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

State of Saldanha Bay & Langebaan Lagoon 2011
Water quality
4 WATER QUALITY
The temperature, salinity (salt content), dissolved oxygen concentration, nutrient levels (specifically
dissolved nitrate – a limiting nutrient for phytoplankton growth), and chlorophyll concentration (a
measure of primary production), 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. Some historic data exist for these three
variables exist for Saldanha Bay but no recent data are available. This historic data has been
presented in previous versions of the State of the Bay report (Anchor Environmental Consultants
2004. 2006, 2008, 2009, 2010, 2011) and are not repeated here. Suffice is to say that there have
been no obvious changes in temperature, salinity, or nutrient or chlorophyll concentrations in the
Bay, but that levels of dissolved oxygen, particularly in Small Bay have declined significantly since the
construction of the Iron Ore Jetty and causeway linking Marcus Island to the mainland. This is
though to be a result of increasing inputs of organic matter, mainly from fish processing factories,
sewage and mussel farms, coupled with the reduced flushing capacity of the bay (particularly Small
Bay) following the development of the port facilities (more detail on this below - §4.1).
Concentrations faecal coliforms (bacteria typically associated with faecal pollution) are
commonly monitored in areas used for human recreation or where marine organisms are harvested
for human consumption. While these organisms themselves do not necessarily pose a risk to human
health, they are a good indicator of levels of other pathogenic organisms in the environment that are
also associated with sewage waste and do pose a risk (e.g. Steptococci and Cholera) but are much
more tricky to quantify. Concentrations of faecal coliforms in waters around the periphery of the
bay have been monitored by the SBWQT since 1999 and are presented below (§4.2).
Information on concentrations of trace metals in the water column are presented in §4.3).
4.1 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 4.1A).
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 4.1A). 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 4.1A). 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.
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