Realised and predicted changes in the invertebrate benthos after restoration of connectivity to the floodplain of a large river

Abstract

1. River–floodplain interactions (i.e. lateral connectivity) are a major driver of the structure and function of the biota in floodplain waterbodies. Therefore, the enhancement of lateral connectivity is a key focus of large river restoration. The Rhône River restoration project in France includes an array of floodplain habitats and restoration measures, offering opportunities to test the predictability of changes in taxon abundance after connectivity restoration. 2. Quantitative samples of benthic macroinvertebrates taken before restoration were used to derive generalised additive models (GAMs) relating the density of taxa to an index of lateral connectivity. The index combined the diversity of sediment grain size, its organic content, vegetation cover and the water’s electrical conductivity. GAMs were used to predict changes in taxon density and between-site (i.e. beta) diversity 2 and 4 years after restoration at 34 sites (14 unrestored, eight reconnected to the river, six dredged, six restored by increasing river flow). 3. Observed post-restoration changes included an increase in the density of rheophilous taxa and an increase of beta-diversity among restored sites. Fifty-four GAMs (27% of the taxa) were selected for post-restoration predictions. Predictions from these models matched observations for a set of Trichoptera and other taxa with large density changes after restoration. The quality of predictions of density changes was poor on average among sites, but higher for sites where large changes in lateral connectivity occurred. Taxon-level predictions partly explained increased beta-diversity in restored sites, suggesting that predictions based on a subset of individual taxa can be used to predict community-level changes. 4. Our models can guide restoration measures in the Rhône and other rivers. Our results also show the benefit of diversifying restoration actions at the scale of entire floodplains. Post-restoration trajectories should be monitored over long periods to improve the identification of the key environmental predictors of biotic changes, especially at the less connected floodplain sites

Hydromorphological conditions in eighteen restored floodplain channels of a large river: linking patterns to processes

“…Such an improvement is suggested by the increased range of the lateral connectivity index values after restoration mentioned in Castella et al (2015) and the corresponding higher beta diversity of macroinvertebrate assemblages across restored floodplain channels. In particular, pre-restoration data were available for 88% of the channels that were reconnected downstream but only 57% of channels that were reconnected at both ends.…”

“…Castella et al, 2015;Paillex et al, 2015) is strongly related to overflow frequency and shear stress (Fig. Castella et al, 2015;Paillex et al, 2015) is strongly related to overflow frequency and shear stress (Fig.…”

“…sediment dynamics, connection frequency, hydraulics) due to heterogeneous and dynamic topographies, multiple connections with the main channel and difficulties of performing measurements during floods (e.g. According to Castella et al (2015), their indirect index potentially integrates all aspects of lateral connectivity, including effects of groundwater supply. Due to the lack of such information for the Rhône floodplain channels, hydrobiologists studying these systems have used indirect descriptions of floodplain channel lateral connectivity defined as a linear combination of organic matter content, water electrical conductivity, horizontal cover by submerged aquatic vegetation and substratum diversity Paillex et al, 2015).…”

Testing predictions of changes in alien and native macroinvertebrate communities and their interaction after the restoration of a large river floodplain (French Rhône)

“…Such models complement taxa-level models as those developed in another article in this Special Issue (Castella et al, 2015). Here, we develop community-level models to predict changes in alien and native macroinvertebrate communities after restoration and to investigate potential interactions between them.…”

Lateral and longitudinal patterns of water physico-chemistry and trace metal distribution and partitioning in a large river floodplain

“…Studies looking for main drivers of community changes in floodplains invariably found hydrological variables describing the connection, respectively the isolation from the main channel to be the most important factors (e.g., bacteria (Lew et al, 2016), phytoplankton (Grabowska et al, 2014), macrophytes (Keruzoré et al, 2013), macroinvertebrates (Castella et al, 2015;Paillex et al, 2007), and fish (Daufresne et al, 2015)). However, hydrological connections were often modified by anthropogenic interventions affecting floodplains and large rivers (e.g., Olivier et al, 2009a;Tockner and Stanford, 2002).…”