Braided gravel-bed rivers are widespread in temperate piedmont and mountain-valley areas. In their pristine state, braided rivers are characterized by a shifting mosaic of channels, ponds, bars, and islands, since both flow and flood pulses create a diversity of habitats with fast turnover rates. Large wood has a major role in determining the geomorphology and ecological functioning of these rivers. Braided river habitats are colonized by a diverse fauna and flora adapted to their dynamic nature, including a significant proportion of highly endangered species. Animals exhibit high mobility, short and asynchronic life cycles, and ethological and phenological plasticity. Braided gravelbed rivers also offer various categories of refugia such as shore areas, hypogeic and hyporheic habitats that are pivotal for maintaining diversity in the face of frequent disturbances. Today, however, most gravel-bed rivers bear little resemblance to their highly dynamic natural state due to anthropogenic modifications, and most braided rivers have been converted into incised single-thread channels. Gravel bars and vegetated islands are among the most endangered landscape elements worldwide. They are very sensitive to channelization, gravel extraction, and flow regulation. Therefore, more than for most other ecosystems, restoring braided rivers and their landscape elements means restoring their underlying hydrogeomorphological dynamics.

Floodplain pond distribution, density, and diversity were investigated along the active river corridor of the Tagliamento River in northeastern Italy, the last major semi-natural river in Central Europe. Along the corridor, ponds peaked in bar-and island-braided floodplains but were absent in constrained sections. Within the main study area, a 1.0-km 2 large braided floodplain, the number of ponds containing water ranged between 18 and 39 depending on the water level in the main channel. Thermal properties and water-level fluctuations were the most important variables determining pond characteristics and heterogeneity. Results from a PCA on environmental variables distinguished four groups of ponds distributed along a hydrologic and thermal gradient. Parafluvial ponds are short-lived, discrete, aquatic ''islands'' within the floodplain matrix. They are expected to contribute disproportionately to aquatic biodiversity; however, they are very sensitive landscape elements that disappear as a consequence of river regulation, wood removal, and flow control.

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Braided river floodplains are highly dynamic ecosystems, where aquatic communities are strongly regulated by the hydrologic regime. So far, however, understanding of how flow variation influences assembly mechanisms remains limited. We collected benthic chironomids and oligochaetes over a year across a lateral connectivity gradient in the semi-natural Tagliamento River (Italy). Four bankfull flood events occurred during the study, allowing the assessment of how flooding and hydrologic connectivity mediate the balance between stochastic and deterministic community assembly. While invertebrate density and richness were positively correlated with connectivity, diversity patterns showed no significant correlation. Species turnover through time increased with decreasing connectivity. Contrary to expectations, hydrologic connectivity did not influence the response of community metrics (e.g. diversity, density) to floods. Invertebrate composition was weakly related to connectivity, but changed predictably in response to floods. Multivariate ordinations showed that faunal composition diverged across the waterbodies during stable periods, reflecting differential species sorting across the lateral gradient, but converged again after floods. Stable hydrological periods allowed communities to assemble deterministically with prevalence of non-random beta-diversity and co-occurrence patterns and larger proportion of compositional variation explained by local abiotic features. These signals of deterministic processes declined after flooding events. This occurred despite no apparent evidence of flood-induced homogenisation of habitat conditions. This study is among the first to examine the annual dynamic of aquatic assemblages across a hydrologic connectivity gradient in a natural floodplain. Results highlight how biodiversity can exhibit complex relations with hydrologic connectivity. However, appraisal of the assembly mechanisms through time indicated that flooding shifted the balance from deterministic species sorting across floodplain habitats, towards stochastic processes related to organisms redistribution and the likely resetting of assembly to earlier stages.

29Braided river floodplains are highly dynamic ecosystems, where aquatic communities are 30 strongly regulated by the hydrologic regime. So far, however, understanding of how flow 31 variation influences assembly mechanisms remains limited. 32 We collected benthic chironomids and oligochaetes over a year across a lateral connectivity 33 gradient in the semi-natural Tagliamento River (Italy). Four bankfull flood events occurred 34 during the study, allowing the assessment of how flooding and hydrologic connectivity 35 mediate the balance between stochastic and deterministic community assembly. 36 While invertebrate density and richness were positively correlated with connectivity, diversity 37 patterns showed no significant correlation. Species turnover through time increased with 38 decreasing connectivity. Contrary to expectations, hydrologic connectivity did not influence 39 the response of community metrics (e.g. diversity, density) to floods. 40 Invertebrate composition was weakly related to connectivity, but changed predictably in 41 response to floods. Multivariate ordinations showed that faunal composition diverged across 42 the waterbodies during stable periods, reflecting differential species sorting across the lateral 43 gradient, but converged again after floods. Stable hydrological periods allowed communities 44 to assemble deterministically with prevalence of non-random beta-diversity and co-45 occurrence patterns and larger proportion of compositional variation explained by local 46 abiotic features. These signals of deterministic processes clearly declined after flooding 47 events. This occurred despite no apparent evidence of flood-induced homogenisation of 48 habitat conditions. 49This study is among the first to examine the annual dynamic of aquatic assemblages across a 50 hydrologic connectivity gradient in a natural floodplain. Results highlight how biodiversity 51 can exhibit complex relations with hydrologic connectivity. However, appraisal of the 52 assembly mechanisms through time indicated that flooding shifted the balance from 53 deterministic species sorting across floodplain habitats, towards stochastic processes related to 54 organisms redistribution and the likely resetting of assembly to earlier stages. 55 56 Introduction 61Braided rivers floodplains are among the most dynamic ecosystems, constituting a shifting 62 mosaic of habitat patches with high spatio-temporal turnover rates [1,2]. The complex 63 interaction between floodplain topography and variation in river flow and sediment transport 64 maintains a distinct gradient of lateral hydrologic connectivity, which facilitates the co-65 existence of numerous aquatic, amphibian, and terrestrial species [2]. This connectivity is 66 defined as the permanent or temporary links between the main stem of the river and the 67 diverse waterbodies across the alluvial floodplain [3]. 68 Since the formulation of the flood-pulse concept and its extension [4,5], there have been 69 substantial efforts to understand how river-floodpla...