Aquatic vegetation and hydrology of a braided river floodplain

The response of aquatic vegetation to the complex hydrology of a river floodplain was investigated on a stretch of former braided channels of the Rhône River near Lyon, France, which is fed by three different water inputs: underground water, surface flow from a small tribulary and river overflow during spates. Water temperature, pH, conductivity, alkalinity, sulfate, nitrate, and phosphate content were measured monthly at seven sampling stations. Principal Component Analysis (PCA) of these data revealed two gradients: a gradient of spatial heterogeneity and a gradient of temporal fluctuations. A PCA of samples based on substrate grain size, flow velocity, channel width and depth, height and shade of bank vegetation suggested the division of the braided system into four zones. Along these former channels, the vegetation varies from communities with Potamogeton coloratus of nutrient-poor water, to meso-eutraphent communities with Callitriche platycarpa, Elodea canadensis, and Ranunculus trichophyllus. Correspondence Analysis (CA) of the floristic data revealed four zones. Regarding the habitat parameters investigated, one zone is assumed to be linked to the instability and the nutrient inputs of a former channel (running water from the tributary and flood disturbances), while another is linked to the stability and nutrient-poor conditions of the other former channel (supplied by underground water). The other two zones are considered as ecotones, each of them occurring in a former channel, upstream of the confluences. The location as well as the floristic and functional differences between these two ecotones are discussed.