Environmental Engineering Reference
In-Depth Information
particles. The total respiration by aggregates (Total R
agg
across a complete
size spectrum was determined by integrating equations (1) and (2) to yield the
expression
r
2
b
3
∗
171
1
r
2.0
−
b
4
+
1
=
=
TotalR
agg
n
r
Q
tot
,
r
dr
(3)
2.0
−
b
4
+
r
1
We used published values of b
3
and b
4
for the diatom bloom reported in
Kiørboe and Jackson [19]. These values are 1 x 10
−
4
and 1.3 x 10
−
3
for b
3
,
and 1.6 and 2.6 for b
4
, respectively. Mean values for b
3
and b
4
are 7.5 x
10
−
4
and 2.1, respectively. The reported aggregate size spectrum ranged from
0.225 mm to 5 mm [20]. The calculated respiration rates vary between 0.1
and 7.7 µMO
2
day
−
1
with a mean of 1.7 µM day
1
. These average oxygen
consumption rates translate into depth-integrated oxygen consumption rates of
52, 96, 172 mmol O
2
m
−
2
day
−
1
for water depths of 30, 50, and 100 m depth,
respectively. Decreasing particle abundances with depth and variations in the
intensity and the temporal and spatial extent of blooms in the Benguela current
[10, 16] introduce uncertainties to our method that are difficult to quantify at
the moment. Yet, this method for the calculation of oxygen consumption is
superior over other approaches such as the apparent oxygen utilization in that
it is independent of variations in the oxygen concentration of the source waters
feeding the upwelling system.
4.2 Anaerobic Water Column Processes
Nitrate is a potential electron acceptor for the respiration of organic matter
in the absence of oxygen and may serve as electron acceptor for the oxidation
of dissolved sulphide, when oxygen is depleted. Thus, the development of
sulphidic bottom waters may require the complete consumption not only of
oxygen, but also of dissolved nitrate. Long-term records of bottom water nitrate
and hydrogen sulphide indicated highly variable nitrate concentrations in the
bottom water of the Namibian shelf. Hydrogen sulphide was detected in the
coastal inshore bottom waters (25m depth) in May 2001 and 2004, and during
these periods, nitrate was absent from the shelf bottom waters (Fig. 4). However,
there were also periods when sulphide was absent and nitrate was very low,
e.g., January 2003, and when sulphide and nitrate co-existed, albeit at very
low concentrations, e.g. September 2003 (Fig. 4). One possible explanation
is that the bottom water was in a transitional stage during these sampling
times.
An extensive data set of the Sea Fisheries Institute of South Africa and the
Ministry of Fisheries, Namibia indicates a permanent deficit of nitrate relative
to phosphate and silicate in the coastal upwelling waters. The nitrate deficit
