Biogeochemical Processes Regulating Nutrient Solubility in a Freshwater Coastal Delta

Authors

Elizabeth Herndon¹* (herndonem@ornl.gov), Matthew Berens¹, Chunlei Wang¹, Geoff Schwaner¹, Andre Rovai², Robert Twilley²

Institutions

¹Oak Ridge National Laboratory, Oak Ridge, TN; ²Louisiana State University–Baton Rouge, LA

Abstract

Coastal wetlands are undergoing rapid change in response to coastal degradation and sea level rise. On the Louisiana Gulf Coast, river management aimed at mitigating flood risk in urban areas has contributed to extensive land subsidence and the submergence of coastal zones. To maintain and restore these coasts, numerous projects are underway to divert river water and associated sediment loads into areas undergoing subsidence and salinization. The objective of this research is to evaluate how phosphorus (P) dynamics in coastal ecosystems are influenced by water management strategies that ultimately shape hydro-biogeochemical gradients. The initial phase of this project focuses on Wax Lake Delta (WLD), a nutrient-rich, freshwater delta that has formed recently as a result of water diversion away from the Mississippi River and into the Atchafalaya River. WLD is one of the few actively growing areas of coastal Louisiana and is an example of ecosystems that could develop because of ongoing and future restoration projects.

The team’s objective is to evaluate how P solubility shifts over time along redox gradients that develop in response to seasonal and tidal fluctuations in water level on the delta. Researchers have established two transects spanning supratidal to subtidal elevations on opposite ends of an emerging deltaic island that are comprised of either established (old) or newly formed (young) soils. Environmental sensors are being deployed to monitor redox potential, pH, and salinity at a high temporal resolution along each transect. Additionally, researchers are measuring a suite of biogeochemical parameters in surface and porewater.

Initial results from late autumn 2022 show that soils along the old transect were reducing at all depth levels and exhibited strong vertical gradients of nutrient solubility. Soluble reactive phosphorus (SRP) and ammonium were relatively low in surface water, <0.06 mg L^-1 and <0.4 µg L^-1, respectively, and increased up to 0.53 mg L^-1 and 6.3 mg L^-1, respectively, by 30 cm in depth beneath the soil surface. In contrast, nitrate and sulfate concentrations were highest in surface waters yet strongly attenuated within the top 10 to 20 cm of soils. Researchers infer that reducing conditions within the soils result in nitrate and sulfate removal through anaerobic microbial metabolism, producing high concentrations of ammonium and sulfide. Moreover, increased phosphate solubility may be driven by reductive dissolution of metal oxides that otherwise bind P under oxidizing conditions. Ongoing evaluation of biogeochemical processes in WLD will improve process-based understanding of nutrient transformation in coastal ecosystems.