Environmental Engineering Reference
In-Depth Information
Table 1.
Summary of the stable isotope composition of suspended and sinking particles in the
Arabian Sea. The δ
15
N of suspended particles is the concentration-weighted mean for the upper
100 or 1500 m of the water column. Where we analyzed suspended particles from multiple
casts (i.e., at 18˚ and 10˚N), the range of means is given. For the sediment traps, mean isotope
compositions (
±
SD) were calculated from multiple intervals, each interval lasting 1 day.
Latitude
Suspended
Suspended
Sinking
Sinking
(
◦
N)
PN (0 - 100 m)
PN (0 - 1500 m)
PN (100 m)
PN (500 m)
21
8.8
9.8
n.d.
n.d.
19
7.8
9.4
n.d.
n.d.
18
8.8-10.0
9.3-10.3
11.2
±
0.58 (N= 7)
5.1
±
1.1 (N= 7)
17
8.9
9.3
n.d.
n.d.
14
8.8
9.5
n.d.
n.d.
12
8.8
9.4
n.d.
n.d.
10
7.8-8.2
9.4-9.6
7.1
±
0.8 (N= 6)
1.9
±
2.7 (N= 6)
9
6.8
8.7
n.d.
n.d.
7
7.5
9.8
n.d.
n.d.
5
8.0
9.7
n.d.
n.d.
3
8.3
9.9
9.7
±
0.7 (N= 2)
n.d.
2
7.9
9.9
n.d.
n.d.
0
10.3
8.2
n.d.
n.d.
and an enrichment in the
15
N content of the residual nitrate. The impact of
this isotopic fractionation has been documented in both laboratory and field
studies of suboxic systems [6, 10, 16, 34, 36, 57]. The residual NO
3
−
exiting
pelagic OMZs can be advected laterally for substantial distances [e.g., 34,
51], potentially transporting the isotopic signature of partial denitrification to
regions well removed from the OMZ proper.
The isotopic signature of denitrification can also propagate upward to the
surface through vertical mixing, especially in a monsoonal system like the
Arabian Sea. The subsequent uptake of isotopically enriched nitrate by phy-
toplankton in the mixed layer will result in formation of suspended particles
with δ
15
N values that are much higher than is typical of waters well removed
from OMZs [1, 5, 38, 41, 56]. Some fraction of the organic matter produced
will sink out of the surface layer, further transporting the isotopic signature of
denitrification activity vertically through the water column and into the deep
sea.
Surface blooms of N
2
-fixing cyanobacteria, primarily
Trichodesmium
,have
been reported in the coastal and open waters of the Arabian Sea in the oligo-
trophic intermonsoon periods [11, 20, 21, 22, 32], making it one of the few
places in the world where both N
2
-fixation and pelagic denitrification are known
to occur simultaneously and at significant rates in the same water column.
