Aquatic macroinvertebrate patterns in the lateral dimension of the floodplain

Results obtained in the previous chapters (2-3-4) highlight the lateral hydrological connectivity as a key determinant of macroinvertebrate diversity in the Rhône River.

Macroinvertebrate metrics (i.e. measurable characteristics of macroinvertebrate communities) were chosen from the literature for their empirical or demonstrated link with the lateral connectivity in a floodplain context. The metrics were also chosen for their potential to characterize and predict the changes that restoration of the lateral connectivity could induce in the biota. Furthermore, macroinvertebrate metrics were tested to select those that were significantly related with the constructed gradient of lateral connectivity. In the order of decreasing coefficient of determination, the following metrics were significantly related with the constructed gradient of lateral connectivity: the taxonomical composition, the macroinvertebrate characteristics, the richness of taxonomic subgroups and the total taxonomic richness.

6.2.1. Does lateral connectivity influence taxonomical composition and richness?

As shown in others studies (Castella et al., 1984; Gallardo et al., 2008;

Reckendorfer et al., 2006), the aquatic macroinvertebrate composition in the Rhône River floodplain appeared strongly related to the lateral connectivity. A gradual and linear change in taxonomical composition was evidenced along the constructed gradient of lateral connectivity (chapter 2). In other terms, a continuous change in taxonomic composition takes place along the gradient of lateral connectivity. This result was in accordance with empirical theories about floodplain biodiversity (Richardot-Coulet and Greenwood, 1993; Robinson et al., 2002). This strong and significant relationship between macroinvertebrate composition and the lateral

connectivity underlines the importance of the lateral dimension of large rivers like the Rhône to sustain aquatic biodiversity.

Taxonomic subgroups (e.g. Trichoptera, Coleoptera, Odonata) exhibited different patterns of richness along the gradient of lateral connectivity and corroborated previous research observations. Originally the EPT richness (Ephemeroptera, Plecoptera and Trichoptera) was used to provide information about the water quality in lotic systems (Lenat, 1988). However this metric is also well correlated with the lateral connectivity (chapter 3) and can inform on the connection frequency of a cut-off channel with the main river. EPT richness increases with the lateral connectivity to reach a maximum in the most connected channels. Their sensitivity to low level of oxygen may explain their pattern along the gradient of lateral connectivity and their limitation in disconnected channels. Indeed, disconnected channels with high abundance of macrophytes may be subject to oxygen depletion at night. Oppositely, Odonata and Coleoptera richness increased from the most connected channel to the most disconnected. Those patterns were also observed in other floodplains (Arscott et al., 2005; Davis et al., 2007; Gallardo et al., 2009a;

Tockner et al., 1999b). The progressive replacement of Odonata and Coleoptera by EPT species when the lateral connectivity increases, reflects the gradual change of composition along the gradient of connectivity. Other groups of species were not identified to a high level and may have similar patterns as those previously described.

For example, Oligochaeta richness should be higher in parapotamal channels where the substrate is diversified and offers a large range of habitat for benthic species (Juget and Lafont, 1994). In the same way, Heteroptera that were identified to genus level in this study may show a gradual increase in species richness to reach high values in disconnected environments (i.e. plesiopotamal channels) (Skern et al., in press).

Subgroup richness

The intermediate disturbance hypothesis, proposed by Connell (1978), is at the core of many theoretical developments in ecology (Shea et al., 2004). It is hypothesised that communities are influenced by the frequency and intensity of disturbance they are subjected to. The hypothesis predicts that richness should be highest at intermediate level of disturbance. We used this hypothesis to test the pattern of total macroinvertebrate richness along the constructed gradient of lateral connectivity. We postulated that high connectivity with the main river implies frequent and intense disturbance by floods, when at the other end of connectivity the flood disturbance is extremely reduced or absent. Competition is expected to occur in the most disconnected channel and to exclude the weaker competitors, therefore, reducing richness. Frequent disturbances by floods are expected to reduce the richness because few species are able to tolerate or escape the frequent disturbances. Despite those expectations, the total macroinvertebrate rarefied richness on the Rhône River did not peak along the gradient of lateral connectivity, but decreased from the most disconnected channels to the most connected ones (chapter 4). Karaus (2004) showed also that the aquatic macroinvertebrate richness decreased with increasing hydrological connectivity on a braided floodplain (Tagliamento). Both negative responses were consistent with the fact that only few species can withstand or tolerate frequently disturbed environments. On the contrary, on a meandering river, Gallardo (2009) underlined that species diversity was consistent with the intermediate disturbance hypothesis. The author underlined that both generalists and specialists coexisted in intermediately disturbed sites. Townsend et al. (1997) also tested the hypothesis on macroinvertebrates in streams that differed in disturbance intensity and their results supported the prediction of a maximum of richness with an intermediate level of disturbance. They underlined that the recovery of the macroinvertebrates after a disturbance (e.g. a flood) may be largely influenced by the presence of refugia within the system. Therefore the intermediate disturbance hypothesis should be tested in parallel with the availability of refugia. In the case of absence of refuge, the recovery Total

richness

of the macroinvertebrates may be low and the species richness may not support a unimodal pattern as predicted. (Connell, 1978) (Karaus, 2004)

Reasons for the absence of a unimodal response on the Rhône River floodplain are not yet elucidated. Nevertheless, we are confident to have accounted for the full range of hydrological disturbances by floods, i.e. from fully connected to disconnected cut-off channels. In this study, only the temporary sites (where water is not permanent) were not taken into account and may provide the “missing” part of the unimodal curve. Another explanation should be that even in the most disconnected channels the disturbance occurred and is sufficient to avoid the development of the exclusive competitors. Indeed, the Rhône River has been described as a mostly braided and dynamic system (Bravard, 1987; Olivier et al., 2009), which supports the idea of a disturbance that may occur also in the most disconnected channels. Whereas in a less

"active" floodplain, disturbance level in the most disconnected channels may be sufficiently lower to allow competition to be exclusive (Gallardo, 2009). Finally, Townsend et al. (1997)’s idea to study the availability of refugia within a system should be considered in the future. As Mackey and Curie (2001) stated, the diversity-disturbance relationship does not always peak as the contemporary consensus would suggest. (Gallardo, 2009) (Townsend et al., 1997)

(Mackey and Currie, 2001)

6.2.2. Taxonomical composition and richness after restoration

On the basis of the above-mentioned results, we were allowed to predict that a modification of the lateral connectivity by restoration would induce a proportional change in macroinvertebrate composition. Indeed, two years after the floodplain restoration, the changes in lateral connectivity were reflected by changes in macroinvertebrate composition. Furthermore, the observed amplitude of change in macroinvertebrate composition appeared proportional to the changes in lateral Taxonomical

composition

connectivity. Few compositional changes were observed in unrestored channels, while larger changes were observed in channels where the lateral connectivity was increased by restoration. However, the changes in macroinvertebrate taxonomical composition after a moderate restoration (such as a dredging) were lower than after a full reconnection to the main channel. In 1992, after the dredging of a channel on the Upper Rhône River, small changes in macroinvertebrate composition was also evidenced (Henry et al., 1995; Rosset, 2006). In this restored channel, more reophilic species were observed after the dredging (Henry et al., 1995) and taxonomical changes in macrophytes were also evidenced (Henry et al., 2002). Currently no result at a large spatial scale seems to have been published elsewhere and we have therefore no comparable results. Only local-scale changes in composition were observed after the raise of the water level in floodplain water bodies or after small-scale restoration measures (Funk et al., 2009; Henry et al., 2002; Simons et al., 2001). (Carron et al., 2007)

Concerning the changes in richness, the results showed the effect of the restoration on Gastropoda and Odonata that decreased with the increase of the lateral connectivity. It concurred with Reckendorfer et al. (2006) on the fact that few gastropod species inhabit lotic environment. Two years after restoration, the increase of the lateral connectivity by restoration induced a predictable increase of the EPT richness (chapter 5), which was consistent with the pre-restoration models. However Coleoptera richness was not linearly influenced by the changes in lateral connectivity by restoration measures. Despite this overall result upon Coleoptera; Carron et al.

(2007) showed that after the restoration of a single site on the Rhône River (BEAR channel), the community of Coleoptera was found to be severely reduced but then quickly recovered. The authors hypothesised that the high mobility of adult Coleoptera and the presence of reservoir-populations nearby may explain the quick recovery.

Subgroup richness

Moreover, this important reduction and the rapid recovery may be also linked to the observed increase in lateral connectivity and a rapid return to a previous situation after the dredging of the channel (see discussion Fig.1). Therefore the recovery of the Coleoptera may be a complex response related to the changes in lateral connectivity, the presence of nearby reservoir populations and the high mobility in the adult phase.

This could explain the absence of a linear effect of restoration measures on Coleoptera richness, even if general models showed Coleoptera richness to be negatively related to the hydrological connectivity (Davis et al., 2007; Paillex et al., 2009).

Concerning the total rarefied richness within sites, no linear changes were induced by restoration measures. Based on the pre-restoration decrease of the rarefied richness along the gradient of lateral connectivity (chapter 4), we predicted a linear and predictable decrease in macroinvertebrate richness with an increase of lateral connectivity by restoration measures. Moderate restoration (e.g. dredging) did not decrease the value of total rarefied richness within channels and corroborated a previous observation of Henry et al. (1995), who highlighted that the dredging of a channel on the Rhône River did not impact the macroinvertebrate richness. Therefore, it appears that total rarefied richness of macroinvertebrates is unable to detect small change in lateral connectivity. In the case of an important change in lateral connectivity (such as after direct reconnection), the total rarefied richness evidenced a decrease and suggested a more constraining environment for the establishment and development of species. The reconnection increased the fluvial dynamic within the channels, therefore the environmental forces (e.g. hydraulic stress) acted as a filter and selected the few species adapted to such constraints. The reduction in rarefied richness after a reconnection is consistent with the fact that few species inhabit narrow eupotamic side channels of braided systems (Karaus, 2004; Paillex et al., 2007;

Tockner et al., 1998) Total

richness

6.2.3. Macroinvertebrate traits along the lateral dimension of a floodplain

When the total taxonomic richness failed to corroborate empirical hypothesis along the lateral dimension of the Rhône River floodplain, the biological characteristics of the taxa (traits) offered a complementary approach (Doledec, 2009;

Doledec et al., 1999; Haddad et al., 2008; Statzner and Beche, 2010). Traits used in this thesis were selected on the basis of the predictions proposed by Townsend and Hildrew (1994) and Poff et al. (2006). Not all the traits proposed by Townsend and Hildrew (1994) were tested, only the traits that were the most relevant to be linked with the lateral dimension of the floodplain and especially those proposed by Poff et al. (2006). We also tested the traits for which we were confident in the quality of the information coded into the available databases. Among all the traits tested (voltinism, drift tendency, feeding habits), some of their categories were significantly and logically correlated to the gradient of lateral connectivity (see Fig.2 for a summary).

The percentage of plurivoltine individuals increased along the gradient of lateral connectivity (chapter 3) to reach a maximum in the most connected sites.

Plurivoltinism is certainly a way to persist in environment frequently disturbed as described by Connell (1978) and explained in a floodplain context in chapter 3. The percentages of passive filter feeders and drifters also increased with lateral connectivity due to the associated increase in flow. Oppositely, the percentage of predators decreased with connectivity (chapter 3), this was also observed in the Parana river floodplain (Zilli et al., 2008) and the Ebro river floodplain (Gallardo, 2009).

Moreover, this pattern of predators within a community of macroinvertebrates is potentially linked with the pattern of total richness (Chase et al., 2002). Predation could participate in the monotonic negative response of the total richness along the gradient of lateral connectivity on the Rhône River, if the predation is recognize to prevent the competition from being exclusive. Indeed, predators have sometimes been shown to reduce competitive strength (Chase et al., 2002). Therefore richness may be higher in little disturbed environments than predicted by Connell (1978).

As explained in chapter 2, further tests are necessary to apply Merritt et al.

(2002)’s trait-based metrics in our temperate floodplain context. These metrics originated in the use of functional feeding groups in running waters (Cummins and Klug, 1979) and were furtherer developed and applied for a tropical floodplain context. These metrics are based on the ratio of different trait categories and used as surrogate for ecosystem attributes (Cummins et al., 2005; Doledec and Statzner, 2010;

Merritt et al., 2002). However, the validity of those calculations as surrogates of ecosystem attributes must be tested before being applied outside their original context.

Merritt et al. (2002) suggested that an external measurement of the ecosystem function is necessary to validate the trait-based metrics as ecosystem surrogates. This does not seem to have been done yet. In nine streams of Parana state (Brazil), Cummins et al.

(2005) tested the trait-based metrics and concluded that these metrics should be calibrated specifically for the region on a larger data set. Similarly, a broader assessment in mid-European floodplain ecosystem should be done to calibrate the range of ratios, the thresholds and to differentiate natural, to semi-natural conditions among floodplains systems.

Figure 2. A: diagramme extracted from Poff et al. (2006) representing macroinvertebrate traits linked to environmental variables. B: summary of the trait-categories and their responses to a gradient of lateral connectivity.

Lateral connectivity predators

plurivoltine

drifters passive filter feeders

% among the community

A B

Functional metrics

6.2.4. Beyond the non-significant results of macroinvertebrate traits along the gradient of lateral connectivity

All the results obtained here underlined that the lateral hydrological connectivity functions as a filter for certain macroinvertebrate traits as suggested in previous researches (Poff, 1997; Poff et al., 2006; Townsend and Hildrew, 1994). In chapters 2, 3 and 4, we highlighted the response of several traits (drift tendency, voltinism, filtering feeders, predators) along the gradient of lateral connectivity (see Fig.2B for a summary). However we failed to evidence the link between the hydrological connectivity and other trait-categories (shredders, scrapers and deposit feeders). Those feeding habits were expected to be also related to the gradient of lateral connectivity. We predicted that an increase in lateral connectivity with the main river would be detected as a decrease of macroinvertebrates feeding upon coarse particulate organic matter, for which the floodplain is a source (Preiner et al., 2008;

Rostan et al., 1987; Tockner et al., 1999a). As Poff et al. (2006) underlined, the feeding habits are expected to be linked to a gradient of food resources rather than to habitat stability (see Fig. 2A), the latter being depicted in the lateral dimension of the floodplain. Therefore such a gradient of food resources needs first to be evidenced at the floodplain scale and independently from a gradient of lateral connectivity (Poff et al., 2006). A composite variable incorporating, among others, the percentage of organic matter in the sediment could be constructed and tested. Such a gradient may evidence the link between the food types and quantities, and macroinvertebrate feeding habits. In parallel, another gradient may be developed to describe the thermal regime of the sites. This was approached in Besacier-Monbertrand et al. (2010), where the authors underlined the response of invasive species to the solar access to the sites.

Solar access may contribute to the thermal balance of floodplain waterbodies in addition to the groundwater inputs that may buffer temperature variations (Brunke and Gonser, 1997).

Within a given floodplain sector, water bodies may exhibit a range of temperature patterns depending upon their size, light availability and the relative balance between surface and groundwater inputs. Such changes in temperature may influence the fauna and limit their development. For example, Poff et al. (2006) suggested that species size and voltinism may be linked both to the habitat stability and the thermal regime. Chessman (2009) showed that the increase of stream water temperature in Australian streams limited the development of cold water macroinvertebrates. On the Rhône River, the effects of such temperature increase on aquatic biodiversity remain to be tested in conjunction with the lateral connectivity.

Concerning restoration works on the Rhône River, the changes in lateral connectivity were sometimes associated with modifications of the surrounding riparian vegetation:

trees and invasive species (e.g. Reynoutria japonica) were cut in places, and riparian trees (e.g. Salix sp.) were planted. Restoration measures also modified the linkage between the surface water and the groundwater through sediment dredging (i.e.

increase of the vertical connectivity). These actions potentially influence the thermal regime of the sites, through the influence of the solar access on the water body and the exchange with the groundwater that buffer the temperature variations. Also the tree cutting may influence the provision of coarse organic matter to the aquatic communities. Both modifications (thermal regime and supply in organic matter) may influence the fauna and limit their development (Boulton, 2007; Pithart et al., 2007;

Poff et al., 2006). The water temperature, the vegetation shade and the organic-matter input in the system should be monitored (see for example Davies-Colley and Payne, 1998): i) to test for patterns of macroinvertebrate traits that were non-significant along the gradient of lateral connectivity, ii) to assess the macroinvertebrate responses to modifications due to restoration measures such as modifications of the water temperature. (Besacier-Monbertrand et al., in press) (Davies-Colley and Payne, 1998) Temperature,

Shade, Organic matter

6.2.5. Functional diversity (FD)

Even if functional diversity, rather than taxonomic diversity, may be regarded as a better reflection of ecosystem functions (Naeem, 2006) and of greater accuracy with less sampling effort (Bady et al., 2005), there is no explicit framework to define which freshwater animal traits will be representative of or participate in a given ecosystem function, such as nutrient cycling (Wallace and Webster, 1996). The value of functional diversity will depend upon which functional traits are used for its calculation (Petchey and Gaston, 2006). Naeem (2006) summarized two ways to classify species traits, firstly by grouping those traits that involve a response to changes in an ecosystem (response-functional) and secondly by grouping traits that are directly related to the ecosystem functions (effect-functional). In a general way, as Diaz and Cabido (2001) wrote, FD may be seen as the value or the range of functional traits of the organisms in a given ecosystem. In our study, as a first approach, we used traits that were supposed to be related to “community ecology” such as competition, traits involved in the colonization and traits involved in the distribution of organisms such as dispersal (chapter 4). We also selected traits known to be related to the variations in lateral connectivity, such as voltinism and dispersal, and for which we were also confident in the quality of the coding-information. Others traits could have been used for the same purpose, but the coded information was often missing and the quality of the coding sometimes doubtful. This approach aimed at calculating FD based upon the selected traits with a Simpson index (Simpson, 1949).

Simpson index was applied to the relative abundances of functional groups in a community; functional groups grouping species by traits or trait categories. Grouping several traits to create functional groups related to an ecological process (e.g.

Simpson index was applied to the relative abundances of functional groups in a community; functional groups grouping species by traits or trait categories. Grouping several traits to create functional groups related to an ecological process (e.g.