Future needs

6.3.1. Restoration measures and non-indigenous aquatic macroinvertebrates

In addition to the generalized threat caused by habitat destruction and alteration, biodiversity is also threatened by invasive species (Chapin et al., 2000; Sala et al., 2000).Since the beginning of the Rhône River restoration programme, we have been observing the arrival of non-indigenous species and the colonization of habitats that were previously devoided of such species (see Table 1). During the pre-monitoring phase of the Upper Rhône River, a list of non-indigenous species was established (Table 1). Those species were identified in the unrestored channels and only 6 sites out of 34 were devoided of non-indigenous species. Before restoration, the non-indigenous species occurring in the Upper Rhône River represented 18% of the known non-indigenous species in French freshwater systems (Devin et al., 2005).

After the pre-monitoring phase, we observed the arrival of two non-indigenous species (firstly Dikerogammarus villosus, secondly Hypania invalida). Dikerogammarus villosus arrived in France in 1997 (Bollache, 2004; Bollache et al., 2004). This species was observed on the Upper Rhône River in the main river channel in October 2004 (Castella et al., 2004). This species was listed among the 100 defined as the most invasive alien species in Europe (Daisie, 2009). Hypania invalida arrived in France in 2000 and was observed on the lower Rhône River in 2002 (Devin et al., 2006). This species was observed for the first time in the Upper Rhône River in spring 2007 (Castella et al., 2008).

After the discovery of these two species, the non-indigenous species in the Upper-Rhône River represent around 25% of the French list. After restoration, in most of the sites we observed an increase of the number of non-indigenous species and an

Post-monitoring

Pre-monitoring

introduction in sites where they were previously unrecorded. Nowadays, only four sites among 34 do not harbour non-indigenous species. This observation needs to be put in the context of the current flux of Ponto-Caspian, Asian and North-American non-indigenous macroinvertebrates at the scale of the entire Rhône River (Devin et al., 2005). The main river channel can be considered as the main route for dispersion in the Upper Rhône River (Besacier-Monbertrand et al., 2010). This phenomenon must be surveyed to establish if the community of macroinvertebrates in the restored channels is resistant to the progressive expansion of the non-indigenous species. On the Loire River, Lasne et al. (2007) stated that native fish species decreased in disconnected channels, while non-native increased. They suggested that restoring the hydrological connectivity should contribute to higher diversity of fish in the Loire and limit the non-native species. Such measure on the Rhône River will increase the potential introduction and establishment of some non-indigenous freshwater macroinvertebrates as underlined by Besacier-Monbertrand et al. (2010) (appendix VI). In this context, we recommend that floodplain pools devoided of non-indigenous species before restoration should be preserved. A complement should be to assess the level of rarity of the pre-restoration community at the scale of the floodplain. If the community is considered as rare and of value, the dredging or the reconnection should be avoided to preserve the community. If the community is common or shows an already high proportion of invasive species, the dredging or the reconnection may be applied. Some unrestored pools within a channel may therefore be preserved to allow rapid recolonization by plants and animals of the other restored parts of the same channel. (Lasne et al., 2007)

Table 1:Non-indigenous species observed in the Upper Rhône River floodplain before restoration (from summer 2003 to summer 2006) and after restoration (from spring 2007 to summer 2008).

Number of colonized sites

Name Before After

Dikerogammarus villosus * 0 14

Hypania invalida 0 5

Orconectes limosus 1 2

Crangonyx pseudogracilis 2 10

Dreissena polymorpha 4 12

Dugesia tigrina 4 17

Corbicula fluminea 8 19

Gyraulus parvus 10 16

Potamopyrgus antipodarum 13 19

Physella.( heterostropha / acuta) 24 22 *observed in the main river channel in 2004

6.3.2. The temporal dimension.

The temporal dimension, which has traditionally been presented as the "fourth dimension" of fluvial hydrosystem functions, is clearly crucial to be taken into account (Cellot et al., 1994; Kondolf et al., 2006; Ward, 1989) in order to assess the biodiversity changes and their consequences (Chapin et al. 2000). Comparatively to spatial dimensions, temporal dimension has received only little attention, probably because of the costs associated with long-term monitoring, and certainly deserves further attention after the ecological restoration of a floodplain (Lake et al., 2007).

Sampling and assessment of the restored channels over time will allow assessment of their fluctuation, succession speed and life duration. The study carried out here two years after restoration, only allowed description of what could be regarded as "first colonizers". The communities described here may be replaced by more diversified ones over time. Currently the three beta diversity (taxonomical composition, rarefied richness, functional diversity) showed neither homogenization nor diversification two years after restoration. Post-restoration monitoring should be longer to assess

diversification or homogenization at least at a decennial timescale (Lake et al., 2007).

Moreover, we do not know how this succession might be affected by the spread of non-indigenous species in the floodplain. It remains to be evaluated if the restored communities will be resilient to this propagation (Jansson et al., 2007; Lake et al., 2007). As Palmer et al. (2005) stated, the restored system should require only minimal interventions after restoration and the resilience of the system should also be increased. The consideration of the floodplain temporal dimension after restoration will allow assessing the success of the restoration programme.

6.3.3. A holistic approach.

A holistic approach of the aquatic components of the floodplain implies the consideration of the overall aquatic diversity. The richness and the functional characteristics of other biotic components may be distributed differently along the lateral dimension of a floodplain (Amoros and Bornette, 2002; Tockner et al., 1999b).

Johnson and Hering (2009) showed that the responses of biological groups (fish, vegetation, diatoms and invertebrates) along a gradient of nutrient enrichment differ between groups. Those patterns may respond differently to the restoration of the ecosystem. For example, Funk et al. (2009) showed that the water enhancement in a secondary channel of the Danube positively impacted Dragonflies and Molluscs, while fishes were not impacted. In parallel to the diversity of the macroinvertebrates considered alone, it might be crucial to assess the overall diversity of other aquatic components of the floodplain for a better understanding of the system and to guide restoration works. The current thesis offers a way to assess the lateral dimension and provides information about macroinvertebrate diversity. The same approach could be used for example with fish and vegetation. In turn, fish-vegetation-macroinvertebrate data may be confronted to better indentify hot-spots of richness and functional diversity. Such a holistic approach would permit to guide the restoration works aiming at a fully functional hydrosystem. (Johnson and Hering, 2009)

CONCLUSION

The lateral hydrological connectivity is not easy to measure directly as such because it encompasses a complex set of interacting effects (shear stress, sedimentation / erosion processes, control upon vegetation quantity and structure, thermal and chemical properties of the water). In the absence of direct and independent measurement of hydrological connectivity on the Rhône River, surrogates variables were used to describe the lateral connectivity and gathered together in a composite variable. This composite variable proved to be a successful surrogate of the lateral connectivity.

The lateral hydrological connectivity between the main river and floodplain water-bodies is a key determinant of aquatic macroinvertebrate diversity in the Rhône River. The macroinvertebrate taxonomical composition, their total richness, the richness of some taxonomical subgroups and the functional characteristics of the communities are strongly related to the gradient of lateral connectivity. Taxonomical composition shows the strongest response to this gradient, with a continuous change along the lateral hydrological connectivity, from reophilic species in connected channels to limnophilic species in disconnected ones.

The total richness and the richness of some taxonomical subgroups also depict the effect of the gradient of lateral connectivity upon the macroinvertebrates. Taxa such as Ephemeroptera, Plecoptera, Trichoptera were mostly present in the main river channel and decreased along the gradient. Coleoptera and Odonata show the opposite pattern with a maximum of richness in the most disconnected environments. The former are progressively replaced by the latter along the gradient of decreasing connectivity and the total richness shows a maximum in the most disconnected environment.

Functional characteristics of the taxa are good indicators of the effects of hydrological connectivity upon macroinvertebrates. Connected channels are inhabited by communities dominated by plurivoltine species, drifters and passive filter feeders.

Those strategies are related to the continuous or frequent presence of a flow through the channels. Disconnected channels are inhabited by a lower frequency of plurivoltine, drifters and passive filter feeders, but by a higher proportion of predators.

This latter trait is interpreted as indicating more complex communities with higher interspecific interactions in disconnected channels.

In the case of the Rhône River, restored channels underwent what can be regarded as "rejuvenation" in the context of floodplain succession, while unrestored channels underwent natural changes with progressive terrestrialization. The enhancement of the lateral hydrological connectivity influenced key physical habitat parameters (e.g.

sediment composition, aquatic vegetation structure, hydraulic stress) and macroinvertebrate taxonomical composition, functional characteristics and subgroups of richness in a predictable way. Among restoration operations, the direct reconnection was found to account for the highest reduction of macroinvertebrate total rarefied richness and functional diversity, followed by dredging, which had a less contrasted impact. Currently no negative side effects of restoration could be observed and the restoration-induced diversification of lateral connectivity between channels prevented a homogenization of the macroinvertebrate diversity. The absence of short-term homogenization for the three diversity metrics, the taxonomical composition, the rarefied richness and the functional diversity, provides an overall positive assessment of the restoration programme after two years.

The results were obtained two years after restoration and it is now important to establish if macroinvertebrate communities are in a temporary and vulnerable state or in a more stable state. In parallel to the restoration programme, the floodplain is also influenced by the constant spread of non-indigenous species, some of them being new for the French upper-Rhône. The main river channel is the principal dissemination route for those species. Attention must be paid to the resilience of restored communities to new disturbances and exposed to the spread of non-indigenous species.

Nowadays, 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 no restoration may lead to increased terrestrialization and the progressive disappearance of aquatic systems. Floodplain reconnection is a widely applied method to restore such ecosystems, 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. Therefore, we recommend that restoration procedures aim at re-creating a diversity of connectivity levels among channels at the floodplain scale, in order to preserve a maximum of biodiversity.

The sampling effort carried out in this study should be maintain in the time to establish the success of restoration measures upon macroinvertebrate diversity. The post-restoration observations were made two years after restoration, it remains to be seen over the long term the evolution of the restored habitats and their associated communities.

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