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Keywords:

  • connectivity;
  • drawdown;
  • exposure;
  • macrophytes;
  • river regulation;
  • riverine vegetation

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References
  • 1
    In recent years, interest has grown in restoring floodplain function of regulated rivers. Successful rehabilitation of riparian systems requires knowledge of how regulation of river flow affects biodiversity and ecosystem function. The effects of changes in the river's low water-level regime on aquatic ecosystems in floodplains has received little attention so far.
  • 2
    The aquatic vegetation of 215 floodplain lakes along the Lower Rhine (the Netherlands) was analysed in relation to lake characteristics and lake water-level fluctuations in 1999–2000.
  • 3
    Vegetation composition was related to lake morphology and age, cattle access to the shoreline, the amount of time the river was in flood, and lake sediment area exposed at low water level (drawdown). Surprisingly, vegetation composition was correlated more strongly with lake age and occurrence of drawdown than the amount of time the river was in flood.
  • 4
    In older lakes, water-level fluctuations are reduced due to an accumulation of clay and silt that ‘seals’ sediment, preventing drawdown during periods of low river levels. Our results suggest that this clay sealing process is a major driving force for aquatic vegetation succession in floodplain lakes along the Lower Rhine, as succession drives from desiccation-tolerant species (e.g. Chara spp.) in young lakes to desiccation-sensitive species (e.g. Nuphar lutea) in old lakes.
  • 5
    Water levels were stable in lakes along a river branch that was impounded below mean flow only. Here, the original low water-level regime has been replaced by an artificial regime with higher water levels than would be expected naturally. Consequently, in these lakes drawdown was rare or absent, and the aquatic macrophyte vegetation was characterized by low species richness and frequent dominance by the invasive species Elodea nuttallii.
  • 6
    Synthesis and applications. Our results show that stabilization of river water levels during low flow may negatively affect vegetation composition and succession in floodplain lakes adjacent to these rivers. A management scheme including incidental temporary lowering of the river water level, which results in drawdown of floodplain lakes, would enhance the ecological status of those rivers with stabilized water levels during low flow.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References

Dams regulate the flow regime in the majority of the world's large river systems. More than 840 000 dams obstruct approximately two-thirds of the fresh water flowing to the oceans (Petts 1984; Dynesius & Nilsson 1994; McCully 1996). In recent years, interest has grown in restoring floodplain function by reconnecting the floodplain with the main channel of regulated rivers. To provide guidelines for the rehabilitation of these riparian systems, knowledge is needed of the way in which regulation of the river flow affects biodiversity and ecosystem function of large rivers. Several studies have focused on the effects of changes in the flooding regime on ecosystem composition and succession of riparian ecosystems (Bravard, Amoros & Pautou 1986; Jansson et al. 2000; Dynesius et al. 2004). However, comparatively little is known about the effects of changes in the river's low water-level regime on aquatic ecosystems in floodplains along regulated rivers.

In floodplains along large lowland rivers, lake systems represent an important component of riparian wetlands. Along unregulated rivers, lakes originate from past geomorphological change, such as meander cut-off (Bravard, Amoros & Pautou 1986). In addition to natural lakes, human interference has created many artificial water bodies along regulated rivers through the extraction of gravel, sand and clay. Young lakes in river floodplains may be colonized rapidly by submerged vegetation, followed by a succession of nymphaeids and helophytes (Van Donselaar 1961). River flooding through surface connection with the main channel may be an important determinant of vegetation composition in floodplain lakes (Van der Voo & Westhoff 1961; Van den Brink et al. 1991; Bornette, Amoros & Chessel 1994; Sparks & Spink 1998). In contrast, the impact of fluctuating lake water levels during isolated stages is largely unknown (Amoros & Bornette 1999). Input of seepage water may affect the vegetation by increasing or decreasing nutrient concentrations in lake water (Bornette, Amoros & Chessel 1994; Bornette, Guerlesquin & Henry 1996; Tockner et al. 1999). Additionally, fluctuating river levels may influence the amplitude of water-level fluctuations in floodplain lakes by groundwater flow. When the water level is low in the main channel, infiltration of lake water into the alluvial aquifer may result in a lowering of lake water levels, which, depending on the morphometry of the lake, may result in lake-bottom exposure (drawdown) (Van Geest et al. in press). Drawdown has been shown to have strong effects on the composition and abundance of aquatic vegetation (Wilcox & Meeker 1991; Wagner & Falter 2002).

River flooding through surface connection with the main channel (Brock, Van der Velde & Van de Steeg 1987; Henry, Bornette & Amoros 1994), river seepage and infiltration-induced water-level fluctuations affect all aquatic vegetation development in floodplain lakes. Their relative importance may depend on what is referred to as ‘hydrological connectivity’ (Tockner, Malard & Ward 2000; Amoros & Bornette 2002), i.e. the degree of hydrological contact between the river and a particular lake in the floodplain. The hydrological connectivity of a floodplain lake under non-flooded conditions may be expressed by the degree to which the lake follows water-level fluctuations of the river. When lakes become older, water-level fluctuations will decrease as the result of decreased hydraulic conductivity of the lake bottom, caused by accumulation of organic matter, silt and clay in the sediment (Wood & Armitage 1997). As a result, sediment exposure will occur less frequently with increasing lake age (Van Geest et al. in press). We hypothesized that a shift in the vegetation composition occurs with lake ageing: desiccation-tolerant macrophyte species dominate in young lakes, whereas desiccation-sensitive species are favoured in later successional stages.

In this study we evaluated the relative importance of drawdown (by infiltration) and flooding (by surface connection) on vegetation composition in 215 lakes situated in the remaining active floodplain of the Lower Rhine in the Netherlands. Further, we analysed vegetation succession in the floodplain lakes by synchronic analysis of differently aged lakes. In addition, the presence in the Lower Rhine of two unimpounded branches and one impounded branch enabled us to evaluate the potential consequences of hydrological regulation measures on aquatic vegetation composition and succession.

Study area

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References

The Rhine, from its source in Switzerland to the outflow to the North Sea, is 1320 km long and has a catchment area of 185 000 km2 (Lelek 1989), of which 25 000 km2 are situated in the Netherlands. Where the Rhine enters the Netherlands, the flow varies between 800 and approximately 12 000 m3 s−1, resulting in a difference between the minimum and maximum water levels of up to 8 m (Middelkoop 1997). Typically, the highest river flows occur in winter and the lowest in late summer and early autumn (Buijse et al. 2002). During the growing season of the vegetation (May–October), periods of extreme low water levels may occur.

After crossing the border of the Netherlands, the Lower Rhine splits into three branches, the Waal, IJssel and Neder-Rijn. No weirs are present along the Waal and IJssel, whereas the lower water levels of the Neder-Rijn became regulated by the construction of three weirs in the 1960s. All weirs are closed when the mean river flow of the Rhine falls below 2200 m3 s−1. Hence, in the Neder-Rijn the construction of weirs has not resulted in changes in the flooding regime, whereas the natural water-level regime, with occasional low river water levels, has been replaced by an artificial distribution, with higher minimum water levels than would be expected naturally (Fig. 1). Consequently, water levels rarely fall below a fixed level in the Neder-Rijn and the floodplain lakes alike. Indeed, lake drawdown is strongly reduced along the impounded Neder-Rijn, whereas drawdown occurs frequently in lakes along the unimpounded Waal and IJssel, especially in young lakes (Van Geest et al., in press). For lakes along the Waal and IJssel in 1999, the mean drawdown area (± SΕ) was 29 ± 7% for lakes 1–29 years old, 18 ± 5% for lakes 30–99 years old and 10 ± 3% for lakes ≥ 100 years old. For lakes along the Neder-Rijn, these values were 2 ± 1%, 2 ± 2% and 8 ± 2%, respectively (Van Geest et al., in press).

image

Figure 1. The mean and minimum water levels (metres above sea level) in the three branches of the Lower Rhine (Waal, IJssel and Neder-Rijn) for the period May–October 1990–99. For the Neder-Rijn, the mean and minimum river water levels (metres above sea level) are also shown at times when the weirs would be open (calculations based on the water quantity model sobek; RIZA, Lelystad, the Netherlands). For each branch, the indicated river kilometres represents the upper and lower limits of the locations of the lakes.

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Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References

In this study, we confined our research to lakes that were shallow (mean depth < 2 m) and disconnected from the main channel during summer. In 1999, submerged and floating-leaved macrophytes were sampled in 100 floodplain lakes along the Lower Rhine. In 2000, an additional 115 lakes was sampled.

sampling and classification of plots

In July–early August 1999, aquatic vegetation was sampled in 917 plots distributed over 100 floodplain lakes along the Lower Rhine. The number of plots per lake varied (4–31), depending on lake size and apparent complexity of the vegetation structure. Plots were selected to represent the range of vegetation types present in the lake. Plot size varied from 1 m2 (submerged vegetation) to 4 m2 (nymphaeid vegetation). All plots were sampled from a boat. In each plot, species composition and cover were determined by combining visual estimates and collection. Species cover in each plot was expressed in one of seven classes (1, < 1%; 2, 1–5%; 3, 6–15%; 4, 16–25%; 5, 26–50%; 6, 51–75%; 7, 76–100%), and total percentage covers of submerged plants, floating plants, helophytes and filamentous algae were estimated in the same way. Two-way indicator species analysis (twinspan; Hill 1979) was used to assign plots to vegetation types. Classes of plant cover were used as pseudospecies cut levels. Eleven vegetation types were distinguished based on differences in species dominance. The vegetation types were named according to their dominant species. In addition, plots with total vegetation cover < 15% were classified as a separate ‘vegetation-poor’ type. Based on the classification of the plots, the area covered by each vegetation type proportional to the total lake surface area was determined for each lake sampled in 1999. In 2000, the cover of vegetation types was estimated visually using the same classification system for an additional 115 lakes. In both years, a vegetation type was regarded as characteristic if it occupied at least 5–10% of the total lake surface area.

sampling of species richness

Preliminary research indicated that calculation of species richness of aquatic macrophytes on the basis of data from the plots alone would result in an underestimation of the species richness in the lakes. Therefore, for the 100 lakes sampled in 1999, species composition and abundance of submerged and floating-leaved macrophyte vegetation of the whole lake area were surveyed by boat until no new species were found. The time spent per lake varied from approximately 30 min for very small lakes (< 0·10 ha) to several hours for lakes with a large surface area or a complex vegetation structure. Species abundances were recorded using the Tansley scale (rare, occasional, frequent, abundant, dominant) (Schaminée, Stortelder & Westhoff 1995), and were converted to an ordinal scale ranging from 1 to 5 for statistical analysis. For the 115 additional lakes sampled in 2000, no equivalent data were obtained for species richness.

abiotic variables

The abiotic variables were selected to represent potentially important factors for macrophyte growth, such as inundation duration by overland flooding, occurrence of drawdown, water depth and water clarity.

For all 215 lakes, estimates of the average amount of time the area was flooded annually by the river, the surface area and the shoreline length were obtained from GIS maps (RIZA, Lelystad, the Netherlands). Water depth in July was established at several locations per lake. The proportion of the surface area of the lake bottom that became exposed between July and October (the drawdown area) was determined by visual estimation. The approximate age of lakes was derived from historical topographical maps. Reliable estimates of the age of the lakes could be made up to 300 years; lakes older than this were classified as being 300 years old. The uncertainty of the lake age estimation was approximately 1 year for lakes originating from 1980 onwards, 3–7 years for lakes originating during the period 1910–80, and 10–25 years for lakes originating before 1910. During fieldwork, the prevailing land use in the adjacent floodplain was recorded (presence of intensive agriculture, presence of adjoining trees and cattle access to shoreline).

In addition, several abiotic variables were determined for lakes sampled in 1999. For all 917 plots in these lakes, we measured water clarity with a small black and white disk called a Secchi disk. The Secchi depth is the vertical distance from the water surface where the Secchi disk disappears from view when it is dropped in the water. Secchi depth was measured once during the period of maximum seasonal development of macrophytes (July and early August). Based on these data, the proportion of the lake bottom within the euphotic depth (the euphotic zone) was calculated as the percentage of the lake surface area where the water depth was equal or less than 1·7 times the Secchi depth (Reynolds 1984). Furthermore, the seasonal water-level trend between July and October 1999 was measured from a marked rod placed in each lake. Because of sampling difficulties, complete abiotic data were available for 94 of the 100 lakes sampled in 1999.

The resulting data set of 215 lakes represented the range of values along the three Rhine branches of potentially important factors for macrophyte growth, such as average inundation duration (1–258 days year−1), lake surface area (0·01–44·6 ha), shoreline length (40–6535 m), lake age (1–> 300 years) and distance to the main channel (10–1400 m). For the subset of 100 lakes sampled in 1999, 36 lakes were of natural origin (e.g. cut-off meanders) and 64 lakes were of artificial origin (e.g. clay pits). Along the Waal, there were 10 natural and 30 artificial lakes, while along the IJssel and Neder-Rijn the numbers were 14 and 12, and 12 and 22, respectively. Overall, artificially created lakes (mean lake age 47 years, n= 64 lakes) were younger than natural lakes (mean age 247 years, n= 36 lakes) (Mann–Whitney U-test; U= 18·00, P < 0·001). However, between these two categories of lakes there were no significant differences in surface area and mean depth (U = 1005·0 and 1037·5, P= 0·29 and 0·41, respectively).

data analyses

Of the 215 lakes, 165 were vegetated (vegetation cover at least 5–10% of lake area; see above). The relationship between the lake area covered by the different vegetation types and abiotic lake characteristics (Table 1) was analysed using canonical correspondence analysis (CCA) (canoco version 4; ter Braak 1998). For each lake, the percentage cover of the 11 classified vegetation types was used as a characteristic of the lake's vegetation. Default parameters were used in the analysis. The approximate significance level for the correlation between each abiotic variable and vegetation composition was computed by a Monte Carlo permutation test (ter Braak 1998). The number of random permutations was 9999. A significant ordination indicated a significant correlation between the ordination of the lakes according to their abiotic characteristics and the ordination of the lakes based on their floristic composition.

Table 1.  Independent variables used for CCA and multiple logistic regression
VariableUnitComments
  • *

    ln(x) transformed.

  • ln(x + 1) transformed for multiple logistic regression analyses only.

Mean lake depth*mCalculated from 4–31 water depth measurements in July 1999 in each lake
Lake surface area*haSurface area of lake at start of growing season
Drawdown area%% of lake surface area in October that has been exposed compared with the surface area in July
Lake age*yearFor accuracy, see the Methods
Inundation duration*day year−1Long year average 1900–95
Presence of intensive agricultureCategoriesSee the Methods
Presence of treesCategoriesSee the Methods
Access of cattle to shorelineCategoriesSee the Methods

Subsequently, a more detailed analysis was carried out on the 100 lakes sampled in 1999. In this analysis, the probability of presence of each vegetation type was related to abiotic lake characteristics by means of multiple logistic regression analysis (Jongman, ter Braak & Van Tongeren 1995). In addition to the abiotic lake characteristics given in Table 1, the variable euphotic zone was included in this analysis. The parameters were estimated by means of the maximum-likelihood principle. The abiotic variables inundation duration, lake age, mean lake depth and lake surface area were ln(x) transformed; the proportion of drawdown area was ln(x + 1) transformed. For each analysis both forward and backward multiple logistic regressions were carried out; only the results of the backward regression are presented because the two produced comparable results.

To determine the effect of stabilization of the river water level on the aquatic macrophyte composition of the lakes, we calculated the occurrence of characteristic vegetation types for each river branch, calculated as the number of lakes where each vegetation type occurred divided by the total number of lakes along each branch. Furthermore, we calculated the species richness (sum of submerged and rooted floating-leaved species) for each lake. Because species richness can be influenced strongly by the number of species that occur in low abundance (Nillson & Nillson 1983), we checked if the same relationships were found when the species with low abundance (the category ‘rare’ according to the Tansley scale) were excluded from the analyses.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References

relationships between vegetation types and abiotic lake characteristics

CCA results indicated that vegetation composition in the sampled floodplain lakes was significantly related to the water-level regime of the river (inundation duration, drawdown area), lake age, morphometry (depth, surface area) and cattle access to the shoreline (Fig. 2). Dominance of benthic filamentous macro-algae, Persicaria amphibia (L.) Gray, Chara vulgaris (L.) and Nymphoides peltata (S.G. Gmel.) Kuntze was positively related to the proportion of drawdown area in the lakes (Fig. 2 and Table 2). Remarkably, inundation through surface flooding had no relationship with characteristic vegetation types in our study (Table 2). Submerged vegetation types dominated by Potamogeton pusillus (L.)/Potamogeton trichoides Cham. & Schltdl., Chara vulgaris and Elodea nuttallii (Planch.) St John predominated in younger lakes (Tables 2 and 3). In contrast, Ceratophyllum demersum L. (Table 3) and Potamogeton lucens L. (Tables 2 and 3) showed the opposite pattern, with maximum occurrence in lakes older than 100 years; the highest occurrence of Potamogeton pectinatus L. occurred in intermediately aged lakes (30–99 years; Table 3). The vegetation types Nuphar lutea (L.) Sm. and Nymphoides peltata were common in lakes that were 30–40 years and older, although Nymphoides peltata also occurred in 8% of the lakes aged 1–30 years. Relatively large (> 5 ha) lakes were usually dominated by nymphaeids or were almost without vegetation, whereas relatively small and shallow lakes were usually dominated by submerged vegetation (Fig. 2). Potamogeton lucens and Nuphar lutea were positively related to mean lake depth, whereas Potamogeton pusillus/Potamogeton trichoides was negatively related to mean lake depth (Table 2). Furthermore, Chara vulgaris, Potamogeton lucens and Potamogeton pectinatus were characteristic of lakes with a large proportion of the bottom within the euphotic depth (Table 2).

image

Figure 2. Distribution of vegetation types in 165 floodplain lakes along the Lower Rhine. CCA ordination diagram with vegetation types, floodplain lakes and environmental variables. The abiotic variables explained 18% of the variation in vegetation composition. Abbreviations of the vegetation types: BA, benthic filamentous macro-algae; CD, Ceratophyllum demersum; CV, Chara vulgaris; EN, Elodea nuttallii; NC, Nitella capillaris; NL, Nuphar lutea; NP, Nymphoides peltata; PA, Persicaria amphibia; PL, Potamogeton lucens; Ppe, Potamogeton pectinatus; Ppt, Potamogeton pusillus/Potamogeton trichoides. E = Eigenvalue.

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Table 2.  Results of backward multiple logistic regression analysis in a set of 94 lakes for dominance of vegetation types of benthic filamentous macro-algae (BA), Chara vulgaris (CV), Elodea nuttallii (EN), Nuphar lutea (NL), Nymphoides peltata (NP), Persicaria amphibia (PA), Potamogeton lucens (PL), Potamogeton pectinatus (Ppe) and Potamogeton pusillus/Potamogeton trichoides (Ppt). Vegetation types of Ceratophyllum demersum (CD) and Nitella capillaris (NC) that occurred in less than 10 lakes were excluded from the analysis. The abiotic variables inundation duration, presence of trees and presence of intensive agriculture did not significantly explain vegetation type occurrence
Vegetation typen lakesSurface area (ha)Mean lake depth (m)Drawdown area (%)Lake age (year)Euphotic zone (%)Cattle grazingConstant
  • *

    P < 0·05;

  • **

    P < 0·01;

  • ***

    P < 0·001.

  • ln(x) transformed.

  • ln(x + 1) transformed.

BA12  0·58**    −3·15
CV101·00** 1·28** 0·05*2·88*−10·8
EN28   −0·34*   
NL27  0·98*   1·39***   −7·46
NP29  0·35*  0·55**   −3·78
PA11  0·93**    −4·23
PL24  1·40*   0·42*0·04***  −5·57
Ppe27     0·41*0·026***  −4·33
Ppt23 −1·15**     −1·29
Table 3.  Percentage occurrence of vegetation types in lakes of different age classes along the unimpounded Waal and IJssel (category U) and impounded Neder-Rijn (category I). For species abbreviations, see Table 2
Age class0–2930–100> 100
River regulation n lakesU 21I 16U 55I 24U 70I 29
BA24 6 0 0 9 0
CD 0 0 5 413 7
CV29 6 4 4 6 0
EN295016501314
NC 5 0 2 8 1 0
NL 0 031 04345
NP14 027 441 7
PA19 0 7 0 7 0
PL14 624 42321
Ppe14 620211114
Ppt383116171121

relationship between sediment exposure and macrophyte vegetation

The probability of the lake bottom becoming exposed was not directly dependent on water depth in July, because lakes showed strongly different degrees of water level decrease during summer. Large areas of the plots containing Persicaria amphibia (94%) and Chara vulgaris (76%) were exposed in October 1999, whereas plots with Nuphar lutea were not exposed (Fig. 3). Potamogeton lucens, Potamogeton pectinatus, Potamogeton pusillus/Potamogeton and Nymphoides peltata occurred in plots with an intermediate incidence of sediment exposure (Fig. 3). Most vegetation types had maximum presence at water depths of 0·5–1·0 m, and were almost absent in water deeper than 2 m. Vegetation types of Persicaria amphibia, Chara vulgaris and Potamogeton pusillus/Potamogeton trichoides showed maximum occurrence in shallow water (< 1 m), whereas optimal conditions for Potamogeton lucens were in relatively deep water (0·5–2·0 m).

image

Figure 3. Occurrence of the main vegetation types in sample plots in relation to the occurrence of drawdown in October 1999. Abbreviations of the vegetation types: BA, benthic filamentous macro-algae (20 plots, 11 lakes); CV, Chara vulgaris (38, 10); EN, Elodea nuttallii (65, 25); NL, Nuphar lutea (84, 27); NP, Nymphoides peltata (80, 27); PA, Persicaria amphibia (16, 10); PL, Potamogeton lucens (55, 22); Ppe, Potamogeton pectinatus (61, 26); Ppt, Potamogeton pusillus/Potamogeton trichoides (50, 13); VP, vegetation-poor (355, 64). Plots with vegetation types of Ceratophyllum demersum or Nitella capillaris were excluded because both types occurred in less than 10 plots.

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differences between river branches

There were significant differences in the occurrence of vegetation types in lakes along the impounded Neder-Rijn and unimpounded Waal and IJssel (χ2 = 58·20, d.f. = 20, P < 0·01). The vegetation types characterized by Chara vulgaris, Nuphar lutea, Nymphoides peltata, Persicaria amphibia and Potamogeton lucens occurred in a lower number of lakes along the Neder-Rijn compared with lakes along the Waal and IJssel, in particular in lakes 30–100 years old (Fig. 2 and Table 3). In contrast, the vegetation type Elodea nuttallii was more common in lakes along the Neder-Rijn than along the other two branches (Fig. 2 and Table 3). In lakes older than 100 years, Nuphar lutea and Potamogeton lucens occurred equally along all three river branches (Table 3). The reduced occurrence of several vegetation types along the Neder-Rijn (see above) resulted in a lower species richness of submerged and rooted floating-leaved species in lakes along this impounded river branch compared with lakes along the free-flowing Waal and IJssel (Fig. 4). Similar results were obtained when species that occurred in very low abundance in the lakes (rare according to the Tansley scale) were excluded from the analysis, indicating the robustness of this relationship.

image

Figure 4. Mean (± SD) of the species number of submerged and rooted floating-leaved macrophyte species in lakes along the impounded Neder-Rijn (n lakes = 34) and unimpounded Waal (n = 40) and IJssel (n = 26) in 1999. Different letters indicate groups that differ in species number with a significance of P < 0·01 (t-test).

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References

Our results indicate that the variation in aquatic macrophyte vegetation between lakes in these floodplains is largely determined by differences in lake morphometry, water transparency, water-level fluctuations and lake age, while within-lake macrophyte distribution depends on depth and the probability of sediment exposure (drawdown) in summer. Surprisingly, macrophyte composition was correlated more strongly with lake age and occurrence of drawdown than with the duration of overland flooding.

Ageing of lakes tends to coincide with a reduction in water-level fluctuations because of sedimentation of fine organic and mineral particles (Rostan, Amoros & Juget 1987; Schwarz, Malanson & Weirich 1996; Middelkoop 1997). The accumulated layer of clay sediments induces a reduced porosity of the lake bottom, thereby reducing the exchange of groundwater between the river and the lake (Wood & Armitage 1997). Indeed, in the set of 100 floodplain lakes sampled in 1999, young lakes had a significantly higher occurrence of sandy sediments and exhibited larger water-level fluctuations compared with older lakes, in which clay sediments prevailed (Van Geest et al., in press). In lakes along other rivers, floods may have a scouring effect by removing the accumulated sediment layer (Bornette, Amoros & Chessel 1994; Henry, Bornette & Amoros 1994). However, because of a lack of erosive currents during flooding of lakes along the Lower Rhine, there is probably a systematic decrease in the amplitude of water-level fluctuations with increasing age.

The decreasing amplitude of water-level fluctuations with lake age may have a strong impact on macrophyte succession. Young lakes tended to be dominated by desiccation-tolerant species such as Chara vulgaris, and old lakes by desiccation-sensitive species such as Nuphar lutea. Amphibious species, such as Persicaria amphibia, show a clear association with alternating exposed and reflooded sites (Partridge 2001), while the abundance of Chara spp. seems positively related with previous drawdown. In addition, the prevalence of Nymphoides peltata was positively related with the probability of sediment exposure. According to Smits, Van Ruremonde & Van der Velde (1989), germination of seeds of Nymphoides peltata requires oxygen, while seeds and seedlings of this species show some resistance to the effects of desiccation. In contrast, Elodea spp. have been reported to be susceptible to desiccation (Martin et al. 1995). In addition, Nuphar lutea was negatively related with the probability of sediment exposure, and germination of Nuphar lutea is stimulated by anoxic conditions in sediments, whereas the seeds and especially juvenile submerged plants of this species are highly susceptible to desiccation (Smits, Van Ruremonde & Van der Velde 1989).

Our evidence for the importance of reduced amplitude of water-level fluctuations with lake age is indirect, because it is based upon a comparison of differently aged lakes. Nevertheless, we observed a clear characterization of species dominating certain age groups of lakes fitting the ‘reduced water-level fluctuation hypothesis’. However, macrophyte succession during lake ageing may be driven by other factors at the same time. For instance, organic matter accumulates in lake sediments over the course of time, thereby changing sediment properties and the nutrient status. Furthermore, macrophyte succession may be influenced by differences in dispersal characteristics of species. In general, early successional species have easily dispersed propagules, such as oospores, plant fragments and turions in Chara vulgaris and Potamogeton pusillus (Haag 1983; Wade 1990). In contrast, late successional species such as Nuphar lutea (Smits, Van Ruremonde & Van der Velde 1989) often possess heavy or large propagules that are transported over relatively short distances (Silvertown 1982), making them poor colonizers. Overall, it can be concluded that the succession of macrophytes shifts from being externally driven in young floodplain lakes (allogenic succession) to internally driven in old lakes (autogenic succession).

Surprisingly, surface flooding had no relationship with species dominance in our study (Table 2), although the total cover of submerged macrophytes declined in lakes with higher duration of surface flooding (Van Geest et al. 2003). Apparently, water-level fluctuations and associated drawdown events overrule the effects of flooding by the river on macrophyte species dominance in these lakes. In contrast, other studies have emphasized the importance of the erosive power of the water current and timing and duration of flooding for macrophyte composition and succession in floodplains (Henry, Bornette & Amoros 1994; Janauer & Kum 1996; Bornette, Amoros & Lamouroux 1998; Tyser et al. 2001). The discrepancy between these studies and our results might be caused by differences in the timing and intensity of flooding. In our temperate regions, floods occur predominantly during winter, when vegetation largely remains underground as dormant structures and the temperature is low. This reduces the ecological effects of floods compared with summer flooding (Brock, Van der Velde & Van de Steeg 1987). In addition, in lakes along the Lower Rhine the erosive force of floods in lakes is relatively weak, which may explain the minor impact of floods compared with other studies.

The impoundment in the 1960s of the Neder-Rijn, one of the three branches of the Lower Rhine, has resulted in strongly stabilized water levels in the main channel and adjacent floodplain lakes (Fig. 1). Hence, the amplitude of water-level fluctuations and occurrence of drawdown are strongly reduced in lakes along the Neder-Rijn compared with the unimpounded Waal and IJssel. Our results suggest a reduced species richness of the lakes along the Neder-Rijn, caused by stabilization of the water level in this river branch. Species richness of macrophytes in lakes along the three branches of the Lower Rhine tended to peak at intermediate amplitude of water-level fluctuations (Van Geest et al., in press). In lakes with small water-level fluctuations, the absence of drawdown events may result in reduced species richness. Disturbances caused by drawdown may prevent competitive exclusion by desiccation-sensitive species, or may stimulate germination of species like Chara (Bonis & Grillas 2002), thereby increasing species richness (Hill, Keddy & Wisheu 1998). This may explain the rare occurrence of desiccation-tolerant species such as Chara vulgaris, Nymphoides peltata and Persicaria amphibia in lakes along the Neder-Rijn (Table 3). The rarity of late-successional species like Potamogeton lucens and Nuphar lutea in 30–100-year-old lakes along the Neder-Rijn (Table 3) is more difficult to explain, but might be due to competition with the desiccation-sensitive Elodea nuttallii. The latter species, which is a rapid colonizer in riverine habitats (Barrat-Segretain, Henry & Bornette 1999; Barrat-Segretain 2001), tended to dominate the vegetation of lakes with stable water levels.

The rarity of characteristic riverine species is in line with patterns found in similar studies along rivers with strong regulation of the water-level regime (Nilsson et al. 1991; Jansson et al. 2000; Robertson, Bacon & Heagney 2001; Johansson & Nilsson 2002). This pattern is often thought to result from changes in the flooding regime (Ward & Stanford 1995). However, along the Neder-Rijn the construction of the weirs did not result in changes in flooding regime, because the weirs are closed below mean river flows only. Thus, our results show that a reduced occurrence of drawdown because of water-level stabilization in the main channel during low river flow can also affect the establishment and growth of characteristic riverine species in floodplain lakes, a factor that has received little attention so far.

The water-level regime has been strongly regulated in the majority of large temperate rivers during the past few centuries. In North America, Europe and the former Soviet Union, 71% of the large rivers have been affected by dams and reservoirs, water diversion and abstraction (Dynesius & Nilsson 1994). In many river systems, the natural low river water levels have been replaced by an artificial distribution with higher water levels than would be expected naturally (McMahon & Finlayson 2003). Our results indicate that, for macrophyte composition and succession of aquatic habitats along regulated rivers, changes in the water-level regime during low river flow may be as important as changes in the flooding regime. Consequently, restoration of aquatic habitats in floodplains along river stretches with stabilized water levels during low discharges may be difficult to achieve. Where possible, the natural water-level regime in the river should be restored to conserve the full successional range of floodplain water bodies, including those with the highest biodiversity. Along most regulated rivers, reinstatement of the natural water-level regime may not be achievable. However, it might be possible to allow low river levels in certain years to promote incidental drawdowns of lakes. Along the unregulated branches of the Lower Rhine, the number of lakes with drawdown varies strongly from one year to another, because of large interannual variation in the minimum water levels of the river. In years with prolonged periods of very low water levels (such as 1959, 1976, 1978 and 1991), 25–60% of the lakes dried out, whereas in other years the proportion of dried-out lakes was small (Coops & Van Geest, in press). Although difficult in practice, incidental temporary lowering of the river water level might be considered a suitable management to enhance the ecological status of the Neder-Rijn river branch as well as other regulated rivers.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References

This project was financed by RIZA Institute for Inland Water Management and Waste Water Treatment. We are grateful to Susan Graas, Bart Groeneveld, Matthijs Rutten and John van Schie for their assistance in the field.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Study area
  5. Methods
  6. Results
  7. Discussion
  8. Acknowledgements
  9. References
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