Environmental Engineering Reference
In-Depth Information
the northern portion of our transect, in the core region of the OMZ, we found
a clear minimum in NO
3
−
concentration between 200 and 400 m. This local
minimum reflects the impact of denitrification (Fig. 1) in removing NO
3
−
from
the water column.
The isotopic composition of suspended particles (δ
15
N) showed relatively
little vertical structure below the surface on this transect, with all subsurface
values falling between 9 and 11‰(Fig. 3). In contrast, we found a well-defined
surface minimum in δ
15
N with values as low as 4‰ near 10
◦
N (Fig. 4). Below
the surface mixed layer, δ
15
N values increased and reached 9‰ between 80
and 100m depth at most stations.
The low δ
15
N values we observed in suspended particles at the surface were
associated with an extensive surface bloom of
Trichodesmium
. This bloom ex-
tended over about 20% of the surface area of the Arabian Sea [12]. The rates of
primary production and N
2
-fixation by
Trichodesmium
were both high within
the bloom, and
Trichodesmium
accounted for roughly 80% of total surface pri-
mary production, and about a quarter of total water column primary production
[12]. As a result of the spatial extent of the bloom and the intense production
by
Trichodesmium
within the bloom, N
2
-fixation appeared to support most of
the new production in the Arabian Sea at the time of sampling [12].
2.3 Sinking Particles
Our sediment trap collections show both vertical and lateral variations in
the δ
15
N of sinking organic matter (Table 1, Fig. 4). In general, the materials
collected in the shallow (100 m) trap had an isotopic composition similar to
(10˚N) or enriched (3˚N, 18˚N) in
15
N relative to the average particle field in the
upper 100 m of the water column. The low δ
15
N of sinking particles collected
by the shallow trap at our 10˚N station coincided with the peak abundances of
Trichodesmium
in the upper water column (Fig. 4).
At the two time series stations at 18˚N and 10˚N, we deployed a second
trap at 500 m depth. Interestingly, sinking particles collected by the deeper
trap had a significantly lower δ
15
N than materials collected by the shallow
trap (Fig. 4). Although materials collected in the 500 m trap at 10˚N showed
substantial variation from day to day (SD = 2.7‰), the mean value over the 6
days of deployment was lower than the surface PN at that station, which was
dominated by
Trichodesmium
(Fig. 4).
3. DISCUSSION
The Arabian Sea, with its well-developed oxygen minimum zone (OMZ)
is one of the major regions of pelagic denitrification in the world ocean and
provides a large midwater habitat for denitrifying bacteria. The consumption
of NO
3
−
by bacteria leads to a deficit in NO
3
−
relative to other nutrients
