Thesis
Reference
Aquatic macroinvertebrate diversity along the lateral dimension of a large river floodplain : application to the Rhône River restoration
PAILLEX, Amael
Abstract
Hydrological connectivity plays a major role in shaping both the habitat conditions and the biota in floodplain ecosystems. Current restoration strategies in large river floodplains often focus on the increase in lateral connectivity of secondary channels. However, the knowledge on the effect of restoration strategies on biodiversity remains limited. In this study, a framework was constructed to assess the level of lateral connectivity in thirteen cut-off channels of two braided sectors of the Rhône River (France). The effect of restoration measures on macroinvertebrate diversity was assessed. Changes were measured within (i.e.
alpha diversity) and between channels (i.e. beta diversity). The coherence of the relationships established for some of the richness and trait-based metrics demonstrated their potential for the development of invertebrate-based tools to predict and monitor river-floodplain changes associated with restoration. At the channel scale, an increase in lateral connectivity induced a significant change in macroinvertebrate composition, a decrease of total richness and functional diversity. It is [...]
PAILLEX, Amael. Aquatic macroinvertebrate diversity along the lateral dimension of a large river floodplain : application to the Rhône River restoration. Thèse de doctorat : Univ. Genève, 2010, no. Sc. 4211
URN : urn:nbn:ch:unige-129388
DOI : 10.13097/archive-ouverte/unige:12938
Available at:
http://archive-ouverte.unige.ch/unige:12938
Disclaimer: layout of this document may differ from the published version.
Institut Forel Dr. Emmanuel Castella
Aquatic Macroinvertebrate Diversity along the Lateral Dimension of a Large River Floodplain.
Application to the Rhône River Restoration.
THÈSE
présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention interdisciplinaire
par
Amael PAILLEX de
Chavannes des Bois (VD)
Thèse No 4211
GENĖVE
Atelier d’impression ReproMail 2010
Paillex, A.:
Terre & Environnement, vol. 100, xviii + 172 pp. (2010)
ISBN 2-940153-99-X
Section des sciences de la Terre et de l'environnement, Université de Genève, 13 rue des Maraîchers, CH-1205 Genève, Suisse Téléphone ++41-22-379.66.28 - Fax ++41-22-379.32.11
Aquatic macroinvertebrate diversity along the lateral dimension of a large river floodplain. Application to the Rhône River restoration.
“Ecological restoration is a recent discipline that should be conducted scientifically and rigorously to move from a trial-and-error process to a predictive science to increase its success and the self- sustainability of restored ecosystems”
Henry and Amoros (1995)
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REMERCIEMENTS
REMERCIEMENTS
Je remercie le Professeur Jean-Bernard Lachavanne de m’avoir accueilli au sein de son laboratoire pour effectuer le présent travail de thèse sous la direction du Dr. Emmanuel Castella. Je remercie vivement le Dr. Emmanuel Castella qui a encadré ce travail de thèse.
Sa passion, son enthousiasme, son dévouement et sa patience ont été précieux pour moi durant ces années de thèse, je lui en suis très reconnaissant.
Je remercie chacun des membres du jury de cette thèse d'avoir accepté d'évaluer le présent travail, Professeur Jean-Bernard Lachavanne (Université de Genève), Professeur Sylvain Dolédec (Université de Lyon), Professeur Beat Oertli (HES-SO Genève), Docteur Jean- Michel Olivier (Université de Lyon), ainsi que le Docteur Emmanuel Castella (Université de Genève).
Ce travail n’aurait pas pu aboutir sans l’aide de nombreuses personnes qui ont participé aux campagnes de prélèvement, aux tris des échantillons et aux déterminations des macroinvertébrés, je remercie D. McCrae, N. Peru, A.L. Besacier-Monbertrand, O. Béguin, F. Play, A. Schutzle, E. Malet, S. André, C. Ilg, ainsi que les membres du bureau de l'ARALEP à Lyon. Je tiens également à formuler mes remerciements posthumes à Gilles Carron qui a déterminé tous les coléoptères aquatiques, et de ce fait laisse un bel héritage aux spécialistes du domaine ainsi que pour le programme de restauration. Je remercie E.
Castella et D. McCrae qui ont tous deux corrigé l’anglais de mes travaux. Mes remerciements s'adressent également à B. Lachal, E. Pampaloni et P. Arpagaus de l’Institut Forel qui m'ont tous aidé pour effectuer diverses mesures de variables environnementales.
Ce travail ne serait pas grand chose sans les collaborations développées pendant 5 ans avec les membres du Laboratoire d’Ecologie des Hydrosystèmes Fluviaux de l’Université de Lyon, je remercie particulièrement Professeur Sylvain Dolédec, Dr. Sylvie Mérigoux, Dr.
Jean-Michel Olivier, ainsi que Dr. Nicolas Lamouroux et Dr. Hervé Piégay, tous deux participant au suivi scientifique du programme de restauration du Rhône.
financement de ce travail; l’Université de Genève, les financeurs du programme décennal de restauration écologique et hydraulique du Rhône : La Compagnie Nationale du Rhône, La Direction Régionale de l’Environnement, l’Agence de l’Eau et de nombreuses collectivités publiques ; je remercie également pour leur soutien lors de congrès : la Société Académique de Genève, le Laboratoire d'Ecologie et de Biologie Aquatique, ainsi que l'Institut Forel.
Mes remerciements s'adressent également aux membres du Laboratoire d’Ecologie et de Biologie Aquatique avec qui j’ai partagé passion, science et la vie de tous les jours pendant de nombreuses années. Je remercie tous les étudiant(e)s et stagiaires avec qui j'ai travaillé lors de ma thèse.
Que les membres du groupe de réflexion de l’APCINT (Association Pour le Corps Intermédiaire de l’Université de Genève), avec qui j’ai partagé de nombreuses réunions à discuter du rôle du doctorant au sein du corps intermédiaire et de la place de ce dernier au sein de notre Université, soient remerciés.
Je remercie mes amis qui ont beaucoup compté tout au long de mes études et de mon travail de doctorat. Vous étiez présents tout au long de ce travail et chacun a une place toute particulière dans l'aboutissement de ce travail.
Enfin, mes remerciements s'adressent à mes proches et à toute ma famille pour leur intérêt et leurs encouragements, particulièrement à Michel, Pascale, Dorane, mes frères Camille, Aurélien et Jonas. J'aimerais remercier chacun d'entre vous de votre présence, de votre générosité et de votre soutien. Et enfin Hélène pour son amour. Merci à Tous.
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RÉSUMÉ
Ensembles d’écosystèmes d’une grande complexité, les plaines alluviales sont des milieux exceptionnels à haute valeur écologique, sociale et économique. Ces écosystèmes permettent le maintien d’une importante biodiversité qui est le produit des habitats diversifiés qui les composent et de la dynamique fluviale qui les régit. Bien qu’étant le siège d’une biodiversité remarquable, les plaines alluviales sont soumises à des altérations de type anthropiques.
La plaine alluviale du Rhône a connu une altération progressive de son fonctionnement suite à la construction de digues et d’usines hydroélectriques. Les aménagements ont progressivement réduit la connectivité latérale des annexes fluviales avec le cours principal du fleuve entrainant une altération des communautés floristiques et faunistiques constitutives de ces annexes. Fort de ce constat, un programme de restauration a été établi pour le Rhône. Sa mission consiste à augmenter la connectivité latérale des annexes fluviales avec le cours principal, par leur reconnexion avec ce dernier, le recreusement de certaines d’entre elles et par une augmentation des débits circulant dans le fleuve court-circuité. Ces actions visent à rétablir une dynamique hydrologique et morphologique au sein des annexes et à restaurer leur biodiversité.
Le présent travail de thèse a pour objectif i) d’établir la distribution et la diversité des macroinvertébrés aquatique le long d’un gradient de connectivité latérale, de décrire les cortèges faunistiques et leurs caractéristiques avant les travaux de restauration, ii) de prédire et d’établir un diagnostic des effets liés à l’augmentation de la connectivité latérale sur les communautés de macroinvertébrés.
Dix-huit annexes fluviales couvrant un large spectre de conditions au sein de la plaine alluviale ont été étudiées. La connectivité latérale de ces annexes avec le cours principal a été établie sur la base de variables représentative de cette connectivité. Quatre groupes de métriques basées sur les macroinvertébrés ont été testées le long de ce gradient i) la composition taxonomique, ii) la richesse taxonomique totale et celle de sous- groupes taxonomiques, iii) la représentation de certains traits fonctionnels, iv) la diversité fonctionnelle. Un total de 576 échantillons de faune ont été prélevés selon un échantillonnage stratifié, répété en été, au printemps et également avant et après restauration. Cinq variables environnementales ont servis à construire une variable synthétique décrivant la connectivité latérale de chaque annexe fluviale.
Avant restauration, les variables environnementales mesurées permettent d’ordonner les annexes le long d’un gradient de connectivité latérale. La composition des communautés de macroinvertébrés reflète ce gradient de connectivité (R2 = 0.73, p < 0.001), les communautés de types rhéophiles étant présentes dans les annexes fréquemment connectées et progressivement remplacées par des communautés de types limnophiles. De plus, la richesse raréfiée diminue le long de ce gradient pour atteindre un minimum dans les annexes les plus connectées au cours principal (R2 = 0.27, p = 0.002). En revanche, la richesse en EPT (Ephemeroptera, Plecoptera, Trichoptera) atteint un maximum dans les annexes connectées (R2 = 0.61, p < 0.001). Le pourcentage d’individus plurivoltins augmente avec la connectivité (R2 = 0.42, p < 0.001), suggérant une dominance des espèces à colonisation rapide dans les annexes fréquemment perturbées. Enfin, les prédateurs sont plus abondants dans les milieux déconnectés (R2 = 0.28, p < 0.01), à l’inverse des filtreurs passifs abondants dans les milieux connectés (R2 = 0.50, p < 0.001).
Deux ans après restauration, les variables de milieu attestent bien d’un "rajeunissement" des annexes fluviales restaurées, tandis que les annexes non-restaurées ont suivi une trajectoire opposée, conforme à la succession naturelle vers l’atterrissement. Lors de reconnexion au cours principal, les communautés subissent une diminution de la richesse raréfiée totale (diversité alpha) (Wilcoxon apparié, p < 0.001), tandis que la diversité entre annexes (diversité beta) reste similaire après restauration. La diversité fonctionnelle au sein des annexes diminue également dans les annexes reconnectées (diversité alpha) (Wilcoxon apparié, p = 0.03), tandis que la diversité fonctionnelle entre annexes reconnectées (diversité beta) reste inchangée après restauration. Les travaux tels que les recreusements n’induisent pas de changements notables des deux paramètres, richesse raréfiée totale et diversité fonctionnelle. En revanche, les changements de connectivité latérale engendrés par les travaux de restauration induisent en général des changements linéaires et prédictibles de la composition faunistique (R2 = 0.74, p < 0.001) et de certaines caractéristiques de la communauté : augmentation du pourcentage d’individus à tendance dérivante (R2 = 0.45, p < 0.01), de la richesse en EPT (R2 = 0.24, p <
0.01) et de la richesse en gastéropodes (R2 = 0.26, p < 0.01). Un point positif du programme de restauration est l’absence d’homogénéisation de la composition taxonomique entre chenaux à l’échelle de la plaine alluviale.
La significativité des relations linéaires entre les métriques basées sur les macroinvertébrés et le gradient de connectivité avant restauration soulignent la composante déterministe de la dimension latérale de la plaine alluviale. Ces relations démontrent également le potentiel des métriques faunistiques pour prédire et suivre les changements naturels ou anthropiques à l’échelle d’une plaine alluviale. Selon les résultats, les travaux de restauration qui influencent la connectivité latérale induisent des changements prédictibles de la diversité des macroinvertébrés. L’analyse hiérarchique de la diversité montre que même si des diminutions au sein des annexes restaurées sont observées (diversité alpha), la diversité entre annexes reste stable (diversité beta) deux ans après restauration.
Les résultats ont été obtenus deux ans après les travaux de restauration, il s’agit maintenant de suivre les effets des travaux dans le temps, afin de déterminer si les communautés de macroinvertébrés sont dans un état transitoire ou ont déjà atteint un état stable. Une attention particulière devra être portée aux communautés restaurées possédant une faible diversité fonctionnelle, face à de nouvelles perturbations et à la progression continue des espèces non-indigènes.
L’effort d’échantillonnage effectué dans ce travail fournit un jeu de données considérable et sans précédent sur l’effet des travaux de restauration sur les communautés de macroinvertébrés en zone alluviale. L’augmentation de la connectivité latérale des annexes fluviales est la méthode la plus répandue pour restaurer ce type d’environnement. Cependant, selon la réponse des communautés de macroinvertébrés, une homogénéisation de la connectivité dans une plaine alluviale avec de multiples annexes peut conduire à une réduction de la biodiversité. Il convient donc de diversifier les niveaux de connectivité entre les annexes, afin de restaurer la biodiversité des plaines alluviales et de ne pas porter atteinte à l’hydrosystème dans son ensemble.
Mots clés : plaine alluviale, macroinvertébrés, biodiversité, connectivité latérale, Rhône, annexes fluviales, restauration.
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ABSTRACT
In large alluvial floodplains, cut-off channels can be permanently connected to the main river or only temporarily during floods. 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 often focus on the increase in lateral connectivity of the secondary channels.
A large-scale hydrological and ecological restoration programme has been initiated on several sections of the French Rhône River, where by-passed hydro-power plants were constructed until the beginning of the 80’s.
Restoration measures include the re-connection or deepening of cut-off floodplain channels and increasing the discharge into the by-passed sections, in order to increase their lateral connectivity with the main channel.
The objectives of the thesis are i) to provide a better understanding of aquatic macroinvertebrate diversity patterns in the lateral dimension of floodplain ecosystems, by an analysis of the taxonomical and the functional response of macroinvertebrates along the gradient of lateral connectivity, ii) to predict and analyse the effects of restoration measures on aquatic macroinvertebrate diversity.
In this thesis, a framework was constructed to assess the gradient of lateral connectivity in eighteen cut-off channels of two braided sectors of the Rhône River (France), and the sensitivity of several macroinvertebrate- based metrics to this gradient was compared. The selected metrics belong to four groups i) taxonomical composition, ii) taxonomic diversity, iii) functional trait representation and iv) functional diversity. The analysis was conducted to describe the pre-restoration state of the cut-off channels and further to monitor the consequences of restoration measures. A total of 576 quantitative samples were collected using a stratified design repeated in summer and spring, before and two years after restoration. Five environmental variables, depicting physical and chemical characteristics, were used to construct a synthetic variable describing the lateral hydrological connectivity of each channel.
Before restoration, the synthetic variable enabled a clear ordering of channels into three types according to levels of lateral hydrological connectivity. Macroinvertebrate composition changed along the gradient of lateral connectivity from lenitophilous to reophilous species (R2 = 0.73, p < 0.001). The total richness decreased along the gradient to reach a minimum in the most connected channels (R2 = 0.27, p = 0.002). Conversely, EPT richness was highest in the most connected channels and increased significantly with lateral connectivity (R2 = 0.61, p < 0.001), thus reflecting the effect of hydraulic disturbance on insect assemblages. The proportion of macroinvertebrates with short life cycles increased with connectivity (R2 = 0.42, p < 0.001), providing evidence of the dominance of rapid-colonizing species in frequently disturbed habitats. The proportion of passive filter feeders increased along the same gradient (R2 = 0.50, p < 0.001), suggesting an increase of flow permitting filtration. The proportion of predators decreased with increasing connectivity (R2 = 0.28, p < 0.01), suggesting a decrease of community interactions with increasing flood disturbance.
did “rejuvenate” the reconnected floodplain channels, while unrestored channels followed the opposite direction. Post restoration monitoring showed a decrease of rarefied richness within reconnected channels (alpha diversity) (Wilcoxon signed rank test, p < 0.001), while the between channels diversity (beta diversity) remained similar after restoration. The functional diversity within reconnected channels also decreased (alpha diversity) (Wilcoxon signed rank test, p = 0.03) whereas it stayed stable between the same channels (beta diversity). As opposed to full reconnection, dredging had less contrasted effects on rarefied richness and functional diversity. Restoration measures induced linear and predictable changes in taxonomical composition (R2 = 0.74, p < 0.001), in the percentage of drifters (R2 = 0.45, p < 0.01), in EPT rarefied richness (R2 = 0.24, p < 0.01) and Gastropoda rarefied richness (R2 = 0.26, p < 0.01). A positive point for the restoration measures is the absence of homogenization of taxonomical composition between channels after restoration (beta diversity).
The significant relationships observed between changes in lateral connectivity and changes in community composition, rarefied richness of taxonomic subgroups and functional characteristics emphasized a strong deterministic component in the lateral dimension of the floodplain. It also demonstrated the potential for the development of invertebrate-based tools to predict and monitor river-floodplain changes, especially those associated with restoration. 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.
Hierarchical analysis of macroinvertebrate diversity underlined that even if within channel diversity (alpha level) decreases are observed after restoration, the between channels diversity may stay stable (beta level).
Our results were obtained two years after restoration and it is now important to establish if macroinvertebrate communities are in a transient state or in a more stable one. Attention must be paid to the resilience of restored communities with lower functional diversity confronted with new disturbances and exposed to the spread of non-indigenous species.
The sampling effort carried out in this study provided an unprecedented large data-set on restoration effects upon macroinvertebrates in a floodplain system. Increasing lateral connectivity of floodplain channels is the most widely applied method to restore such systems, but according to the observed responses of macroinvertebrate communities, homogenization of connectivity levels in multi-channel systems, may lead to a reduction of biodiversity. Therefore, it is recommended that floodplain-scale restorations focus on diversification of the lateral hydrological connectivity of channels, thereby, conserving a maximum of biodiversity.
Key words: floodplain, large river, macroinvertebrates, biodiversity, lateral connectivity, Rhône River, secondary channels, restoration.
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CONTENTS
Pages
Remerciements iv-v
Résumé viii-ix
Abstract xii-xiii
1. Introduction 1-22
1.1 The scope 2
1.2 Floodplains 2-5
1.3 Aquatic macroinvertebrates 5-7
1.4 Functional characteristics 7-9
1.5 Floodplain restoration 9-11
1.6 The Rhône River restoration programme 11-13
1.7 Objectives of the study 13-15
1.8 References 16-21
2. Aquatic macroinvertebrate response along a gradient of lateral 23-42 connectivity in river floodplain channels
2.1 Introduction 25-27
2.2 Methods & Results 27-34
2.3 Does lateral connectivity influence 34-37
taxonomic richness and composition ?
2.4 Functional group ratios along the connectivity gradient 37 2.5 Comparison of the metrics and perspectives 37-38
2.6 Literature cited 38-40
2.7 Appendix 41-42
3. Large river floodplain restoration: predicting species richness
and trait responses to the restoration of hydrological connectivity 43-60
3.1 Introduction 45-46
3.2 Methods & Results 46-49
3.3 The significance of hydrological connectivity for 49-50 aquatic macroinvertebrate diversity
3.4 Are the characteristics of the macroinvertebrate 50-51 communities predictable ?
3.5 Restoration of hydrological connectivity and non-native species 51 3.6 Implication for future floodplain restoration 51-52
3.7 Supplementary material 53-60
4. The Intermediate Disturbance Hypothesis: the role of voltinism 61-86 and dispersal in aquatic macroinvertebrates
4.1 Abstract and Introduction 63-67
4.2 Methods & Results 68-76
4.3 Discussion and Conclusion 77-83
4.4 References 84-86
5. Floodplain restoration provides contrasted pictures of changes 87-108 in taxonomical composition, richness and functional diversity
5.1 Abstract and Introduction 89-91
5.2 Methods & Results 92-101
5.3 Discussion 102-105
5.4 Supporting material 106
5.5 References and Notes 107-108
6. Discussion: How do macroinvertebrates respond to lateral 109-136 hydrological connectivity and its modification induced by restoration?
6.1 Lateral dimension of the floodplain
6.1.1 Lateral hydrological connectivity 111-113
6.1.2 Lateral connectivity after restoration 113-115 6.2 Aquatic macroinvertebrate patterns in the lateral dimension
of the floodplain
6.2.1 Does lateral connectivity influence taxonomical 116-119 composition and richness ?
6.2.2 Taxonomical composition and richness after restoration 119-121 6.2.3 Macroinvertebrate traits along the lateral dimension 122-123
of a floodplain
6.2.4 Beyond the non-significant results of macroinvertebrate 124-125 traits along the gradient of lateral connectivity
6.2.5 Functional diversity (FD) 126-127
6.2.6 Functional characteristics and diversity after restoration 128-129 6.2.7 Alpha and beta diversity among macroinvertebrate metrics 130-132 6.3 Future needs
6.3.1 Restoration measures and non-indigenous 133-135 aquatic macroinvertebrates
6.3.2 The temporal dimension 135-136
6.3.3 A holistic approach 136
7. Conclusion 137-139
8. References 140-146
9. Appendixes I to VI 148-172
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CHAPTER 1
INTRODUCTION
INTRODUCTION
Biodiversity means the diversity of what constitutes life and encompasses genetic diversity, species diversity and ecosystem diversity. Biodiversity is not evenly distributed on earth, some areas being rich and others devoided of life. Where it exists, it provides ecological services (e.g. maintenance of soil, water and air quality, decomposition of wastes, provision of food). Everywhere biodiversity is under pressure of human activities that are responsible for its degradation. The most obvious manifestation of biodiversity erosion or loss is the extinction of species. Habitat destruction, spread of invasive species, increasing quantities of pollutants and climatic change are among the most prominent reasons for biodiversity erosion. Nowadays, and certainly more than in the past, biodiversity needs to be protected or restored when and where it has been altered. This thesis deals with biodiversity changes in floodplain ecosystems, which have been widely and worldwide altered by human activities and are, for some of them, currently subject to large restoration programmes. This thesis attempts to characterize floodplain habitats of the Rhône River and their related aquatic biodiversity, in order to provide a better understanding of this set of ecosystems and sound restoration recommendations. It focuses on the lateral dimension of the floodplain, because it is along this dimension that man interacts to restore hydrological and ecological processes.
Floodplains and floodplain drivers
According to Junk et al. (1989), floodplains are defined as the area periodically inundated by lateral overflow, precipitation or groundwater acting on the organisms which develop adaptations. Floodplains encompass terrestrial and aquatic environments, developing on river sediment and subject to flooding. They are complex ecosystems with a mosaic of habitats and a high spatio-temporal variability (Cellot et al., 1994; Tockner and Stanford, 2002). They are four dimensional systems (Amoros and Bornette, 2002; Ward, 1989). Their functions and heterogeneity are developed along the longitudinal, the lateral, the vertical and the temporal dimensions (see fig.1).
The longitudinal dimension covers the pathway from the upstream parts of the river to
adjacent floodplain, while the vertical dimension is related to the exchanges between groundwater and the floodplain surface. Finally, the temporal dimension is related to the changes occurring at various times scales. Among others, it incorporates both daily and seasonal discharge (and hence water level) fluctuations, and sucessional trends taking place in the floodplain over decades or centuries.
Figure 1: Three dimensions of a floodplain. Longitudinal (A), lateral (B), and vertical (C) dimensions. Adapted from Amoros & Petts (Hydrosystèmes fluviaux, 1993).
In this set of interacting ecosystems, water plays the main role to connect the various floodplain components and operates along the four dimensions mentioned above (Amoros and Petts, 1993). Hydrological connectivity is therefore one of the main drivers of floodplain ecosystems and it contributes to the creation and maintenance of floodplain habitats (Amoros and Bornette, 2002). Floodplains include a wide range of habitats from aquatic to terrestrial (Ward and Tockner, 2001) and, within the aquatic realm, from lotic to lentic and temporary (Ward and Stanford, 1995). The aquatic part of the floodplain contains the main river channel, which carries permanently the highest discharge and secondary channels that are permanently or temporarily linked to the main channel. Slope, bankfull discharge and sediment bedload define the geomorphologic type of the channels (i.e. straight, braided, meandering) (Amoros and Petts, 1993). The main river channel and the connected channels represent the lotic part of the system, while disconnected waterbodies represent the lentic one. The lateral dimension of the river encompasses this diversity of channels (Amoros and Bornette, 2002) from lotic to lentic, the latter being generally located at the margin of the floodplain (see fig.2). Therefore all floodplain waterbodies can be arranged along a gradient of lateral hydrological connectivity (Amoros et al., 1982).
A
B C
Figure 2: Decrease of the hydrological connectivity along the lateral dimension of a large river floodplain. Eupotamal channel (A), parapotamal channel (B), plesiopotamal channel (C), paleopotamal channel (C). Adapted from Amoros & Petts (Hydrosystèmes fluviaux, 1993).
Floodplains are highly dynamic and fluctuating systems. They can be considered as being always either in an expanding or a contracting state, depending on the trend of the water stage (Junk et al., 1989; Thomaz et al., 2007; Tockner et al., 2000). A floodplain can contain eupotamal channels that are permanently connected to the main river channel (Amoros et al., 1982); parapotamal channels, which remain permanently connected at their downstream end and disconnected upstream (Amoros et al., 1982) and plesiopotamal channels, which are totally disconnected from the main river channel and composed of isolated pools at average water level (Amoros et al., 1982).
Channels may have temporary parts with various hydroperiod durations. Finally, at the end of the aquatic succession, channels may become totally terrestrial. During high water periods all these channel types may be fully reconnected to the main river channel (Junk et al., 1989; Thomaz et al., 2007). Water acts as a vector for transport of organisms, nutrients, mineral and organic matter from connected channels to isolated pools at raising water stage, and vice versa during flood recession (Preiner et al., 2008;
Tockner et al., 1999a).
The diversity of channel types governed by hydrological connectivity provides a diversity of conditions. For example, in eupotamal channels, the constant flow prevents the establishment of vegetation and maintains a coarse mineral sediment, while in disconnected channels the aquatic vegetation is more abundant and the sediment becomes finer and incorporates a higher organic fraction (Amoros and
A B
C
D
permanent flow or during floods that enhance the rejuvenation of the channels, while the absence of hydraulic stress increases sediment deposition and therefore the terrestrialization of the channels. Natural and dynamic floodplains are subject to both rejuvenation and terrestrialization acting simultaneously at various spatial and temporal scales (Ward and Tockner, 2001). At the scale of a floodplain, a balance between the two processes exists and sustains a diversity of successional stages.
Floodplains are therefore dynamic systems that evolve in time. They are the support of adapted animal and vegetal communities (Amoros and Bornette, 2002; Tockner et al., 1999b).
Freshwater fauna and macroinvertebrates
Freshwater animals (fish, macroinvertebrates, zooplankton) and plants (macrophytes, algae and phytoplankton) inhabiting large river floodplain are highly diversified and build up complex communities (Amoros and Bornette, 2002; Castella et al., 1991;
Malmqvist, 2001; Ward and Tockner, 2001). Macroinvertebrates are defined as animals without back bones, larger than 3-5 mm at the last stage of their life (Tachet et al., 2000). Among floodplain animals, macroinvertebrates are the most diversified group of taxa. Their composition varies among the four dimensions of the river system and they cover all the floodplain conditions. In the River Continuum Concept (Vannote et al., 1980), the authors underlined that macroinvertebrates are not only taxonomically diversified along the longitudinal gradient of the river, but also functionally. They reflect the differences of conditions that exist from the upstream to the downstream parts of the river. Macroinvertebrates also cover the full range of habitats that exists in the lateral dimension of the floodplains and have been shown to be 'describers' of morphological and hydrological conditions (Castella et al., 1984).
The hydrological connectivity strongly influences the composition of macroinvertebrate assemblages. Rheophilous species inhabit the main river and connected channels, while lenitophilous species inhabit disconnected channels. This observation was sustained by different researches on different riverine floodplains (Arscott et al., 2005; Gallardo et al., 2008; Paillex et al., 2007; Reckendorfer et al., 2006).
Macro - invertebrates
many insects (Hayden and Clifford, 1974; Smock, 1994). Some river species depend upon the existence of floodplain waterbodies to complete their life cycle. During flood periods secondary channels may be connected to the main river channel (Junk et al., 1989) and nymphs have the opportunity to move from the main river channel to floodplain pools. Macroinvertebrates can also take refuge in the floodplain during floods, before returning to their initial habitat by drift or aerial recolonization (Wallace, 1990). Emergences of river insects may occur in floodplain pools where vegetation is abundant and the conditions less stressful. Afterwards, adult fly back to the river to reproduce and lay eggs.
Despite the movement of some species, Tockner et al. (1999b) underlined that the species richness on the Danube River was recorded as maximal in a channel with an intermediate degree of connectivity. This result supported the general idea that species diversity is maximum at an intermediate level of disturbance (Connell, 1978). In a floodplain context, frequent connections by floods imply high disturbance, on the contrary less frequent connections induce the reduction or absence of disturbance.
Connell (1978) hypothesised that the highest diversity is maintained at an intermediate level of disturbance (see fig.3), which allows the co-existence of specialist and generalist species. Colonization by specialists occurs in frequently disturbed environment, while competition between species occurs in less frequently disturbed environment. The species that are excluded by disturbance may persist and grow in less disturbed environments. Therefore, a gradient between colonization and competition exists along the gradient of disturbance with a maximum of species in habitats with an intermediate level of disturbance. However, this pattern remains poorly demonstrated in floodplain systems (Ward and Tockner, 2001) and contrasted response may occur along a gradient of disturbance (Mackey and Currie, 2001).
Figure 3: Idealised distribution of the diversity along a gradient of disturbance, from frequently disturbed to less disturbed. Maximal diversity is reached for an intermediate level of disturbance.
Extract from Connell (1978).
aquatic and terrestrial vertebrate and other invertebrate consumers. They are among the most widespread and important food for running-water fish and only few groups of fish do not feed on invertebrates (Wallace and Webster, 1996). Many aquatic macroinvertebrates are also predators and therefore influence their prey numbers and locations (Covich et al., 1999). They release nutrients, accelerate the decomposition of organic matter and nutrient transfer to adjacent riparian zones (Cummins and Klug, 1979). Finally macroinvertebrates are regularly used as tools for monitoring, for example to assess the success of conservation or restoration measures (Bonada et al., 2006; Dziock et al., 2006; Rosenberg and Resh, 1993).
Functional characteristics and diversity (Southwood, 1977)
Aquatic macroinvertebrates present a large range of biological and ecological characteristics (also called traits) that allow them to live and grow in contrasted environments (Townsend and Hildrew, 1994; Verberk et al., 2008b). It is recognized that species' characteristics are constrained by environmental forces, which play the role of selective filters (Lytle and Poff, 2004; Poff, 1997; Statzner et al., 1997;
Verberk et al., 2008a). Townsend and Hildrew (1994) adapted the Habitat Templet concept of Southwood (1977) to a spatio-temporal habitat framework that can be applied to floodplains: the River Habitat Templet (RHT). The habitat is assumed as a templet on which evolution forged life history strategies. Townsend and Hildrew (1994) established predictions about species traits expected in particular habitats of the RHT. The predictions were influenced by the potential investment of species in tactics for escaping disturbances in time or space. The proposed framework and the associated predictions were tested on the Rhône River, but the results gave little agreement with the predictions (Richoux, 1994; Tachet et al., 1994; Usseglio-Polatera, 1994). The high spatio-temporal heterogeneity of the floodplain was supposed to be responsible for the discrepancies between predictions and observations. Later, Poff et al. (2006) selected what they considered to be “robust traits useful for a predictive community ecology”. They proposed that size, voltinism and drift propensity should be related to habitat stability (see fig.4), which in turn is related to the lateral connectivity of floodplain channels. Only few studies demonstrated the direct link between macroinvertebrate traits (considered for the entire community) and Traits
Only some limited groups of taxa were tested for the response of their traits along the lateral connectivity gradient. This is the case of the well documented work of Reckendorfer et al. (2006) about freshwater molluscs. Few studies used the entire community of macroinvertebrates to test the trait responses to a gradient of lateral connectivity (Paillex et al., 2009). Complex combinations of species traits were also proposed as surrogates for ecosystem attributes (Cummins et al., 2005; Merritt et al., 2002). The developed trait-based ratios provided information about the type of organic matter, the physical stability of the habitat and the availability of macroinvertebrate to higher tropic levels (such as fish or waterbirds). These authors concluded that this approach was useful to identify the need for restoration of remnant oxbow of southwest Florida. However, this approach was never tested in a European floodplain system (Paillex et al., 2007).
Figure 4: Conceptual framework proposed by Poff et al. (2006). Species characteristics (size, voltinism, drift, thermal preferences and trophic habit) are linked to environmental variables (habitat stability, thermal regime and food resources). Specific characteristics are influenced by one or two environmental variables.
(Falk et al., 2006)
The calculation of Functional Diversity (FD) indices builds upon the trait approach to supplement the use of “traditional” taxonomic diversity measurements. FD may be defined as the diversity of functional traits in a species assemblage represented in a site or an ecosystem (Diaz and Cabido, 2001). It is suggested that FD reflects some aspect of the ecosystem functions, its stability or its resilience in response to a disturbance. Moreover, according to Falk et al. (2006) functional diversity rather than taxonomical diversity is the most relevant component of biodiversity to assess ecosystem functions. However, this assumption is largely dependent on the traits that are considered in the calculation of the FD. Two types of traits can be recognized i) traits that are directly related to ecosystem functioning (effect traits) and ii) traits that involve responses to changes in an ecosystem (response traits) (Naeem, 2006).
Alternative methods for calculating functional diversity also exist in the current Functional
diversity
functional groups within a community (Simpson, 1949). Such functional groups can for example be based upon traits that are related to ecosystem functioning. Rao diversity (a generalized form of Simpson index) is one of the few indices that consider both the species abundances and distances for several traits (Botta-Dukat, 2005;
Pavoine and Bonsall, 2009; Rao, 1982). Several authors proposed a framework for the measurement of FD (Bady et al., 2005; Botta-Dukat, 2005; Laliberte and Legendre, in press; Pavoine et al., 2009), but no consensus was found and currently different methods are applied in individual case studies. Even if the concept of FD remains complex due to the multiple questions about its definition and the absence of consensus on how to measure it (Petchey and Gaston, 2006), FD is an important measurement of biodiversity as exemplified in the measurement of the effect of land use on biodiversity (Flynn et al., 2009; Laliberte et al., 2010). Nowadays, FD may help in a better ecosystem assessment and provides a tool for environmental managers to assess the success of restoration measures.
Necessity of floodplain restoration and existing results
The extinction rate of freshwater fauna in north America is predicted to be five times higher than that of terrestrial fauna (Ricciardi and Rasmussen, 1999). Land use is predicted to be the major cause of biodiversity changes in streams and lakes for the year 2100 (Sala et al., 2000). Worldwide, both longitudinal and lateral fragmentation of large river systems are major threats to running-water ecosystems (Dynesius and Nilsson, 1994). More than half of the northern hemisphere large river systems were dammed to produce electricity (Nilsson et al., 2005). Moreover, according to Tockner and Stanford (2002), up to 90% of European and North American rivers are altered with consequences in floodplain structures and functionalities. Therefore, it remains important to restore floodplain ecosystems to maintain biodiversity and reduce the rate of extinction of freshwater fauna. (Tockner and Stanford, 2002)
(Ward et al., 2001)
Ward et al. (2001) stated that successful restoration measures and programmes require a strong understanding of the underlying natural processes. They also stated that a strong conceptual foundation was necessary. This statement entails the need to identify the forces that structure the diversity in order to provide sound restoration
ecological processes often implies the improvement of the hydrological connectivity (Jansson et al., 2007; Kondolf et al., 2006). The restoration of lateral and longitudinal connectivities is a common goal of floodplain restoration measures, due to their high influence on biotic composition. Based upon the existing knowledge of the forces constraining the aquatic biodiversity, there are many ways to restore floodplain waterbodies (Amoros, 2001; Ardón and Bernhardt, 2009; Henry et al., 2002). The restoration of the lateral connectivity can be applied through the floodplain reconnection, the removal of levies or alluvial plugs, the increase of minimal base flow in the by-passed section of the river (Amoros, 2001; Ardón and Bernhardt, 2009;
Jansson et al., 2007; Kondolf et al., 2006). However, the effect of such restoration works on biodiversity remains poorly documented at the scale of an entire floodplain.
(Merigoux et al., 2009) (Lamouroux et al., 2006) (Coops et al., 2006)
The main river channel and floodplain waterbodies are differently restored due to the differences of forces that structure the biodiversity. Mérigoux et al. (2009) and Lamouroux et al. (2006) underlined the potential effects upon macroinvertebrates and fish of an increase of the minimal base flow in the main river channel. In addition to the minimal base flow increase, Coops et al. (2006) underlined that the reconnection of secondary channels is crucial for aquatic macroinvertebrates that use different habitat during their life cycles. At the scale of the floodplain, such restoration measures may increase the habitat heterogeneity and the associated biodiversity, but this still needs to be demonstrated.
Several authors made predictive models before restoration about the distribution of biodiversity in the lateral dimension of floodplains (Gallardo et al., 2009b; Lasne et al., 2007; Paillex et al., 2007; Reckendorfer et al., 2006; Tockner et al., 1999b), but only few studies demonstrated the effect of restoration measures on macroinvertebrate biodiversity (Funk et al., 2009; Paillex et al., 2009). Paillex et al. (2009) observed that the increased in lateral connectivity shifted the restored sites away from their predicted state. Funk et al. (2009) revealed that the water enhancement in an Austrian channel of the Danube significantly impacted the mollusc and the dragonfly communities, but not the fish. Despite those first results, the macroinvertebrate biodiversity changes related to floodplain restoration such as an increase of the lateral connectivity are currently scarcely documented. Indeed, Bernhardt et al. (2005) showed that only 10% of the Restoration
of the main channel and floodplain waterbodies
few possibilities to know if restoration measures have been relevant to restore communities or functions at the scale of a floodplain. (Bernhardt et al., 2005)
The Rhône River restoration programme
In Europe, the Rhône River flows across two countries (Switzerland and France). The French part of the Rhône River is 522 km long from the Swiss border to the Mediterranean Sea (Bravard, 1987). The Rhône is mostly a braided river with few fossil meandering sectors. The Upper part of the French Rhône was degraded at the end of the 19th century by the construction of embankments and in the middle of the 20th by the construction of several hydroelectric power plants (Roux et al., 1989). The low slope of the river and the important width of the floodplain necessitated the construction of channels and dams that by-passed the natural floodplain (Olivier et al., 2009) (see appendix I for an example of by-passed floodplain). Most of the flow goes to the hydroelectric power plant through an artificial canal and a minimal flow is maintained through the natural floodplain. When the maximal production capacity of the hydroelectric power plant is exceeded, the remnant flow goes through the by- passed section.
Embankments and dams lead to a rapid terrestrialization and disconnection of the floodplains and in turn to a reduction of aquatic ecosystems function and biodiversity (Dolédec et al., 1996; Henry et al., 2002; Roux et al., 1989). In 1993, a single channel was successfully restored to bring ecological succession back to a semi-lotic and mesotrophic stages (Henry et al., 1995; Henry et al., 2002). This channel was dredged to increase the ground water supply and flooding frequency (see fig.5). In 2000, three channels in the sector of Pierre-Bénite were restored by an enhancement of the hydro- geomorphological dynamics. This restoration aimed at restoring processes and was not species oriented (Amoros, 2001). Since 2003, an important restoration project is being carried out along the entire French Rhône River. On the upper Rhône River, 22 floodplain channels were restored with the aim to reduce their terrestrialization by an increase of their connectivity with the main river channel (i.e. the lateral connectivity).
This increase was obtained by (i) raising the minimal base flow in the by-passed sections of the floodplain and (ii) through floodplain channel dredging and/or
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 (see appendix II for examples). The aim of this programme also is to restore processes within the floodplain, rather than specific communities (e.g. fig.6)
Figure 5: Conceptual scheme of restoration of a channel on the Upper Rhône River in 1993.
The channel is dredged in order to restore vertical connectivity and the frequency of flash floods. Nowadays, the same scheme was used when the cut-off channels were dredged.
Extract from Henry et al. (2002)
Since 2003, 18 secondary channels in two by passed floodplains were monitored before and after restoration (see appendices III and IV). Macroinvertebrates were sampled within the sites and the samples were accompanied by the measurement of a series of habitat characterisitics and physico-chemical variables. Macroinvertebrates were used as monitoring tools because they inhabit a large range of conditions and they integrate the anthropogenic alterations (Rosenberg and Resh, 1993). Nowadays, little is known about the effect of such restoration measures on macroinvertebrates diversity. The restoration programme on the Rhône River provides a unique case to study such effects. To date there is no agreement on what constitutes a successful restoration programme. To face this gap, Palmer et al. (2005) proposed five criteria to evaluate the ecological success of restoration measures. The monitoring of macroinvertebrates in the case of this study will permit to implement Palmer et al.
(2005) propositions to establish if the Rhône River restoration is a success or not.
(Palmer et al., 2005) Monitoring
Figure 6: Conceptual framework of a three dimensional hydrosystem adapted from Kondolf et al. (2006) This example was establish for the Pite River and may be applied also for the Rhône River. The solid arrows represent degradation, whereas the dashed arrows represent the restoration trajectory of a river system. This example suggests that restoration may regain as certain amount of historical component of the floodplain. 1 to 2 represents the effect of embankment construction on connectivity. 2 to 3 represent the effect of dam constructions; 3 to 4 represent conjointly the restoration of the vertical and lateral connectivity. The circles represent the annual flow variability and we can observe its decrease after the completion of dams.
Objectives of the study and structure of the content
Monitoring and research on the lateral dimension of floodplains are crucial, because nowadays this dimension is at the core of many floodplain restoration programmes.
Most of floodplain restoration programmes aim to restore the lateral connectivity of floodplain waterbodies by an increase of their connectivity with the main river channel. The first thesis objective is to provide a better understanding of aquatic macroinvertebrate diversity patterns along the lateral dimension of floodplain ecosystems. This objective incorporates assessment of the lateral connectivity and characterisation of relevant macroinvertebrate metrics. The second objective is to analyse the effects of restoration measures upon aquatic macroinvertebrate metrics and assessment of the predictability of such effects.
2. First degradation of the river system after embankment
construction, for example to increase fluvial shipping or protect the crops from floods events 3. Second
degradation by the construction of dams to produce hydroelectricity
1. Pre-human intervention restore lateral and vertical
connectivity without to remove dams and therefore without restoration of the longitudinal connectivity
a large river floodplain (the Rhône River). Secondly, to compare the responses of selected aquatic macroinvertebrate metrics along the lateral connectivity. We focused our comparison on i) the total taxonomic richness, ii) the taxonomic composition, iii) a set of functional trait-based metrics proposed by Merritt et al. (2002) as surrogates for ecosystem processes.
The aims of the second chapter are (1) to model the relationship between the gradient of lateral connectivity and a range of macroinvertebrate-based metrics in 28 alluvial sites before restoration and (2) to assess the level of post-restoration hydrological connectivity in four sites in order to compare the expected and the actual macroinvertebrate assemblages. We computed macroinvertebrate metric related to different taxonomic richness, species status and biological traits. This chapter highlights the effect of restoration in four restored sites and gives recommendations for future floodplain restoration.
The third chapter addresses the following questions: i) does the unimodal diversity pattern proposed by the IDH (Connell, 1978) exist in large river floodplain and ii) what mechanisms generate the observed pattern? The aims of this chapter are (1) to test whether the IDH could be validated in the Rhône River floodplain waterbodies and (2) to test whether species richness and macroinvertebrate density could be related to the relative representation of competition and colonization strategies on the one hand, or to dispersal strategies on the other hand.
The fourth chapter provides the results about the effects of lateral connectivity enhancement by restoration measures upon macroinvertebrates diversity. This chapter looks at the question of the enhancement of the lateral hydrological connectivity and its influence upon key physical habitat parameters (e.g. sediment composition, aquatic vegetation) and in turn upon macroinvertebrate taxonomical composition, richness and functional diversity.
The last chapter synthesises the information provided by the four preceding chapters.
It provides a criticism of the way lateral connectivity has been assessed, establishes the effects of restoration measures upon macroinvertebrate diversity and provides recommendations for environmental management and for future research upon floodplain waterbodies in a restoration perspective.
chapter
Second chapter
Third chapter
Fourth chapter
Fifth chapter
A paper focusing on the distribution and ecological niche modelling of alien macroinvertebrates along the lateral dimension of the Rhône floodplain is also provided in the appendices. (Henry and Amoros, 1995)
Appendices
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