  Nitrogen (N) removal from estuaries is driven in part by sedimentary
microbial processes. The processes of denitrification and anaerobic ammonium
oxidation (anammox) remove N from estuaries by producing di-nitrogen gas, and
each can be coupled to N recycling pathways such as nitrification and
dissimilatory nitrate reduction to ammonium (DNRA). Environmental conditions in
estuaries influence sedimentary N cycling processes; therefore, seawater
intrusion may affect the coupling of N cycling processes in the freshwater
portions of estuaries. This study investigated the potential effects of
seawater intrusion on these process couplings through a comparative modeling
approach. We applied environ analysis, a form of ecological network analysis,
to two N cycling mass-balance network models constructed at freshwater
(oligohaline) and saltwater (polyhaline) sites in the Cape Fear River Estuary,
North Carolina. We used a space-for-time substitution to predict the effects of
seawater intrusion on the sedimentary N cycle. Further, we conducted an
uncertainty analysis using linear inverse modeling to evaluate the effects of
parameterization uncertainty on model results. Nitrification coupled to both
denitrification and anammox was 2.5 times greater in the oligohaline model,
while DNRA coupled to anammox was 2.7 times greater in the polyhaline model.
However, the total amount of N2 gas produced relative to the nitrogen inputs to
each network was 4.7% and 4.6% at the oligohaline and polyhaline sites,
respectively. These findings suggest that changes in water chemistry from
seawater intrusion may favor direct over coupled nitrogen removal, but may not
substantially change the N removal capacity of the sedimentary microbial
processes.
