Vertical segregation among pathways mediating nitrogen loss (N<sub>2</sub> and N<sub>2</sub>O production) across the oxygen gradient in a coastal upwelling ecosystem
The upwelling system off central Chile (36.5° S) is seasonally subjected to oxygen (O<sub>2</sub>)-deficient waters, with a strong vertical gradient in O<sub>2</sub> (from oxic to anoxic conditions) that spans a few metres (30–50 m interval) over the shelf. This condition...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-10-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/14/4795/2017/bg-14-4795-2017.pdf |
| Summary: | The upwelling system off central Chile (36.5° S) is seasonally
subjected to oxygen (O<sub>2</sub>)-deficient waters, with a strong vertical
gradient in O<sub>2</sub> (from oxic to anoxic conditions) that spans a few metres
(30–50 m interval) over the shelf. This condition inhibits and/or
stimulates processes involved in nitrogen (N) removal (e.g. anammox,
denitrification, and nitrification). During austral spring (September 2013)
and summer (January 2014), the main pathways involved in N loss and its
speciation, in the form of N<sub>2</sub> and/or N<sub>2</sub>O, were studied using
<sup>15</sup>N-tracer incubations, inhibitor assays, and the natural abundance of
nitrate isotopes along with hydrographic information. Incubations were
developed using water retrieved from the oxycline (25 m depth) and bottom
waters (85 m depth) over the continental shelf off Concepción, Chile.
Results of <sup>15</sup>N-labelled incubations revealed higher N removal activity
during the austral summer, with denitrification as the dominant N<sub>2</sub>-producing pathway, which occurred together with anammox at all times.
Interestingly, in both spring and summer maximum potential N removal rates
were observed in the oxycline, where a greater availability of oxygen was
observed (maximum O<sub>2</sub> fluctuation between 270 and
40 µmol L<sup>−1</sup>) relative to the hypoxic bottom waters
( < 20 µmol O<sub>2</sub> L<sup>−1</sup>). Different pathways were
responsible for N<sub>2</sub>O produced in the oxycline and bottom waters, with
ammonium oxidation and dissimilatory nitrite reduction, respectively, as the
main source processes. Ammonium produced by dissimilatory nitrite reduction
to ammonium (DNiRA) could sustain both anammox and nitrification rates,
including the ammonium utilized for N<sub>2</sub>O production. The temporal and
vertical variability of <i>δ</i><sup>15</sup>N-NO<sub>3</sub><sup>−</sup> confirms that multiple
N-cycling processes are modulating the isotopic nitrate composition over the
shelf off central Chile during spring and summer. N removal processes in this
coastal system appear to be related to the availability and distribution of
oxygen and particles, which are a source of organic matter and the fuel for
the production of other electron donors (i.e. ammonium) and acceptors (i.e.
nitrate and nitrite) after its remineralization. These results highlight the
links between several pathways involved in N loss. They also establish that
different mechanisms supported by alternative N substrates are responsible
for substantial accumulation of N<sub>2</sub>O, which are frequently observed as hotspots
in the oxycline and bottom waters. Considering the extreme variation in
oxygen observed in several coastal upwelling systems, these findings could
help to understand the ecological and biogeochemical implications due to global
warming where intensification and/or expansion of the oceanic OMZs is
projected. |
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| ISSN: | 1726-4170 1726-4189 |