Surface water temperature impacts on coastal wetland denitrification: Implications for river reconnection

Louisiana, located in the southeast United States, is home to 40% of the continental US's coastal wetlands yet accounts for 80% of the nation's coastal wetland loss. This loss is generally attributed to decreased sediment supply, hydrologic alteration from levees, channelization, subsidence, sea-level rise, and wave and tidal induced marsh edge erosion. The Mid-Barataria Sediment Diversion is a US $1.3 billion coastal restoration project that will divert up to 2100 m³ s^-1 of sediment-laden Mississippi River water directly into Barataria Basin. The influx of colder, nutrient-rich, springtime river water could negatively impact water quality of the receiving basin. We quantified the effects of colder, surface water temperature on the nitrate (NO₃^-) reduction rate in vegetated marsh and open water bay sediments. Colder water limited NO₃^- removal processes averaging 17.1 mg N m^-2 d^-1 in the range of 5-14 °C, before increasing almost 3-fold in the 20 °C treatments at 50.6 mg N m^-2 d^-1. Low N removal rates, especially near the project inflow where temperatures will be coldest will favor transport of NO₃^- further into Barataria Basin where eutrophic conditions could become expressed. These results will inform coastal managers around the world of the potential ecosystem response to coastal restoration aimed at river reconnection where colder waters enter warmer, shallow basins.