Denitrification Rates in the Sediments of a Shallow Estuarine Lake That Receives Diverted Mississippi River Water.

The Bonnet Carré Spillway is used as a major release valve when the lower Mississippi River flood stage threatens New Orleans and downstream communities, diverting up to 20.8% of the river into the shallow, estuarine Lake Pontchartrain.  During a one-month diversion event in 2008, ~8 km³ of water containing 10,000 mt of NO₃-N was discharged into Lake Pontchartrain.  The diversion created a freshwater plume that covered ~40% of the lake and much of the N load was transported to the coastal ocean.  Following the diversion event, the remaining nitrate from the plume area did not mix with the non-plume area, but instead “collapsed” via phytoplankton uptake and denitrification.  Here, we investigated the potential for denitrification in Lake Pontchartrain sediments along a ~30 km transect extending from the spillway inflow to the lake’s center by measuring sediment characteristics and simulating high nitrate flood events in intact sediment cores.  Sediment moisture, organic matter, and microbial biomass were positively correlated with distance from the spillway.  Simulated flood events using intact sediment cores resulted in significantly greater (p < 0.05) total nitrate loss rates for lake center sediments (16.5 mg NO₃-N m^-2 d^-1) associated with greater carbon availability and a greater likelihood of anaerobic conditions in the sediment surface than for sandier spillway inflow sediments (12.4 mg NO₃-N m^-2 d^-1).  Maximum nitrate reduction rates (13.6-38.7 mg NO₃-N m^-2 d^-1), however, were not significantly different across sites, indicating that sediments spanning the transect had similar denitrification potential.  Based on the rates observed here, sediment denitrification within a plume area covering ~40% of Lake Pontchartrain has the potential to remove up to 23.71 mt NO₃-N d^-1, resulting in significant removal of the remaining nitrate following diversion events.