Range 0–50.4 Sand (%) Median 52.5 b Range 0–90.0 Mud (%) Median 24.6 b Range 0–100 Clay (%) Median 2.3 b Range 0–84.5 Peat (%) Median 0 Range 0–63.6 Substratum homogeneity Median 0.81 b Range 0.42–1.00
Group 2 (n = 37)
Group 3 (n = 4)
Group 4 (n = 31)
Group 5 (n = 13)
Group 6 (n = 6)
1.16 b 0.14–4.90
0.64 a 0.26–0.98
3.50 c 1.16–5.08
1.36 a 1.06–5.26
2.44 c 1.35–2.59
0.67 a 0.30–3.86
0.63 a 0.52–0.84
1.29 b 0.67–7.16
0.89 ab 0.50–2.03
1.16 ab 0.90–2.21
0.23 0.05–0.53
0.27 0.20–0.36
0.20 0.04–0.96
0.23 0.07–1.15
0.27 0.14–0.44
32 a 8–78
59 b 30–65
58 b 16–140
47 ab 13–136
40 ab 34–68
373 a 86–1666
276 a 61–443
702 ab 241–3000
633 ab 152–2800
511 ab 201–680
1.6 0–82.8
0.0 0–5.3
3.1 0–50.0
8.1 0–48.3
6.7 0.4–17.2
9.7 b 0–64.8
0.0 a 0–1.3
19.4 b 0–78.1
5.2 ab 0.8–49.4
16.5 b 3.5–51.4
64.5 b 15.5–96.6
84.1 b 41.1–93.6
33.1 a 3.3–84.0
41.0 ab 17.0–89.2
30.5 ab 10.2–49.4
4.2a 0–83.9
4.7 ab 0–46.7
2.5 b 0–83.0
16.9 b 2.5–60.3
47.4 b 2.9–64.4
0a 0–13.0
1.4 ab 0–5.6
0.9 ab 0–21.3
0a 0–2.4
2.1 b 0–13.6
0 0–26.5
6.5 4.7–8.3
0 0–17.6
0 0–2.3
0 0–0
0.70 a 0.30–0.91
0.77 ab 0.46–0.84
0.77 ab 0.55–0.95
0.71 a 0.61–0.86
0.75 ab 0.53–0.82
a Group 1: S. emersum-community. Group 2: Callitriche-community. Group 3: M. alterniflorum-community. Group 4: Potamogeton-community. Group 5: G. maxima-community. Group 6: B. aquatile-community. n = number of stream sites. Values with different letters are significantly different (p < 0.05; Kruskal–Wallis test). There was no significant differences in water velocity, proportion of stone and peat among the groups.
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Table 4 Distribution of sites among semi-quantitative and qualitative categorial types (four independent classifications) for six community typesa
Bank steepness 1/high Bank steepness 2/intermediate Bank steepness 3/low Soil type 1/coarse sandy soil Soil type 2/fine sandy soil Soil type 3/clay-mixed-sand soil Soil type 4/sand-mixed-clay soil Land use 1/cultivated Land use 2/reed swamp Land use 3/ruderal Land use 4/ fertilized grazed meadow Land use 5/forest Land use 6/scrub Land use 7/heath Land use 8/tall herb fen Land use 9/ unfertilized grazed meadow Land use 10/bog Region 1/West Jutland Region 2/North Jutland Region 3/East Jutland Region 4/Island of Funen Region 5/Island of Zealand
Group 1 (n = 114)
Group 2 (n = 37)
Group 3 (n = 4)
Group 4 (n = 31)
Group 5 (n = 13)
Group 6 (n = 6)
19/27 70/61 25/26 58/65 19/12 18/17 18/19 7/13 0/2 2/4
10/9 19/20 8/8 29/21b 1/4 3/5 4/6 7/4 0/1 3/1
2/1 2/2 0/1 4/2 0/0 0/1 0/1 0/0 0/0 0/0
8/7 12/17 11/7 11/18 2/3 8/5 10/5b 8/3 2/0b 0/1
7/3b 4/7 2/3 10/7 0/1 1/2 2/2 0/1 1/0b 2/0
2/1 3/3 1/1 5/3 0/1 0/1 1/1 1/1 0/0 0/0
45/36b 4/4 3/4 0/0 38/32
7/12 1/1 1/1 0/0 7/10
1/1 0/0 0/0 0/0 0/1
7/10 2/1 1/1 0/0 7/9
4/4 0/0 2/1 0/0 2/4
0/2 0/0 1/0 0/0 3/2
14/14 1/6 18/18 34/21b 43/53 14/12 5/10
5/5 6/2b 12/6 1/7 21/17 3/4 0/3
1/1 2/0 3/0 0/1 1/2 0/0 0/0
4/4 0/2 0/5 2/6 13/15 3/18 13/3b
1/2 1/0 0/2 0/2 12/6b 1/1 0/1
1/1 0/0 0/1 0/1 6/3 0/1 0/1
a Observed/expected values shown, where expected values were determined as the total number of sites in each community type multiplied by the proportion of all sites in the appropriate category. There was a significant difference in number of observations of the variables among groups of stream sites in all four variables (read table horizontally; χ 2 -test, p < 0.05). The number of stream sites in group 3 and 6 were too low to test differences of number of observations of the categories among stream sites. Group 1: S. emersum-community. Group 2: Callitriche-community. Group 3: M. alterniflorum-community. Group 4: Potamogeton-community. Group 5: G. maxima-community. Group 6: B. aquatile-community, n = number of stream sites. b The number of observations of the occurrence of the different categories of the qualitative variables in each group was tested, and significant difference between categories in the category with highest score (read table vertically; χ 2 -test, p < 0.05).
the qualitative variables between groups of stream sites were tested and showed a significant difference among the groups in all four variables (χ 2 -test, p < 0.05; read Table 4 horizontally). In each group of stream sites, differences between observed and predicted number of observations in the categories of the qualitative data were tested (χ 2 -test, p < 0.05; read Table 4 vertically). If the test showed significant differences the category with the largest difference between observed and expected was marked. Some general characteristics of environmental properties appeared in the six groups of stream sites representing the different plant communities. Stream sites with the Sparganium emersum-community had high light attenuation in the water, a relatively low proportion of gravel, high proportion of mud and clay and a high substratum homogeneity (Table 3). There
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was a higher number of observations in North Jutland and in areas with fertilized meadows than expected by chance (Table 4). The Callitriche-community was disproportionately well represented at shallow and narrow stream sites with a low alkalinity, a low light attenuation coefficient, a low proportion of mud and a low substratum homogeneity (Table 3). On coarse sandy soil and in bogs it was present in the catchment more times than expected (Table 4). Only four stream sites with the M. alterniflorum-community were present, and those had a relatively low alkalinity and a high proportion of sand (Table 3). Stream sites with the Potamogeton-community had a high alkalinity, a high light attenuation, a high proportion of gravel, but a relatively low proportion of sand (Table 3). They were located on sand-mixed-clay soils in Region 5, and the catchment included reed swamps more times than expected (Table 4). Stream sites with G. maxima-community were deep and wide, contained a relatively high proportion of mud and had a high substratum homogeneity (Table 3). Land use included reed swamp and sites were located in region 3 more times than expected (Table 4). Finally, stream sites with the B. aquatile-community exhibited high alkalinity, intermediate size, and a high proportion of gravel, mud and clay (Table 3). The sites were all located in Region 3 (Table 4). There was a significant intercorrelation between most of the environmental variables (Spearman rank correlation test, p < 0.05; Table 5). Taking alkalinity as an example, it correlates positively to light attenuation, proportion of mud and clay, substratum homogeneity, bank steepness, soil type and region, and correlates negatively to water velocity, proportion of sand and peat, and land use. As alkalinity correlates very well with the geographical location, and intercorrelates with most other variables as well, it is possible to describe some characteristics of the different regions. Region 1 differs most from Region 4 and 5. Alkalinity is lowest in Region 1 (0.75 ± 0.27 (SD) meq l−1 ), and light attenuation coefficient, proportion of mud and clay, substratum homogeneity, bank steepness and soil type are also relatively low in this region, while water velocity and proportion of sand and peat are relatively high. On the other hand, alkalinity is highest in Region 4 (4.92 ± 0.40 meq l−1 ) and 5 (3.55 ± 0.52 meq l−1 ), and light attenuation coefficient, proportion of mud and clay, substratum homogeneity, bank steepness and soil type are relatively high in these regions, while water velocity and proportion of sand and peat are relatively low. Region 2 and 3 are intermediate with respect to environmental variables. 4. Discussion 4.1. Plant communities and the importance of environmental variables Alkalinity appears to be the most important measured environmental variable distinguishing plant communities in Danish lowland streams. Previous studies have shown alkalinity to be an important factor in the distribution of aquatic plants in streams (Wiegleb, 1984, Robach et al., 1996; Sabbatini et al., 1998), but the influence of alkalinity and alkalinity related factors are even more prominent in lakes (e.g. Spence, 1967; Jackson and Charles, 1988). The correlation between alkalinity and plant distribution in stream communities, therefore, probably result from the interconnection of lakes and streams in water systems,
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and the dispersal of plants from lakes into the streams as well as a direct influence of alkalinity on plant performance and survival in the streams. In lakes some species benefit from their physiological ability to utilize bicarbonate in addition to carbon dioxide in photosynthesis (e.g. Potamogeton sp.), while other species only use carbon dioxide or have a low affinity for bicarbonate (e.g. M. alterniflorum; Spence et al., 1985; Madsen and Sand-Jensen, 1991). In most streams the concentration of carbon dioxide is high (Rebsdorf et al., 1991; Sand-Jensen and Frost-Christensen, 1998), and the ability to use bicarbonate is therefore less important for the photosynthetic performance of stream plants relative to lake plants. Nonetheless, high concentrations of bicarbonate in high-alkaline streams are still important for bicarbonate-users to maintain high photosynthesis throughout the day despite profound depletion of carbon dioxide at noon and in the afternoon in streams with dense macrophyte stands (Sand-Jensen, 1983; Sand-Jensen and Frost-Christensen, 1998). Although the DCA-analysis shows alkalinity to be the most important factor to the distribution of stream vegetation, it also shows light attenuation, width, depth, proportion of most substratum types, substratum homogeneity, bank steepness, soil type, land use and regions to have significant influence on the segregation of plant communities. Apart from water depth, width, and proportion of stone and gravel, all these variables are correlated with alkalinity and the importance of the different factors in separating the communities is, therefore, hard to evaluate. Those variables that are most clearly related to the segregation of communities (i.e. soil type and region) are also most closely correlated to alkalinity. However, because the depth and width of the streams have a significant correlation to the segregation of communities and do not correlate to alkalinity, it is possible to state that stream size is an important variable in addition to the alkalinity related variables. The Callitriche-community is mainly present in small streams and the Potamogeton-community is mainly present in larger streams, and this pattern might reflect a preference to higher temperatures of Potamogetons compared to species of Callitriche. This agrees with observations of frequent presence of Potamogeton species in the warmer, downstream reaches in streams and in outflow of lakes (Pedersen, 1976). The results show indications of weed cutting in the streams to be responsible for the presence of the S. emersum-community on more than half of the 208 observed stream sites, although the frequency and magnitude of weed cutting were not included as a variable in the study. The species S. emersum and E. canadensis together constitute almost 50% of the total frequencies of plants at the 114 sample sites housing the community. The two species are also the most common species in Danish streams constituting 19 and 11% of total plant cover in the 208 studied localities. Both S. emersum and E. canadensis are probably favoured by the extensive cutting of vegetation taking place in most of the larger Danish streams. The established life strategy of the species has been designated as competitive-ruderal (Grime et al., 1988) with fast growth, efficient dispersal and resistance to disturbance. Elodea canadensis has negatively buoyant shoot fragments which are released in great numbers during weed cutting and are able to settle on the stream bottom and regrow (Nichols and Shaw, 1986). Sparganium emersum has well-developed rhizomes and a basal meristem allowing regrowth after weed cutting of above-ground parts (Nielsen et al., 1985; Sand-Jensen et al., 1989). In contrast, competitor-species such as the broad-leaved species of Potamogeton (e.g. P. lucens and P. perfoliatus) are likely to be limited by disturbance caused by weed cutting, and these species only reach a small total plant cover on these stream sites.
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Stream sites with S. emersum-communities are present in all the five geographical regions along with a widespread practice of weed cutting. The low proportion of gravel, the high proportion of mud and the high substratum homogeneity on the stream sites holding the S. emersum-community, are accomplished likely by rough managing and dredging during the last decades, which have promoted a wide stream profile and reduced the current velocity. The two dominating species are known from other countries to grow preferentially at sites with low current velocity and muddy substrata (Hynes, 1970; French and Chambers, 1996). 4.2. Community segregation in lowland streams Differences in species composition between plant communities is relatively small in Danish streams compared to stream plant communities in other countries with stronger environmental gradients (e.g. Holmes et al., 1998) and to plant communities in Danish lakes (Vestergaard and Sand-Jensen, 1999). At least four conditions can be expected to reduce differences among stream plant communities in general and differences within Denmark in particular. First, segregation can result from different water chemistry in the stream systems, and variability among the species in their tolerance and optimum requirements to the chemical environment. Submerged plants in streams include obligatory aquatic species, amphibious species and predominantly terrestrial species (e.g. Holmes and Whitton, 1977; Henry and Amoros, 1996). Among these three groups, only the obligatory aquatic species are dependent on the characteristic chemistry of the stream water throughout their life span. Therefore, in the total vegetation only the obligatory aquatic species are expected to exhibit a clear segregation relative to water chemistry, while the other two life forms might have a less distinct distribution among sample sites. This hypothesis will be tested later. Second, the longitudinal connectance of the streams means that water and plant species are dispersed from upstream to downstream reaches simultaneously reducing the differences in water chemistry and plant composition within specific stream systems (Johansson and Nielsson, 1993). Third, due to the constant input of new drainage water from land along the stream course, concentrations of dissolved inorganic nutrients are usually high in streams reducing the likelihood of nutrient limitation and the tendency of forming specific oligotrophic or eutrophic plant communities. Fourth, the lack of complete vegetation cover in streams reduces the probability that adaptation of certain plant species to the particular water chemistry will result in competitive exclusion of species finding the water chemistry suboptimal. The small environmental gradients in Denmark is another possible reason why plant communities in Danish streams do not segregate more markedly. In countries and geographical regions with large environmental gradients such as between lowland and mountain ranges, marked differences in stream plant communities can be observed because of high variability of stream slope, bottom substrata, temperature, alkalinity and nutrient richness. An extensive study of the vegetation in more than 1500 stream sites in Great Britain resulted in the classification of plants into four floral communities ranging from highland, torrential, oligotrophic streams to lowland, eutrophic streams (Holmes et al., 1998). The communities segregated much more strongly, likely because of the wider range of geological, altitudinal and nutritional conditions in Great Britain compared to Denmark. The Danish communities
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all fit into the lowland eutrophic stream community. While the two most distinct Danish communities had 17 species in common, no species overlapped in the four British stream communities (Holmes et al., 1998). The segregation in terms of Bray–Curtis similarity in the plant communities is also much more distinct in Danish lakes than streams (t-test, p < 0.05). A study of the vegetation in 82 Danish lakes classified the lakes in five groups with alkalinity being the most important factor responsible for the segregation (Vestergaard and Sand-Jensen, 1999). The lake communities showed an average Bray–Curtis similarity of 51% (±1 S.E.) while this study showed average Bray–Curtis similarity among stream communities of only 25% (±5 S.E.). Lake communities are probably more distinct than stream communities because of the more stable growth environment contrary to streams with frequent flow disturbance, movement of the stream bed and formation of open space available for plant colonisation. In lakes competition for resources increases the adaptation and segregation of species to adjust to the given physical and chemical conditions. Moreover, the greater physical isolation of many lakes compared to the longitudinal connectance in stream systems likely increases the differences of plant communities among lakes relative to streams.
5. Conclusions Six distinct plant communities were distinguished in Danish streams although the differences in the species composition among the communities were small. Indirect gradient analyses suggested alkalinity to be the most important factor responsible for separating the six communities. However, the effect of alkalinity was correlated to several other variables and thus it was hard to separate the effect of alkalinity from the effect of the correlated variables. Among communities the Myriophyllum alterniflorum-community is associated with low alkalinities, Sparganium-, Callitriche- and Batrachium-communities are associated with medium alkalinities and the Potamogeton-community is associated with high alkalinity. The indirect gradient analyses also suggest stream size to be important in segregating the communities. Thus, the Potamogeton-community and a Glyceria maxima-community containing several reed swamp species are present in relatively large streams, and the Callitriche-community is present in small streams. The Batrachium aquatile-community was associated with stone and gravel substratum. Frequent vegetation cutting in the streams is probably important for the segregation of the most common stream community dominated by E. canadensis and S. emersum but this aspect was not tested. A direct influence of the environmental conditions in the riparian zone on plant distribution in the streams was not apparent, because of correlations between alkalinity and the variables characterising the riparian zone. Plant communities are less distinct in Danish streams compared to stream communities in other regions with larger environmental gradients, as well as compared to plant communities in Danish lakes. These differences are probably due to: (1) the integrity of streams with respect to water chemistry and species dispersal, and (2) the disturbed environment of streams leading to open space and smaller differences in species composition as a function of variable water chemistry and hydrology among separate stream systems.
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Acknowledgements We thank the Danish Natural Science Research Council (grant 11-7795) and the Danish Research Academy for financial support. We thank Jens Borum for helpful comments on the manuscript and Anne Garde for linguistic improvements. We also thank Statician Søren E. Larsen for help. References Butcher, R.W., 1933. Studies on the ecology of rivers I. On the distribution of macrophytic vegetation in the rivers of Britain J. Ecol. 21, 58–91. Carpenter, S.R., Lodge, D.M., 1986. Effects of submersed macrophytes on ecosystem processes Aquat. Bot. 26, 341–370. Clarke, K.R., Warwick, R.M., 1994. Change in marine communities: An approach to statistical analysis and interpretation. Plymouth Marine Laboratory, Plymouth. French, T.D., Chambers, P.A., 1996. Habitat partitioning in riverine macrophyte communities Freshwat. Biol. 36, 509–520. Gleason, H.A., 1926. The individualistic concept of the plant association Bull. Torrey bot. Club 53, 1–20. Grasmück, N., Haury, J., Léglize, L., Muller, S., 1995. Assessment of the bio-indicator capacity of aquatic macrophytes using multivariate analysis Hydrobiologia 300/301, 115–122. Grime, J.P., Hodgson, J.G., Hunt, R., 1988. Comparative Plant Ecology: A Functional Approach to common British Species. Allen and Hyman, London. Hansen, K., 1981. Dansk feltflora (in Danish). Nordisk Forlag, Copenhagen. Harper, D., 1961. Approaches to the study of competition. In: Milthorpe, F.L. (Ed.), Mechanisms in Biological Competition, Symposium of the Society for Experimental Biology 15, pp. 1–39. Haslam, S., 1978. River Plants. Cambridge University Press, Cambridge. Henry, C.P., Amoros, C., 1996. Are the banks a source of recolonization after disturbance: An experiment on aquatic vegetation in a former channel of the Rhône River Hydrobiologia 330, 151–162. Henry, C.P., Amoros, C., Bornette, G., 1996. Species traits and recolonization processes after flood disturbances in riverine macrophytes Vegetatio 122, 13–27. Holmes, N.T.H., Whitton, B.A., 1977. Macrophytic vegetation of the River Swale, Yorkshire Freshwat. Biol. 7, 545–558. Holmes, N.T.H., Boon, P.J., Rowell, T.A., 1998. A revised classification system for British rivers based on their aquatic plant communities Aquat. Conser. Marine Freshwat. Ecosys. 8, 555–578. Hutchinson, G.E., 1957. A Treatise On Limnology, vol. 1. Wiley and Sons, New York. Hynes, H.B.N., 1970. The Ecology of Running Waters. Liverpool University Press, Liverpool. Jackson, S.T., Charles, D.F., 1988. Aquatic macrophytes in Adirondack (New York) lakes: Patterns of species composition in relation to environment Can. J. Bot. 66, 1449–1460. Jacobsen, D., 1993. Herbivory of invertebrates on submerged macrophytes in freshwater, Ph.D. Thesis. Freshwater Biological Laboratory, University of Copenhagen, Denmark. Johansson, M.E., Nielsson, C., 1993. Hydrochory, population dynamics and distribution of the clonal aquatic plant Ranunculus lingua J. Ecol. 81, 81–91. Kent, M., Coker, P., 1992. Vegetation Description and Analysis. A Practical Approach. Wiley, UK, p. 354. Kern-Hansen, U., Dawson, F.H., 1978. The standing crop of aquatic plants of lowland streams in Denmark and their inter-relationship of nutrients in plant, sediment and water. In: Proceedings of European Weed Research Society Fifth Symposium on Aquatic Weeds, Amsterdam, The Netherlands, pp. 143–150. Kirk, J.T.O., 1983. Light and Photosynthesis In Aquatic Ecosystems. Cambridge University Press, Cambridge. Madsen, T., Sand-Jensen, K., 1991. Photosynthetic carbon assimilation in aquatic macrophytes Aquat. Bot. 41, 5–40. McCune, B., Mefford, M.J., 1995. PC-Ord. Multivariate analysis of ecological data, version 2.0. MjM Software Design, Gleneden Beach, Oregon, USA.
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T. Riis et al. / Aquatic Botany 66 (2000) 255–272
Moeslund, B., Løjtnant, B., Mathiessen, H., Mathiessen, L., Pedersen, A., Thyssen, N., Schou, J. C., 1990. Danske Vandplanter (in Danish). Environmental News No. 2. Danish Environmental Protection Agency, Copenhagen. Nichols, S.A., Shaw, B.H., 1986. Ecological life histories of the three aquatic nuisance plants, Potamogeton crispus and Elodea canadensis Hydrobiologia 131, 3–21. Nielsen, L.W., Nielsen, K., Sand-Jensen, K., 1985. High rates of production and mortality of submerged Sparganium emersum Rehman during its short growth season in an eutrophic Danish stream Aquat. Bot. 22, 325–334. Nørrevang, A., Meyer, T.J., 1969. Danmarks Natur (in Danish). Politikens Forlag, Copenhagen. Pedersen, A., 1976. Najadernes, Zannichelliaceernes og Zosteraceernes udbredelse i Danmark (in Danish) Bot. Tidsskr. 70, 203–262. Rebsdorf, A., Thyssen, N., Erlandsen, M., 1991. Regional and temporal variation in pH, alkalinity and carbon dioxide in Danish streams in relation to land use Freshwat. Biol. 25, 419–436. Robach, F., Thiébaut, G., Trémoliéres, M., Muller, S., 1996. A reference system for continental running waters: Plant communities as bioindicators of increasing eutrophication in alkaline and acidic waters in north-east France Hydrobiologia 340, 67–76. Sabbatini, M.R., Murphy, K.J., Irigoyen, J.H., 1998. Vegetation-environment relationships in irrigation channel systems of southern Argentina Aquat. Bot. 60, 119–133. Sand-Jensen, K., 1983. Photosynthetic carbon sources of stream macrophytes J. Exp. Bot. 34, 198–210. Sand-Jensen, K., Frost-Christensen, H., 1998. Photosynthesis of amphibious and obligately submerged plants in CO2 -rich lowland streams Oecologia 117, 31–39. Sand-Jensen, K., Jeppesen, E., Nielsen, K., van der Bijl, L., Hjermind, L., Nielsen, L.W., Iversen, T.M., 1989. Growth of macrophytes and ecosystem consequences in a lowland Danish stream Freshwat. Biol. 22, 15–32. Sand-Jensen, K., Lindegaard, C., 1996. Økologi i søer og vandløb (in Danish). G.E.C. Gads Forlag, Copenhagen, 188 pp. Spence, D.H.N., 1967. Factors controlling the distribution of freshwater macrophytes with particular reference to the lochs of Scotland J. Ecol. 55, 147–170. Spence, D.H.N., Maberly, S., 1985. Occurrence and ecological importance of HCO3 - use among aquatic higher plants. In: Lucas, W.J., Berry, J.A., (Eds.), Inorganic Carbon Uptake By Aquatic Photosynthetic Organism. American Society of Plant Physiologists, University of California, Davis, pp. 125–143. ter Braak, C.J.F., 1992. CANOCO - a FORTRAN program for Canonical Community Ordination. Microcomputer Power, Itacha, New York, USA. Vestergaard, O., Sand-Jensen, K., 1999. Alkalinity and trophic state regulate aquatic plant distribution in lakes. Aquat. Bot. (submitted). Westlake, D.F., 1975. Macrophytes. In: Whitton, B.A. (Ed.), River Ecology. Blackwell, Oxford. Wiegleb, G., 1984. A study of habitat conditions of the macrophytic vegetation in selected river systems in Western Lower Saxony (Federal Republic of Germany) Aquat. Bot. 18, 313–352. White, K.E., 1978. Dilution methods, in: R.W. Herschy (Ed.), Hydrometry. Wiley, New York. Økland, R.H., 1990. Vegetation ecology: Theory, methods and applications with reference to Fennoscandia. Sommerfeltia (Suppl. 1). University of Oslo, Oslo, pp. 1–233.
Plant communities in lowland Danish streams: species composition and environmental factors Tenna Riis ∗ , Kaj Sand-Jensen, Ole Vestergaard Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød, Denmark Received 5 February 1999; received in revised form 16 July 1999; accepted 23 August 1999
Abstract We studied the quantitative composition of the vegetation at 208 stream sites distributed throughout Denmark with the purpose of identifying distinct plant communities, using cluster analysis, and their relationship to environmental conditions using indirect gradient analysis. Six plant communities were defined in the analysis, although the differences in species composition among them were small. The plant communities were mainly related to differences in water alkalinity with a Potamogeton-community being associated with high alkalinities, Sparganium-, Callitriche- and Batrachium-communities with medium alkalinities and a Myriophyllum alterniflorum-community associated with low alkalinity. Also stream size turned out to be an important variable to the separation of plant communities, with the Callitriche-community mainly found in small streams and the Potamogeton-community and a community of amphibious helophytes mainly found in larger streams. The most common species were Sparganium emersum and Elodea canadensis forming the widespread Sparganium-community at half of the stream sites throughout Denmark, a pattern most likely stimulated by the common practice of weed cutting. Though communities of stream plants could be defined, the overlap of species among them was very substantial. Communities are less distinct in Danish streams compared to streams in other regions including wider environmental gradients of temperature, current velocity, substratum, alkalinity and nutrient richness. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Stream plant communities; Alkalinity; Stream size; Weed cutting; Denmark
1. Introduction Distribution of plant species in an ecosystem can be viewed as a continuum, because plant species respond individually to variation in environmental factors and these vary con∗ Corresponding author. Present adress: National Environmental Research Institute, Vejlsøvej 25, P.O. Box 314, DK-8600 Silkeborg, Denmark. Tel.: +45-8920-1499; fax: +45-8920-1414. E-mail address: [email protected] (T. Riis)
0304-3770/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 9 ) 0 0 0 7 9 - 0
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tinuously in time and space (Gleason, 1926). Nonetheless, an operational way to describe the vegetation is to define plant communities as the collection of plant species growing together in a particular location and showing a definite association with each other (Kent and Coker, 1992). The reason why a certain plant community is present in a given environment is usually explained by the fact that the plant species in the community have similar environmental requirements (Kent and Coker, 1992). The focus of this study was a nationwide quantitative investigation of plant distribution in Danish streams. In streams different plant communities may arise as a result of differences in environmental factors such as stream size, water chemistry, flow velocity and substratum composition (e.g. Robach et al., 1996; Holmes et al., 1998). The assemblages of plants also are influenced by dispersal of the species by shoot fragments and turions drifting with the water, or by lateral ingrowth from the permanent amphibious populations on the banks (Johansson and Nielsson, 1993; Henry and Amoros, 1996). Therefore, the dispersal regime and environmental conditions in the riparian zone (e.g. bank steepness, soil type and land use) might influence species composition in the streams. The purpose of this work was to determine the composition of plant communities in Danish streams in relation to the environmental conditions. To ensure a proper evaluation of the differences in plant assemblages and their relationship to environmental variables, we included a large number of separate stream systems (29) and individual stream reaches (208) in the analysis and studied the vegetation and the environmental variables quantitatively. Numerical classification methods were used to group stream reaches into classes on the basis of their floristic composition, while indirect gradient analyses were used to evaluate the variation in plant composition in relation to environmental conditions. Our analysis was confined to Danish waters. However, as the species pool and physico-chemical conditions of Danish streams are very similar to those of other European lowland regions, we expect the patterns to reflect general mechanisms and regulations. The first specific objective was to determine if distinct plant communities in lowland streams exist. The second objective was to evaluate the relationships between the composition of stream plant communities and environmental conditions. 2. Methods Vegetation and environmental variables were studied in the summers of 1996 and 1997 at 208 unshaded stream sites each of 100 m length. Stream sites were located in 29 stream systems chosen so they were distributed throughout Denmark and representing different environmental and morphometric characteristics. The sites were selected on a map beforehand because they had to be close to a road for practical reasons. However, a site was excluded if it was shaded or if there was no vegetation at all. Mean width of the reaches varied from 0.6 m at upper reaches to 30 m at lower reaches. Danish stream systems are generally small, and 82% of the sites were between 0.6 and 10.0 m wide. 2.1. Sampling of vegetation data On each 100 m long stream site 10 transects were placed equally (one for each 10 m) across the stream. Along each transect, quadrats (25 × 25 cm) were placed side by side from
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one bank to the other. In each quadrat, the presence of vascular plant species and bryophytes was recorded. Observations were made by means of a viewing tube. Plants were identified to species when possible (Hansen, 1981; Moeslund et al., 1990). Species were not identified within the genera Callitriche and Epilobium, because identification is only certain when the individuals have flowers or seeds. Flowering is also required to identify some of the Batrachium species, but flowers were common. When flowers were absent, Batrachium aquatile, B. baudotii and B. peltatum could not be distinguished, and they were registered as Batrachium sp. Relative frequencies of species, or species groups, on each stream site were calculated from the number of quadrats with the species present and total number of quadrats with plants. A minimum of 100 quadrats with plants per site were analysed to ensure accurate calculations of frequencies. If 10 transects did not provide 100 quadrats with plants, additional transects were included. 2.2. Environmental data and their selection Environmental variables for each stream site were: mean depth and width, frequency of different bottom substrata, homogeneity of the spatial distribution of substrata, water velocity, light attenuation, alkalinity, bank steepness, soil type, land use and geographical region. Mean depth and width were chosen to characterise the size of the streams. Depth was measured in each quadrat included in the vegetation analysis, and mean depth of a stream site was calculated from all the quadrats examined. Mean stream width on a site was calculated as the mean of the widths of the 10, or more, transects. Proportions of different substrata were quantified because of their proposed importance for the development of plant species (Butcher, 1933, Carpenter and Lodge, 1986). The substrata were divided into stone (>30 mm diameter), gravel (3–30 mm diameter), sand (0.1–3 mm diameter), mud (<0.1 mm, black), hard clay and peat. Coverage of each substratum type on a reach was calculated as the relative frequency of all the quadrats examined. Homogeneity of the spatial distribution of substrata was quantified, because high homogeneity of an area is likely to restrict the number of niches and species richness (Harper, 1961). Substratum homogeneity was quantified from the spatial distribution of substratum types on the stream sites, as: P10 Pni 1, if x = y i=1 j =2 I (xj , xj −1 ) ; (x, y) = SH = P10 Pni 0, otherwise 1 i=1
j =2
where SH is substratum homogeneity, i are transects and j are quadrats. The value expresses how often two neighbouring quadrats differ from each other with respect to the substratum. The value can take any number between 0 and 1. The closer the value is to 1, the more often are two neighbouring quadrats alike, and the higher is substratum homogeneity. High water velocity and poor light penetration can restrict the growth of some submerged stream plants (Westlake, 1975). Therefore, these parameters were measured at each stream site. Mean water velocity on the stream site was measured by an integration method of dilution gauging (White, 1978). Mean water velocity was calculated from the time elapsed for a 5 l salt water solution (12% w/w) to pass through the 100 m reach. The salt water solution was added upstream of the reach at time 0, while conductivity was measured continuously
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at the 100 m point downstream of the reach. As the solution passed the downstream point, conductivity first increased and later decreased until the initial level was reestablished. By means of integration of conductivity over time, it was possible to determine the time at which half of the salt water solution had passed the downstream point. The mean water velocity on the stream site was assumed to represent the mean water velocity in the growing season. The annual variation in water velocity is not considered. Light attenuation of photosynthetically available radiation (PAR, 400–700 nm) on each stream site was determined by measurements of the photon flux at 10 cm depth intervals with a flat cosine-corrected sensor (Li-Cor 190). The vertical, diffusive attenuation coefficient (Kd ) was found by linear regression of the exponential decrease of photon flux density on logarithmic scale plotted against depth (Kirk, 1983). Alkalinity is often considered to be the water chemical variable which is most closely related to the distribution of submerged plants in lakes (Jackson and Charles, 1988; Vestergaard and Sand-Jensen, 1999), and may also be of importance for the distribution of stream plants (e.g. Holmes and Whitton, 1977; Haslam, 1978). Alkalinity has a close relationship to pH and conductivity, and therefore, these variables are not included. Alkalinity was determined by potentiometric end-point titration with HCL on 100.0 ml samples of stream water (Hutchinson, 1957). The following four variables: bank steepness, soil type, land use and geographical region are qualitative variables, which were described by different categories in the field. Each category was given a number, which made it possible to evaluate these variables together with the above-listed quantitative variables. The vertical profile of the stream bank was determined, because the steepness may influence the ability of amphibious species on the bank to colonise into the stream. Bank steepness in a distance of 0–0.5 m from the stream was measured and given a value from 1 to 3, with 3 representing a steep bank and 1 representing a less steep bank. If the two stream banks along the reach differed, the value selected was from the side of the stream holding the highest species richness. Several researchers have suggested that catchment soil type in the stream and the adjacent land can be a suitable predictor of the distribution of stream plants (e.g. Grasmück et al., 1995; Holmes et al., 1998). Soil type in the vicinity of stream reaches was found on soil maps. Four different categories were used: coarse sandy soil (category 1), fine sandy soil (category 2), clay-mixed-sand soil (category 3), and sand-mixed-clay soil (category 4). Land use in the area surrounding the stream reach was included in the analysis because many species, especially in small streams, derive from the banks, which in turn are colonised from the surrounding areas (Henry and Amoros, 1996; Henry et al., 1996). Modes of land use were divided into 10 groups representing increasing diversity of terrestrial plant species (Table 4). The relative differences in diversity was estimated from Nørrevang and Meyer (1969). The lowest plant diversity was observed in arable land and reed swamps, and the highest diversity was found in low and high meadow vegetations. We distinguished between five geographical regions in Denmark (Fig. 1) to reflect differences in alkalinity due to changes in clay and calcium carbonate content of the subsoils. The regions may also differ in discharge pattern and in summer temperature of the streams, and the geographical location may influence the immigration history of the species. Region
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Fig. 1. Map of Denmark showing the five geographical regions and the boundary of the ice cap during the last Ice Age ending about 11,000 years ago.
1 has sandy fluvial deposits from the last Ice Age (ending about 11,000 years ago) and predominantly old moraine deposits from the previous Ice Age (ending about 130,000 years ago). Region 2 has clay moraine from the last Ice Age and raised sea beds about 7000–8000 years old. Region 3 has clay moraine from the last Ice Age. Region 4 (island of Funen), and region 5 (island of Zealand) have mainly clay moraine soils. Nitrogen and phosphorus were not included in the analysis because they typically display high concentrations in lowland Danish streams (>3 mg total-N l−1 and >0.1 mg total-P l−1 ; Sand-Jensen and Lindegaard, 1996). Previous studies in Danish streams showed no correlation between species distribution of plants, tissue content of nitrogen and phosphorus in the plants and nutrient concentration in the stream water (Kern-Hansen and Dawson, 1978). Moreover, tissue content of nitrogen and phosphorus are generally above those levels likely to constrain plant growth rate (Jacobsen, 1993). 2.3. Data analysis First, a hierarchical agglomerative cluster analysis based on Bray–Curtis similarity measures was used to describe the percentage similarity in species distribution between localities
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(Clarke and Warwick, 1994). Relative frequencies of each species on each locality were not transformed or normalised. Rare species were downweighted by omitting those occurring at less than five localities. This operation was done because rare species are unlikely to be adequately described and may act as outliers in a multivariate treatment of the data (Økland, 1990). Moreover, species of Lemna were omitted because of their free-floating growth form. A final 75 taxa remained for further analysis. Groups of localities with a more or less similar floristic composition were defined based on the percentage similarity in species distribution between localities. These species present in the different groups of localities were defined as different plant communities. Second, environmental gradients related to the sample groups defined by the cluster analysis were looked for. Detrended correspondence analysis (DCA) was carried out by CANOCO (ter Braak, 1992) followed by a Spearman rank correlation analysis between environmental parameters and sample scores in the DCA analysis. Significant differences between the groups in the DCA ordination diagram were tested by multi-response permutation procedures in PC-Ord statistical package (McCune and Mefford, 1995). The quantitative environmental parameters were not normally distributed, and differences between the medians of the groups of sample sites were tested by Kruskal–Wallis test. For each group of sample sites differences in number of observations of the categories of qualitative variables (bank steepness, soil type, land use and region) were tested by χ 2 -tests. Differences between frequencies of the qualitative environmental variables and between groups of localities were also tested by χ 2 -tests. Correlations among the environmental variables were tested with a Spearman rank correlation test. 3. Results 3.1. Numerical classification The cluster analysis separated six groups of sample sites including 205 of the 208 sites. Similarities in species composition within the groups were small as the mean Bray–Curtis similarity in the groups was 25% (±5% SE; range 16–51%). However, the groups were significantly different from each other (MRPP-test, p < 0.01). Vegetation distribution and abundance in each group of sample sites were analysed. The 30 most abundant taxa in each group of sample sites were used to describe different communities (Table 1 ). Only 23 taxa were available in Group 3. The overall list of abundant taxa contained 56 taxa, implying a high overlap of species among communities. Six taxa were present in all six communities (Callitriche sp., Epilobium sp., Myosotis palustris/Myosotis laxa ssp. caespitosa, Phalaris arundinacea, Sparganium emersum and Sparganium erectum), and five taxa were present in five communities (Berula erecta, Elodea canadensis, Glyceria maxima, Potamogeton crispus and Ranunculus repens). Each community was characterised by dominant and exclusive species. A Sparganiumcommunity (Group 1) was dominated by S. emersum and E. canadensis, and only Sagittaria sagittifolia was restricted to this community. A Callitriche-community (Group 2) was dominated by species of Callitriche and B. peltatum, and three taxa were exclusively present in this community (Equisetum palustre, Montia fontana and Polygonum amphibium). A
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Table 1 The relative frequency of plant species in the six communities based on the presence in 208 localities. The values of the dominating. Species in the groups are in bold. Group 1: S. emersum-community. Group 2: Callitriche-community. Group 3: M. alterniflorum-community. Group 4: Potamogeton-community. Group 5: G. maxima-community. Group 6: B. aquatile-community, n = number of stream sites Species
Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 (n = 114) (n = 37) (n = 4) (n = 31) (n = 13) (n = 6)
Saggittaria sagittifolia L. 0.28 Agrostis stolonifera L. 0.46 Batrachium circinatum (Sibth.) Spach 0.65 Potamogeton praelongus Wulf. 0.60 Potamogeton pectinatus L. 1.29 Potamogeton natans L. 2.02 Butomus umbellatus L. 2.21 Nuphar lutea (L.) Sm. 0.59 Phragmites australis Trin. 0.48 Batrachium baudotii (Godron) F. Schultz 1.96 Glyceria fluitans (L.) R. Br. 1.95 Batrachium peltatum (Schrank) Presl. 1.02 Batrachium sp. 2.23 Potamogeton perfoliatus L. 0.55 Solanum dulcamara L. 0.63 Veronica anagallis-aquatica L. 0.95 Equisetum fluviatile L. 0.30 Scirpus lacustris L. 0.47 Alopecurus geniculatus L. 0.40 Elodea canadensis L. C. Rich 17.56 Berula erecta Hudson (Coville) 6.50 Ranunculus repens L. 0.28 Glyceria maxima (Hartman) Holmberg 5.96 Potamogeton crispus L. 1.16 Callitriche sp. 7.38 Epilobium sp. 0.88 Myosotis palustris L. / laxa Lehm ssp. caespitosa (C.F. Schultz) Hyl. 0.82 Phalaris arundinacea L. 4.81 Sparganium emersum Rehman 29.23 Sparganium erectum L. 2.48 Equisetum palustre L. Montia fontana Cham. ssp. Fontana Polygonum amphibium L. Agrostis sp. Myriophyllum alterniflorum D. C. Stellaria alsine Grimm Zannichellia repens Boenn. Cardamine amara L. Nasturtinum sp. Carex rostrata Stokes Mentha aquatica L. Poa sp. Deschampsia caespitosa (L.) Beauv. Potentilla palustre (L.) Scop.
0.63
0.36 17.75
2.39 19.89 4.75 1.54 0.51 1.13
15.74 6.18
0.33 1.86 16.39 0.79 4.02 1.13 0.36 0.46 0.28 1.28 20.84
2.14 0.42
2.21 0.84 2.95 2.34 1.38 1.46 7.13 2.36 1.51 4.17 0.55 2.32
4.62 2.17 2.08 0.32 0.26 0.39
3.57 4.56
0.43 8.00 2.22 0.64 3.50 18.32 2.30 0.28
25.78 0.50 1.10 1.14
4.86 0.27 8.48 0.27 6.30 0.86
0.15 0.08 7.34 0.55
0.39 3.14 4.91 2.69
0.72 5.39 1.36 10.48
1.99 1.04 5.25 5.39
3.46 3.55 0.54 5.29 2.17 20.79 0.98
6.30 0.30 0.26
4.07 2.55 5.48 2.80 0.37 1.15 2.29 0.19 0.39 0.53 0.71 1.76 2.58 0.66 1.04 1.11
0.45 22.05 0.76 0.24 0.13 0.07 4.24 4.06 0.64 0.09 1.92
1.21 1.05 1.57
1.29
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Table 1 (Continued). Species Carex sp. Juncus effusus L. Potamogeton lucens L. Iris pseudacorus L. Batrachium aquatile (L.) Wimmer Alisma plantago-aquatica L. Myriophyllum spicatum L. Typha latifolia L. Carex acutiformis Ehrh. Veronica beccabunga L. Catabrosa aquatica (L.) Beauv. Fontinalis antipyretica Hedw.
Group 1 (n = 114)
Group 2 (n = 37)
Group 3 (n = 4) 0.45 0.90
Group 4 (n = 31) 0.25 0.45 0.65 0.39
Group 5 (n = 13)
Group 6 (n = 6)
1.10 1.57 2.92 50.44 0.55 3.71 9.38 0.75 0.69
0.63 0.07 0.21 0.27
Myriophyllum alterniflorum-community (Group 3) was dominated by M. alterniflorum, Potamogeton natans and G. fluitans and two taxa (Deschampsia caespitosa and Potentilla palustre) were restricted to this community. A Potamogeton-community (Group 4) was dominated by three species of Potamogeton (P. perfoliatus, P. crispus and P. pectinatus) and Potamogeton lucens was restricted to this community. A G. maxima-community
Fig. 2. Ordination diagram by DCA showing the scores of 205 localities based on the relative frequencies of 75 plant species. The groups of stream sites representing the plant communities defined by Cluster analysis are marked by different symbols and framed. Eigenvalue of Axis 1 was 0.53 and of Axis 2 was 0.48, and the gradient lengths were 4.54 and 3.65 SD units for Axis 1 and 2, respectively.
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(Group 5) was dominated by G. maxima and had three exclusive species (Alisma plantagoaquatica, Myriophyllum spicatum, and Typha latifolia). A B. aquatile-community (Group 6) was dominated by B. aquatile, while Catabrosa aquatica and Fontinalis antipyretica were restricted to this community. The DCA-analysis of 205 samples with 75 taxa revealed that the groups of sample sites separated along two axes (Fig. 2). Along Axis 1, Group 2 and 3 were located to the left in the diagram, Group 1, 5 and 6 in the center and Group 4 farthest to the right. This pattern indicates the existence of an underlying environmental gradient between the plant communities at different stream sites. The eigenvalue of Axis 1 was 0.53, and the gradient length was 4.54 standard deviation (SD) units, which indicate a full turn over of species on the localities farthest away from each other along Axis 1 (Økland, 1990). Spearman rank correlation analysis between sample scores from the DCA analysis and the environmental variables revealed that alkalinity had the highest correlation with the sample sites. Besides alkalinity, also stream width, stream depth, light attenuation coefficient, proportions of mud in the substratum, soil type and region correlated positively to Axis 1, while the proportions of peat and sand, and the land use correlated negetatively to this axis (Spearman rank correlation, p < 0.05; Table 2). Thus, the analysis shows that alkalinity is higher in sample sites from Group 4 than Group 1, 5 and 6, which in turn have a higher alkalinity than sites in Group 2 and 3. Along Axis 2, Group 3 was located at the bottom of the ordination diagram, Group 1, 2 and 4 in the middle, and Group 5 and 6 at the top. The Axis-2 eigenvalue was 0.48 and the gradient length was 3.65 SD units. The proportion of stone and gravel and geographical region were positively correlated to Axis 2, while substratum homogeneity and bank steepness were correlated negetatively to this axis (Table 2).
Table 2 Spearman rank correlations coefficients between the sample scores on the DCA axis 1, 2 and 3 and environmental variables (p < 0.05) Variable Alkalinity Attenuation coefficient Water velocity Depth Width Stone Gravel Sand Mud Clay Peat Substratum homogeneity Bank steepness Soil type Land use Region
Axis 1
Axis 2
0.49 0.38
0.35 −0.26 −0.14 −0.15
0.35 0.42
−0.35 0.34 0.20 −0.36 0.25 0.40 −0.24 0.44
Axis 3
0.23 0.33 −0.14 −0.20 −0.17 −0.32 −0.21
0.34
0.20
0.19 0.21
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3.2. Environmental properties The indirect gradient analysis suggested the existence of differences in environmental variables among groups of stream sites containing the different communities. To test this pattern, medians of the quantitative variables were compared between groups of stream sites (Kruskal–Wallis test; Table 3). Differences in the number of observations of the categories of Table 3 Environmental parameters of the six groups of stream sites representing the six plant communities. The total number of sample sites is 205a Group 1 (n = 114) Alkalinity (meq l−1 ) Median 2.36 c Range 0.59–5.78 Light att. coef. (m−1 ) Median 1.00 ab Range 0.20–2.80 Water velocity (m s−1 ) Median 0.24 Range 0.01–0.72 Mean depth (cm) Median 57 b Range 15–157 Mean width (cm) Median 583 b Range 124–2500 Stone (%) Median 0.5 Range 0–35.9 Gravel (%) Median 4.6 b Range 0–50.4 Sand (%) Median 52.5 b Range 0–90.0 Mud (%) Median 24.6 b Range 0–100 Clay (%) Median 2.3 b Range 0–84.5 Peat (%) Median 0 Range 0–63.6 Substratum homogeneity Median 0.81 b Range 0.42–1.00
Group 2 (n = 37)
Group 3 (n = 4)
Group 4 (n = 31)
Group 5 (n = 13)
Group 6 (n = 6)
1.16 b 0.14–4.90
0.64 a 0.26–0.98
3.50 c 1.16–5.08
1.36 a 1.06–5.26
2.44 c 1.35–2.59
0.67 a 0.30–3.86
0.63 a 0.52–0.84
1.29 b 0.67–7.16
0.89 ab 0.50–2.03
1.16 ab 0.90–2.21
0.23 0.05–0.53
0.27 0.20–0.36
0.20 0.04–0.96
0.23 0.07–1.15
0.27 0.14–0.44
32 a 8–78
59 b 30–65
58 b 16–140
47 ab 13–136
40 ab 34–68
373 a 86–1666
276 a 61–443
702 ab 241–3000
633 ab 152–2800
511 ab 201–680
1.6 0–82.8
0.0 0–5.3
3.1 0–50.0
8.1 0–48.3
6.7 0.4–17.2
9.7 b 0–64.8
0.0 a 0–1.3
19.4 b 0–78.1
5.2 ab 0.8–49.4
16.5 b 3.5–51.4
64.5 b 15.5–96.6
84.1 b 41.1–93.6
33.1 a 3.3–84.0
41.0 ab 17.0–89.2
30.5 ab 10.2–49.4
4.2a 0–83.9
4.7 ab 0–46.7
2.5 b 0–83.0
16.9 b 2.5–60.3
47.4 b 2.9–64.4
0a 0–13.0
1.4 ab 0–5.6
0.9 ab 0–21.3
0a 0–2.4
2.1 b 0–13.6
0 0–26.5
6.5 4.7–8.3
0 0–17.6
0 0–2.3
0 0–0
0.70 a 0.30–0.91
0.77 ab 0.46–0.84
0.77 ab 0.55–0.95
0.71 a 0.61–0.86
0.75 ab 0.53–0.82
a Group 1: S. emersum-community. Group 2: Callitriche-community. Group 3: M. alterniflorum-community. Group 4: Potamogeton-community. Group 5: G. maxima-community. Group 6: B. aquatile-community. n = number of stream sites. Values with different letters are significantly different (p < 0.05; Kruskal–Wallis test). There was no significant differences in water velocity, proportion of stone and peat among the groups.
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Table 4 Distribution of sites among semi-quantitative and qualitative categorial types (four independent classifications) for six community typesa
Bank steepness 1/high Bank steepness 2/intermediate Bank steepness 3/low Soil type 1/coarse sandy soil Soil type 2/fine sandy soil Soil type 3/clay-mixed-sand soil Soil type 4/sand-mixed-clay soil Land use 1/cultivated Land use 2/reed swamp Land use 3/ruderal Land use 4/ fertilized grazed meadow Land use 5/forest Land use 6/scrub Land use 7/heath Land use 8/tall herb fen Land use 9/ unfertilized grazed meadow Land use 10/bog Region 1/West Jutland Region 2/North Jutland Region 3/East Jutland Region 4/Island of Funen Region 5/Island of Zealand
Group 1 (n = 114)
Group 2 (n = 37)
Group 3 (n = 4)
Group 4 (n = 31)
Group 5 (n = 13)
Group 6 (n = 6)
19/27 70/61 25/26 58/65 19/12 18/17 18/19 7/13 0/2 2/4
10/9 19/20 8/8 29/21b 1/4 3/5 4/6 7/4 0/1 3/1
2/1 2/2 0/1 4/2 0/0 0/1 0/1 0/0 0/0 0/0
8/7 12/17 11/7 11/18 2/3 8/5 10/5b 8/3 2/0b 0/1
7/3b 4/7 2/3 10/7 0/1 1/2 2/2 0/1 1/0b 2/0
2/1 3/3 1/1 5/3 0/1 0/1 1/1 1/1 0/0 0/0
45/36b 4/4 3/4 0/0 38/32
7/12 1/1 1/1 0/0 7/10
1/1 0/0 0/0 0/0 0/1
7/10 2/1 1/1 0/0 7/9
4/4 0/0 2/1 0/0 2/4
0/2 0/0 1/0 0/0 3/2
14/14 1/6 18/18 34/21b 43/53 14/12 5/10
5/5 6/2b 12/6 1/7 21/17 3/4 0/3
1/1 2/0 3/0 0/1 1/2 0/0 0/0
4/4 0/2 0/5 2/6 13/15 3/18 13/3b
1/2 1/0 0/2 0/2 12/6b 1/1 0/1
1/1 0/0 0/1 0/1 6/3 0/1 0/1
a Observed/expected values shown, where expected values were determined as the total number of sites in each community type multiplied by the proportion of all sites in the appropriate category. There was a significant difference in number of observations of the variables among groups of stream sites in all four variables (read table horizontally; χ 2 -test, p < 0.05). The number of stream sites in group 3 and 6 were too low to test differences of number of observations of the categories among stream sites. Group 1: S. emersum-community. Group 2: Callitriche-community. Group 3: M. alterniflorum-community. Group 4: Potamogeton-community. Group 5: G. maxima-community. Group 6: B. aquatile-community, n = number of stream sites. b The number of observations of the occurrence of the different categories of the qualitative variables in each group was tested, and significant difference between categories in the category with highest score (read table vertically; χ 2 -test, p < 0.05).
the qualitative variables between groups of stream sites were tested and showed a significant difference among the groups in all four variables (χ 2 -test, p < 0.05; read Table 4 horizontally). In each group of stream sites, differences between observed and predicted number of observations in the categories of the qualitative data were tested (χ 2 -test, p < 0.05; read Table 4 vertically). If the test showed significant differences the category with the largest difference between observed and expected was marked. Some general characteristics of environmental properties appeared in the six groups of stream sites representing the different plant communities. Stream sites with the Sparganium emersum-community had high light attenuation in the water, a relatively low proportion of gravel, high proportion of mud and clay and a high substratum homogeneity (Table 3). There
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was a higher number of observations in North Jutland and in areas with fertilized meadows than expected by chance (Table 4). The Callitriche-community was disproportionately well represented at shallow and narrow stream sites with a low alkalinity, a low light attenuation coefficient, a low proportion of mud and a low substratum homogeneity (Table 3). On coarse sandy soil and in bogs it was present in the catchment more times than expected (Table 4). Only four stream sites with the M. alterniflorum-community were present, and those had a relatively low alkalinity and a high proportion of sand (Table 3). Stream sites with the Potamogeton-community had a high alkalinity, a high light attenuation, a high proportion of gravel, but a relatively low proportion of sand (Table 3). They were located on sand-mixed-clay soils in Region 5, and the catchment included reed swamps more times than expected (Table 4). Stream sites with G. maxima-community were deep and wide, contained a relatively high proportion of mud and had a high substratum homogeneity (Table 3). Land use included reed swamp and sites were located in region 3 more times than expected (Table 4). Finally, stream sites with the B. aquatile-community exhibited high alkalinity, intermediate size, and a high proportion of gravel, mud and clay (Table 3). The sites were all located in Region 3 (Table 4). There was a significant intercorrelation between most of the environmental variables (Spearman rank correlation test, p < 0.05; Table 5). Taking alkalinity as an example, it correlates positively to light attenuation, proportion of mud and clay, substratum homogeneity, bank steepness, soil type and region, and correlates negatively to water velocity, proportion of sand and peat, and land use. As alkalinity correlates very well with the geographical location, and intercorrelates with most other variables as well, it is possible to describe some characteristics of the different regions. Region 1 differs most from Region 4 and 5. Alkalinity is lowest in Region 1 (0.75 ± 0.27 (SD) meq l−1 ), and light attenuation coefficient, proportion of mud and clay, substratum homogeneity, bank steepness and soil type are also relatively low in this region, while water velocity and proportion of sand and peat are relatively high. On the other hand, alkalinity is highest in Region 4 (4.92 ± 0.40 meq l−1 ) and 5 (3.55 ± 0.52 meq l−1 ), and light attenuation coefficient, proportion of mud and clay, substratum homogeneity, bank steepness and soil type are relatively high in these regions, while water velocity and proportion of sand and peat are relatively low. Region 2 and 3 are intermediate with respect to environmental variables. 4. Discussion 4.1. Plant communities and the importance of environmental variables Alkalinity appears to be the most important measured environmental variable distinguishing plant communities in Danish lowland streams. Previous studies have shown alkalinity to be an important factor in the distribution of aquatic plants in streams (Wiegleb, 1984, Robach et al., 1996; Sabbatini et al., 1998), but the influence of alkalinity and alkalinity related factors are even more prominent in lakes (e.g. Spence, 1967; Jackson and Charles, 1988). The correlation between alkalinity and plant distribution in stream communities, therefore, probably result from the interconnection of lakes and streams in water systems,
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and the dispersal of plants from lakes into the streams as well as a direct influence of alkalinity on plant performance and survival in the streams. In lakes some species benefit from their physiological ability to utilize bicarbonate in addition to carbon dioxide in photosynthesis (e.g. Potamogeton sp.), while other species only use carbon dioxide or have a low affinity for bicarbonate (e.g. M. alterniflorum; Spence et al., 1985; Madsen and Sand-Jensen, 1991). In most streams the concentration of carbon dioxide is high (Rebsdorf et al., 1991; Sand-Jensen and Frost-Christensen, 1998), and the ability to use bicarbonate is therefore less important for the photosynthetic performance of stream plants relative to lake plants. Nonetheless, high concentrations of bicarbonate in high-alkaline streams are still important for bicarbonate-users to maintain high photosynthesis throughout the day despite profound depletion of carbon dioxide at noon and in the afternoon in streams with dense macrophyte stands (Sand-Jensen, 1983; Sand-Jensen and Frost-Christensen, 1998). Although the DCA-analysis shows alkalinity to be the most important factor to the distribution of stream vegetation, it also shows light attenuation, width, depth, proportion of most substratum types, substratum homogeneity, bank steepness, soil type, land use and regions to have significant influence on the segregation of plant communities. Apart from water depth, width, and proportion of stone and gravel, all these variables are correlated with alkalinity and the importance of the different factors in separating the communities is, therefore, hard to evaluate. Those variables that are most clearly related to the segregation of communities (i.e. soil type and region) are also most closely correlated to alkalinity. However, because the depth and width of the streams have a significant correlation to the segregation of communities and do not correlate to alkalinity, it is possible to state that stream size is an important variable in addition to the alkalinity related variables. The Callitriche-community is mainly present in small streams and the Potamogeton-community is mainly present in larger streams, and this pattern might reflect a preference to higher temperatures of Potamogetons compared to species of Callitriche. This agrees with observations of frequent presence of Potamogeton species in the warmer, downstream reaches in streams and in outflow of lakes (Pedersen, 1976). The results show indications of weed cutting in the streams to be responsible for the presence of the S. emersum-community on more than half of the 208 observed stream sites, although the frequency and magnitude of weed cutting were not included as a variable in the study. The species S. emersum and E. canadensis together constitute almost 50% of the total frequencies of plants at the 114 sample sites housing the community. The two species are also the most common species in Danish streams constituting 19 and 11% of total plant cover in the 208 studied localities. Both S. emersum and E. canadensis are probably favoured by the extensive cutting of vegetation taking place in most of the larger Danish streams. The established life strategy of the species has been designated as competitive-ruderal (Grime et al., 1988) with fast growth, efficient dispersal and resistance to disturbance. Elodea canadensis has negatively buoyant shoot fragments which are released in great numbers during weed cutting and are able to settle on the stream bottom and regrow (Nichols and Shaw, 1986). Sparganium emersum has well-developed rhizomes and a basal meristem allowing regrowth after weed cutting of above-ground parts (Nielsen et al., 1985; Sand-Jensen et al., 1989). In contrast, competitor-species such as the broad-leaved species of Potamogeton (e.g. P. lucens and P. perfoliatus) are likely to be limited by disturbance caused by weed cutting, and these species only reach a small total plant cover on these stream sites.
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Stream sites with S. emersum-communities are present in all the five geographical regions along with a widespread practice of weed cutting. The low proportion of gravel, the high proportion of mud and the high substratum homogeneity on the stream sites holding the S. emersum-community, are accomplished likely by rough managing and dredging during the last decades, which have promoted a wide stream profile and reduced the current velocity. The two dominating species are known from other countries to grow preferentially at sites with low current velocity and muddy substrata (Hynes, 1970; French and Chambers, 1996). 4.2. Community segregation in lowland streams Differences in species composition between plant communities is relatively small in Danish streams compared to stream plant communities in other countries with stronger environmental gradients (e.g. Holmes et al., 1998) and to plant communities in Danish lakes (Vestergaard and Sand-Jensen, 1999). At least four conditions can be expected to reduce differences among stream plant communities in general and differences within Denmark in particular. First, segregation can result from different water chemistry in the stream systems, and variability among the species in their tolerance and optimum requirements to the chemical environment. Submerged plants in streams include obligatory aquatic species, amphibious species and predominantly terrestrial species (e.g. Holmes and Whitton, 1977; Henry and Amoros, 1996). Among these three groups, only the obligatory aquatic species are dependent on the characteristic chemistry of the stream water throughout their life span. Therefore, in the total vegetation only the obligatory aquatic species are expected to exhibit a clear segregation relative to water chemistry, while the other two life forms might have a less distinct distribution among sample sites. This hypothesis will be tested later. Second, the longitudinal connectance of the streams means that water and plant species are dispersed from upstream to downstream reaches simultaneously reducing the differences in water chemistry and plant composition within specific stream systems (Johansson and Nielsson, 1993). Third, due to the constant input of new drainage water from land along the stream course, concentrations of dissolved inorganic nutrients are usually high in streams reducing the likelihood of nutrient limitation and the tendency of forming specific oligotrophic or eutrophic plant communities. Fourth, the lack of complete vegetation cover in streams reduces the probability that adaptation of certain plant species to the particular water chemistry will result in competitive exclusion of species finding the water chemistry suboptimal. The small environmental gradients in Denmark is another possible reason why plant communities in Danish streams do not segregate more markedly. In countries and geographical regions with large environmental gradients such as between lowland and mountain ranges, marked differences in stream plant communities can be observed because of high variability of stream slope, bottom substrata, temperature, alkalinity and nutrient richness. An extensive study of the vegetation in more than 1500 stream sites in Great Britain resulted in the classification of plants into four floral communities ranging from highland, torrential, oligotrophic streams to lowland, eutrophic streams (Holmes et al., 1998). The communities segregated much more strongly, likely because of the wider range of geological, altitudinal and nutritional conditions in Great Britain compared to Denmark. The Danish communities
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all fit into the lowland eutrophic stream community. While the two most distinct Danish communities had 17 species in common, no species overlapped in the four British stream communities (Holmes et al., 1998). The segregation in terms of Bray–Curtis similarity in the plant communities is also much more distinct in Danish lakes than streams (t-test, p < 0.05). A study of the vegetation in 82 Danish lakes classified the lakes in five groups with alkalinity being the most important factor responsible for the segregation (Vestergaard and Sand-Jensen, 1999). The lake communities showed an average Bray–Curtis similarity of 51% (±1 S.E.) while this study showed average Bray–Curtis similarity among stream communities of only 25% (±5 S.E.). Lake communities are probably more distinct than stream communities because of the more stable growth environment contrary to streams with frequent flow disturbance, movement of the stream bed and formation of open space available for plant colonisation. In lakes competition for resources increases the adaptation and segregation of species to adjust to the given physical and chemical conditions. Moreover, the greater physical isolation of many lakes compared to the longitudinal connectance in stream systems likely increases the differences of plant communities among lakes relative to streams.
5. Conclusions Six distinct plant communities were distinguished in Danish streams although the differences in the species composition among the communities were small. Indirect gradient analyses suggested alkalinity to be the most important factor responsible for separating the six communities. However, the effect of alkalinity was correlated to several other variables and thus it was hard to separate the effect of alkalinity from the effect of the correlated variables. Among communities the Myriophyllum alterniflorum-community is associated with low alkalinities, Sparganium-, Callitriche- and Batrachium-communities are associated with medium alkalinities and the Potamogeton-community is associated with high alkalinity. The indirect gradient analyses also suggest stream size to be important in segregating the communities. Thus, the Potamogeton-community and a Glyceria maxima-community containing several reed swamp species are present in relatively large streams, and the Callitriche-community is present in small streams. The Batrachium aquatile-community was associated with stone and gravel substratum. Frequent vegetation cutting in the streams is probably important for the segregation of the most common stream community dominated by E. canadensis and S. emersum but this aspect was not tested. A direct influence of the environmental conditions in the riparian zone on plant distribution in the streams was not apparent, because of correlations between alkalinity and the variables characterising the riparian zone. Plant communities are less distinct in Danish streams compared to stream communities in other regions with larger environmental gradients, as well as compared to plant communities in Danish lakes. These differences are probably due to: (1) the integrity of streams with respect to water chemistry and species dispersal, and (2) the disturbed environment of streams leading to open space and smaller differences in species composition as a function of variable water chemistry and hydrology among separate stream systems.
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Acknowledgements We thank the Danish Natural Science Research Council (grant 11-7795) and the Danish Research Academy for financial support. We thank Jens Borum for helpful comments on the manuscript and Anne Garde for linguistic improvements. We also thank Statician Søren E. Larsen for help. References Butcher, R.W., 1933. Studies on the ecology of rivers I. On the distribution of macrophytic vegetation in the rivers of Britain J. Ecol. 21, 58–91. Carpenter, S.R., Lodge, D.M., 1986. Effects of submersed macrophytes on ecosystem processes Aquat. Bot. 26, 341–370. Clarke, K.R., Warwick, R.M., 1994. Change in marine communities: An approach to statistical analysis and interpretation. Plymouth Marine Laboratory, Plymouth. French, T.D., Chambers, P.A., 1996. Habitat partitioning in riverine macrophyte communities Freshwat. Biol. 36, 509–520. Gleason, H.A., 1926. The individualistic concept of the plant association Bull. Torrey bot. Club 53, 1–20. Grasmück, N., Haury, J., Léglize, L., Muller, S., 1995. Assessment of the bio-indicator capacity of aquatic macrophytes using multivariate analysis Hydrobiologia 300/301, 115–122. Grime, J.P., Hodgson, J.G., Hunt, R., 1988. Comparative Plant Ecology: A Functional Approach to common British Species. Allen and Hyman, London. Hansen, K., 1981. Dansk feltflora (in Danish). Nordisk Forlag, Copenhagen. Harper, D., 1961. Approaches to the study of competition. In: Milthorpe, F.L. (Ed.), Mechanisms in Biological Competition, Symposium of the Society for Experimental Biology 15, pp. 1–39. Haslam, S., 1978. River Plants. Cambridge University Press, Cambridge. Henry, C.P., Amoros, C., 1996. Are the banks a source of recolonization after disturbance: An experiment on aquatic vegetation in a former channel of the Rhône River Hydrobiologia 330, 151–162. Henry, C.P., Amoros, C., Bornette, G., 1996. Species traits and recolonization processes after flood disturbances in riverine macrophytes Vegetatio 122, 13–27. Holmes, N.T.H., Whitton, B.A., 1977. Macrophytic vegetation of the River Swale, Yorkshire Freshwat. Biol. 7, 545–558. Holmes, N.T.H., Boon, P.J., Rowell, T.A., 1998. A revised classification system for British rivers based on their aquatic plant communities Aquat. Conser. Marine Freshwat. Ecosys. 8, 555–578. Hutchinson, G.E., 1957. A Treatise On Limnology, vol. 1. Wiley and Sons, New York. Hynes, H.B.N., 1970. The Ecology of Running Waters. Liverpool University Press, Liverpool. Jackson, S.T., Charles, D.F., 1988. Aquatic macrophytes in Adirondack (New York) lakes: Patterns of species composition in relation to environment Can. J. Bot. 66, 1449–1460. Jacobsen, D., 1993. Herbivory of invertebrates on submerged macrophytes in freshwater, Ph.D. Thesis. Freshwater Biological Laboratory, University of Copenhagen, Denmark. Johansson, M.E., Nielsson, C., 1993. Hydrochory, population dynamics and distribution of the clonal aquatic plant Ranunculus lingua J. Ecol. 81, 81–91. Kent, M., Coker, P., 1992. Vegetation Description and Analysis. A Practical Approach. Wiley, UK, p. 354. Kern-Hansen, U., Dawson, F.H., 1978. The standing crop of aquatic plants of lowland streams in Denmark and their inter-relationship of nutrients in plant, sediment and water. In: Proceedings of European Weed Research Society Fifth Symposium on Aquatic Weeds, Amsterdam, The Netherlands, pp. 143–150. Kirk, J.T.O., 1983. Light and Photosynthesis In Aquatic Ecosystems. Cambridge University Press, Cambridge. Madsen, T., Sand-Jensen, K., 1991. Photosynthetic carbon assimilation in aquatic macrophytes Aquat. Bot. 41, 5–40. McCune, B., Mefford, M.J., 1995. PC-Ord. Multivariate analysis of ecological data, version 2.0. MjM Software Design, Gleneden Beach, Oregon, USA.
272
T. Riis et al. / Aquatic Botany 66 (2000) 255–272
Moeslund, B., Løjtnant, B., Mathiessen, H., Mathiessen, L., Pedersen, A., Thyssen, N., Schou, J. C., 1990. Danske Vandplanter (in Danish). Environmental News No. 2. Danish Environmental Protection Agency, Copenhagen. Nichols, S.A., Shaw, B.H., 1986. Ecological life histories of the three aquatic nuisance plants, Potamogeton crispus and Elodea canadensis Hydrobiologia 131, 3–21. Nielsen, L.W., Nielsen, K., Sand-Jensen, K., 1985. High rates of production and mortality of submerged Sparganium emersum Rehman during its short growth season in an eutrophic Danish stream Aquat. Bot. 22, 325–334. Nørrevang, A., Meyer, T.J., 1969. Danmarks Natur (in Danish). Politikens Forlag, Copenhagen. Pedersen, A., 1976. Najadernes, Zannichelliaceernes og Zosteraceernes udbredelse i Danmark (in Danish) Bot. Tidsskr. 70, 203–262. Rebsdorf, A., Thyssen, N., Erlandsen, M., 1991. Regional and temporal variation in pH, alkalinity and carbon dioxide in Danish streams in relation to land use Freshwat. Biol. 25, 419–436. Robach, F., Thiébaut, G., Trémoliéres, M., Muller, S., 1996. A reference system for continental running waters: Plant communities as bioindicators of increasing eutrophication in alkaline and acidic waters in north-east France Hydrobiologia 340, 67–76. Sabbatini, M.R., Murphy, K.J., Irigoyen, J.H., 1998. Vegetation-environment relationships in irrigation channel systems of southern Argentina Aquat. Bot. 60, 119–133. Sand-Jensen, K., 1983. Photosynthetic carbon sources of stream macrophytes J. Exp. Bot. 34, 198–210. Sand-Jensen, K., Frost-Christensen, H., 1998. Photosynthesis of amphibious and obligately submerged plants in CO2 -rich lowland streams Oecologia 117, 31–39. Sand-Jensen, K., Jeppesen, E., Nielsen, K., van der Bijl, L., Hjermind, L., Nielsen, L.W., Iversen, T.M., 1989. Growth of macrophytes and ecosystem consequences in a lowland Danish stream Freshwat. Biol. 22, 15–32. Sand-Jensen, K., Lindegaard, C., 1996. Økologi i søer og vandløb (in Danish). G.E.C. Gads Forlag, Copenhagen, 188 pp. Spence, D.H.N., 1967. Factors controlling the distribution of freshwater macrophytes with particular reference to the lochs of Scotland J. Ecol. 55, 147–170. Spence, D.H.N., Maberly, S., 1985. Occurrence and ecological importance of HCO3 - use among aquatic higher plants. In: Lucas, W.J., Berry, J.A., (Eds.), Inorganic Carbon Uptake By Aquatic Photosynthetic Organism. American Society of Plant Physiologists, University of California, Davis, pp. 125–143. ter Braak, C.J.F., 1992. CANOCO - a FORTRAN program for Canonical Community Ordination. Microcomputer Power, Itacha, New York, USA. Vestergaard, O., Sand-Jensen, K., 1999. Alkalinity and trophic state regulate aquatic plant distribution in lakes. Aquat. Bot. (submitted). Westlake, D.F., 1975. Macrophytes. In: Whitton, B.A. (Ed.), River Ecology. Blackwell, Oxford. Wiegleb, G., 1984. A study of habitat conditions of the macrophytic vegetation in selected river systems in Western Lower Saxony (Federal Republic of Germany) Aquat. Bot. 18, 313–352. White, K.E., 1978. Dilution methods, in: R.W. Herschy (Ed.), Hydrometry. Wiley, New York. Økland, R.H., 1990. Vegetation ecology: Theory, methods and applications with reference to Fennoscandia. Sommerfeltia (Suppl. 1). University of Oslo, Oslo, pp. 1–233.