Flooding and hydrologic connectivity modulate community assembly in a dynamic river-floodplain ecosystem

Abstract

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.

Spatiotemporal diet shifting in a neotropical fish species: The role of riparian vegetation and seasonal inundation

“…Fish species are able to shift their diet according to changes in resource availability (Goulding 1980;Hahn et al, 1997;Agostinho et al, 2007). In river floodplains, resource abundances are regulated by spatial and temporal variations in environmental conditions due to variations in forest cover and seasonal hydrological changes in the lateral extent of the river channel (Junk and Wantzen, 2004;Arantes et al, 2019;Larsen et al, 2019).…”

Spatially variable hydrological and biological processes shape diverse post‐flood aquatic communities

“…However, most of the floodplains are isolated from river mainstream at low flow when most studies are conducted, and the concept of the connectivity is not well defined. In fact, aquatic communities in floodplain waterbodies are not only influenced by the environmental variation that is observed at low flow; they are also strongly influenced by the hydrological and biological processes during flood and flood recession periods (Larsen et al, 2019;Thomaz et al, 2007). For example, flow pulses may differently modify the physical and chemical environment of each waterbody, thereby altering the suitability of habitats for aquatic organisms over time and space (Tockner et al, 2000;Ward et al, 1999).…”

Metacommunities from bacteria to birds: stronger environmental selection in mediterranean than in tropical ponds

“…In this sense, some studies found that temperate communities were more environmentally controlled than tropical ones, which were more spatially structured (Myers et al 2013; Souffreau et al 2015). In addition, massive floods during the rainy season increase the aquatic connectivity in the tropical regions (Junk et al 1989; Bunn et al 2006; Larsen et al 2019), reducing dispersal limitation and homogenizing local communities (Thomaz et al 2007; Brasil et al 2020), and contrasting with the more intense isolation of the Mediterranean region.…”