Effects of a decennial spate flood on substrate and aquatic vegetation of riverine wetlands

Spate floods control sediment, nutrient and biodiversity patterns in river floodplains, but the diachronic effects of spate floods on these compartments are rarely assessed. The effects of a 10-year flood spate on substrate characteristics and aquatic vegetation (community composition and life-history traits) were assessed before and after the spate in eleven former river channels that had become wetlands and varied in flood frequency. The effects of spate flooding on sediment and vegetation were expected to be greater in the less frequently flooded channels. For life-history traits, we expected a greater effect of the flood on tall evergreen species with high leaf area, while species with ruderal traits were expected to be less affected. For sediments, the flood reduced fine sediment cover and organic carbon content, but did not affect nutrient content. Vegetation cover decreased particularly in the most and least frequently flooded channels. Species richness and functional indices were little affected. The disturbance had a significant effect on plant traits, reducing tall evergreen species. This effect on plant characteristics increased as the frequency of overflow increased. The results highlight the key role of erosive floods in maintaining poorly competitive aquatic plants and limiting successional processes in riverine wetlands. Given the importance of plants in maintaining spate floodplain biodiversity, these erosive spate floods are essential to maintain or restore in a context of extreme river regulation.