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Sites with new channel meanders
Two stream sections, Flume (48°14’36.8″N 1°46’43.6″W) and Malville (47°55’06.0″N 2°24’36.4″W) draining farmlands and which had previously been channeled away from their original thalwegs to create space for farmlands were restored to their original thalwegs (Figure 10). Malville (Figure 11a) was restored in summer 2019 while Flume (Figure 11b) was restored in autumn 2019. The new thalwegs were monitored for macroinvertebrate recolonization on a fine temporal scale for one year.
Three sampling points were established on the new thalwegs in each stream. A sampling point at least 200 meters upstream of the thalweg was established as a control or reference site in each stream. We equally established a point just downstream of the thalweg in each case for comparison (Figure 10). Macroinvertebrate samples were taken using artificial substrates enclosed in a big mesh (5cm square mesh) plastic bowl and set as a trap at the sampling point. 6 replicates were used at each sampling point and every sampling campaign. Each stream was monitored for 1 year after the creation of the thalwegs. One of the streams, Flume, was monitored at 1 week (T1), 4 weeks (T4), and 1 year (T10) after restoration. The second stream, Malville, was monitored at 1 week (T1), 2 weeks (T2), 4 weeks (T4), 6 months (T8), and 1 year (T10) after restoration.
Figure 11: Aerial view through a drone photo of the new thalwegs in a) Malville and b) Flume. Yellow stars represent sampling points.
General description of the B-A-C-I design as applied in this study
For each of the indicators assessed on the sites with barrier removal, measurements were made one year (n-1) before restoration works and two years after restoration works in a Before-After-Control-Impact (BACI) design (Figure 9 above). The BACI design permits the measurement of impacts by comparing a perturbed system with a control or reference condition (Downes, 2002). A control site was located about 100 meters upstream of the impacted section on each of Malville and Traou Breuder and about 200 meters upstream of the impacted section in Pontplaincoat (Figure 12a). The selection of the control site on the same stream was to standardize for catchment variables and analyze the impact of the barrier. We also selected a site downstream on each stream to determine the extent of the impact downstream.
For streambed oxygenation, in addition to the impacted, control, and downstream sites, we also took measurements at four other transects (L1, L2, L3, and L4; Figure 12b) of about 10 meters apart in each of Malville and Traou Breuder and about 20 meters apart in Pontplaincoat to determine the spatial gradient in the depth of streambed oxygenation along the zone of sedimentation upstream of the barrier.
Figure 12: a) A general illustration of the sampling sites for each measurement and b) a zoom on the impacted section showing additional sampling transects (L1, L2, L3, and L4) for the evaluation of streambed oxygenation gradient along the zone of sediment accumulation upstream of barriers.
This chapter seeks to address the first question of this thesis that aims to determine the response of macroinvertebrate community indices to hydromorphological restoration. Indices of fauna communities, including macroinvertebrates, have been widely used as indicators of environmental changes in streams with great success. However, in the evaluations of in-stream restorations, results from the deployment of macroinvertebrate community indices as bio-indicators have been inconclusive with very scanty evidence for success. This study aims to determine if this will be different in headwater streams and particularly according to the type of in-stream restoration studied (i.e. suppression of nozzle). We monitored three headwater stream reaches where artificial structures (i.e. nozzle) constituting hydromorphological impairments to the streams were removed. We collected macroinvertebrate samples from the impacted sections of the streams and control sites established on the streams. Samples were collected before and after the restoration activities in a before-after-control-impact (BACI) study design. We used two macroinvertebrate-based multimetric tools (I2M2 and ERA) to evaluate the ecological status of the streams based on macroinvertebrate community indices and to identify the relative contributions of watershed anthropic pressures to the ecological status. We found that the removal of the artificial structures and the restoration of natural flow were successful in reducing clogging. However, only taxonomic richness shows a positive significant change in streams. Results showed that the presence of other confounding factors even after restoration may have been responsible for this minimal success in biodiversity recovery. In addition, though the multimetric assessment tools were able to differentiate between the streams and potent in disentangling the effects of the multiple pressures contributing to degradation in the streams, they showed limitations at scales below the watershed scale. Our result showed that for a better outcome on biodiversity improvement, methods of in-stream restorations must consider the multiple pressures contributing to the degradation of fauna communities in watersheds.
Keywords: biodiversity, restoration, assessment tools, macroinvertebrate
Ce chapitre vise à répondre à la première question de cette thèse qui cherche à analyser la réponse des indices des communautés de macroinvertébrés suite à une restauration hydromorphologique. Les indices faunistiques des communautés, notamment basés sur les macroinvertébrés, ont été largement utilisés comme indicateurs des changements environnementaux dans les cours d’eau avec succès. Cependant, dans les évaluations des opérations de restaurations des cours d’eau, les résultats basés sur des indices de communautés de macroinvertébrés en tant que bio-indicateurs sont souvent peu concluants et peu d’études montrent des résultats génralisables.
Cette étude vise à déterminer si dans les cours d’eau de tête de bassin versant les évaluations des opérations de restauration sur la base d’indice de commuanutés de macroinvertébrés présentent des réponses plus marquées, en particulier pour un certain type de restauration (i.e. le débusage). Nous avons suivi trois tronçons de cours d’eau de tête de bassin versant où des structures artificielles (des buses) constituant des altérations hydromorphologiques des cours d’eau ont été retirées. Nous avons collecté des échantillons de macroinvertébrés dans les sections impactées et dans des sections de contrôle établies. Les échantillons ont été collectés avant et après les activités de restauration dans le cadre d’une étude avant-après-contrôle-impact (BACI). Nous avons utilisé deux outils multimétriques basés sur les macroinvertébrés (I2M2 et ERA) pour évaluer l’état écologique des cours d’eau sur la base des communautés de macroinvertébrés et pour quantifier les contributions relatives des pressions anthropiques des bassins versants.
Nos résultats monternt que la suppression des buses et la restauration de l’écoulement naturel ont réussi à réduire le colmatage. Cependant, seule la richesse taxonomique montrent un changement significatif positive. Les résultats montrent que la présence d’autres facteurs confondants peut avoir été responsable du peu de succès dans la récupération de la biodiversité. En outre, bien que les outils d’évaluation multimétriques aient été capables de différencier les cours d’eau et d’aider à démêler les effets des multiples pressions contribuant à la dégradation des cours d’eau, ils ont montré des limites à fine échelle, c’est à dire à une échelle inférieure de celle du bassin versant. Nos résultats monternt que pour favoriser la biodiversité, les méthodes de restauration des cours d’eau doivent considérer les multiples pressions contribuant à la dégradation des communautés fauniques dans les bassins versants.
Mots-clés: biodiversité, restauration, outils d’évaluation, macroinvertébrés
Anthropogenic activities have resulted in widespread degradation of ecosystems worldwide with the attendant alteration to their ecological status (Vitousek et al., 1997; Dobson, Bradshaw
& Baker, 1997). The natural balance between spatial and temporal species occurrences is also being altered across all major ecosystems by human activities in concert with the global change in climate (Dirzo et al., 2014). The lotic systems, in particular, have seen an increasingly severe impact as a result of extensive land-use changes and river modifications (Newson et al., 1992; Gleick, 2003; Allan, 2004; Dudgeon et al., 2006; Stoll et al., 2016).
According to the European Water Framework Directive (WFD), almost 60% of river water bodies in Europe do not meet the criteria for good ecological status (European Environmental Agency, 2018). Dams and other forms of hydraulic structures built across flow channels have severely altered the natural ecology of rivers (Bednarek, 2001; Cooper et al., 2017). For example, with the reduction in flow current occasioned by these structures, there is impairment to sediment transport with the consequent clogging of interstitial spaces just upstream ((Hazel Jr. et al., 2006)). Migratory species are impeded and species with an affinity for high flow current are replaced by species that have a preference for low flow current (Drinkwater & Frank, 1994; Stanford et al., 1996).
To reverse this trend and achieve the goal of good ecological status for European streams, the European WFD required member states of the EU to implement appropriate management and restoration programs on impacted streams (Heiring & Plachter, 1997; Voulvoulis, Arpon & Giakoumis, 2017). Consequently, the past decades have documented an increasing number of restoration works on hitherto degraded streams (Bernhardt et al., 2005; Smith, Clifford & Mant, 2014; Verdonschot et al., 2016).
Ecological restoration can be described as the process of assisting the recovery of damaged, degraded, or destroyed ecosystems (Hobbs & Cramer, 2008; Suding, 2011). Depending on the source of the degradation and the size (or type) of targeted stream reach, restoration activities may differ, but the primary focus of most schemes has been the in-stream restoration of habitats (Palmer et al., 2005; Bernhardt & Palmer, 2007; Miller, Budy & Schmidt, 2010a; Smith et al., 2014). The underlining assumption is that habitat restoration will lead to an increase in biodiversity and ultimately, the improvement of the ecological ‘health’ of such streams (Kail & Hering, 2009; Palmer, Menninger & Bernhardt, 2010; England & Wilkes, 2018). Consequently, biodiversity metrics of fauna have been the chief ecological indicators deployed for the assessment of restoration in streams (Mondy et al., 2012; Dolédec et al., 2015; Teresa & Casatti, 2017). While post-restoration assessments are generally not widespread (Bernhardt et al., 2005; Miller et al., 2010a), the few built on this assumption have recorded limited recovery of biodiversity (Palmer et al., 2010; Friberg et al., 2014).
In restored headwater streams, due to the dearth of information and the peculiarities of the nature of degradation (Levi and McIntyre 2020), the nature of ecological recovery remains uncertain (Sarriquet, Bordenave & Marmonier, 2007; Levi & McIntyre, 2020). Indeed, most post-assessment work on restored streams focused on large streams and rivers that represent only a small number of sites restored each year. For example, (Zaidel et al., 2021) reported that in the United States, while there are more than 90,000 dams in the country but that when smaller dams are taken into account, the number probably reaches two million. In France, the national database on obstacles to water flow references 121,540 obstacles in 2021 (https://www.sandre.eaufrance.fr/). 0.5% have a waterfall height of more than 10 meters and 74% of them have a waterfall height of less than 1 meter. Consequently, the vast majority of hydromorphological restoration operations are carried out in the headwaters on small structures. These operations are rarely studied both in terms of impact and in terms of response to restoration (Poff & Hart, 2002; Liu et al., 2014; Fencl et al., 2015).
From available records, assessment strategies have mostly relied on structural metrics as bio-indicators of the impact of environmental stressors on ecosystems (Bailey, Norris & Reynoldson, 2004; Roni, Hanson & Beechie, 2008; Barnes, Vaughan & Ormerod, 2013). Structural metrics such as indicator species, species diversity, richness or composition of communities, including those of macroinvertebrates, are frequently used for bio-assessment because species can be lost or replaced in response to environmental stressors (Miller et al., 2010a; Clapcott et al., 2012; Kail et al., 2015; Verdonschot et al., 2016). However, most assessment efforts have only reported scanty changes in diversity resulting from restoration and diversity metrics have shown little difference between restored and unrestored streams (Lepori et al., 2005; Palmer et al., 2010; Louhi et al., 2011; Dolédec et al., 2015; Verdonschot et al., 2016; Stoll et al., 2016). We set out to determine whether this will be different in headwater streams in response to hydromorphological restoration.
Following the European WFD, new monitoring tools emerged on macroinvertebrates (Birk et al., 2012). The WFD requires that bioassessment methods implicitly evaluate the ecological status of water bodies, by comparing biological quality elements between an observed versus a reference situation (Wright, Furse & Moss, 1998; Morandi et al., 2014). In France, the IBGN method (Indice Biologique Global Normalisé) has been used at the national scale since 1992 and revised in 2004. However, it was no longer satisfying due to severe inconsistencies with WFD (for further details, see Mondy et al. 2012). Mandated by the French Ministry of Environment (MEDDTL), Mondy et al. (2012) designed a new multimetric index (I2M2) for the invertebrate-based ecological assessment of French wadeable streams. This index should be able to identify impaired reaches for 17 anthropogenic pressure categories potentially leading to water quality alteration or habitat degradation and considering both taxonomic characteristics and biological traits of benthic macroinvertebrates. As expected, later studies showed that this index responds efficiently to the effects of both physical, chemical, and hydromorphological stressors (Villeneuve et al., 2015) as a proxy of site ecological status (Corneil et al., 2018) and it is more robust than the IBGN (Wiederkehr et al., 2016). This index also allows taking into account the nested spatial scales driving stream functioning in the description of ecological status by highlighting the importance of the site and reach scales in explaining stream biological condition (Villeneuve et al., 2018). Today, managers urgently need tools that can support them in the decision-making process for protecting and restoring river ecosystems, either for biodiversity conservation or amelioration of anthropic pressures. It is particularly true for restoration programs concerning small artificial obstacles in headwater streams which represent the vast majority of restoration operations.
Table of contents :
GENERAL INTRODUCTION
1.0. Ecological degradation in streams
2.0. Physical alterations
3.0. What is restoration?
4.0. Main types of restoration in headwater streams
5.0. Indicators of restoration success
6.0. Study objectives and hypotheses
7.0. Organization of the thesis sections
CHAPTER 1: Materials and Methods
1.0. General description of study area and sites
2.0. General description of the B-A-C-I design as applied in this study
CHAPTER 2: The response of macroinvertebrate community structure to barrier removal
Article 1: Responses of macroinvertebrate communities to hydromorphological restoration of headwater streams in Brittany
CHAPTER 3: Ecological processes as functional indicators of ecological recovery
Article 2: In-stream variability of litter breakdown and consequences on environmental monitoring.78
Article 3: The recovery of key ecological processes in headwater streams following hydromorphological restoration: the examples of litter decomposition and streambed oxygenation
GENERAL DISCUSSION
