A floodplain spring: An ecotone between surface water and groundwater

Ecotones may be viewed as boundaries separating adjacent resource patches. They are dynamic components of the aquatic landscape and in the alluvial floodplain they occur as both diffuse and concentrated zones between surface and groundwater environments. This study is focused on a floodplain spring whose water supplies a backwater of the upper Rh6ne River alluvial floodplain (Iles Nouvelles). This paper analyses the community structure on both sides of the physical boundary separating the two ecosystems in relation to hydrological characteristics. The contact between two different environments (surface water and water-saturated sediment) creates a physical discontinuity which is of great importance for the distribution of aquatic fauna. Two types of organization are described: (1) within the sediment body there are mixed communities (epigean and hypogean) which can move with hydrological variations (biological ecotone) and (2) within the surface-water body there is a typical epigean fauna which displays a ‘border effect’. The ecological implications of the various patterns of biological responses are discussed as well as the regulatory aspect of the ecotone which minimizes the washout of hypogean fauna towards the surface ecosystem.