Environmental Engineering Reference
In-Depth Information
enriched NO
3
−
upward into the euphotic zone. In any case, the two, monsoon-
associated mixing events each year would be enough to lead to considerable
15
N
enrichment of the particle pool at the surface through assimilation of residual
NO
3
−
mixed upward from the zone of active denitrification in the upper part of
the OMZ.
3.2 Surface Processes
As noted above, the δ
15
N values we measured in suspended particles (Fig.
3) are quite high relative to values typical of particles in regions of the open
ocean well removed from suboxic water masses [1, 5, 38, 41, 56]. In addi-
tion to creating a characteristic isotopic signature, denitrification in the OMZ
significantly alters the relative abundance of nutrients in the water column by
removing NO
3
−
independently of PO
4
3
−
. This contrasts strongly with assimi-
latory processes, which remove both NO
3
−
and PO
4
3
−
from solution in roughly
Redfield proportions and therefore have minimal impact on the relative con-
centrations of the two ions. The net result of denitrification activity in the OMZ
is that subsurface waters in the Arabian Sea show a clear deficit in NO
3
−
avail-
ability relative to PO
4
3
−
(Fig. 5). This departure from Redfield stoichiometry
can be described using the N* parameter [25], which quantifies the degree to
which NO
3
−
:PO
4
3
−
concentration ratios depart from the oceanic mean. The
nutrient distributions within the Arabian Sea yield uniformly negative N* val-
ues, with a minimum below -12.5 µmol kg
−
1
in the core of the OMZ (Fig. 5).
As a result, vertical mixing processes can promote primary production but will
induce nitrogen limitation of production because of the NO
3
−
deficit arising
through denitrification.
The intermonsoon period we sampled is characterized by little wind stress
and dissolved nutrient concentrations were at or below the limit of detection
in the surface mixed layer throughout our transect (Fig. 4). These conditions
are ideal for bloom formation by
Trichodesmium
, and our cruise track passed
through one of the largest such blooms ever documented [12].
N
2
-fixation produces organic matter with an isotopic composition depleted
in
15
N relative to atmospheric N
2
, and therefore a significantly lower δ
15
N than
average marine sources of nitrogen [38]. This isotopic contrast is accentuated in
the Arabian Sea, where the isotopic fractionation associated with denitrification
leads to an overall increase in the δ
15
N of organic matter. Not surprisingly,
samples of suspended particles collected within the upper 10 m of the water
column in the bloom show significantly lowerδ
15
N values than surface particle
samples collected north and south of the densest portion of the bloom (Fig. 4,
upper panel). Although
Trichodesmium
biomass was most concentrated near
the surface, our vertical profiles of the δ
15
N of suspended particles indicate
that nitrogen fixation made a measurable contribution to the nitrogen budget
