Redox Response to Hydrologic Variability in an Aggrading Coastal Delta
Authors
Elizabeth Herndon1* (herndonem@ornl.gov), Matthew Berens1, Geoff Schwaner1, Andre Rovai2,3, Robert Twilley3
Institutions
1Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN; 2U.S. Army Engineer Research and Development Center, Vicksburg, MS; 3Louisiana State University, Baton Rouge, LA
URLs
Abstract
Coastal ecosystems serve as the interface between land and oceans, processing material that is delivered by rivers and buffering interior lands against sea level rise and hurricanes. The biogeochemical function of these ecosystems is not well represented in Earth system models, limiting prediction of how ecosystem processes vary over time and respond to chronic and episodic disturbance. The project’s objective is to understand how soil biogeochemistry varies across redox gradients shaped by landscape features and variable hydrology. Here, researchers evaluate how redox processes respond to changing water levels on the freshwater Wax Lake Delta, one of the few aggrading areas of coastal Louisiana. The team examines these processes across two elevation transects spanning supra- to subtidal zones that are located on older and younger portions of a deltaic island. Sensor networks continuously record water level, soil redox potential, soil moisture, pH, and conductivity. Soil and pore water are collected seasonally to evaluate carbon and (micro)nutrient storage and mobilization. Researchers determine that soil redox (Eh) responds to water tables that fluctuate with the seasons and tides. On the old transect, surface soils (<20 cm) in supra- and intertidal zones transitioned from reducing (<0 millivolts at pH 7) to oxidizing conditions during summer as water tables dropped >0.5 m below the soil surface. On the young transect, Eh in supra- and intertidal surface soils fluctuated with tides. Subtidal soils and deeper soils were persistently reducing. Soil water contained high dissolved iron (Fe2+), manganese (Mn), and ammonium but low sulfate and nitrate, indicating microbial reduction of Fe and Mn oxides, sulfate, and nitrate.
Dissolved phosphate also increased with depth and was correlated with dissolved Fe, indicating phosphate release from dissolving Fe oxides. This work provides insight into the spatial and temporal variability of biogeochemical processes in a freshwater wetland.