Extreme water-level fluctuations determine aquatic vegetation in modified large-river floodplains.

Abstract

Water bodies in large-river floodplains are typically dominated by aquatic vegetation. The proportion of macrophyte-dominated lakes varies between years, depending on annually different water-level fluctuations within the lakes. We reanalysed aquatic-vegetation inventories made over five decades in floodplain lakes along the Lower Rhine, and related them to summer inundation and drawdown events. We observed a lower probability of submerged macrophyte dominance (lake cover > 20 %) after inundation in summer, with contrasting responses for different species. Lake-bottom exposure during prolonged low water stages had an opposite effect, resulting in an increased probability of dominance by submerged vegetation and a decrease of nymphaeid vegetation. Our results indicate that under the current, hydromorphologically modified, conditions a small proportion of water bodies in the floodplains has abundant aquatic vegetation, unless new sites are repeatedly created which provide opportunities for colonization. The results are important for management as they help in defining the ecological status of floodplain lakes.