AbstractsBiology & Animal Science

Large wood (LW) forms a key element in the morphology of river channels. Its presence can significantly influence the hydromorphological processes shaping the river channel by increasing hydraulic roughness, and thus modify prevailing flow patterns and sediment transport. The surface of LW has often been described as a hotspot of macroinvertebrate diversity and abundance, since it represents a hard and stable substrate suitable for animal attachment and oviposition, and it may additionally offer more diverse food resources. Such effects of LW are expected to be especially conspicuous in lowland sandy rivers, where LW logs represent the main roughness elements within the river channel, and also the unique stable substratum for colonization within the otherwise homogeneous and loose sandy or muddy river bottom. Despite this, previous studies on LW have focused on mountain streams with coarser sediments, and did thereby also hardly consider the effects of LW on the benthic macroinvertebrates colonizing the surrounding riverbed sediments. This thesis thus analysed the effects of the presence of in-channel pieces of LW on the physical habitat conditions and macroinvertebrate benthic assemblages of lowland sandy rivers. The study was based on field observations in river channels that were subjected to little human disturbances only, and were hence naturally rich in LW. The field records of hydraulic, morphological and biotic variables allowed for the identification of the mechanistic linkages between the changes in the physical and biotic components of the river ecosystems that were induced by the presence of LW. The effects of LW on macroinvertebrate communities were analysed in terms of taxonomic and functional composition, and of alpha and beta diversity. Results show that in the studied lowland rivers LW induced consistent patterns of habitat variability within the otherwise homogeneous sandy river channel, as e.g. a 57%-126% increased coefficient of variation of key abiotic variables such as mean flow velocity, turbulence, median grain size and organic content of the sediments. In particular, the presence of LW was associated with the appearance of both erosion and sedimentation areas, which resulted in the formation of habitat patches with distinct combinations of sediment composition, organic matter content and flow patterns. E.g., median sediment grain size decreased by up to 53% in the sedimentation patches whereas it increased by up to 243% in scouring patches, compared to adjacent river sections without LW. The changes in abiotic conditions and the persistence of the LW logs as stable substrata also increased the diversity of the trophic resources available for macroinvertebrate assemblages by enhancing both autochthonous primary production and the retention of allochthonous organic matter. This resulted in a 1.9-5.5 fold increased contributions of autochthonous primary production and a 1.3 fold increased contribution of allochthonous organic matter to the diet of local macro¬invertebrate assemblages. The…