PHYSICAL AND CHEMICAL CHARACTERISTICS OF WATER IN SALDANHA BAY AND LANGEBAAN LAGOON

Summary

In April 1974 the Sea Fisheries Branch commenced a detailed survey of the physical and chemical characteristics of the water in Saldanha Bay and Langebaan Lagoon to establish base-line conditions in the region prior to the development of the bay as a major port and industrial centre. The thermohaline characteristics of the water resemble those of the Benguela Current, although in summer surface heating and evaporation tend to raise the temperature and salinity somewhat. This is most marked in the Langebaan Lagoon where surface temperatures and salinities in excess of 24 °C and 37 %0 were recorded during the latter part of summer. A variety of chemical parameters were determined. These include a number of heavy and transition elements, dissolved oxygen, nitrate/nitrite, inorganic phosphorus, total phosphorus, reactive silica, pH, chlorophylls, chemical oxygen demand and oxygen absorbed. Data for some of these parameters are tabulated, and indicate that the bay is at present relatively unpolluted. Prior to 1974 organic pollution from the fish factories in Hoedjies Bay was serious, but the situation has shown a marked improvement since the installation of the dry off-loading systems for fish. The circulation pattern in Saldanha Bay is complex. The bay and lagoon are tidal and currents in the upper 5 m are highly dependent on wind speed and direction. Surface currents in the region have a magnitude typically in the range 10–20 cm/sec. Currents in the mouth of the bay are tidal and are not appreciably influenced by wind. Tidal currents of up to 1 m/sec were recorded at the entrance to Langebaan Lagoon. The construction of the jetty and breakwater is altering the circulation pattern. The physical and chemical data indicate the existence of three systems, viz. Bay System, Lagoon System and Benguela System. The interchange between the bay and the open sea is small and on the basis of a simple model the removal time of pollutants from the bay was calculated as 20 days. The available data indicate that the discharge of noxious and toxic effluents into the bay should be discouraged. The beach near the residential area of Saldanha Bay and the Donkergat peninsula will probably be polluted on occasions once the harbour comes into operation, and it is not impossible that Langebaan Lagoon will also become polluted.