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 N2 gas, and each can be coupled to N recycling pathways such as nitrification and dissimilatory nitrate reduction to ammonium (DNRA). Environmental conditions such as seawater intrusion influence sedimentary estuarine N cycling processes. This study investigated the potential effects of seawater intrusion on N cycling processes and their couplings through a comparative modeling approach. We applied environ analysis, a form of ecosystem network analysis, to 2 N cycling mass-balance network models constructed at oligohaline and polyhaline sites in the Cape Fear River Estuary, North Carolina, USA. We found that nitrification coupled to both denitrification and anammox was 2.5 times greater at the oligohaline site, while DNRA coupled to anammox was 2.7 times greater at the polyhaline site. However, the total amount of N2 gas produced relative to the N inputs to each network was 4.7 and 4.6% at the oligohaline and polyhaline sites, respectively, as direct removal was greater at the polyhaline site. An uncertainty analysis using linear inverse modeling indicated that our results are relatively robust to the effects of parameterization uncertainty. These results suggest that changes in water chemistry from seawater intrusion may favor direct over coupled N removal, but may not substantially change the N removal capacity of sedimentary microbial processes.