State of the Bay Report 2010-Final - Anchor Environmental

5 WATER QUALITY
Anchor Environmental
The temperature, salinity (salt content) and dissolved oxygen concentration occurring in
marine waters are the variables most frequently measured by oceanographers in order to
understand the origins, physical and biological processes impacting on, or occurring within a body of
sea water. Long-term data series of these three variables exist for Saldanha Bay and these are
discussed in some detail below. Other measurable physical and chemical variables such as nutrient
levels (specifically dissolved nitrate – a limiting nutrient for phytoplankton growth), chlorophyll
concentration (a measure of primary production), current strengths and circulation patterns have
been reported on in various studies and the key findings or trends are also summarised in this
chapter.
5.1 Water temperature
Water temperature records for Saldanha Bay and Langebaan Lagoon were first collected
during 1974-75 as part of a detailed survey by the then Sea Fisheries Branch, Department of
Industries (now Marine and Coastal Management, Department of Environmental Affairs and
Tourism). The survey was initiated to collect baseline data of the physical and chemical water
characteristics prior to the development of the Bay as an industrial port. The findings of this survey
were published in a paper by Shannon and Stander (1977). Surface water temperatures prior to the
construction of the iron ore/oil jetty and Marcus Island causeway varied from 16-18.5 ° C during
summer (January 1975) and 14.5-16 ° C during winter (July 1975). During both periods, higher
temperatures were measured in what is now the northern part of Small Bay and within Langebaan
Lagoon, whilst cooler temperatures were measured at sampling stations in Outer Bay and Big Bay.
The water column was found to be fairly uniform in temperature during winter and spring (i.e.
temperature did not change dramatically with depth) and the absence of a thermocline (a clear
boundary layer separating warm and cool water) was interpreted as evidence of wind driven vertical
mixing of the shallow waters in the Bay. A clear shallow thermocline was observed at about 5 m
depth, during the summer and autumn months at some deeper stations and was thought to be the
result of warm lagoon water flowing over cooler sea water. The absence of a thermocline at other
shallow sampling stations was once again considered evidence of strong wind driven vertical mixing.
Shannon and Stander (1977) suggested that there was little interchange between the relatively sunwarmed
Saldanha Bay water and the cooler coastal water through the mouth of the Bay, but rather
a “slopping backwards and forwards tidal motion”.
The Sea Fisheries Research Institute continued regular monitoring (quarterly) of water
temperature (and other variables) in Saldanha Bay until October 1982. These data were presented
and discussed in papers by Monteiro et al. (1990) and Monteiro and Brundrit (1990). The
temperature time series for Small Bay and Big Bay is shown in Figure 5.1. This expanded data series
allowed for a better understanding of the oceanography of Saldanha Bay. The temperature of the
surface waters was observed to fluctuate seasonally with surface sun warming in summer and
cooling in winter, whilst the temperature of deeper (10 m depth) water shows a smaller magnitude,
non-seasonal variation, with summer and winter temperatures being similar (Figure 5.1). In most
years, a strong thermocline separating the sun warmed surface layer from the cooler deeper water
was present during the summer months at between 5-10 m depth. During the winter months, the
thermocline breaks down due to surface cooling and increased turbulent mixing, and the water
column becomes nearly isothermal (surface and deeper water similar in temperature) (Figure 5.1).
Unusually warm, deeper water was observed during December 1974 and December 1976 and was
attributed to the unusual influx of warm oceanic water during these months (Figure 5.1).
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 85