Benthic Nitrogen Dynamics in Response to Deltaic Succession, Anthropogenic Fertilization and Hurricane Events in an Emerging Coastal Deltaic Floodplain within the Mississippi River Delta Plain

Abstract

Increasing concentrations of nitrogenous fertilizer (NO3-) in rivers have potentially altered benthic nitrogen (N) cycling in coastal deltaic floodplains at the continental margins of major rivers. Here I evaluated the N removal capacity of a newly emergent coastal deltaic floodplain [Wax Lake Delta (WLD)] within the Mississippi River Delta Plain using continuous flow-through incubations. I investigated the response of benthic N dynamics in WLD to deltaic succession, anthropogenic fertilization and hurricane events. The estimated annual NO3- removal of 896 Mg N yr-1 in WLD accounts for 10 to 27% of total NO3- load to WLD, most of which is converted to N2 through denitrification. WLD is a continuously emerging ecosystem where the capacity of N removal increases by 0.2 to 2% per year prior to riverine NO3- is export to coastal ocean. Organic matter content increased from younger to older deltaic sediments from subtidal to supratidal hydrogeomorphic zones, which were coupled with increasing benthic NO3- uptake and N2 production. Direct denitrification, as the dominant N pathway in WLD, was one to two magnitudes larger than coupled nitrification-denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox). Increased NO3- loading due to anthropogenic fertilization enhanced denitrification rates but inhibited heterotrophic N2 fixation rates. N2 fixation was comparable to denitrification under historically lower NO3- concentrations (10 µM) and less than 32% of N removed via denitrification was from riverine NO3- loading. In contrast, currently higher overlying NO3- concentration (100 µM) makes denitrification the dominated benthic N pathway compared to N2 fixation and over 61% of the removed N via denitrification was from riverine NO3- loading. Hurricane Barry generated a decrease of denitrification, enhanced proportion of DNRA to total NO3- loss and increased NH4+ effluxes in most experimental sites in WLD. These hurricane effects lead to a decreased capacity of coastal deltaic floodplain to process eutrophic riverine water. This research highlights the increasing capacity of an emerging coastal deltaic floodplain in processing elevated riverine NO3- and clarifies the contribution of denitrification, N2 fixation, DNRA and anammox to benthic N dynamics at different stages of deltaic development and anthropogenic NO3- loading.