BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

harbour. No benthos sampling results have been reported for the Ben Schoeman Dock
Basin or the outer areas of the harbour that would allow a determination of the extent of
the apparent pollution effect on benthos distributions. AVS values do tend to decline in
these areas and may be zero indicating that the sediments are oxic. If, as is likely,
anoxia is the controlling variable it is probable that there will be benthos in the sediments
with low to zero AVS levels. Distortion levels in community composition due to pollution
effects or other stressors such as frequent surficial sediment suspension are, however,
unknown.
Outside the harbour, large numbers of juvenile rock lobsters can be found on the vertical
faces of the outer harbour wall (Hazell et al. 2002). Other major components of the wall
community are encrusting corallines, the barnacle Notomegabalanus algicola, the sea
urchin Parechinus angulosus, the ribbed mussel Aulacomya ater, and sponges (Mayfield
1998). The relatively low coverage and biomass of mussels and the high cover of
encrusting corallines at the wall contrast with nearby natural subtidal rocky shores, for
example at Mouille Point, where mussels cover approx. 80% of the rocks and encrusting
coralline cover is low (Mayfield 1998, Table 1).
Harbours are a typical location for the introduction of alien species through ships calling
at the port which may transport organisms on their hulls or in their ballast waters.
Examples in the Port of Cape Town are the European shore crab Carcinus maenas, the
anemone Metridium senile, which grows on loose, silt-laden boulders or other objects,
and on the soft sediments on the floor of the Victoria and Alfred Basins, the red algae
Schimmelmannia elegans, and the ascidian Ciona intestinalis (Robinson et al. 2005).
Table 1: Average percentage cover and wet biomass (± standard error) of benthic
organisms at the outer harbour wall and at Mouille Point. Percentage cover includes
secondary cover (e.g. barnacle cover) and can thus exceed 100% (after Mayfield 1998).
Benthos
Harbour Wall
%cover / g wet mass/m 2 ± SE
Mouille Point
%cover / g wet mass/m 2 ± SE
Coralline algae 75 / 2235 ± 289 20 / 506 ± 192
Foliar algae