State of the Bay Report 2011-Final.pdf - Anchor Environmental

State of Saldanha Bay & Langebaan Lagoon 2011
Water quality
The currents and circulation of Saldanha Bay subsequent to the construction of the Marcus
Island causeway and the iron ore/oil Terminal 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 Terminal
(Figure 4.1B). 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 terminal 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.
Construction of the iron ore terminal and the Marcus Island causeway altered the wave
exposure zones evident in the Bay. Prior to harbour development in Saldanha Bay, Flemming (1977)
distinguished four wave-energy zones in the Bay, defined as being a centrally exposed zone in the
area directly opposite the entrance to the Bay, two adjacent semi-exposed zones on either side and
a sheltered zone in the far northern corner of the Bay (Figure 4.1A). The iron ore terminal essentially
divided the Bay into Small Bay and Big Bay and altered the wave energy and exposure patterns
within the Bay. The causeway increased the extent of sheltered and semi-sheltered zones in Small
Bay with no semi-exposed degree of wave energy being present in this area (Luger et al. 1999).
Wave exposure in Big Bay was altered less dramatically, however, the extent of sheltered and semisheltered
wave exposure areas increased after harbour development (Luger et al. 1999).
4.2 Microbiological monitoring
Faecal pollution contained in, for example, untreated sewage or storm water runoff, may introduce
disease-causing micro-organisms into coastal waters. These pathogenic micro-organisms constitute
a threat to recreational water users and consumers of seafood. Bacterial indicators are used to
detect the presence of faecal pollution. These bacterial indicators, however, only provide indirect
evidence of the possible presence of water borne pathogens and may not accurately represent the
risk to water users (Monteiro et al. 2000). Historically, the DWAF (1995) and (1996b) guidelines for
inland and coastal waters respectively, have been used to assess compliance in respect of human
health criteria. However as of 2011, these have been replaced with the South African Water Quality
Guidelines for Coastal Marine Waters Volume 2: Interim Guidelines for Recreational Use
(Department of Environmental Affairs, 2011).
4.2.1 DWAF 1995 and 1996 guidelines
The DWAF (1995) and (1996b) guidelines for inland and coastal waters respectively, identified three
recreational user groups; full-contact, intermediate-contact and non-contact recreation. Full contact
recreation included swimming and diving among other activities. Partial-contact recreation covered
59
ANCHOR
e n v i r o n m en t a l