Nitrate flux into the sediments of a shallow oligohaline estuary during large flood pulses of Mississippi River water.

Lake Pontchartrain is a large, oligohaline estuary located in coastal Louisiana that receives episodic diversions of nitrogen-rich Mississippi River water via the Bonnet Carré Spillway to alleviate flood threats to the city of New Orleans. These events may be linked to expressions of eutrophication, and it is therefore important to investigate pathways of nitrate (NO) loss. Nitrate flux into the sediments of Lake Pontchartrain was investigated using two independent methods: (i) simulating high NO flood events under aerobic and anaerobic incubations in intact sediment cores collected during 2010 and (ii) in situ field measurements of the vertical profiles of dissolved inorganic nitrogen species at the sediment-water interface during the 2011 Bonnet Carré Spillway opening. Mean rates of NO flux into sediments based on mass transfer in intact cores collected in 2010 and in situ porewater measurements in 2011 were -17.4 and -1.4 mg NO-N m d, respectively, for water column NO concentrations observed in situ in 2011. During the laboratory incubations, there was no significant difference in NO flux between oxygen treatments. We estimate that NO flux into sediments accounted for up to 3.1% (309 Mg NO-N) of water column NO loss during the 2008 Bonnet Carré Spillway event. Sediment characteristics, field measurements, and results from the laboratory experiment suggest that denitrification is the primary pathway for NO reduction. Even though there is significant NO reduction occurring in Lake Pontchartrain sediments during Mississippi River diversion events, this pathway of NO loss from the water column plays a relatively minor role in the transformation of the very large amount of NO received during these times.