OOS 11-8 - Macroinvertebrates in the lateral dimension of a river floodplain: Challenges for restoration and conservation

Tuesday, August 4, 2009: 10:30 AM
Aztec, Albuquerque Convention Center
Amael Paillex, University of Geneva and Emmanuel Castella, University of Geneva
Background/Question/Methods

In large alluvial floodplains, cut-off channels can be permanently or only temporarily connected to the main river during floods. As a result, lateral hydrological connectivity plays a major role in shaping both the habitat conditions and the biota of floodplain ecosystems. This role is often strongly impacted by human activities leading to the progressive and rapid disconnection of secondary channels. Current restoration strategies in large river floodplains focus on the increase in lateral connectivity of the secondary channels. In this study, we constructed a framework to assess a gradient of lateral connectivity in thirteen cut-off channels of two braided sectors of the Rhône River (France), and compared the sensitivity of several invertebrate-based metrics to this gradient. The metrics were based, either upon taxonomic diversity or upon functional traits. Comparisons between predicted and observed values of the metrics in restored channels, allowed us to assess the effect of an increase in the hydrological connectivity on the biological characteristics of macroinvertebrate assemblages

Results/Conclusions

Six environmental variables, depicting physical and chemical characteristics, were used to construct a synthetic variable describing the hydraulic connectivity of each channel.

This synthetic variable enabled a clear ordering of channels into three types according to levels of hydrological connectivity. Moreover, it permitted us to compare the invertebrate-based metrics. Indeed, EPT richness was highest in the disturbed channels and significantly increased with lateral connectivity (R2 = 0.67, p < 0.001), thus reflecting the effect of hydraulic disturbance on insect assemblages. Increasing proportions of macroinvertebrates with short life cycles along the connectivity gradient (R2 = 0.43, p < 0.001), provided evidence of the dominance of rapid-colonizing species in frequently disturbed habitats. The proportion of predators decreased along the same gradient (R2 = 0.27, p = 0.004), suggesting a decrease of community interactions with increasing flood disturbance. Post-restoration sampling revealed deviations of the assemblages, in restored channels, from their expected state. A large proportion of colonizers (percentage of plurivoltine species) were favoured by restoration operations, while at the same time, non-native species percentages increased. The coherence of the relationships established for some of the richness and trait-based metrics demonstrates their potential for the development of invertebrate-based tools to predict and monitor river-floodplain changes, especially those associated with restoration. We recommend that floodplain-scale restorations focus on diversification of the lateral hydrological connectivity of channels, thereby, conserving a maximum of functional characteristics in macroinvertebrate assemblages.

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