State of the Bay Report 2010-Final - Anchor Environmental

Anchor Environmental
likely as a result of naturally occurring conditions, however, the ecological functioning of the system
could be further compromised by organic pollutants entering the Bay. There is evidence of anoxia in
localised areas of Small Bay (e.g. under the mussel rafts, within the yacht basin) that is caused by
excessive organic inputs. Monteiro et al. (1997) identified the effluent from a pelagic fish processing
factory as the source of nitrogen that resulted in an Ulva seaweed bloom in Small Bay.
5.4 Currents and waves
Circulation patterns and current strengths prior to the development (1974-75) in Saldanha
Bay were investigated using several techniques (drogues, dye-tracing, drift cards and sea-bed
drifters). Surface currents (within the upper five meters) are complex and appeared to be
dependent on wind strength and direction as well as the tidal state. Within Small Bay, currents were
weak (5-15 cm.s -1 ) and tended to be clockwise (towards the NE) irrespective of the tidal state or the
wind (Figure 5.5A). Greater current strengths were observed within Big Bay (10-20 cm.s -1 ) and
current direction within the main channels was dependent on the tidal state (Figure 5.5A). The
strongest tidal currents were recorded at the mouth of Langebaan Lagoon (50-100 cm.s -1 ), these
being either enhanced or retarded by the prevailing wind direction (Figure 5.5A). Currents within
the main channels in Langebaan Lagoon were also relatively strong (20-25 cm.s -1 ). Outside of the
main tidal channels, surface currents tended to flow in the approximate direction of the prevailing
wind with velocities of 2-3 % of the wind speed (Shannon and Stander 1977). Current strength and
direction at 5 m depth was similar to that at the surface, but was less dependent on wind direction
and velocity and appeared to be more influenced by the tidal state. Currents at 10 m depth at the
mouth of the Bay were found to be tidal (up to 10 cm. s -1 , either eastwards or westwards) and in the
remainder of the Bay, a slow (5 cm.s -1 ) southward or eastward movement, irrespective of the tidal
state, was recorded.
The currents and circulation of Saldanha Bay subsequent to the construction of the Marcus
Island causeway and the iron ore/oil jetty were described by Weeks et al. (1991a). Historical data of
drogue tracking collected by the Sea Fisheries Research Institute during 1976-1979 were analysed in
this paper. This study confirmed that wind is the primary determinant of surface currents in both
Small Bay and Big Bay; although tidal flows do influence currents below the thermocline and are the
dominant forcing factor in the proximity of Langebaan Lagoon. Weeks et al. (1991a) noted that
because much of the drogue tracking was conducted under conditions of weak or moderate wind
speeds, the surface current velocities measured (5-20cm.s -1 ), were probably underestimated. The
authors concluded that the harbour construction had constrained water circulation within Small Bay,
enhancing the general clockwise pattern and increasing current speeds along the boundaries,
particularly the south-westward current flow along the iron ore/oil jetty (Figure 5.5B). More recent
data collected during strong NNE wind conditions in August 1990 revealed that greater wind
velocities do indeed influence current strength and direction throughout the water column (Weeks
et al. 1991b). These strong NNE winds were observed to enhance the surface flowing SSW currents
along the ore jetty in Small Bay (out of the Bay), but resulted in a northward replacement flow (into
the Bay) along the bottom, under both ebb and flood tides. The importance of wind as the
dominant forcing factor of bottom, as well as surface, waters was further confirmed by Monteiro
and Largier (1999) who described the density driven inflow-outflow of cold bottom water into
Saldanha Bay during summer conditions when prevailing SSW winds cause regional scale upwelling.
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 91