The Atchafalaya River Basin (ARB) in southcentral Louisiana, USA, is a structurally and biotically diverse floodplain of Atchafalaya River (AR), which is the largest distributary of the Mississippi River. Annual floodplain inundation facilitates the exchange of nutrients and organic material between the AR and its floodplain, giving rise to the high productivity of the river-floodplain system. Production within the ARB is driven by periphytic algae, phytoplankton, and aquatic macrophytes, however, very little is known about periphytic algal assemblages in floodplain systems or how loss of annual flooding impacts these assemblages. In this study, we use artificial substrates to sample periphytic algae bi-weekly (January 2019 – September 2019) from ARB sites with active river connections and from a permanently-isolated floodplain system (Lake Verret). Our results showed that connection to the river caused spatiotemporal shifts in periphytic algal assemblages in the ARB. Overall, ARB sites had a higher density of algal cells compared with non-ARB sites, and for ARB sites with more active river connections, total algal density was greater nearer to river inputs, particularly for cyanobacteria and centric diatoms, with diatoms dominating periphyton assemblages year-round. In contrast, the river-isolated system was dominated largely by chlorophytes. In both isolated and connected systems, sites with heavy macrophyte cover showed increased densities of euglenoids, chrysophytes, and xanthophytes. Shifts in periphytic algal assemblages due to floodplain alterations, such as the disconnection of a floodplain from its river source, could impact higher trophic levels and should be considered in future wetland management decisions.

The Atchafalaya River Basin (ARB) in southcentral Louisiana, USA, is a structurally and biotically diverse floodplain of Atchafalaya River (AR), which is the largest distributary of the Mississippi River. Annual floodplain inundation facilitates the exchange of nutrients and organic material between the AR and its floodplain, giving rise to the high productivity of the river-floodplain system. Production within the ARB is driven by periphytic algae, phytoplankton, and aquatic macrophytes, however, very little is known about periphytic algal assemblages in floodplain systems or how the annual flooding impacts these assemblages. In this study, we use artificial substrate to sample periphytic algae bi-weekly (January 2019 – September 2019) from ARB sites with active river connections and from a permanently-isolated floodplain lake (Lake Verret). Our results showed that annual river flood pulses caused spatiotemporal shifts in periphytic algal assemblages in the ARB. Overall, ARB sites had a higher abundance of algal cells compared with non-ARB sites and community assemblage differed substantially. Specifically, in ARB sites with active river connections, total algal abundance was greater nearer to river inputs, particularly cyanobacteria and centric diatoms, and diatoms dominated the periphyton assemblages year-round. In contrast, the river-isolated lake was dominated largely by chlorophytes. In both isolated and connected systems, sites with heavy macrophyte cover showed increased abundances of euglenoids, chrysophytes, and xanthophytes. Shifts in periphytic algal assemblage due to floodplain alterations, such as the disconnection of a floodplain from its river source, could impact higher trophic levels and should be considered in future wetland management decisions.

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