Testing predictions of changes in benthic invertebrate abundance and community structure after flow restoration in a large river (French Rhône)

Abstract

1. Principal threats to running waters are linked to human-made discharge modifications, but tools to predict the quantitative consequences of flow restoration for benthic invertebrates in large rivers remain untested. 2. Quantitative benthos samples from two bypassed reaches (Pierre-B enite -PBE -and Chautagne -CHAU) of the French Rhône River were collected during four years each before and after minimum flow increases (from 10 to 100 m 3 s À1 at PBE and from 10 to 50 m 3 s À1 at CHAU). These samples provided observed ln-density changes for the 50 and 62 (PBE and CHAU, respectively) most abundant taxa (typically species or genera). For about half of the 'model' taxa among them, distinct preference models for bottom shear stress categories were available from four reaches of the Upper Rhône River and from various German rivers. Linking these preference models with a statistical hydraulic model predicting frequencies of shear stress categories for any given discharge, we predicted ln-density changes of the model taxa after restoration. 3. Community structure of the abundant taxa changed clearly and rapidly after restoration at PBE but less clearly at CHAU. Our predictions explained a considerable amount of mean ln-density changes of our model taxa observed after restoration (75 and 30% at PBE and CHAU, respectively). They also explained (67 and 40% at PBE and CHAU, respectively) the model taxa scores along the principal components analysis axis that summarised the community variations of all abundant taxa before and after restoration. For taxa not identified to species (assessed as genus, tribe or family), the predicted ln-density changes were sometimes inaccurate at PBE (a Lower Rhône site), suggesting that the transferability of preference models for taxonomic levels above that of species can be problematic. 4. If further developed, statistical habitat models focussing on ecologically relevant physical variables (in our case shear stress) should enable reliable quantitative assessments of associations between flow restoration efforts and achievable ecological improvement.

The generality of changes in the trait composition of fish and invertebrate communities after flow restoration in a large river (French Rhône)

“…The two taxonomic articles concerning the main channels of bypassed reaches (datasets hereafter named CHAN INV and CHAN FISH for channel invertebrate and fish communities, respectively) involved three reaches with fish data (Chautagne, Belley, Pierre‐Bénite; Lamouroux & Olivier, ) and two with invertebrate data (Chautagne and Pierre‐Bénite; Mérigoux et al ., ). These reaches were subjected to an increase in the daily minimum flow after restoration (10‐fold at Pierre‐Bénite, fivefold at Chautagne and slightly more than twofold at Belley).…”

“…The approach acknowledges the variability in traits that often occurs among different life stages or populations of an organism (Blanck & Lamouroux, ). Moreover, based on the taxonomic levels used by authors in this issue, especially for the identification of invertebrates (Castella et al ., ; Mérigoux et al ., ), fuzzy coding was the only way to account for within‐species, within‐genus or even within‐family variation (especially for Diptera not identified beyond tribe level). As an example of the coding, Caenis luctuosa was given an affinity of 3 for the ‘plurivoltine’ category and 1 for the ‘univoltine’ category for the trait of the maximum number of reproductive cycles per year.…”

“…Multiple surveys of fish and invertebrate taxa were conducted before and after restoration in the main channels of restored reaches, and invertebrates were monitored in the floodplain channels. In addition, habitat models predicting changes in density after restoration (that is, after‐ minus before‐restoration average values) of several fish and invertebrate taxa were detailed in three articles of this Special Issue (fish of the main channel, Lamouroux & Olivier, ; invertebrates of the main channel, Mérigoux et al ., ; invertebrates of reconnected floodplain channels, Castella et al ., ; hereafter called ‘taxonomic’ articles). These predictions involved a subset of modelled taxa, which occurred in reaches before restoration and for which preference models (relating taxon density to physical habitat) were available in the taxonomic articles.…”

Débits écologiques : la place des modèles d’habitat hydraulique dans une démarche intégrée

“…Les modèles d'habitat ont par ailleurs été utilisés pour prédire, dans le temps, les effets biologiques de restauration de débits (Jowett & Biggs, 2006 ;Sabaton et al, 2008 ;Bradford et al, 2011 ;. Lorsque les modifications hydrauliques liés à la restauration des débits étaient fortes (c'est le cas de tronçons du Rhône où des débits minimaux ont été multipliés par des facteurs de 5 à 10 ; Mérigoux et al, 2015), les modèles ont permis de prédire avec précision des modifications d'abondance des espèces (poissons et invertébrés) ou de structure de communauté (ex. : proportions de groupes d'espèces multipliées par 2 ou 3).…”

“…Labels correspond to different fish and invertebrate taxa. From , Mérigoux et al (2015). (Saraeva & Hardy, 2009 ;Snelder et al, 2011 ;Wilding et al, 2014).…”