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of the entire upper 200 m of the water column (Fig. 3). An isotope mixing
model revealed that diazotroph nitrogen constituted about a quarter of the
total pool of suspended particulate nitrogen in the upper 100 m of the water
column [12]. On a larger scale, the low δ
15
N values of surface particles and the
integrated biomass of the upper water column (0 - 100 m, Table 1) in the region
of the
Trichodesmium
bloom demonstrates that N
2
-fixation made a significant
contribution to the overall nitrogen budget between roughly 12˚N and 3˚N (Fig.
4).
3.3 Interactions Between Suspended and Sinking
Particles
Studies in other oceanic areas have demonstrated that the isotopic signature
of N
2
-fixation can propagate into other components of the pelagic ecosystem
[37, 38], though the actual mechanism of transfer of diazotroph nitrogen into the
food web is not yet well understood. In the Arabian Sea, the surface N
2
-fixation
signature had a clear impact on the field of sinking organic matter sampled by
our sediment traps (Fig. 4, middle panel), and a more modest effect on the
mean δ
15
N of suspended particles in the upper 100 m of the water column
(Fig. 4, upper panel). Under calm sea and wind conditions,
Trichodesmium
tends to aggregate at the surface as a result of the positive buoyancy imparted
to healthy trichomes by their internal gas vesicles [13]. This behavior may
have contributed to the absence of a clear N
2
-fixation signal in the isotopic
composition of suspended particles below 200 m in the core of the bloom at
10˚N (Fig. 3).
Our time series at 10˚N is particularly interesting because we deployed
traps at both 100 and 500 m at this station, which was located within the
densest part of the
Trichodesmium
bloom (Figs. 3 and 5). The sinking particles
collected by the 100 m trap are strongly depleted in
15
N relative to the sinking
particles collected at this depth at 18˚N and 3˚N outside the core of the bloom.
Interestingly, the material sinking into the 100 m trap had a mean δ
15
N only
15
N of particles in the upper 100 m of the water
column (Fig. 4, Table 1) and higher than the δ
slightly lower than the mean δ
15
N of material collected right
at the surface (Fig. 4), suggesting that this trap effectively sampled the entire
mixed layer.
In contrast, the trap at 500 m collected material with a substantially lower
mean δ
15
N of sinking particles with
depth has been observed in other oceanic areas [e.g., 2, 56], albeit over a much
greater depth range than the 500 m in our study. Although we cannot rule
out the possibility that the difference in δ
15
N (Fig. 4, Table 1). A decrease in the δ
15
N between traps reflects temporal
15
N of material
collected in our trap at 500 m could also arise from preferential remineral-
15
N of sedimenting organic matter, the low δ
changes in the δ
