Hurricane Effects on Benthic Nitrogen Cycling in an Emerging Coastal Deltaic Floodplain Within the Mississippi River Delta Plain

Hurricanes are one of the most common natural events that disturb estuarine and coastal wetlands along the Gulf of Mexico. The episodic and energetic events of hurricanes modify wetland hydrodynamics, sedimentation, and vegetation structure, which can impact the connectivity of coastal deltaic floodplains in processing riverine nutrients. Hurricane effects on benthic nitrogen dynamics and their fluxes during ecosystem recovery following an event were investigated at three experimental sites with distinct sediment organic matter (SOM) concentrations (lower-, intermediate-, and higher-SOM) in Wax Lake Delta (WLD), Louisiana. Intact sediment cores were incubated with 15NO3- enrichment prior to, 1 month, 2 years, and 3 years post Hurricane Barry. The disturbance of Hurricane Barry on benthic nitrogen dynamics was most significant at the higher-SOM site, where SOM concentrations significantly decreased together with a 50% reduction in rates of direct denitrification, coupled nitrification-denitrification, and dissimilatory nitrate reduction to ammonium (DNRA). Shifts in SOM and benthic nitrogen dynamics as result of hurricane sedimentation followed the linear function between increased denitrification (or NO3- fluxes) with greater SOM concentrations previously established by sampling along SOM gradients in WLD. The estimated NO3- removal capacity decreased by 8.5% (76 Mg N year-1) compared to pre-hurricane conditions due to lower SOM concentrations associated with mineral sedimentation at the most disturbed site. The disturbed site had not recovered to pre-hurricane conditions in terms of SOM concentrations and benthic nitrogen dynamics three years after Hurricane Barry. NO3- removal capacity of an active delta such as WLD is a combination of SOM increase from ecological succession and SOM decrease associated with events that stimulate mineral deposition. Frequency of high energy events such as floods and storms along with wetland feedback in soil organic development and delta expansion will influence NO3- removal capacity.