Environmental Engineering Reference
In-Depth Information
Figure 4.
Time series of bottom water oxygen, nitrate, and hydrogen sulphide at SEDLAB
Station 1 from May 2001 until May 2004.
N = 16[PO
4
3
−
] - [NO
3
−
] is most extreme in the in-shore areas in
the Northern Benguela near 23
◦
S [38]. Tyrell and Lucas [38] interpreted this
deficit as the anaerobic respiration of organic matter with nitrate as electron
acceptor, i.e., denitrification. However, direct tracer measurements using
15
N-
labelled nitrate and ammonium do not support this interpretation. Anammox, a
recently discovered anaerobic bacterial process in anoxic water columns, which
catalyzes the consumption of nitrate/nitrite together with ammonium to form
N
2,
appears to be the main process for nitrogen removal in the oxygen deficient
water column of the Namibian shelf [21]. Irrespective of which bacterial process
is ultimately responsible for nitrogen removal, our current data suggest that
complete bottom water nitrate depletion is a common corollary of sulphidic
bottom waters.
The only direct measurements of bacterial sulphate reduction with radiola-
belled
35
S-SO
4
2
−
in bottom waters (3 m above ground) were performed at four
shelf stations in March 2004. During this period, bottom waters were anoxic,
but did not contain hydrogen sulphide. The experimental rates ranged from
0.2 to 6.8 nmol l
1
defined as
∆
day
−
1
. At the highest rates, it would require 147 days to
