PERIRHEIC MIXING AND FLOODPLAIN BIOGEOCHEMISTRY WITHIN THE ATCHAFALAYA SWAMP

Floodplains in the Southeastern Coastal Plain play a crucial role in ecosystem services and biogeochemical processing. Fluvial systems in the region are characterized by low gradient, high discharge channels with large, spatially complex floodplains that experience prolonged inundation. These characteristics create ideal conditions for exchange of materials between the floodplain and river network, often resulting in a hotspot for biogeochemical processing which potentially decreases the export of excess nutrients to receiving water bodies (i.e. the Gulf of Mexico). In this study, we examined biogeochemical processing within two contrasting floodplain wetlands over the course of an annual flood pulse along the Atchafalaya River, a large distributary of the Mississippi River in Southern Louisiana. Both wetlands are highly managed through the use of levees and represent flow through and backwater hydrology, respectively. Synoptic sampling campaigns were conducted in both wetlands during the rising limb, peak, and falling limb of the hydrograph. Using a combination of conservative and nonconservative tracers, we identified three dominant processes that occurred over the course of the flood pulse: flushing (rising limb), advective transport (peak), and organic matter accumulation (falling limb). The two wetlands had similar signals during the peak, but during the rising and falling limbs, water in the backwater wetland experienced much greater residence time. This led to the accumulation of dissolved organic matter and dissolved phosphorus. There were also elevated dissolved organic carbon to nitrate ratios (DOC:NO₃^-) in the backwater area, suggesting nitrogen removal was limited by the amount of NO₃^- transported into the floodplain. These results highlight the importance of hydrologic connectivity, residence time, and perirheic mixing within large, spatially complex floodplains.