Environmental Engineering Reference
In-Depth Information
The mooring data indicate a transient, sluggish bottom water layer of up to 30
m thickness. This layer appears to be stable for months. The slower the merid-
ional current component, the longer the residence time on the shelf, the more this
layer becomes oxygen- and nitrate-depleted, and ultimately sulphidic. At coast-
parallel bottom current speeds of 2 and 4 cm s
−
1
, water is transported within
approximately 600 and 300 days, respectively, from the Angola-Benguela front
to 26
◦
S. Hence, this period can be taken as an approximate residence time for
shelf bottom water reaching 26
◦
S.
4. PATHWAYS, RATES, AND AMOUNT OF WATER
COLUMN RESPIRATION
4.1 Aerobic Water Column Respiration
Generally, oxygen concentrations decrease rapidly below the thermocline
on the shelf as a result of the aerobic respiration of sinking organic material.
Video observations from a remotely operated vehicle during RV METEOR
Expedition M57-3 in March 2003 suggest that particulate organic material in
the water column over the shelf consists mostly of aggregates [42]. We have
used a new approach to estimate the rate of oxygen consumption in the water
column by combining volume-specific oxygen consumption rates of diatom
aggregates with abundances and size spectra of aggregates determined by in-
situ video observation during a diatom bloom in the southern Benguela system
[20]. Size-dependent rates of diffusive oxygen uptake in diatom aggregates
have been determined by [30]. Diffusive oxygen uptake varied as a function of
aggregate size, and was described by the relationship Q
tot
,
vol
= 65.8(vol)
0.67
,
where Q
tot
,
vol
is the total oxygen consumption (nmol agg
−
1
h
−
1
) as a function
of aggregate volume (vol) [30]. The above equation can be recast as a function
of aggregate radius, which yields the expression
171
r
2.0
(1)
where Q
tot
,
r
has the unit nmol O
2
cm
−
3
h
−
1
. Extrapolation of the aggregate
radius-specific respiration rate to a respiration rate per volume seawater requires
that the aggregate size spectrum in the water column is known. We used the
empirical relationship reported in Kiørboe and co-workers [20] for a diatom
bloom in the southern Benguela. The aggregate size spectrum was described
by the power function
Q
tot
,
r
=
b
3
r
−
b
4
n
r
=
(2)
where n
r
represents the volume-normalized number of aggregates, the sub-
script r is the aggregate radius, b
3
is a particle concentration coefficient and
b
4
describes the slope of the spectrum [19]. The larger b
4
, the smaller are the
