Environmental Engineering Reference
In-Depth Information
During our German JGOFS cruise, we encountered a massive bloom of
Tri-
chodesmium
[12], which had a clear impact on the water column nitrogen
budget and the δ
15
N of suspended particles at the surface.
3.1 Subsurface Processes in the Arabian Sea
The Arabian Sea is contained within a single, roughly triangular basin ex-
tending from about 25
◦
N to the Carlsberg ridge in the south with a maximal
depth of about 4000m. The hydrology of intermediate waters in the Arabian
Sea reflects significant inputs of Indian Deep Water from the South with as
well as warm, salty water from both the Persian Gulf in the northwest and the
Red Sea to the southwest [55, 58]. The slow and tortuous path followed by
the northward-flowing Indian Deep water along with the low initial O
2
con-
centration of waters entering the Arabian Sea from the north lead to extensive
development of suboxic conditions in the water column of the Arabian Sea
[55]. The core of the OMZ is about 800 m thick in the central Arabian Sea as
far south as about 14˚N, and low O
2
concentrations (
<
10 µmole L
−
1
) extend
to about the same latitude (Fig. 2).
The elevated δ
15
N signature characteristic of particles (Fig. 3) in the Ara-
bian Sea is consistent with the effects of denitrification within the OMZ. The
relative uniformity of the δ
15
N we measured in suspended particles below the
surface mixed layer suggests that the isotopic composition of the particle field
is determined primarily by processes occurring either at the surface (e.g., pri-
mary production) or within the upper reaches of the OMZ (e.g., heterotrophic
production), rather than by processes occurring in situ within the core of the
OMZ proper.
A number of lines of evidence indicate that the highest rates of denitrification
occur just below the nitracline in the upper margin of the OMZ, a scenario
consistent with our isotope data. For example, the portion of the water column
between 300 and 400 m depth is characterized by high electron transport system
activity [43], high N
2
O concentrations [44], and elevatedδ
15
N values for NO
3
−
[10]. Although this characteristic isotopic signature originates at depth within
the OMZ, vertical mixing, particularly during the strong Southwest Monsoon,
can transport isotopically enriched NO
3
−
into the surface mixed layer. This
vertical mixing and recharge of the surface layer with nutrients through mixing
associated with the monsoon presumably occurs twice a year, a time scale that
matches well with the 1 year ventilation time estimated by Naqvi and Shailaja
based on the rate of metabolic activity within the OMZ [43]. Finally, a high
temporal resolution study in the eastern central Arabian Sea revealed internal
wave oscillations of several tens of meters beneath the thermocline which
were strongest during the period immediately after onset of the Southwest
Monsoon [50]. Such internal wave activity could also help to move isotopically
