Lateral Connectivity In Large River Floodplains: From Invertebrate Community Prediction To Local Food Web Transfers

Abstract

The particularly rich aquatic biodiversity of large river floodplains is structured by river dynamics and hydrological connectivity. Subjected to very high pressure from river uses and engineering (e.g. hydropower, dams, dykes), large rivers undergo physical (loss of diversity of floodplain habitats) and functional (loss of lateral hydrological connectivity) changes in their floodplain habitats. The significant degradation of these habitats and the associated biodiversity has led to a desire to limit the impacts of human activities and restore floodplain environments. The aim of the present thesis is to explore the relationship between hydrological connectivity and biological processes (taxonomic diversity and trophic transfers) in large river floodplains. To this end, a multidisciplinary (geomorphology, biology and ecology), multi-compartment (plankton, invertebrate and fish) and multi-scale (channel vs. floodplain) approach was adopted. Two floodplains of the French Upper Rhone comprising restored channels characterized by variable connectivity with the river were selected as study sites. The results confirm the strong influence of hydrological connectivity on the composition of invertebrate assemblages at both the individual channels and the entire floodplain. They also confirmed the aquatic invertebrate diversity varies in response to two components of the hydrological connectivity (upstream overflows with hydraulic constraints and so-called connections) at both spatial scales. Both flow components also proved to be effective to predict invertebrate diversity changes along a progressive alluviation and disconnection sequence after the reconnection with the main river of a previously isolated channel. At the floodplain scale, relationships between changes in the composition of invertebrate assemblages and thermal inertia in stagnant floodplain channels show that the benthic diversity of floodplains is also generated by the diversity of the water sources that feed them (surface water vs. ground water). Isotope analyses in juvenile fish revealed the dietary adaptation of these organisms to the fluctuations and scarcity of trophic resources in two floodplain fish nurseries. These results justified that environmental conditions at the base of the food web have a large influence on the YOY diet composition. Our study also demonstrated that changes in temporal patterns in water level variations (i.e. dam hydropeaking) can alter key ecosystem processes such as planktonic production and energy transfer through food webs. An innovative approach integrating light trapped adult caddisflies and statistical mixed models was proposed to decipher the role of species traits and habitats in mercury accumulation and transfer in a large river floodplain. This study revealed that variation of mercury concentrations between species was best explained by the larval feeding type, whereas the contributions of larval micro-habitat and forewing length (used as surrogate of adult body size) were minor. Moreover, caddisfly adults associated with the floodplain larval macrohabitat were, on average, 3 times less contaminated than those associated with the main river channel. These results demonstrated the strong effect of lateral connectivity and the likely impact of the composition of the dissolved organic matter in floodplains (that reduces the mercury bioavailability for biota) on mercury bioaccumulation and transfers. Overall, the findings of the present work revealed the influence of lateral hydrological connectivity on (i) the distribution of benthic invertebrates in the floodplain, (ii) the functionality of fish nurseries and (iii) the transfer of contaminants through the emergence of aquatic insects. These outcomes have major implications for the improvement of the existing practices of floodplain ecological restauration. Namely, they allow to anticipate taxonomic diversity in relation to natural evolution or abrupt change (e.g. by reconnection) in the level of hydrological connectivity of the floodplain channel. The influence of thermal inertia in floodplain invertebrate diversity offers important prospects for restoring vertical connectivity (raising the groundwater table level, limiting the bed incision, etc.). Our results also show that dams can contribute to the restoration effort. Flow releases from dams can be manipulated in order to mimic elements of natural flow regimes (upstream overflow and slow connections) in floodplains. Conversely, a low flow variability is a prerequisite for favourable plankton production and juvenile feeding in floodplain fish nurseries. Finally, our work highlighted certain links between lateral hydrological connectivity and biogeochemical cycles (organic matter, nutrients, contaminants) in restored floodplain channels.