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Medientyp:
E-Artikel
Titel:
δ15N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation
Beteiligte:
Wells, Naomi S.;
Eyre, Bradley D.
Erschienen:
Wiley, 2019
Erschienen in:Limnology and Oceanography
Sprache:
Englisch
DOI:
10.1002/lno.11080
ISSN:
0024-3590;
1939-5590
Entstehung:
Anmerkungen:
Beschreibung:
<jats:title>Abstract</jats:title><jats:p>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<jats:sup>15</jats:sup>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<jats:sup>15</jats:sup>N and δ<jats:sup>13</jats:sup>C) and inorganic (nitrate and ammonium, δ<jats:sup>15</jats:sup>N and δ<jats:sup>18</jats:sup>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<jats:sup>−1</jats:sup>, moderate: 150 kg N d<jats:sup>−1</jats:sup>, degraded: 630 kg N d<jats:sup>−1</jats:sup>). The difference in the inorganic vs. organic pool δ<jats:sup>15</jats:sup>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 N<jats:sub>2</jats:sub>O concentrations, NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> dual isotope values, and offsets between “measured” and “mixing expected” δ<jats:sup>15</jats:sup>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.</jats:p>