R.C.M. Verdonschot

Contrasting the roles of section length and instream habitat enhancement for river restoration success: a field study of 20 European restoration projects

Restoration of river hydromorphology often has limited detected effects on river biota. One frequently discussed reason is that the restored river length is insufficient to allow populations to develop and give the room for geomorphological processes to occur. We investigated ten pairs of restored river sections of which one was a large project involving a long, intensively restored river section and one represented a smaller restoration effort. The restoration effect was quantified by comparing each restored river section to an upstream non-restored section. We sampled the following response variables: habitat composition in the river and its floodplain, three aquatic organism groups (aquatic macrophytes, benthic invertebrates and fish), two floodplain-inhabiting organism groups (floodplain vegetation, ground beetles), as well as food web composition and land-water interactions reflected by stable isotopes. For each response variable, we compared the difference in dissimilarity of the restored and nearby non-restored section between the larger and the smaller restoration projects. In a second step, we regrouped the pairs and compared restored sections with large changes in substrate composition to those with small changes. When comparing all restored to all non-restored sections, ground beetles were most strongly responding to restoration, followed by fish, floodplain vegetation, benthic invertebrates and aquatic macrophytes. Aquatic habitats and stable isotope signatures responded less strongly. When grouping the restored sections by project size, there was no difference in the response to restoration between the projects targeting long and short river sections with regard to any of the measured response variables except nitrogen isotopic composition. In contrast, when grouping the restored sections by substrate composition, the responses of fish, benthic invertebrates, aquatic macrophytes, floodplain vegetation and nitrogen isotopic composition were greater in sections with larger changes in substrate composition as compared to those with smaller changes. Synthesis and applications. The effects of hydromorphological restoration measures on aquatic and floodplain biota strongly depend on the creation of habitat for aquatic organisms, which were limited or not present prior to restoration. These positive effects on habitats are not necessarily related to the restored river length. Therefore, we recommend a focus on habitat enhancement in river restoration projects. The effects of hydromorphological restoration measures on aquatic and floodplain biota strongly depend on the creation of habitat for aquatic organisms, which were limited or not present prior to restoration. These positive effects on habitats are not necessarily related to the restored river length. Therefore, we recommend a focus on habitat enhancement in river restoration projects.

The role of benthic microhabitats in determining the effects of hydromorphological river restoration on macroinvertebrates

Despite the large number of river restoration projects carried out worldwide, evidence for strong and long-term positive ecological effects of hydromorphological restoration on macroinvertebrates is scarce. To improve the understanding of the success and failure of restoration measures, a standardized field study was carried out in nineteen paired restored and degraded river sections in mid-sized lowland and mountain rivers throughout Europe. We investigated if there were effects of restoration on macroinvertebrate biodiversity, and if these effects could be related to changes in microhabitat composition, diversity and patchiness. Effects were quantified for all taxa combined, as well as Ephemeroptera, Plecoptera and Trichoptera separately. Additionally, species trait classifications of microhabitat preference types were used as a functional indicator. Restoration had no overall positive effects on the selected macroinvertebrate metrics. Rather, we did find positive relationships between the macroinvertebrate responses and the effect of restoration on the diversity and patchiness of microhabitats. Furthermore, the effects on macroinvertebrates could be related to changes in the cover of specific substrate types in the restored sections. We conclude that the limited effect of restoration on macroinvertebrate diversity overall reflected, at least in part, the limited effect of most restoration measures on microhabitat composition and diversity.

Effects of river restoration on riparian ground beetles (Coleoptera: Carabidae) in Europe

Studies addressing the effects of river and floodplain restoration on riparian ground beetles mainly focus on single river sections or regions. We conducted a large-scale study of twenty paired restored and degraded river sections throughout Europe. It was tested (i) if restoration had an overall positive effect on total species richness, Shannon–Wiener diversity and richness of riparian, wetland and floodplain forest specialists, and (ii) if the effects depended on river and project characteristics as well as habitat differences caused by restoration. Groupwise comparison of the restored and degraded river sections showed that restoration had a significant positive effect on one out of the five metrics investigated (the number of riparian specialists), and pairwise comparison of the restored sections with the corresponding degraded sections revealed an additional positive effect of restoration on total species richness. These positive effects were related to a co-occurring set of environmental variables, with the effects being more apparent in widened river sections of high-gradient cobble/gravel-bed rivers where restoration decreased riparian woody vegetation and increased sparsely vegetated banks. These results clearly indicate that the effect of restoration on riparian ground beetle richness depends on the creation of such pioneer habitats.

The role of macrophyte structural complexity and water flow velocity in determining the epiphytic macroinvertebrate community composition in a lowland stream

Habitat structural complexity provided by aquatic macrophytes in lowland streams affects the associated epiphytic macroinvertebrate assemblages in both direct (increased microhabitat diversity, refuge against predation) and indirect ways (e.g. current attenuation by physical structures). In a correlative field study carried out in two different years in a Belgian stream, we investigated the effects of the factors macrophyte identity, macrophyte complexity (represented as fractal complexity) and current velocity on the composition of the macroinvertebrate community associated with monospecific macrophyte patches, consisting of plants with differing structural complexity; Sparganium emersum Rehmann (least complex), Potamogeton natans L. (intermediate) and Callitriche obtusangula Le Gall (most complex). In addition to significantly lower within-patch current velocity being observed, vegetation stands consisting of complex macrophytes also harboured significantly richer macroinvertebrate communities than stands of simpler macrophytes. A significant part of the variation in the macroinvertebrate community composition could be explained by plant identity, macrophyte complexity and current velocity. However, it was not possible to determine the relative importance of these three factors, because of their high degree of intercorrelation. Additionally, the explanatory power of these factors was higher under conditions of high current velocity, suggesting a role of macrophyte patches as instream flow refugia for macroinvertebrates.

Substrate homogenization affects survival and fitness in the lowland stream caddisflies Micropterna sequax and Potamophylax rotundipennis: a mesocosm experiment

Loss of substrate heterogeneity or patchiness is common in lowland streams with disturbed hydrological regimes. At the reach scale, peak discharges tend to homogenize the stream bed and decrease the availability of specific microhabitat types. This spatial shift in habitats toward a more homogeneous landscape could have large negative effects on species that perform essential ecosystem processes. We conducted an aquatic mesocosm experiment to test the effect of habitat homogenization on survival and fitness of 2 species of Trichoptera (Micropterna sequax and Potamophylax rotundipennis). We used caddisflies as model organisms because of their abundance in lowland streams, their representativeness of the total shredder community, and their important role in leaf-litter decomposition. We reared larvae in artificial recirculating channels containing leaf and sand patches in 3 spatial configurations, differing in homogeneity of substrates, varying from few large patches to many small patches. We used emergence rate as a measure of survival and biomass and wing span of the adults as fitness correlates. For M. sequax, survival was lower in the homogeneous than in the heterogeneous configurations, but patch configuration did not affect fitness. For P. rotundipennis, spatial configuration of the patches did not affect survival, but the longest forewings for both males and females were found in the homogeneous configuration. Our results suggest that both species experience intraspecific resource competition arising from the spatial distribution of patches, expressed as an investment in wing development (e.g., dispersal capacity) in P. rotundipennis and resulting in lower survival rates in M. sequax. Our results indicate the importance of knowledge of trait-based responses and highlight the effects of the configuration of stream bottom substrate for its inhabitants on microscale.

Biotic interactions enhance survival and fitness in the caddisfly Micropterna sequax (Trichoptera: Limnephilidae)

Patches of coarse particulate organic matter in lowland streams are inhabited by many different macroinvertebrate species, yet knowledge of interactions among the members of these assemblages is scarce. In a mesocosm experiment we aimed to determine the effect of interspecific interactions on species survival and fitness of two caddisfly species. It was hypothesized that, as a result of positive interactions, mixed species populations would yield higher survival and fitness than single species populations. Larvae of two caddisfly species, Micropterna sequax and Potamophylax rotundipennis, were reared in single species and mixed species populations. Emergence rate was recorded and adult fitness was measured in terms of wingspan and biomass. We found that in mixed populations, emergence rate, wing length and biomass of M. sequax were higher than in single species populations. P. rotundipennis was only significantly, yet negatively, affected in terms of biomass of the male individuals. This study showed that occurring together with other species holds advantages for M. sequax, and emphasizes the importance of species diversity in streams. Furthermore, the observed positive effects on survival and fecundity might influence population sizes of the interacting species, in turn affecting macroinvertebrate-mediated ecosystem processes such as leaf litter decomposition.

Reintroduction of freshwater macroinvertebrates: challenges and opportunities: Reintroduction of freshwater macroinvertebrates

Species reintroductions – the translocation of individuals to areas in which a species has been extirpated with the aim of re‐establishing a self‐sustaining population – have become a widespread practice in conservation biology. Reintroduction projects have tended to focus on terrestrial vertebrates and, to a lesser extent, fishes. Much less effort has been devoted to the reintroduction of invertebrates into restored freshwater habitats. Yet, reintroductions may improve restoration outcomes in regions where impoverished regional species pools limit the self‐recolonisation of restored freshwaters. We review the available literature on macroinvertebrate reintroductions, focusing on identifying the intrinsic and extrinsic factors that determine their success or failure. Our study reveals that freshwater macroinvertebrate reintroductions remain rare, are often published in the grey literature and, of the attempts made, approximately one‐third fail. We identify life‐cycle complexity and remaining stressors as the two factors most likely to affect reintroduction success, illustrating the unique challenges of freshwater macroinvertebrate reintroductions. Consideration of these factors by managers during the planning process and proper documentation – even if a project fails – may increase the likelihood of successful outcomes in future reintroduction attempts of freshwater macroinvertebrates.

Oxygen drives benthic-pelagic decomposition pathways in shallow wetlands

Oxygen availability is perceived as an important environmental factor limiting POM decomposition. In shallow wetlands, however, the impact of commonly observed anoxic conditions in the benthic layer on the relative contribution of microbes and invertebrates to POM decomposition remains largely unknown. Therefore, the aim of this study was to determine if dissolved oxygen drives benthic-pelagic decomposition pathways in shallow wetlands. Dissolved oxygen concentration, invertebrate community composition, microbial decomposition and invertebrate consumption were measured in the benthic and pelagic layer of 15 permanent drainage ditches. We showed that an increased duration of anoxic conditions in the benthic layer of the ditches was related to increased microbial decomposition in this layer, while invertebrate consumption decreased in the benthic layer and increased in the pelagic layer. The increased invertebrate consumption in the pelagic layer was related to the presence of amphipods. We concluded that anoxic conditions in the benthic layer of shallow wetlands relate to an increase in microbial decomposition and a decrease in invertebrate consumption, as detritivorous invertebrates move to the pelagic layer to consume particulate organic matter. This illustrates that environmental conditions, such as dissolved oxygen, may drive the relative importance of aquatic organisms to ecosystem functioning.