RIVER RESEARCH AND APPLICATIONS
River Res. Applic. 30: 557–570 (2014) Published online 25 July 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.2597
SHORT-TERM IMPACTS OF LATERAL HYDROLOGICAL CONNECTIVITY RESTORATION ON AQUATIC MACROINVERTEBRATES A.-L. BESACIER-MONBERTRANDa*, A. PAILLEXa,b AND E. CASTELLAa b
a Laboratory of Ecology and Aquatic Biology, University of Geneva, Geneva, Switzerland Aquatic Ecology Group, Department of Zoology, University of Cambridge, Cambridge, UK
ABSTRACT In floodplain ecosystems, the lateral hydrological connectivity between the main river channel and the secondary channels plays a major role in shaping both the habitat conditions and the macroinvertebrate diversity. Among other threats, human activities tend to reduce the lateral connectivity, which increases floodplain terrestrialization and induces a loss of aquatic biodiversity. Consequently, the restoration of lateral connectivity is of growing concern. We studied four secondary channels of the Rhône floodplain that were subjected either to no restoration or to three different restoration measures (river flow increase only, flow increase plus dredging and flow increase plus reconnection to the river). Macroinvertebrate and environmental data were analysed one year before and during a period of five years after restoration. We expected a progressive increase of lateral connectivity according to the type of restoration. Changes in macroinvertebrate assemblages were predicted to be towards more rheophilic communities and proportionally related to the changes in lateral connectivity. In the reconnected channel, lateral connectivity increased and remained high five years after restoration. In the dredged channel, the immediate increase of the lateral connectivity metric induced by sediment removal was followed by a rapid decrease. In the unrestored channel and the channel only influenced by flow increase, the metric remained constant in time. The macroinvertebrate composition and the rarefied EPT richness changes were proportionally related to the changes in lateral connectivity. Alien species richness and densities increased progressively in all channels after restoration. Our results showed that modifications of the lateral connectivity lead to predictable changes in macroinvertebrate diversity. Synergistic interactions between restoration and longer-term changes (e.g. climatic change, invasion of alien species) encourage long-term monitoring to assess the durability and trends of restoration measures. Copyright © 2012 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. key words: hydrological restoration; lateral connectivity; ecological succession; macroinvertebrate metrics; riverine floodplain; alien species; Rhône River Received 16 June 2011; Revised 4 May 2012; Accepted 7 June 2012
INTRODUCTION Through their intrinsic biodiversity and the economic and environmental services provided, riverine floodplains represent ecosystems of high value (Tockner and Stanford, 2002). Hydrological and biotic interactions across four dimensions (longitudinal, lateral, vertical and temporal) play major roles in floodplain structure and value (Ward, 1989). Nonetheless, because of human activities and consequent degradations (e.g. pollution, channelization, straightening and introduction of alien species), the functions of riverine systems and the dynamics of hydrological connections between the main river channel and the floodplain have been greatly altered since the 19th century (Kondolf et al., 2006; Jansson et al., 2007). The awareness of the necessity to protect such ecosystems increased during the last decades of the 20th century and the restoration of floodplain ecosystems is currently a worldwide *Correspondence to: A.-L. Besacier-Monbertrand, Institute F. A. Forel, Earth and Environmental Sciences, University of Geneva, Institute for Environmental Sciences (ISE), 7 route de Drize, CH-1227 Carouge, Switzerland. E-mail:
[email protected]
Copyright © 2012 John Wiley & Sons, Ltd.
concern (e.g. Harrison et al., 2004; Nakano et al., 2008). There is a growing body of literature about case studies and reviews on the methodology applied to floodplain restoration programmes (Moerke and Lamberti, 2004; Jansson et al., 2007; Lake et al., 2007; Spänhoff and Arle, 2007; Woolsey et al., 2007). Such restoration programmes can involve diverse objectives, from species conservation to the restoration of ecological processes and can occur at different spatial scales (Bash and Ryan, 2002; Woolsey et al., 2007). According to the objectives, the restoration can be performed at a small scale [less than one km of river length (Bernhardt et al., 2005)] or at a large scale [landscape-level restoration (Holl et al., 2003)]. In this context, restoring the hydrological connectivity at the scale of an entire floodplain is among the larger scale objectives. At the floodplain scale, lateral connectivity refers to exchanges of, for example, surface water, sediment and biota between the main river and the floodplain (Amoros and Bornette, 2002). It plays an important role in the ecosystem processes and the maintenance of a diversity of habitats from lotic to lentic conditions (Ward and Stanford, 1995). In natural
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ecosystems, the level of connectivity between floodplain channels and the main river ranges from eupotamal, to parapotamal and plesiopotamal channels (Amoros et al., 1987). Eupotamal channels are permanently connected to the main river channel; parapotamal channels remain permanently connected downstream and disconnected at their upstream end, whereas plesiopotamal channels are totally disconnected from the main river channel at the mean annual average water level (Amoros et al., 1987). Flow regulation by the implementation of dams leads to the disruption of natural disturbance dynamics and affected habitat heterogeneity and successional trajectories of floodplain channels (Ward and Stanford, 1995). Consequently, the restoration and enhancement of lateral connectivity intend to rejuvenate floodplain channels and implies several processes. These may include the reconnection of floodplain channels to the main river, the increase of minimum discharge in the bypass section of the river or the removal of levees and embankments (Jansson et al., 2007). At present, increasing experience is being acquired about the restoration of the longitudinal connectivity at small and large scales (e.g. Harrison et al., 2004; Robinson et al., 2004b; Lamouroux et al., 2006; Mérigoux et al., 2009), whereas the restoration of lateral hydrological connectivity and its effects in space and time are much less studied (but see Reckendorfer et al., 2006; Lasne et al., 2007). In restoration strategy, the time frame of assessment for restoration measures is important. However, monitoring and assessment before and after restoration is not frequently incorporated in restoration programmes (Bash and Ryan, 2002; Bernhardt et al., 2005; Jansson et al., 2007). The time frame of monitoring is also a topic seldomly addressed in restoration (Bash and Ryan, 2002). It should be defined according to the objectives, the indicators selected and the life time of the restored environment (Holl and Cairns, 2002). In a floodplain ecosystem, ecological succession takes place at the scale of decades to centuries (Amoros and Bornette, 2002). In this context, monitoring durations shorter than 10 years can be considered as short term. Among the floodplain biota, aquatic macroinvertebrates play a key role in ecosystem processes (i.e. energy flow and nutrient cycling) (Wallace and Webster, 1996; Richardson and Jackson, 2002; Vanni, 2002; Strayer, 2006). They are frequently used as indicators of the efficiency of restoration measures (Spänhoff and Arle, 2007). Indeed, aquatic macroinvertebrates have the ability to integrate temporal and spatial changes of their environment. The impacts of small scale restoration on aquatic macroinvertebrates seem to be more frequently studied than the effects of large scale restoration (Harrison et al., 2004; Nakano et al., 2008). A restoration programme is currently being implemented along the entire course of the French Rhône River (i.e. 530 km) and its floodplain (Olivier et al., 2009). During the 20th century, this river encountered the implementation of Copyright © 2012 John Wiley & Sons, Ltd.
several dams that disrupted the natural floodplain dynamic and resulted in an increased terrestrialization of the channels. Since 2005, selected floodplain channels have been restored with the aim of slowing down their terrestrialization, by increasing their lateral hydrological connectivity with the main river channel. The present study focuses on the effects of lateral connectivity changes on macroinvertebrate assemblages in floodplain channels and addresses the following: (i) the effects of different types of restoration measures (flow increase, dredging and reconnection); and (ii) the analysis of temporal changes in the early successional stages of the channels after restoration. First, we hypothesized that the restoration operations would influence key physical habitat parameters representative of the lateral connectivity (e.g. vegetation abundance, sediment composition and hydraulic stress) and predictably influence macroinvertebrate diversity. We used four metrics to assess changes in macroinvertebrate assemblages: (1) the macroinvertebrate taxonomic composition; (ii) the rarefied Ephemeroptera, Plecoptera and Trichoptera (EPT) richness; (iii) the alien species richness; and (iv) the alien density. These metrics were chosen as they were demonstrated to be related to the lateral connectivity, and its changes induced by restoration operations (e.g.(Friberg et al., 1994; Harrison et al., 2004; Paillex et al., 2009). Alien species, defined as not originating from the study sector, were studied as they may induce biotic and abiotic changes, and their consideration in restoration programmes is critical (Jansson et al., 2007). According to previous studies, we predicted the changes in macroinvertebrate composition to be related to changes in the lateral connectivity associated to the different restoration measures (Tockner et al., 1999; Reckendorfer et al., 2006). Given the predominantly rheophilic nature of the EPT and the alien species, we predicted an increase of these taxa with an increase of lateral connectivity by restoration measures (Usseglio-Polatera, 1994; Besacier-Monbertrand et al., 2010). We expected maximal changes of all metrics in the channels fully reconnected with the river. In the channels submitted to the flow increase in the bypass river, including the dredged ones, we expected intermediate levels of change in the lateral connectivity, the taxonomic composition and EPT richness. In the channels where dredging created ‘new’ habitats available for colonization, the increase in alien species density and richness was expected to be higher than in the channels submitted only to the flow increase. Changes were expected to be comparatively very limited in the unrestored channel, except for alien species that can be expected to continue to spread in the system independently from restoration operations. In summary, we expect the faunal metrics after restoration to be unchanged in unrestored channels (except for alien species) and to progressively increase with lateral connectivity according to the type of restoration (i.e. flow increase < flow increase + dredging River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
CHANGES IN LATERAL CONNECTIVITY AND MACROINVERTEBRATES
flow increase + reconnection). Finally, we predicted that temporal changes in macroinvertebrate metrics would be predictability related to changes in lateral hydrological connectivity.
METHODS Study channels The four floodplain channels sampled (named according to their type of restoration: unrestored, flow increase, dredging and reconnection) are located along a 15-km stretch of the French Upper-Rhône River in the sector of Belley (Figure 1). These channels represent different levels of connectivity to the main river channel: eupotamal, parapotamal and plesiopotamal
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(Amoros et al., 1987) (Table I). In 1982, the sector was equipped with a diversion dam, which altered the river function and accelerated the terrestrialization of the floodplain channels (Roux et al., 1989). We selected two sites (one upstream and one downstream) in each channel to consider the habitat diversity occurring in parapotamal and plesiopotamal channels (Richardot-Coulet et al., 1983). In two instances, a third site was considered in the middle of the channel length. Existing floodplain sites were submitted only to the bypass flow increase (site codes 3, 4 and 7); others were additionally dredged (site codes 5 and 6, Table I) or were completely reconnected to the main channel (site codes 8, 9 and 10). Within the restored channels, two sites were newly created in the dry thalweg (site codes 6 and 9). An unrestored channel (site codes 1 and 2), located immediately downstream of the junction between the tail-race and the bypass section, was not influenced by the increase of the river discharge. The mechanical restoration (dredging or reconnection) occurred during the winter 2004–2005, whereas the flow increase occurred 27 July 2005. We considered the period between the mechanical restoration event and the flow increase event as a transitional period. Lateral connectivity gradient
5
Dredging
6
7
3
Flow increase
Bypass section
4
8 9
Reconnection 10
1
Unrestored
Five environmental variables expressing the lateral connectivity [water specific conductance, diversity of the mineral substrate, submerged vegetation cover, organic matter of the upper sediment (5-cm upper layer), ammonia nitrogen concentration] and known to reflect the influence of the main river upon floodplain channels were assessed in each site, as described in Paillex et al. (2007). The ammonia nitrogen concentration was measured in winter to reduce the effect of nutrient consumption by plants. The sediment organic matter content was also measured at each site in winter, after the major autumnal incorporation of dead plant material. The three other variables were measured in each quadrat during the spring and summer faunal samplings (Table II). The variables were log-transformed to ensure normality when necessary. The five environmental variables referred to above, were summarized by a principal component analysis (PCA). The first factorial axis of this analysis, yielding most of the between-sites variability, was used as a surrogate for lateral connectivity (Paillex et al., 2007). The site scores along this factorial axis were rescaled between 0 (lowest connectivity) and 1 (highest connectivity) and the average channel scores (i.e. average for the two or three sites per channel) were used as dimensionless measures of the relative level of lateral connectivity between a channel and the main river.
2
Figure 1. Location of the four floodplain channels (unrestored,
flow increase, dredging and reconnection) and their respective sites (1–10) in the study sector of Belley (France)
Copyright © 2012 John Wiley & Sons, Ltd.
Macroinvertebrate sampling In each site, four sampling points (0.5 0.5 m quadrat delimited by a metal frame) were distributed randomly along River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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Table I. Characteristics of the channels, type of restoration, lateral connectivity state (eupotamal, parapotamal, plesiopotamal and terrestrial) before (pre) and after restoration (post). Channels were named according to the type of restoration work implemented Site
Channel
Unrestored Flow increase Dredging Reconnection
Connectivity level
Restoration works
position
code
Pre
Post
Upstream Downstream Upstream Downstream Upstream Middle (new) Downstream Upstream Middle (new) Downstream
1 2 3 4 5 6 7 8 9 10
Eupotamal Eupotamal Eupotamal Eupotamal Plesiopotamal Terrestrial Parapotamal Plesiopotamal Terrestrial Parapotamal
Eupotamal Eupotamal Eupotamal Eupotamal Plesiopotamal Plesiopotamal Parapotamal Eupotamal Eupotamal Eupotamal
a 30 m stretch. Aquatic macroinvertebrates were sampled before (2003 and 2004) and after restoration (2005, 2006, 2007 and 2009) in spring and summer to account for seasonal variations of macroinvertebrate composition (Table II). In summer 2005, the macroinvertebrates were sampled in the mechanically restored channels (i.e. dredged and reconnected) one week before the flow increase into the bypass section. Macroinvertebrates were collected within each quadrat using a hand net (opening: 9.5 14.5 cm; mesh size: 500 mm) by disturbing the vegetation and the substrate and were preserved in 70% ethanol. Later, in the laboratory, in most of the case, only three of four samples were sorted, and the macroinvertebrates were identified to genus or species level using a dissecting microscope. The Diptera were identified to the family level, whereas the Oligochaeta and Hydrachnidia were not taken into account because of the limited taxonomic level of their identification and their ubiquity at that level of identification (both represented less than 2% of total abundance). Therefore, the data resulting from the processing of 221 quadrats were employed in this study.
Unrestored Unrestored Flow increase Flow increase Dredged + flow increase Dredged + flow increase Flow increase Reconnected + flow increase Reconnected + flow increase Reconnected + flow increase
Macroinvertebrate metrics Four macroinvertebrate metrics were considered: the taxonomic composition of the entire community, the richness of EPT and the richness and density of alien species. We computed the densities of aquatic macroinvertebrates for each sampling unit (0.25 m2) and log (x + 1)—transformed the values prior to further analyses. To compare the macroinvertebrate composition between channels and dates, we performed a between-class correspondence analysis (CA) (Dolédec and Chessel, 1989; Baty et al., 2006). The classes of the between-class analysis are the channel*date units. Only the taxa represented by at least three individuals were kept in this analysis. The channel scores along the first axis of the between-class CA were used as the metric measuring changes in taxonomic composition between the channels. Both PCA and between-class CA calculations were performed using the ade4 package (Chessel et al., 2004) in the R software (R Development Core Team, 2009). The EPT richness was rarefied to avoid any bias relating to differences in abundances between samples (Heck et al.,
Table II. Sampling dates for each site before (pre) and after restoration (post) (spring: sp, summer: su; 03 to 09: years from 2003 to 2009) Channel
Unrestored Flow increase Dredging Reconnection
Sampling date
Site position
Upstream Downstream Upstream Downstream Upstream Middle (new) Downstream Upstream Middle (new) Downstream
Pre
Post
sp04, su04 sp04, su04 su03, sp04 su03, sp04 su03, sp04 Terrestrial su03, sp04 su03, sp04 Terrestrial su03, sp04
- - sp07, su07, sp09, su09 - - sp07, su07, sp09, su09 - - sp07, su07, sp09, su09 - - sp07, su07, sp09, su09 su05*, sp06, sp07, su07, sp09, su09 su05*, sp06, sp07, su07, sp09, su09 su05*, sp06, sp07, su07, sp09, su09 su05*, sp06, sp07, su07, sp09, su09 su05*, sp06, sp07, su07, sp09, su09 su05*, sp06, sp07, su07, sp09, su09
*Flow increase occurred 27 July 2005. Copyright © 2012 John Wiley & Sons, Ltd.
River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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discharge was raised to 70.5 m3 s-1, and six flood events higher than 400 m3 s-1 were recorded (Figure 2).
1975). For each channel*date unit, we simulated random samples of similar minimal abundance (211 individuals for this study). The rarefied EPT richness was expressed with a standard error. Calculations were performed using the vegan package (Oksanen et al., 2011) in the R software (R Development Core Team, 2009). Alien species were considered as such for species that did not originate from the Upper-Rhône floodplain and were introduced during the past 200 years (Devin et al., 2005). The density of alien species was expressed as the number of individuals per sampling unit (0.25 m2) and log (x + 1)— transformed. Wilcoxon tests were applied to evaluate differences in densities after restoration. The changes in macroinvertebrate metrics and in lateral connectivity after restoration were expressed as after-before differences for all possible pairs of dates for the same season. Relationships between changes in macroinvertebrate metrics and changes in connectivity were tested with ordinary least square regressions at the site level.
Lateral connectivity before and after restoration The first axis of the PCA generated by the five environmental variables summarized 44% of the total variability between channel*date units. All the variables appeared to correlate well, with the first axis (Table III). The mineral substrate diversity and the ammonia nitrogen concentration were positively correlated to this axis, whereas the three other variables were negatively correlated. The first PCA axis was subsequently used as a surrogate for the lateral hydrological connectivity. Table IV links dimensionless lateral connectivity scores, grouped in three classes [(0–0.33), (0.33–0.66), (0.66–1)], to field values of two environmental variables well related to lateral connectivity, namely, the organic matter of the sediment (%) and the submerged vegetation cover (%). The unrestored channel and the flow increase channel demonstrated limited changes in their lateral connectivity (≤ 0.25 units), both between seasons and after restoration (Figure 3). They maintained their scores in the upper part of the lateral connectivity gradient. The restored channels (dredged and reconnected) evidenced larger changes (> 0.35 units). After an initial increase in its lateral connectivity metric following the mechanical restoration, the dredged channel showed a gradual decrease over time and a return to prerestoration values (Figure 3). The reconnection led to an increase in the lateral connectivity gradient, which persisted after restoration (Figure 3). For these two channels, an important increase of connectivity occurred in spring 2004 (i.e. after the January 2004 flood but before restoration).
RESULTS Hydrological context Three hydrological phases could be distinguished during the study (Figure 2). First, from April to October 2003, an exceptionally high discharge (around 250 m3 s-1) was recorded in the bypass section because of maintenance operations at the power plant. Second, from November 2003 to the end of July 2005 (i.e. before the restoration increased the discharge in the bypass section), the median discharge was 38.2 m3 s-1, with two flood events above 400 m3 s-1. Third, after restoration (from August 2005), the median
Before restoration
After restoration
800
400
su09
sp09
su07
sp07
sp06
su05
Transitional period
su04
sp04
600
su03
Discharge (m3s-1)
Flow increase
200
0 2003
2004
2005
2006
2007
2008
2009
Sampling date Figure 2. Sampling dates and temporal fluctuations of discharge in the bypassed section of the Rhône River in the sector of Belley before and after restoration works. The minimum discharge through the bypass section of the river was increased in this sector at the end of July 2005. The dashed zone represents the transitional period between before and after restoration Copyright © 2012 John Wiley & Sons, Ltd.
River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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Table III. First two principal component analysis (PCA) axes of five environmental variables (subdiv: mineral substrate diversity, N-NH3: ammonia nitrogen concentration, omsed: organic matter content of the sediment, vegsub: submerged vegetation cover, cond: water specific conductance): % of variability explained by the first two axes and correlations between the variables and the axes (Pearson’s correlation tests: p < 0.001*, NS = not significant) PCA
Axis 1
Axis 2
%variability subdiv N-NH3 omsed vegsub cond
44 0.65* 0.53* 0.54* 0.79* 0.75*
20 0.39* 0.75* 0.51NS 0.03NS 0.15*
Changes in macroinvertebrate taxonomic composition and Ephemeroptera, Plecoptera and Trichoptera richness The unrestored channel and the flow increase channel showed limited changes in their macroinvertebrate composition (< 0.25 units) both before and after restoration. They maintained high scores (Figure 4) associated with a rheophilic taxonomic composition, which included species such as Ancylus fluviatilis (Gastropoda), Theodoxus fluviatilis (Gastropoda) and Heptagenia sulphurea (Ephemeroptera) (see Appendix S1). Temporal changes in the dredged channel were lower than 0.45 units, and the channel maintained low scores associated with limnophilic taxonomic composition including species such as Hydroglyphus geminus (Coleoptera) and Ilyocoris cimicoides (Heteroptera) (see Appendix S1). The reconnected channel demonstrated changes higher than 0.8 units in the sampling period. The values after restoration, with scores higher than 0.5, corresponded to taxa typical for large rivers such as Rhyacophila sp. (Trichoptera) and Rhithrogena sp. (Ephemeroptera) (see Appendix S1). Spring values showed lower changes than summer values (Figure 4). The effect of
Table IV. Values of organic matter and submerged vegetation cover for three classes of lateral connectivity measured by a PCA. Each class represent a third of the variability measured along the first axis of the PCA. The extreme values (min. and max.) and the mean are given for each class. Classes: I (0–0.33) ; II (0.33–0.66); III (0.66–1) Connectivity class
Environmental variable
Organic matter (%) Submerged vegetation (%)
Min. Mean Max. Min. Mean Max.
Copyright © 2012 John Wiley & Sons, Ltd.
I
II
III
2.2 3.4 4.9 42.5 69.0 95.0
0.6 1.8 4.9 1.2 35.3 73.0
0.08 1.2 3.1 0.0 7.0 20.0
the January 2004 flood event was also highlighted in the dredged and reconnected channels, with more rheophilous communities in spring 2004 than summer 2003. This was similar to the situation observed post-restoration in spring 2006. Overall, the rarefied EPT richness per channel*date unit varied between a minimum of 1.4 0.6 (SE) and a maximum of 17 1.1 (SE) species (based on a calculation for 211 individuals) (Figure 5). Comparing values before and after restoration for the same season, limited changes were observed concerning both the unrestored and the flow increase channels. The rarefied EPT richness in the flow increase channel experienced a small increase over time (from 14.9 0.9 species in spring 2004 to 17 1.1 in spring 2009 after restoration) and a higher increase in summer (from 8 species in summer 2003 to 15 species in summer 2009). For the two other channels, contrasting responses were observed. For the dredged channel, whereas spring values were stable in time, summer values showed an increase after restoration (from 2.2 0.8 species in summer 2003 to 9.2 1.3 in summer 2007) and then a decrease in 2009 (1.9 0.6 species ). In the reconnected channel, summer values increased significantly after restoration (from 1.4 0.5 species in 2003 to 12 1.0 in 2009) (Figure 5).
Changes in alien species All the channels harboured alien species before restoration (from 0.75 to 27 individuals on average per quadrat) (Figure 6). The densities of alien species increased significantly after restoration (Wilcoxon test, p < 0.001) in all types of channels, including at the unrestored channel. In the case of the reconnected channel, a significant continuous increase of alien species density was observed between 2005 and 2009 (Wilcoxon test, p < 0.001) (Figure 6). The increase in alien density in all the channels after restoration was accompanied by an increase in the number of alien species (Table V). Before restoration, six alien species were collected: Dreissena polymorpha (Pallas, 1771) and Corbicula fluminea (Müller, 1774) (Bivalva), Dugesia tigrina (Girard, 1850) (Turbellaria), and Physella acuta (Draparnaud, 1805), Potamopyrgus antipodarum (Gray, 1843) and Gyraulus parvus (Say, 1817) (Gastropoda). Since summer 2007, the number of alien species increased to 11. The five additional alien species found after restoration were as follows: Crangonyx pseudogracilis (Bousfield, 1958) and Dikerogammarus villosus (Sowinski, 1894) (Amphipoda), Orconectes limosus (Rafinesque, 1817) (Decapoda), Hypania invalida (Grube, 1860) (Polychaeta) and Hemimysis anomala (Sars, 1907) (Mysida). Before restoration, the highest number of alien species found in a channel reached 5 (unrestored, spring and summer 2004), whereas this number increased to 10 (unrestored, summer 2007) after restoration. River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
Lateral connectivity
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
CHANGES IN LATERAL CONNECTIVITY AND MACROINVERTEBRATES
563
Unrestored
Flow increase
Dredging
Reconnection
Flood event
Before
After
su03 sp04 su04 sp05 su05 sp06 su06 sp07 su07 sp08 su08 sp09 su09
Sampling date Figure 3. Temporal changes in the lateral connectivity in each channel type before and after restoration. The dashed zone represents the transitional period between before and after restoration. The vertical axes are site scores along the first axis of the environmental PCA, rescaled between 0 and 1. They represent the lateral connectivity with the main river from 0 (disconnected site) to 1 (fully connected site). The flood of January 2004 is highlighted by a small arrow
Lateral connectivity and macroinvertebrate metrics The relationships between the amplitude and direction of change in lateral connectivity and the amplitude and direction of change in both the macroinvertebrate taxonomic composition and the rarefied EPT richness were significantly positive (R2 = 0.35, p < 0.001; R2 = 0.14, p = 0.02, respectively) (Figure 7). This implies that the amplitude of change in faunal metrics reflected changes in the physical environment (as expressed by the lateral connectivity variables). The direction of this change following restoration went toward communities dominated by more rheophilic taxa and higher EPT richness. The changes in density and richness of alien species did not show any significant relationship with changes in lateral connectivity (R2 = 0.008, p = 0.58 and R2 = 0.0002, p = 0.9 respectively). DISCUSSION Does lateral connectivity restoration influence macroinvertebrate metrics? As underlined by several authors (Junk et al., 1989; Heiler et al., 1995; Leigh and Sheldon, 2009; Paillex et al., 2009), Copyright © 2012 John Wiley & Sons, Ltd.
lateral hydrological connectivity is prominent in determining the environmental conditions and the distribution of aquatic macroinvertebrates in river–floodplain systems. One of the major aims of the French Rhône River restoration programme was to enhance this connectivity and to improve habitat and biotic diversity (Olivier et al., 2009). Given the range of restoration procedures, we expected that the habitat variables would reflect, gradually, the lateral connectivity changes. Restoration by reconnection increased the mineral substrate diversity and reduced both the submerged vegetation cover and the organic matter content of the sediment. In the dredged channel, the increase of the lateral connectivity metric (increase of mineral substrate diversity and decrease of vegetation and organic matter) immediately after restoration reflected the mechanical effect of the dredging. The unrestored and the flow increase channels showed smaller changes in the three previously described variables, demonstrating a limited change in their lateral connectivity. In accordance with Paillex et al. (2009), we can underline that the habitat variables chosen as surrogates for lateral connectivity were appropriate to survey the changes induced by restoration measures. In the initial phase after restoration, the sediment and vegetation River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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Macroinvertebrate composition
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
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Unrestored
Flow increase
Dredging
Reconnection
Flood event
Before
After
su03 sp04 su04 sp05 su05 sp06 su06 sp07 su07 sp08 su08 sp09 su09
Sampling Date Figure 4. Temporal changes in the macroinvertebrate composition in each channel type before and after restoration. The dashed zone represents the
transitional period between before and after restoration. The vertical axes are site scores along the first axis of the between-class CA, rescaled between 0 and 1. They represent the macroinvertebrate composition from 0 (most limnophilous community) to 1 (most rheophilous community). The flood of January 2004 is highlighted by a small arrow
variables reflect mostly the consequences of mechanical dredging. In the course of time, they can be used to monitor the rate of succession in the restored channels under their new connectivity level. Although dimensionless values of the lateral connectivity surrogate are not directly readable for managers or decision makers, a link can be established with direct measures of environmental variables in the field, thus expressing the lateral connectivity gradient. This link gives a direct indication of how environmental conditions have changed in a channel after restoration and may allow managers to assess if changes have reached pre-defined objectives. Evidently, more work still needs to be carried out to link observed values of environmental variables (e.g. sediment characteristics, quantity of vegetation) and operational measures (i.e. dredging depths, connection levels and durations) to effectively guide restoration procedures. River restoration success depends upon numerous parameters (Palmer et al., 2005; Woolsey et al., 2007) and the scale at which restoration measures are performed seems a crucial point for aquatic macroinvertebrates. Several studies showed that large scale restoration procedures should be preferred to small scale restoration because the latter fail to Copyright © 2012 John Wiley & Sons, Ltd.
address the restoration objectives (Harrison et al., 2004; Lepori et al., 2005; Jähnig et al., 2009). In this study, restoration was performed at a large scale (i.e. the scale of 15 km-long river stretch), and macroinvertebrate diversity changed as expected. Concerning macroinvertebrate composition and rarefied EPT richness, changes proved to be proportionally related to the changes in the lateral connectivity. After restoration, the reconnected channel demonstrated a more rheophilic macroinvertebrate composition than the dredged channel. Therefore, the restoration programme reached the objectives of rejuvenating several channels at the floodplain scale and favouring rheophilous communities. Similarly, Simons et al. (2001) revealed that the number of rheophilous taxa, as well as their density, increased after the connection of a man-made secondary channel to the main river channel. Our results also showed that, even with a restricted set of study sites, a significant part of the faunal changes can be predicted from the amplitude of connectivity change induced by restoration. Decisions to implement a restoration programme can have important environmental, economic and social impacts (Buijse et al., 2002; Jansson et al., 2007; Woolsey et al., 2007). Consequently, simulations of the connectivity changes River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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Unrestored
Flow increase
5 0 15 10
Dredging
Reconnection
5
10
15
0
5
EPT rarefied richness
10
15
0
5
10
15
CHANGES IN LATERAL CONNECTIVITY AND MACROINVERTEBRATES
Flood event
0
Before
su03
sp04
su04
After
sp05
su05
sp06
su06
sp07
su07
sp08
su08
sp09
su09
Sampling date Figure 5. Temporal changes in the EPT rarefied richness (based on a calculation for 211 individuals) ( standard error) in each channel type before
and after restoration. The dashed zone represents the transitional period between before and after restoration. The flood of January 2004 is highlighted by a small arrow
and of their effects on biodiversity metrics could be assessed before undertaking the restoration measures. Alien species The question of the spread of alien species, in the context of freshwater ecological restoration, is a growing concern (Jansson et al., 2007). Our hypothesis was that the highest increases in the alien species richness and density would correspond with the highest increase in the lateral connectivity (i.e. full reconnection). Our results indicated a significant increase in alien macroinvertebrate density and richness after restoration in all channels (including the unrestored one that is fully connected to the main river channel). However, we found no links between changes in lateral connectivity and in alien species richness and density. It appears that, during the sampling period, the main channel itself was undergoing an increasing flux of lotic alien species originating either from the upstream (Lake Geneva) or the downstream (the Saône River) parts of the Rhône River catchment (Besacier-Monbertrand et al., 2010). Similarly, Simons et al. (2001) indicated that macroinvertebrate alien species found in a newly created secondary channel Copyright © 2012 John Wiley & Sons, Ltd.
of the River Waal (the main branch of the River Rhine in the Netherlands in its downstream part) were mainly originating from the Danube River. The fact that large rivers are interconnected (e.g. for navigation) facilitates the propagation of ‘new’ alien species in floodplains subject to restoration. Among the five species new to the sector that were collected during our study, four of them had been collected for the first time only recently in the French Upper-Rhône (Crangonyx pseudogracilis in 2004, Dikerogammarus villosus in 2005, Hypania invalida in 2007 and Hemimysis anomala in 2009, unpublished data). The fifth one, Orconectes limosus, is an ancient invader in the system where it was first observed in 1976 (Besacier-Monbertrand et al., 2010). Paillex et al. (2009) and Besacier-Monbertrand et al. (2010) predicted favourable effects of restoration on the alien species dispersal in the Upper Rhône River. In our current study, this favorable effect of restoration was confirmed by the results obtained for alien species metrics in the dredged and reconnected channels. Consequently, we can assume that, although restoration measures are not directly responsible for the propagation of alien species in the restored channels, because of the independent arrival of ‘new’ alien species in River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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Unrestored
Flow increase
4 2 8 0 4
6
Dredging
Reconnection
4
6
8 0
2
Density of alien species (log)
6
8 0
2
4
6
8
A.-L. BESACIER-MONBERTRAND ET AL.
2
Flood event
0
Before
After
su03 sp04 su04 sp05 su05 sp06 su06 sp07 su07 sp08 su08 sp09 su09
Sampling date Figure 6. Temporal changes in the density (log-transformed) of alien species by sampling unit (0.25 m2) in each channel type before and after
restoration. The dashed zone represents the transitional period between before and after restoration. The boxes represent the interquartile range (Q75–Q25) around the median value. The upper and lower whiskers represent respectively the values Q75 + 1.5*(Q75–Q25) (upper datum of the upper quartile) and Q25–1.5*(Q75–Q25) (lowest datum of the lowest quartile). The flood of January 2004 is highlighted by a small arrow
the main channel, they do seem to accelerate the introduction and spread of such species along the lateral dimension of the floodplain. Finally, the recent arrival of alien species may not allow those species to occupy their entire potential niche. The limited time scale of the colonization process may explain the poor relation between changes in lateral connectivity and alien species spread. Furthermore, different populations of the same alien species can follow different long-term demographic trajectories (Strayer and Malcom, 2006) and extreme
events, such as floods or the 2003 heat wave, can locally enhance their development in a stochastic way (Daufresne et al., 2007), reinforcing the need to give particular attention to the fluctuations of alien species in future decades. Durability of restoration and time scale for monitoring Although limited to five years, the duration of the postrestoration monitoring allowed changes in physical and biological characteristics to show. According to the lateral
Table V. Number of alien species in each channel before (pre) and after restoration (post). Species richness is given for each sampling date ( : no sampling) Pre
Unrestored Flow increase Dredging Reconnection Total
Post
su03
sp04
su04
su05
sp06
sp07
su07
sp09
su09
— 2 1 1 2
5 3 2 0 6
5 — — — 5
— — 2* 4* 4
— — 6 7 8
8 7 7 4 9
10 7 6 6 10
7 8 6 8 9
6 7 6 9 11
*Flow increase occurred 27 July 2005. Copyright © 2012 John Wiley & Sons, Ltd.
River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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8
R2= 0.14 4 2 0
EPT rarefied richness
0.6 0.4
-4
0.2
-2
0.8
6
R2= 0.35
-6
0.0
Macroinvertebrate composition
1.0
CHANGES IN LATERAL CONNECTIVITY AND MACROINVERTEBRATES
0.0
0.2
0.4
0.6
0.8
1.0
Connectivity
0.0
0.2
0.4
0.6
0.8
1.0
Connectivity
Figure 7. Relationships between the changes in connectivity after restoration (Δ Connectivity) and the changes in faunal composition
(Δ Macroinvertebrate composition) and in EPT rarefied richness (Δ EPT rarefied richness). Changes are calculated per site as after-before difference for all possible pairs of dates for the same season
connectivity and the macroinvertebrate composition metrics, restoration by reconnection enabled the maintenance of the post-restoration conditions during the monitoring period. On the other hand, restoration by dredging led to a return to the pre-restoration conditions after one to two years. Indeed, the dredged channel rapidly reached the prerestoration state, as expressed by the connectivity metric. In this instance, the length of the monitoring study might not be sufficient to assess the duration of the restoration effects. Nevertheless, in such cases, deepening a stagnant pool within a channel is intended primarily to increase its duration as a permanent water body (i.e. to slow down terrestrialization processes). Therefore, the rapid return to pre-restoration values of the metrics might not be a negative sign, and the real question concerns the longer term duration of this condition, which cannot be assessed here. Furthermore, restoration by dredging in an alluvial environment results in an increase of vertical connectivity with the groundwater (Boulton, 2007). The removal of organic and fine surface sediment increases the groundwater input into the water body and might be crucial to increase the permanency of the water. Henry et al. (2002) showed that five years after restoration by dredging, the groundwater supply increased in a parapotamal channel of the Rhône, and, according to vegetation data, the restored channel exhibited a less advanced succession stage than before restoration. The durability of restoration measures also imposes the time scale of monitoring. Holl and Cairns (2002) indicated that monitoring over space and time should be performed according to the objectives of restoration measures and the ecosystems being monitored. Time scale of monitoring depends also of the selected indicators. Aquatic macroinvertebrates in large scale restoration processes have shown to respond quickly to restoration measures (Simons et al., Copyright © 2012 John Wiley & Sons, Ltd.
2001; Paillex et al., 2009). According to our study, a monitoring time frame of five years allowed trends to be seen. However, the life time of floodplain channels is greater than five years. Floodplain channels successional rate depends on the interaction of various processes (e.g. the autochthonous productivity, erosion versus sedimentation balance, riverbed incision). These processes can alternatively compensate for succession or lead to disconnection (Piegay et al., 2000; Amoros and Bornette, 2002). Terrestrialization can occur from a few decades to more than one century (Amoros et al., 2005). We thus recommend long-term monitoring (i.e. at least over 10 years) to assess the durability of restoration. Although channels as the unrestored one allow, in a short-term monitoring, distinguishing natural changes from restoration induced changes, a long-term monitoring should also be performed. To assess fluctuations and trends in macroinvertebrate alien species and to allow a better distinction between natural fluctuations and trends because of restoration (Henry et al., 2002) or other influences (e.g. climate change) (Daufresne et al., 2004), only on a long-term monitoring period is this relevant. In this respect, the single pre-restoration year that was imposed in this study can be regarded as too short to depict correctly pre-restoration fluctuations. Flood events and restoration measures Results showed that the effects of restoration measures could be of a similar intensity to that of a natural phenomenon such as the flood event that occurred in January 2004. The combination of flow increase and restoration measure (especially reconnection) led to post restoration values of the lateral connectivity and the macroinvertebrate composition metrics, similar to those observed prior to restoration in spring 2004, after the flood of January. In both instances, after flood and River Res. Applic. 30: 557–570 (2014) DOI: 10.1002/rra
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after restoration, the macroinvertebrate composition shifted to rheophilic communities. The flood episodes that occurred after restoration seem to have had less conspicuous effects on the macroinvertebrate composition because the shift towards rheophilic taxa had already happened. Taking this into account, the effectiveness of restoration measures on connectivity and faunal metrics could be demonstrated by the degree to which they reach the same level of change, as those determined, after flood episodes. Flood pulses in river-floodplain ecosystems are responsible for the diversity of habitats in the floodplain (Junk et al., 1989). At present, most regulated rivers have lost their natural flow regime (Tockner and Stanford, 2002), and only important flood events can have an impact on floodplain channels and their associated biota. To restore the natural function of the system, an alternative strategy could be the implementation of artificial floods (Poff et al., 1997). Compared with dredging or reconnection, restoration by artificial floods avoids direct human intrusion and disturbance to the environment. Consequently, the comparison of the effectiveness of restoration measures (e.g. reconnection) with iterated artificial floods in a system would be highly valuable. Finally, most published studies on experimental floods consider the longitudinal impacts of floods in the main stream channel (e.g. Robinson et al., 2003), and future research could valuably focus on the lateral dimension of floodplains (but see Robinson et al., 2004a). Conclusion and future perspectives Because river and floodplain restorations are worldwide applied and frequent, the approaches and indicators used for measuring the success of restoration measures have been debated (Palmer et al., 2005; Ardón and Bernhardt, 2009). According to the objectives, the scale at which restoration measures are performed, the time scale of monitoring and the ecosystem component considered (vegetation, macroinvertebrates, sediments, etc.), make conclusions about the effectiveness of restoration, highly variable (e.g. Jähnig et al., 2009). In this study, we highlighted a predictable relationship between lateral connectivity changes and macroinvertebrate responses to restoration. Given the important impacts and costs of restoration measures (Jansson et al., 2007; Woolsey et al., 2007), this aspect opens perspectives for modeling the effects of the different restoration strategies on lateral connectivity and macroinvertebrate metrics. According to Palmer et al. (2005), restoration projects that do not lead to better self-sustaining conditions cannot be considered as ecological restoration. Therefore, dredging has to be scrutinized to assess the real long-term effectiveness of such a restoration technique. In this study, only a lotic unrestored channel was implemented as a reference. For a better Copyright © 2012 John Wiley & Sons, Ltd.
assessment of dredging restoration measure, more than one reference channel (see Underwood, 1994) and notably lentic unrestored channels should also be taken into account. The process of evaluation of the various restoration techniques is an important step because the homogenization of lateral connectivity levels, at the floodplain scale, could reduce macroinvertebrate diversity. We therefore suggest that the maintenance of different levels of lateral connectivity at the floodplain scale should be a major objective in large scale restoration projects. Additional factors, such as climate change, the propagation of alien species (Daufresne et al., 2007) can interfere, and the confounding effects between natural fluctuations and restoration-induced trends may occur. Long-term monitoring studies at least over 10 years should be carried out to assess the durability and impacts of restoration measures. ACKNOWLEDGEMENTS
We are thankful to J.-M. Olivier and N. Lamouroux for the coordination of the French Rhône River restoration programme. We are grateful to A. Fremier, S. Dolédec and four anonymous reviewers for their useful comments on the manuscript. We thank D. McCrae for editing the English. We are also thankful to O. Béguin, H. Mayor and D. McCrae who participated in the field programmes, the sorting and the identification of macroinvertebrates. This research was jointly funded by the Compagnie Nationale du Rhône, the Agence de l’Eau Rhône Méditerranée et Corse, the Direction Régionale de l’Environnement, the Région Rhône-Alpes and the University of Geneva.
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