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

Abstract

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...

Should I stay or should I go: Hydrologic characteristics and body size influence fish emigration from the floodplain following an atypical summer flood

“…Over evolutionary time scales, many aquatic organisms have adapted advantageous life‐history strategies linked to environmental cues of predictable floods (e.g., changes in discharge, water level, temperature; Lytle & Poff, 2004; Winemiller, 2005; Winemiller & Rose, 1992). However, not all floods are predictable, and frequency of unpredictable floods is expected to increase with climate change (Poff, 2002), requiring additional research to sufficiently characterise ecological responses to atypical floods (Larsen et al, 2019).…”

Riverine bacterioplankton and phytoplankton assembly along an environmental gradient induced by urbanization

“…For instance, Wang et al (2020) found that stochastic processes were less pronounced for archaeal than for bacterial communities in a subtropical river‐bay system, and Gad et al (2020) found that deterministic processes were primarily governing the microeukaryotic habitat specialists while stochastic processes were more important for habitat generalists in an anthropogenically impacted river. Changes in hydrological regimes could also influence the relative importance of deterministic and stochastic processes, as stable hydrological periods tend to allow aquatic communities to assemble deterministically (Isabwe et al 2018; 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).…”