Environmental Engineering Reference
In-Depth Information
Standard sediment echosounder is designed for operation in water depths down
to 400 m. The main advantages of the SES96 echosounder are its high spa-
tial resolution and the information contained in raw data of both HF and LF
channels. This is particularly important for acoustic sediment classification in
connection with the results of sediment physical property measurements.
3. HYDROGRAPHIC EFFECTS ON OXYGEN
LEVELS IN THE COASTAL UPWELLING ZONE
The general hydrography of the Benguela Current system has been reviewed
in [16, 34, 35]. More recent on-board and moored measurements have provided
detailed insight into the dynamics of the regional hydrography. Two advection
processes are involved in the ventilation of the subsurface water on the Namib-
ian shelf. South Atlantic Central Water (SACW) is transported southward by
the poleward undercurrent (Fig. 3a). This water originates from the area of
the Angola gyre along the Namibian shelf and carries water of low oxygen
(
<
undercurrent appears to be controlled by remote forcing in the eastern tropical
Atlantic and to a lesser extent by the local wind. The second water mass is
the East South Atlantic Central Water (ESACW), which is transported by the
Benguela current from the area of the Agulhas retroflection zone to the north
[31]. The ESACW is well-ventilated and has a lower nutrient concentration
than the SACW. Sub-thermocline cross-shelf circulation advects ESACW in
depths between 20 and 70 m from the area off the shelf break onto the shelf
(Fig. 3b). This transport is mainly driven by the local alongshore wind stress
and compensates for the Ekman offshore transport in the surface layer (Fig.
3b). Since the oxygen concentration of the ESACW water is higher, advection
by cross-shelf circulation ventilates the intermediate waters on the shelf much
more efficiently than the advection by the poleward undercurrent. The oxygen
concentration of the subsurface water on the shelf is very sensitive to the local
balance of consumption and ventilation processes, which both shape the pattern
of oxygen concentration and its variation in space and time.
Upwelling-favourable winds on the Namibian shelf are strong in the Cape
Frio and Luderitz upwelling cells and weak in the area between the upwelling
cells. While the northern part of the Benguela is strongly affected by the advec-
tion of SACW with the poleward undercurrent, with increasing distance from
the Angola-Benguela front, the cross-shelf circulation in the subsurface shelf
water gradually increases the proportion of ESACW. Cross shelf circulation
intensifies in the L uderitz upwelling cell such that well-ventilated ESACW
becomes the dominating water mass on the shelf and limits the southward ex-
tension of SACW to the latitude of Luderitz, where the subsurface shelf waters
consist entirely of ESACW. Upwelled water of the L uderitz cell is also ad-
45 µM) and high nutrient concentrations [25]. The intensity of the poleward
