Groundwater-surface water ecotones at the upstream part of confluences in former river channels

Abstract

Two slow flowing former channels of the Rh6ne River, which are supplied by nutrient-poor groundwater and subject to backflows of nutrient-rich water from the Rhtne, were studied in order to demonstrate the occurrence of an ecotone located upstream to their confluences with the main channel. In one of the channels, the long-term stability of the boundary zone compared to the other sections was investigated by examining floristic changes over the period of a decade, demonstrating its high temporal stability compared to the adjacent systems. This stability is seen to be a result of both external and internal processes: backflows of river water rich in suspended sediment favour alluvial deposition of silt in the ecotone area, which in turn allows the growth of numerous hydrophyte species, many of which have specific environmental requirements. These macrophytes promote sediment retention and deposition (a positive feed-back mechanism), thus acting as a filter for the surface water flow. Finally, flash floods regularly scour fine sediment deposits and rejuvenate these areas on a patch by patch basis. The occurrence of boundary zones is demonstrated in the areas located upstream from the confluence in both channels under study. The seasonal chemical variability of the channels and the RhOne itself, as compared to the backflow area, was analysed using chemical variables that are influenced by water fluxes. The fluctuations of the chemical parameters of the boundaries are seen to be intermediate between those of the adjacent systems in one case and higher in the other. With reference to recent definitions in the scientific literature, the overall characteristics of these boundary areas lead us to conclude that they can be considered as ecotones between two water fluxes.

Response of aquatic plants to abiotic factors: a review

“…Sparganium emersum (Bornette et al 1994a). These contrasting phenologies would probably separate species into different niches, and partly explain the contrasting biodiversity patterns observed in groundwater-supplied ecosystems (Bornette and Large 1995;Jansson et al 2007).…”

Surface water–groundwater exchanges in three dimensions on a backwater of the Rhône River

“…1). It is regularly inundated by the Rhône River during flash floods when river discharge exceeds 1000 m 3 s –1 (the mean discharge is 620 m 3 s –1 ), and total inundation lasts 40 days a year ( Bornette & Large 1995;Bornette & Amoros 1996). Except during these floods, the arm is supplied by oligo‐mesotrophic groundwater ( Reygrobellet et al .…”

Coupling groundwater modeling and biological indicators for identifying river/aquifer exchanges

“…Aquatic vegetation is strongly governed by the groundwater/surface water connectivity in aquatic ecosystems (Bornette and Amoros 1991; Bornette and Large 1995; Bornette et al 1998b). The distribution of aquatic plant species is related to the nutrient levels (Carbiener et al 19901995 ; Onaindia et al 1996; Britto et al 2001) and seasonal thermal variation in the water (Bornette and Puijalon 20092011).…”

“…The distribution of aquatic plant species is related to the nutrient levels (Carbiener et al 19901995 ; Onaindia et al 1996; Britto et al 2001) and seasonal thermal variation in the water (Bornette and Puijalon 20092011). In aquatic habitats, there is a strong correlation between the intensity of groundwater exfiltrations and seasonal temperature variability (Sakura 1993; Bornette and Large 1995). The various water sources that supply aquatic habitats usually strongly contrast in terms of nutrient levels (Bornette et al 1998a; Bornette and Arens 2002).…”

“…Cool, well-oxygenated groundwater supplies limit plant growth and competition processes but also allow evergreen species to grow throughout the year (Bornette and Large 1995; Greulich and Bornette 2003). Consequently, the distribution of aquatic plants is partly ruled by water temperature.…”