Estimating the effects of seawater intrusion on an estuarine nitrogen cycle by comparative network analysis

Abstract

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.