The denitrification potential of eroding wetlands in Barataria Bay, LA, USA: Implications for river reconnection

Abstract

Expressions of eutrophication have led to increased stress on coastal ecosystems around the world. The nitrogen (N) removal potential of coastal wetland ecosystems is important due to increased loading of N to the coast. In Louisiana, there is rapid coastal wetland loss due primarily to the presence of river levees, which have isolated the coastal basins, and a high relative sea level rise. Ecosystem managers are planning to construct the Mid-Barataria sediment diversion which will reconnect the Mississippi River with Barataria Basin to build new wetlands and nourish existing marsh. The sediment diversion will deliver large amounts of nitrate into the surface waters of Barataria Bay. This research sought to quantify the nitrate removal potential of three bay zones; vegetated marsh, submerged peat fringe, and bay-bottom muddy estuarine sediment in intact soil cores incubated with a 2 mg L−1 N-NO3 water column. We noted: i) The areal nitrate reduction rates for the marsh, fringe, and estuary zones were 29.29 ± 3.28, 18.83 ± 1.31, and 10.83 ± 0.62 mg N m−2 day−1, respectively; ii) the majority (~93%) of NO3 was converted to N2O, indicating denitrification was the major NO3 reduction pathway; iii) the submerged, eroded marsh soils (peat fringe zone) will play a large role in nitrate reduction due to increased contact time with the surface water. These findings can inform the predictive numerical models produced and utilized by ecosystem managers to better quantitatively understand how the coastal basin will respond to nutrient loading from river reconnection. In a broader context, the current relative sea level rise in coastal Louisiana is within the range of eustatic sea level rise that most stable coastlines will experience within the next 65–85 years. Therefore, these findings can serve as an example of potential future impacts to coastal wetland systems, globally, within the next century.