Ecology of Periphyton in a Subtropical River Floodplain

Abstract

The Atchafalaya River Basin (ARB) in southcentral Louisiana, USA, is a large and biologically diverse floodplain surrounding the Atchafalaya River (AR), which is the largest distributary of the Mississippi River, receiving 30% of the combined daily discharge of the Mississippi and Red Rivers. Annual flooding facilitates exchange between the AR and its floodplain and is thought to give rise to the high productivity of the river-floodplain system. Primary production within the aquatic ARB is driven by periphytic algae, phytoplankton, and aquatic macrophytes, however, very little is known about periphytic algal assemblages in floodplain systems. In this study, artificial substrates were used to sample periphytic algae from several areas across the ARB between 2017- 2019, and environmental variables measured along a spatiotemporal gradient were used to better understand the factors influencing periphytic algal assemblages. Lake Verret, which did not have active AR connections, was also sampled to provide a natural control for the purpose of investigating the effect of an annual flood pulse on periphyton assemblage composition. Algae were classified into seven groups that included chrysophytes, centric diatoms, pennate diatoms, chlorophytes, euglenoids, xanthophytes, and cyanobacteria. Periphytic algal assemblages were dominated by chlorophytes in 2017, but then shifted to mainly diatoms in 2018 and 2019. Canonical correspondence analyses (CCAs) indicated this may have been due to differences in macrophyte abundance among years. Overall, algal densities were higher early in the year and lower during the summer, likely related to greater nutrient levels associated with inundation of the floodplain by the AR. CCAs also indicated spatial effects on ARB periphyton with diatoms dominating assemblages near river water inputs and other groups such as chlorophytes increasing in relative abundance further into the floodplain. This is likely related with associated spatial gradients in nitrogen sources, particularly ammonium. During 2019, the ARB experienced intense flooding, which lasted an unprecedented 329 days. Total algal cell abundance increased substantially during the high magnitude flood but diminished as floodwaters receded during the summer. Although variable across years, the AR flood pulse strongly influenced the composition and dynamics of the ARB periphyton assemblage, with spatial and temporal patterns reflecting interactions of nutrient availability, temperature, distance from source water, macrophyte abundance and composition, and shading.