River Connectivity Affects Submerged and Floating Aquatic Vegetation in Floodplain Wetlands

The submerged and floating plant communities in floodplain wetlands of the Upper Columbia River have never been described. To explore mechanisms behind the influence of the annual flood pulse on vegetation, we investigated how species group into flood response guilds whose distributions vary along a connectivity gradient between 44 floodplain wetlands and the river, how connectivity influences water and sediment, and to what degree the effect of connectivity on vegetation is mediated by its effects on these environmental variables. We characterised assemblages with cluster and indicator species analysis, as well as non-metric scaling ordination and tested a structural equation model, which defined the relationship between assemblage composition, sediment and water quality, and connectivity to the river. We found four assemblages, each associated with different water and sediment conditions, and positioned at differing degrees of connectivity. The model provided a good fit to the data. We conclude that in highly connected floodplain wetlands the direct effect of flooding supersedes the influence of water and sediment quality in structuring vegetation assemblages. Yet where flooding is less intense, these environmental variables resume their structuring role.