Large river floodplain restoration: predicting species richness and trait responses to the restoration of
FLOODPLAIN RESTORATION PROVIDES CONTRASTED PICTURES OF CHANGES IN TAXONOMICAL COMPOSITION, RICHNESS
AND FUNCTIONAL DIVERSITY
ABSTRACT
Floodplains are species rich environments worldwide, but largely impacted by human activities. Current floodplain restorations aim at recovering dynamic, healthy and sustainable ecosystems. In a temperate large river floodplain, we investigated the effect of the restoration of fluvial dynamics upon macroinvertebrates, a key component of aquatic biodiversity. Our results showed a predictable effect of restoration measures upon macroinvertebrate composition, functional characteristics and components of richness. A reduction of both rarefied richness and functional diversity was induced by restoration, however these changes were not linearly correlated with the intensity of restoration. Taxonomical beta diversity showed no reduction after restoration, while functional beta diversity increased. The latter observation was largely due to natural increase in controls, rather than an increase induced by restoration measures. The analyses conducted here lead to a first evaluation with contrasted changes in taxonomical composition, richness and functional diversity within restored channels (alpha diversity), rather than contrasted changes between channels (beta diversity). The effect of the restoration measures on macroinvertebrate diversity needs to be surveyed in the time. A diversification of fluvial dynamics in floodplain channels is necessary to preserve a maximum of macroinvertebrate biodiversity and to avoid biotic homogenization and loss of function.
Keywords: Large river floodplain, lateral connectivity, restoration, aquatic macroinvertebrates, taxonomical composition, rarefied richness, functional diversity, Rhône River.
INTRODUCTION
Riverine floodplains are complex ecosystems encompassing a mosaic of habitats and a high spatio-temporal heterogeneity, governed by floods (1). They are four dimensional systems, with a lateral dimension that encompasses a sequence of aquatic to terrestrial habitats and their associated biodiversity (2). Even if floodplains are among the most productive, species-rich and endangered ecosystem on earth, they have been embanked, drained, controlled by human activities (3-4) and they continue to be dammed to produce electricity in emerging countries (5). In Europe and North America, up to 90% of floodplains have become extensively regulated, occupied, and polluted, with major consequences for ecosystem structure and function (5). Worldwide the overall biodiversity of floodplains has been reduced and altered, leading to the simplification of communities and increased vulnerability to invasive species (6).
Macroinvertebrates represent the major biotic component in floodplain waterbodies in terms of richness and functionality. They play key roles through organic matter cycling and as food for upper trophic levels (7-9). To sustain the functional integrity of floodplain ecosystems and to reduce or stop the erosion of their biodiversity, ecological restoration has been recognized as a necessity and is currently applied worldwide (10-12).
Floodplain restoration efforts increased since the end of the 20th century and billions of dollars were injected in such programmes (5, 13). Diverse techniques are used to restore floodplains and one of the most frequent is the reconnection of floodplain channels with the main river (10). However, most restoration projects are not monitored and only a limited amount of results comes from small-scale studies with poor significance for larger programmes (13). A synthesis of river restoration efforts in the United States underlined that 90% of the operations were not scientifically surveyed, providing few possibilities to establish the effects of restoration on biodiversity (11). To provide sound restoration recommendations, it is important to carry out an initial monitoring incorporating hydro-morphological and ecological components (14-15) and
to compare its results with post restoration responses. Five criteria were proposed to assess the success of a river floodplain restoration (13). But the knowledge about biodiversity changes induced by restoration measures remains currently limited, even if such changes may be critical as drivers or witnesses of changes in ecosystems processes (16).
In Europe, the Rhône River flows across two countries (Switzerland and France). It is a well documented case of a large river modified since the 19th century by the construction of embankments and later of hydro-power plants that by-passed the natural floodplain (17). Both lead to a rapid terrestrialization and to the disconnection of the floodplains and in turn to a reduction of aquatic ecosystems function and biodiversity (18-19). Since 2005, selected floodplain channels have been restored with the aim to slow down their terrestrialization by an increase of their connectivity with the main river (i.e. the lateral connectivity). This increase was obtained: i) by raising the minimal base flow in the by-passed sections of the river and a dredging of the channel sediments, ii) in some instances through the direct reconnection of floodplain channels with the main river. Using a large scale data set, we report the changes in the lateral connectivity induced by restoration works and the associated responses of aquatic macroinvertebrates. The changes after restoration were measured within (i.e. alpha diversity) and between (i.e. beta diversity) the channels. Macroinvertebrates were used because they inhabit a large array of habitat conditions and they integrate the anthropogenic alterations (20).
The restored floodplain channels were compared with control channels (i.e. unrestored) to determine restoration effects on macroinvertebrate composition and diversity. We hypothesized that the enhancement of the lateral hydrological connectivity would influence key physical habitat parameters (e.g. sediment composition, aquatic vegetation structure, hydraulic stress) and in turn macroinvertebrate taxonomical composition, richness and functional diversity in a predictable way.
MATERIAL AND METHODS
Eighteen floodplain channels were monitored, covering the full spectrum of conditions occurring at the scale of the floodplain from totally disconnected to permanently connected channels. Eight unrestored channels were used as controls and 10 channels were restored, either moderately by sediment dredging and increase of the river minimal base flow, or strongly by a deeper dredging and a direct reconnection to the main river channel. Changes in biodiversity in the restored channels were expected i) to differ from those occurring in control channels and ii) to be proportional in their magnitude to the changes in habitat condition (i.e. changes in the synthetic variable used as surrogate for lateral connectivity, see below).
Macroinvertebrate sampling
The floodplain channels were monitored before and two years after restoration. Each comprised two sampling sites, one at each extremity to represent the diversity of habitats occurring in those channels (21). A sampling site was a 30-metre long stretch, within which four sampling points were randomly selected. At each point, macroinvertebrates were collected with a hand net (mesh size: 500 µm) within a quadrat (area: 0.25 m2). Sampling was repeated in summer and spring to account for seasonal variations. Samples were preserved in 100% alcohol. The resulting 576 samples were sorted in the laboratory. Oligochaeta and Chironomidae were omitted from the following calculations because of the limited taxonomic level of their identification and their ubiquity at that level of identification.
Lateral connectivity assessment
Similarly to previous studies (15, 22), we computed an index of lateral connectivity for each site using five environmental variables known to integrate the level of lateral connectivity of floodplain channels with the main river. Three variables were expected to decrease with increasing lateral connectivity: (i) water conductivity, (ii) organic content of the sediment upper layer, and (iii) aquatic vegetation horizontal cover. Two variables were expected to increase accordingly: (i) diversity of the mineral sediment grain size expressed with a Simpson index (23), and (ii) NH3-N concentration in the water. Vegetation cover, diversity of mineral substrate, water conductivity, ammonia nitrogen concentration (NH3-N) and organic matter in the sediment were measured in the floodplain channels at the site or sampling point scales, as described in previous study (15, 22). The five connectivity variables were summarized with a principal component analysis (PCA). This produced synthetic variables (the factorial axes), which were used as surrogates for the level of lateral connectivity between the floodplain channel sites and the main river channel before and after restoration. PCA and the corresponding graphical outputs were computed with the ade4 library (24) in the R 2.8.1 freeware (R Development Core Team 2008).
Macroinvertebrate composition
Densities (individuals/m2) were log (x + 1) transformed prior to analyses, and only those taxa represented by > 2 individuals in all samples combined were used. We used between-class correspondence analysis (bCA) to compare the taxonomic composition of the site assemblages.
Between-class ordinations are a special case of constrained ordination methods where the constraining variable is categorical, not continuous. Therefore, bCA ordinates sites under the constraint that the ordination maximally separates sites in the various classes of the constraining variable (24). The differences of the site positions along the first axis were further calculated to measure their deviations induce by restoration. Statistics were computed with the ade4 package (24) implemented in the R 2.8.1 freeware (R Development Core Team 2008).
Macroinvertebrate richness
Four taxonomic richness indices were calculated at each site as the number of taxa for: (i) the total community, (ii) the Ephemeroptera, Plecoptera and Trichoptera (EPT), (iii) the Odonata, and (iv) the Gastropoda. In each case, the richness was calculated as the number of taxa for the two seasons combined. A rarefaction procedure (using the vegan package for R 2.8.1 freeware) was performed for the four indices at each site to avoid biased comparisons due to differences in abundances (25-26). The number of non-native species was also calculated at each site. The non-native species were identified as such in accordance with a published list of non-native species in the French freshwater systems (27).
Macroinvertebrate traits and functional diversity
We focused on four biological trait-categories of the macroinvertebrates known to be associated with the lateral connectivity (15, 22): plurivotinism, predators, passive filter feeders and drifters. Trait data were extracted from published sources (28-31). For each site we calculated the percentage of the trait-categories among the community as in previous studies (15, 22). The percentage was calculated for the sites before and after restoration. The functional diversity for each site was calculated with Rao’s index (32), which includes species abundances and species distances computed with more than one trait (33-34). The distance between species was calculated as an euclidean distance based upon the selected trait-categories. Statistics were computed with the ade4 package (24) implemented in the R 2.8.1 freeware (R Development Core Team 2008).
Statistical analysis
We investigated the relationships between changes in macroinvertebrate metrics (taxonomical composition, rarefied richness, functional characteristics and diversity) and changes in lateral connectivity after restoration using linear regression. This approach was applied to assess if the changes in lateral connectivity had a predictable effect on macroinvertebrate metrics. We classified the changes in macroinvertebrate metrics within sites according to the three main types of restoration (i.e. control, moderately restored and strongly restored). Wilcoxon rank sum test was applied to assess differences in macroinvertebrate metrics between restoration types. Wilcoxon signed rank test was applied to assess differences in macroinvertebrate metrics between restoration periods.
Beta (β) diversity
We computed beta diversity (i.e. changes between-sites) for the metrics presented above. For the taxonomical composition, a within-period (i.e. pre- and post-restoration) correspondence analysis was used for calculation of dissimilarities between sites. Between-site differences were compared between the two periods with a Wilcoxon signed rank test. This was computed with the ade4 package (24) implemented in the R 2.8.1 freeware (R Development Core Team 2008).
A beta rarefied richness was computed for each of the two periods following (1-2, 35) as:
β= [(γ/ mean α) – 1]/(N-1) x 100
where γ is the rarefied richness of all channels pooled together, “mean α” is the mean rarefied richness per site and N is the number of sites. We used the same number of individuals to rarefy the richness, i.e. the lowest abundance found between all sites and restoration periods.
Finally, a beta functional diversity was computed with a double principal coordinate analysis (38), from which we extracted site-to-site distances before and after restoration. A Wilcoxon signed rank test was used to assess differences of distance values between restoration periods.
This analysis was performed with the dpcoa analysis in ade4 package (24) implemented in the R 2.8.1 freeware (R Development Core Team 2008).
RESULTS
The channels, both in their pre- and post-restoration conditions, were ordinated along the gradient defined by the five variables used as surrogates for the lateral connectivity (Fig. S1 in supporting material). The control channels significantly decreased in their lateral connectivity two years after restoration (Fig. 1, V = 117, p < 0.001). The moderately restored channels had no significant changes (Fig. 1, V = 31, p = 0.56), while the strongly restored channels significantly increased in lateral connectivity (Fig. 1, V = 2, p = 0.023). (Paillex et al., 2009)
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Figure 1. Distribution of lateral connectivity changes within control channels (i.e. unrestored, n = 16), moderately restored (n = 12), and strongly restored (n = 8). Groups a, b and c are significantly different from each other.
control moderately strongly restored restored
Lateral connectivity (∆)
a
b
c
Taxonomic composition
The relationship between the changes in lateral connectivity after restoration and the associated changes in community composition proved to be highly linearly significant (Fig. 2A, R2 = 0.74, p < 10-10). The highest changes in taxonomical composition were observed in the reconnected channels (i.e. strongly restored channels, see Fig. 2B). Overall between-site differences in taxonomical composition (i.e. beta-diversity) were not modified after restoration (Wilcoxon signed rank test, p = 0,086). However, a reduction of beta diversity within the controls (Wilcoxon signed rank test, p < 10-7) was compensated by a beta diversity increase within the strongly restored channels (Wilcoxon signed rank test, p < 10-6).
-0.2 0.0 0.2 0.4
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Changt_connect
Changt_faune
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Figure 2. A) Changes in macroinvertebrate composition as a function of lateral connectivity changes.
[Linear regression: macroinvertebrate composition (∆) = 0.14 +0.87 X (lateral connectivity (∆)); R2= 0.74, n = 36 sites]. A point represents a site and summarizes the information of 16 macroinvertebrate samples. B) Distribution of taxonomical composition changes within control channels (i.e. unrestored, n
= 16), moderately restored (n = 12), and strongly restored (n = 8). Groups a, b and c are significantly different from each other.
Lateral connectivity (∆)
Composition (∆) Composition (∆)
control moderately strongly restored restored a
b
c
A B
Taxonomic richness
The changes in total taxonomic richness within the sites (i.e. α diversity) were not linearly related to the changes in lateral connectivity caused by restoration (Table 1). However, α diversity decreased significantly in the channels restored by a direct reconnection (i.e. strongly restored, Fig 3, Wilcoxon signed rank test, p < 0.001). In reconnected channels, lenitophilous species (e.g. gastropoda) were disfavoured by restoration and replaced by reophilous ones (e.g.
EPT: Ephemeroptera, Plecoptera and Trichoptera) that were favoured by an increase of the lateral connectivity (Table 1). The number of non-native species not increased linearly with the changes in lateral connectivity (Table 1). In the absence of formal test, the slight increase in beta diversity between channels after restoration (before: 2.35 and after: 2.63) can be regarded as negligible.
Table 1. Dependence of changes in rarefied richness (except T: observed richness) with lateral connectivity changes, as determined by simple linear or o quadratic regression.
NS, p > 0.05 ; *, p < 0.05 ; * *, p < 0.01.
Regression parameters Response variable R2 p
Total 0.02 NS
EPT 0.24 **
Gastropoda 0.26 **
Odonatao 0.29 *
Non-nativeT 0.07 NS
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Figure 3. Distribution of total rarefied richness changes within control channels (i.e. unrestored, n = 16), moderately restored (n = 12), and strongly restored (n = 8).
Functional diversity
For three functional categories, the changes in percentual abundance per site were not linearly related to the changes in lateral connectivity. Only the drifters increased linearly with increasing lateral connectivity (Table 2). Changes in functional diversity (Rao’s index) were also not linearly related to the changes in lateral connectivity (Table 2). However, a reduction of functional diversity was observed within the strongly restored channels (Fig.4, Wilcoxon signed rank test, p = 0.03). The overall functional beta diversity between channels increased after restoration (Wilcoxon signed rank test, p < 10-15). However, it did so only within the control channels (Wilcoxon signed rank test between controls, p < 10-9) and no changes occurred in the restored ones (Wilcoxon signed rank test, moderately restored p = 0.38, strongly restored p = 0.93).
Total Rarefied Richness (∆)
control moderately strongly restored restored
Table 2. Dependence of changes in trait-categories and functional diversity upon lateral connectivity changes, as determined by linear regression. NS, p > 0.05 ; * *, p < 0.01.
Regression parameters
Response variable R2 p
Plurivoltine 0.06 NS
Passive filter feeders 0.08 NS
Predators 0.08 NS
Drifters 0.45 **
FD 0.086 NS
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Figure 4. Distribution of functional diversity changes within control channels (i.e. unrestored, n = 16), moderately restored (n = 12), and strongly restored (n = 8).
Functional diversity (∆)
control moderately strongly restored restored
DISCUSSION
Lateral hydrological connectivity is not easy to measure directly as such because it encompasses a complex set of interacting effects including shear stress, sedimentation / erosion processes, control upon vegetation quantity and structure, thermal and chemical properties of the water. These effects are largely integrated into the synthetic surrogate variable we proposed. The significant relationships observed between changes in this variable and changes in community composition, subgroups of rarefied richness and functional characteristics emphasized a strong deterministic nature of the floodplain functions. Therefore, as shown by our results, restoration actions that directly impinge upon lateral connectivity are likely to have a direct and largely predictable influence upon macroinvertebrate diversity.
The results underlined that the changes in lateral connectivity caused by restoration operations rejuvenated the reconnected floodplain channels, while unrestored channels followed the opposite direction. This result was in accordance with the expectations at the origin of restoration measures. In addition, the modifications of the lateral connectivity influenced the macroinvertebrate diversity within sites. The management practices induced a predictable change of the taxonomical composition, the rarefied richness of some taxa groups and the percentages of drifters within the communities. These results are in accordance with pre-restoration models established for the Rhône River (15, 22) but also for the Danube River (2, 14). However, our results showed that restoration induced no linear and predictable changes in the total rarefied richness and the functional diversity. Despite this absence of linear response, these two metrics decreased in the sites completely reconnected with the main river channel, leading to restored sites being less diversified than in their previous situation. The reconnection increased the fluvial dynamic within the channels, therefore the environmental forces (e.g.
hydraulic stress) acted as a filter and selected the few species adapted to such constraints.
Theoretical studies suggested a diversity decrease with increased environmental constraints, species being more similar to one another in a harsher environment (36). Despite the observed reduction of diversity in the strongly restored channels, the moderate restoration type had a less contrasted effect upon the total rarefied richness and functional diversity. Both metrics stayed unchanged for a moderate restoration, as well as in the absence of lateral connectivity changes.
Moreover, the changes in macroinvertebrate total rarefied richness and functional diversity in the moderately restored channels were not significantly different from those in unrestored channels. In the group of moderately restored channels, the vertical connectivity is also enhanced through the reduction of the sediment colmation. This, in turn could be expected to lead to changes in macroinvertebrate diversity (37).
The absence of between channels homogenization (i.e. reduction in beta diversity) based on taxonomical composition is a successful point of the restoration programme. The diversification of the restoration types allowed to compensate for the homogenization that occurred in the unrestored channels. The absence of homogenization was also reflected by the close values of beta diversity calculated for the rarefied total richness. However, it remains necessary to find a statistical way to test the difference of beta diversity based on rarefied richness. A second salient point was the overall increase of functional beta diversity after restoration. However this result should be taken with caution because of the significant increase in the unrestored channels and not in the restored ones. In turn, it underlined natural changes rather than restoration effects. When compared, the beta taxonomical and functional diversity behave contrastingly. The first decreased in the unrestored channels, while the second increased. However, the reason of this difference remains undefined. For instance, the calculations of beta diversities were developed to implement the alpha analysis applied to a metric (e.g. taxonomical composition, richness) (1, 38). Nowadays comparisons between results of beta diversities are poorly documented and it remains to find a way to homogenize the calculations of beta diversities to make them comparable.
Currently, in the Rhône River as in many other large rivers, restoration of the lateral connectivity is considered as a necessity to rejuvenate the floodplain and mitigate habitat loss, while status quo (i.e. no restoration) may lead to increased terrestrialization and the progressive disappearance of aquatic systems. Floodplain reconnection is the most widely applied method to restore such systems (39), but according to the observed responses of macroinvertebrate communities, homogenization of connectivity levels in multi-channel systems, may lead to a reduction of biodiversity and therefore to a potential loss of ecosystem processes (16).
Therefore, we recommend that restoration procedures aim at maintaining or re-creating a diversity of connectivity levels among channels at the floodplain scale, in order to preserve a maximum of biodiversity (40).
The restoration programme on the Rhône River meets 3 out of 5 criteria defined for a successful restoration programme (13): i) the restoration programme aimed to re-establish
The restoration programme on the Rhône River meets 3 out of 5 criteria defined for a successful restoration programme (13): i) the restoration programme aimed to re-establish