Environmental Engineering Reference
In-Depth Information
near surface biota [37, 38, 57] and benthic sediments [4, 23]. In effect, isotopic
discrimination creates a natural isotopic perturbation that can potentially be
exploited in studies of the impact of denitrification on an ecosystem and global
level. Here, we will use data from the German Arabian Sea JGOFS program
as an example of the impact of denitrification and N
2
-fixation on the stable
isotopic composition of organic nitrogen in the water column in and around a
major oceanic OMZ.
2. ARABIAN SEA FIELD PROGRAM
As part of the German JGOFS program, we collected samples of suspended
and sinking particles in 1995 during a meridional transect through the cen-
tral Arabian Sea. This cruise occurred in the intermonsoon period just before
the Southwest Monsoon and our data allow us to evaluate the horizontal and
vertical propagation of isotopic signatures associated with processes occurring
within the OMZ as well as in the surface mixed layer above. Our data provide
interesting isotopic insights into the regional nitrogen cycle, particularly with
respect to the interaction between denitrification and N
2
-fixation and the prop-
agation of the isotopic signature associated with the OMZ southward into areas
well removed from the site of active denitrification.
2.1 Sample Collection and Analysis
We collected samples during a German JGOFS cruise to the Arabian Sea
in May 1995 (cruise ME 32-3, 5 May - 5 June 1995). Sampling stations were
located at 1 degree intervals along a transect at 65
◦
E longitude from 21
◦
N
to the equator (Fig. 2). Samples were collected from the upper 1500 m of the
water column with a CTD-rosette system equipped with Neil Brown sensors
and twelve 10L Niskin bottles. In addition to the standard upper water column
sampling at thirteen transect stations, we carried out time-series studies and
vertical flux measurements at 18
◦
N (D1, 8 days), 10
◦
N (D2, 7 days) and 3
◦
N
(D3, 3 days).
Multiple casts were required to obtain enough water for all the chemical
and biological analyses carried out at each station. At the time-series stations,
a large rosette equipped with six 30L water bottles (Hydrobios) was used to
supplement the standard rosette, allowing rapid collection of large volumes of
water from selected depths. However, a post-cruise comparison of nutrient and
DIC data from the two rosettes showed a systematic mismatch in regions of the
water column with strong gradients in chemical properties. We attribute this
to a design flaw in the 30L bottles, which have a mouth aperture significantly
smaller than the diameter of the bottle itself, leading to poor flushing of the bottle
volume. To correct for this sampling bias, we used the nutrient concentrations
