Δ15N patterns in three subtropical estuaries show switch from nitrogen "reactors" to "pipes" with increasing degradation. (19th November 2018)

• Record Type: Journal Article
• Authors: Wells, Naomi S.; Eyre, Bradley D.
• Is Part Of: Limnology and oceanography. Volume 64:Number 3(2019)
• Page Start: 860
• Page End: 876
• Publication Date: 2018-11-19
• Subjects: Limnology -- Periodicals; Oceanography -- Periodicals; Océanographie; Limnologie; Limnology; Oceanography; Computer network resources; Périodique électronique (Descripteur de forme); Ressource Internet (Descripteur de forme); Periodicals; 551.4805
• DOI: 10.1002/lno.11080
• Languages: English
• ISSNs: 0024-3590

Abstract

Abstract: Ongoing alterations to estuaries by inland agricultural intensification and coastal development could affect their capacity to regulate the flux of excess terrestrial nitrogen (N) to the coastal ocean. Here, a new multiform δ 15 N metric was developed to measure how "pristine, " moderately impacted, and highly degraded estuaries recycle (assimilation, mineralization) and remove (denitrification, anaerobic ammonium oxidation) N. Organic (dissolved and particulate, δ 15 N and δ 13 C) and inorganic (nitrate and ammonium, δ 15 N and δ 18 O) N forms were measured over the salinity gradient in the wet and dry season in subtropical estuaries receiving increasing terrestrial N loads (pristine: 16 kg N d −1, moderate: 150 kg N d −1, degraded: 630 kg N d −1 ). The difference in the inorganic vs. organic pool δ 15 N composition increased between the pristine (0 ± 2‰), moderate (10 ± 6‰), and degraded (20 ± 8‰) systems, indicating that N recycling decreased as degradation increased. The N2 O concentrations, NO3 − dual isotope values, and offsets between "measured" and "mixing expected" δ 15 N values further revealed that microbial processes removed up to 30% of the N load entering the moderately degraded estuary, but only 9% in the highly degraded estuary. Hydrologic differences (depth and flushing times [FTs]) could not fully explain these shifts in N fate between the estuaries and seasons, which instead aligned with nonlinear increases in phytoplankton biomass and light penetration with increasing N loads. These isotopic indicators provide direct evidence that estuaries switch from "reactors" that assimilate and remove terrestrial N to "pipes" that transport N directly to sea as degradation increases.