Benthic fluxes of dissolved oxygen and nutrients across hydrogeomorphic zones in a coastal deltaic floodplain within the Mississippi River delta plain

Abstract

We tested the hypothesis that benthic fluxes will increase spatially in a coastal deltaic floodplain as sediment organic matter increases in response to developing hydrogeomorphic zones along a chronosequence of the active Mississippi River Delta. A continuous flow-through core system was used to incubate intact sediment cores from three hydrogeomorphic zones along a chronosequence in the emerging Wax Lake Delta (WLD). Organic matter content increased from younger to older deltaic sediments from subtidal to supratidal hydrogeomorphic zones, which were coupled with increasing benthic oxygen and nitrogen fluxes. Mean net denitrification rate in spring was 100 µmol N₂–N m⁻² h⁻¹ with significantly lower rates occurring in the younger intertidal zones (T4 transect, − 22 µmol N₂–N m⁻² h⁻¹) and higher rates occurring in the older supratidal zones (T2 and T1 transects, 330 and 262 µmol N₂–N m⁻² h⁻¹, respectively). Mean net denitrification rate in summer was 397 µmol N₂–N m⁻² h⁻¹ without significant site-to-site variability except for the supratidal-T2 site (911 µmol N₂–N m⁻² h⁻¹) showing higher denitrification rate than the other sites. Based on seasonal temperature and inundation time, annual rates of benthic NO₃⁻ removal varied from − 0.5 to − 3.4 mol m⁻² y⁻¹ and N₂–N production rates varied from 1.0 to 3.2 mol N m⁻² y⁻¹ across WLD. The subtidal zone had the lowest fluxes associated with lower organic matter content, but was the hydrogeomorphic zone with the largest area and longest flood duration, and therefore contributed over half of N removal in WLD. The estimated annual NO₃⁻ removal of 896 Mg N y⁻¹ in WLD accounts for 10 to 27% of total NO₃⁻ load to WLD, most of which is converted to N₂ through denitrification. As a small prograding coastal deltaic floodplain under early stages of delta development, 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. These results highlight the contribution of the coastal deltaic floodplain in an active coastal basin in processing elevated riverine NO₃⁻ at continental margins with coastal ocean. The potential loss of this ecosystem service in N removal may increase in global significance as delta areas decline as result of accelerated relative sea level rise and decreased sediment loading in major river basins around the world.