Impact of river reconnection for coastal restoration on nitrate reduction in brackish marsh soils and bay-bottom sediments in coastal Louisiana, USA

Abstract

Wetlands provide important ecosystem services, including improving surface water quality through nutrient removal. Louisiana has experienced ~ 4800 km² of coastal wetland loss between 1932 and 2016 due to high relative sea level rise and reduced sediment from the Mississippi River due to levees. The 2023 LA Coastal Master Plan aims to restore Louisiana’s degraded coastline through restoration projects, including sediment diversions or river reconnection. The Mid-Barataria Sediment Diversion Project will reconnect the river sediment-laden water with the coastal wetlands of Barataria Basin to nourish degrading marshes. However, the diversion will also deliver substantial nitrate (NO₃⁻) to the basin, potentially negatively impacting water quality. We quantified NO₃⁻ reduction rates at these high (2 mg/L) and low (0.5 mg/L) water column concentrations for marsh and submerged estuarine sediments using intact cores and a laboratory incubation. An additional treatment where 2 cm of mineral river sediment was placed over the organic marsh soil as a future, post-diversion scenario to simulate sediment deposition on the marsh once the river is reconnected. We hypothesized that NO₃⁻ reduction rates would decrease once mineral sediment is deposited on the organic marsh soil. For an aerobic water column, nitrate reduction rates for the vegetated marsh, post-diversion marsh, submerged eroded marsh, and estuarine sediment zones were 71.1 ± 2.7, 27.8 ± 4.5, 19.7 ± 1.2, and 13.0 ± 0.75 mg N m⁻² d⁻¹, respectively. Thus, the post-diversion marsh NO₃⁻ reduction rate decreased by ~ 60% compared to the current vegetated marsh. However, we predict the newly deposited sediment will increase NO₃⁻ removal by 1.17 × in the eroded marsh and estuarine sediment zones, which are always flooded and will receive river sediment. The marsh is only flooded 31–48% of the time, lessening the impact of the reduction. These findings can improve predictive water quality models used to assess nutrient loading and fate more accurately across the basin under the river reconnection scenario and inform other deltaic regions as freshwater flows are restored to coastal systems globally.