Environmental Engineering Reference
In-Depth Information
3.4 Advection
Biological processes are critical in driving the vertical movement of material
and isotopic signatures, but physical processes dominate the lateral movement
of organic matter and its associated stable isotope signature on a regional scale.
Most research to date on the hydrography of the Arabian Sea has focused
on the monsoonal wind patterns and the induced alternating surface flows.
Comparatively little has been published about mid and deep water circulation
[53], despite its obvious importance in determining the distribution of physical
and chemical properties below the surface mixed layer.
As a result of the monsoons, the surface circulation changes seasonally in
the Arabian Sea. In summer the Southwest Monsoon drives coastal upwelling
in the western Arabian Sea and surface currents are anticyclonic (clockwise)
with flows from the northwest to the southeast in the eastern Arabian Sea and to
the north along the western boundary of the basin. During the winter, the flow
is driven by the Northeast Monsoon and water moves in the opposite direction
[55]. Interestingly, the depth to which these monsoon-driven currents extend
has not been well studied though it is clearly an important determinant of
the effectiveness of advection in altering the horizontal distribution of organic
matter and dissolved constituents in the Arabian Sea.
Our isotopic measurements suggest that advection plays a critical role in
distributing the elevatedδ
15
N values generated by denitrification in the OMZ to
regions well removed from the OMZ itself. For example, theδ
15
N of suspended
particles is much higher than values typical of non-OMZ regions all the way to
the southern end of our transect at the equator, a pattern that reflects advective
transport of both NO
3
−
and POM. In the Arabian Sea, low O
2
concentrations
extend to only to about 10˚N (Fig. 2), so the highδ
15
N of particles further south
must reflect advective transport of isotopically enriched NO
3
−
and/or organic
matter.
4. CONCLUDING REMARKS
Although the isotope budget of the Arabian Sea is clearly dominated by
the effect of denitrification within the large OMZ in the north-central basin,
N
2
-fixation in the surface layer also makes an important contribution to the
isotopic systematics of the system. The interaction between denitrification and
N
2
-fixation clearly plays a critical role in controlling the relative abundance of
NO
3
−
and PO
4
3
−
in the ocean on long time scales, but to our knowledge, massive
blooms of N
2
-fixers have not been observed above any of the other major
OMZs in the world ocean. The seasonal oligotrophy of surface waters in the
Arabian Sea, in combination with the strong nitrogen limitation arising through
denitrification below the surface, produce conditions that strongly favor N
2
-
fixation as a strategy for growth. With respect to both the mass and the isotopic
