Perirheic mixing and biogeochemical processing in flow‐through and backwater floodplain wetlands

Inundation hydrology and associated processes control biogeochemical processing in floodplains. To better understand how hydrologic connectivity, residence time, and intrafloodplain mixing vary in floodplain wetlands, we examined how water quality of two contrasting areas in the floodplain of the Atchafalaya River—a flow-through and a backwater wetland—responded to an annual flood pulse. Large, synoptic sampling campaigns occurred in both wetlands during the rising limb, peak, and falling limb of the hydrograph. Using a combination of conservative and reactive tracers, we inferred three dominant processes that occurred over the course of the flood pulse: flushing (rising limb), advective transport (peak), and organic matter accumulation (falling limb). Biogeochemistry of the two wetlands was similar during the peak while the river overflowed into both. However, during the rising and falling limbs, flow 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 ratios of dissolved organic carbon to nitrate in the backwater wetland, suggesting nitrogen removal was limited by nitrate transported into the floodplain there. Collectively, our results suggest inclusion of a temporal component into the perirheic concept more fully describes inundation hydrology and biogeochemistry in large river floodplain.

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