δ15N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

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 δ¹⁵N 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, δ¹⁵N and δ¹³C) and inorganic (nitrate and ammonium, δ¹⁵N and δ¹⁸O) 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 δ¹⁵N composition increased between the pristine (0 ± 2‰), moderate (10 ± 6‰), and degraded (20 ± 8‰) systems, indicating that N recycling decreased as degradation increased. The N₂O concentrations, NO₃− dual isotope values, and offsets between “measured” and “mixing expected” δ¹⁵N 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.