Nitrate Flux Into Sediments in the Pontchartrain Estuary During Mississippi River Diversion Events.

Lake Pontchartrain is a large, oligohaline estuary located in coastal Louisiana, U.S.A. that receives episodic diversions of nutrient-rich Mississippi River water via the Bonnet Carré Spillway to alleviate flood threats to the city of New Orleans.  These events may lead to expressions of eutrophication in the estuary and it is therefore critical to understand nitrogen transformation pathways.  Potential for denitrification in the sediments of Lake Pontchartrain was investigated by: (i) simulating high NO₃^- flood events in intact sediment cores under aerobic and anaerobic incubations, (ii) in-situ measurements of the vertical profiles of dissolved inorganic nitrogen species at the sediment-water interface during the 2011 Bonnet Carré Spillway opening, and (iii) acetylene-block batch incubation experiments.  Maximum rates of NO₃^- flux into the sediments for intact cores were 26.4-61.9 mg NO₃-N m^-2 d^-1 and 24.3-53.0 mg NO₃-N m^-2 d^-1 for aerobic and anaerobic treatments, respectively, with no significant differences between treatments.  Mass transfer coefficients ranged between 0.009-0.019 m d^-1.  Flux rates by Fickean diffusion based on in situ porewater measurements averaged 29.7 mg NO₃-N m^-2 d^-1 and were remarkably similar to rates determined based on intact core experiments.  The acetylene-block batch incubations suggest a greater activity of denitrifying microorganisms in sediments near the Spillway inflow that are exposed seasonally to Spillway leakage compared to those in the lake center.  Based on measured rates and a simple model, we estimate that denitrification accounted for ~4.2% (422 mt NO₃-N) of water column NO₃^- loss during the entire 2008 Bonnet Carré Spillway diversion event (10,000 mt NO₃-N).  Denitrification in Lake Pontchartrain appears to play a relatively minor role in the removal of nitrate received during Spillway freshwater diversion events.