Amael Paillex

Aquatic macroinvertebrate response along a gradient of lateral connectivity in river floodplain channels

Abstract Large river floodplains potentially include the full range of freshwater ecosystems from permanently flowing channels to temporary pools and springs. Attempts to restore such complex systems require tools adapted to assess restoration success. In an analysis of invertebrate assemblages in the Rhône River floodplain (France), taxonomic-based indices (rarefied richness and assemblage composition) were compared with functional metrics using trait-based ratios as surrogates of ecosystem processes. Their ability to respond to a gradient of hydrological connectivity was assessed in 7 cut-off channels. The sampling design included 2 sites/channel (upstream and downstream), 4 randomly chosen sampling points (0.5 × 0.5-m quadrats)/site, and 2 sampling seasons (spring and summer). Water physicochemical and habitat variables were recorded when invertebrates were sampled. Environmental variables, including water conductance, [NH3-N], submerged vegetation cover, diversity of sediment grain size, and organic matter content of the sediment, were used to construct a synthetic variable describing the hydrological connectivity of each site with the main river channel. A quadratic regression of rarefied taxonomic richness and the connectivity gradient was not quite significant, but assemblage composition was strongly related to the gradient. Four of 8 trait-based metrics were correlated with the connectivity gradient. Values of metrics that are surrogates for top-down control of assemblage structure and habitat stability (based on functional feeding groups) declined along the gradient from disconnected sites to more connected sites. Values of metrics that are surrogates for voltinism and food supply for water-column-feeding fish increased with connectivity. Top-down control and voltinism surrogates suggested a decline in predator–prey relationships and lower habitat stability, respectively, in the more connected sites. Assemblage composition and some of the trait-based metrics were sensitive to a flood that occurred before one of the sampling dates. Some of the trait-based metrics showed potential for explaining floodplain invertebrate assemblages and for monitoring postrestoration conditions in floodplain water bodies. However, the metrics were developed initially for studies of lotic systems and their use in heterogeneous floodplain water bodies will require further investigation, e.g., delineation of reference conditions for trait-based metrics.

Functional diversity in a large river floodplain: anticipating the response of native and alien macroinvertebrates to the restoration of hydrological connectivity

Summary 1. Rivers and floodplains are among the most threatened ecosystems. Hydroelectric power plants and embankments have reduced the hydrological connectivity between rivers and their floodplain channels, leading to loss of freshwater habitats and biological communities. To prevent and reverse such loss, numerous restoration programmes have aimed at rejuvenating the lateral hydrological connectivity between rivers and floodplain channels. Despite consider- able global attention, we know remarkably little about the ecological benefits of floodplain restoration programmes. 2. We analysed the functional diversity of different macroinvertebrate groups (natives and aliens) along a gradient of lateral hydrological connectivity on the Rhone river in France. We used 36 sampling sites to describe the functional diversity (Raos quadratic entropy) before and after the enhancement of the lateral hydrological connectivity by restoration. The effects of restoration on functional diversity were tested for each macroinvertebrate group and at multiple spatial levels (alpha and beta). 3. Before restoration, alpha functional diversity of the entire macroinvertebrate community peaked in sites with a high lateral connectivity. The contribution of the native groups to func- tional diversity was higher than that of the alien group. The latter was not constrained by high values of lateral hydrological connectivity and reached a maximum in highly connected sites. 4. After restoration, within-site functional diversity (alpha FD) declined linearly following the enhancement of lateral hydrological connectivity. The restoration operations increased the contribution of the aliens to functional diversity and reduced the contribution of a group of native taxa. In addition, among-sites functional diversity (beta FD) was successfully enlarged by restoration. 5. Synthesis and applications. The lateral hydrological connectivity (LHC) represents a key parameter for explaining the functional diversity (FD) of macroinvertebrates in a floodplain ecosystem. Our results demonstrate that restoration-induced changes to functional diversity can be predicted. Controversially, restoration-induced enhancement of lateral hydrological connectivity increased the functional diversity of the alien macroinvertebrates. However, these species contributed only to a small part of the total macroinvertebrate functional diversity. We recommend that restoration programmes diversify the levels of lateral hydrological con- nectivity among the channels to ensure an optimal functional diversity at the floodplain scale.

Alien aquatic macroinvertebrates along the lateral dimension of a large floodplain

Floodplains are simultaneously among the most species-rich and the most threatened ecosystems. Alien aquatic macroinvertebrates contribute to this threat but remain scarcely studied in the lateral dimension of floodplains. We modelled the realized ecological niches of the alien species occurring in 24 floodplain channels of the Rhône River. Environmental variables depicting the ecological niches were associated to the lateral hydrological connectivity and light availability, both being modified during floodplain restoration works. Eight alien species were observed and they demonstrated either ubiquity or a restricted niche, with no link to the date of introduction. For most of them, the main river channel appeared as an important dispersal route in the lateral dimension of the floodplain. An increase of both lateral connectivity and light availability favoured most of the modelled species. Consequently, we recommend that sector-scale restoration programmes preserve varying levels of lateral connectivity for floodplain channels to prevent the expansion of alien aquatic macroinvertebrates.