N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)
We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (February–March 2015). Using a Lagrangian strategy, we sampled the same water mass for...
| Main Authors: | , , , , , , , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-05-01
|
| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/2565/2018/bg-15-2565-2018.pdf |
| Summary: | We performed nitrogen (N) budgets in the photic layer of three
contrasting stations representing different trophic conditions in the western
tropical South Pacific (WTSP) Ocean during austral summer conditions
(February–March 2015). Using a Lagrangian strategy, we sampled the same
water mass for the entire duration of each long-duration (5 days) station,
allowing us to consider only vertical exchanges for the budgets. We
quantified all major vertical N fluxes both entering (N<sub>2</sub> fixation,
nitrate turbulent diffusion, atmospheric deposition) and leaving the photic
layer (particulate N export). The three stations were characterized by a
strong nitracline and contrasted deep chlorophyll maximum depths, which were
lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B)
than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station
LD C). N<sub>2</sub> fixation rates were extremely high at both LD A
(593 ± 51 µmol N m<sup>−2</sup> d<sup>−1</sup>) and LD B
(706 ± 302 µmol N m<sup>−2</sup> d<sup>−1</sup>), and the diazotroph
community was dominated by <i>Trichodesmium</i>. N<sub>2</sub> fixation rates
were lower (59 ± 16 µmol N m<sup>−2</sup> d<sup>−1</sup>) at LD C, and
the diazotroph community was dominated by unicellular N<sub>2</sub>-fixing
cyanobacteria (UCYN). At all stations, N<sub>2</sub> fixation was the major source
of new N (> 90 %) before atmospheric deposition and upward nitrate
fluxes induced by turbulence. N<sub>2</sub> fixation contributed circa 13–18 %
of primary production in the MA region and 3 % in the
SPG water and sustained
nearly all new primary production at all stations. The <i>e</i> ratio
(<i>e</i> ratio = particulate carbon export ∕ primary production) was
maximum at LD A (9.7 %) and was higher than the <i>e</i> ratio in most studied
oligotrophic regions (< 5 %), indicating a high efficiency of the WTSP
to export carbon relative to primary production. The direct export of
diazotrophs assessed by qPCR of the <i>nifH</i> gene in sediment traps
represented up to 30.6 % of the PC export at LD A, while their
contribution was 5 and < 0.1 % at LD B and LD C, respectively. At the
three studied stations, the sum of all N input to the photic layer exceeded
the N output through organic matter export. This disequilibrium leading to N
accumulation in the upper layer appears as a characteristic of the WTSP
during the summer season. |
|---|---|
| ISSN: | 1726-4170 1726-4189 |