State of the Bay Report 2010-Final - Anchor Environmental

contaminants. Furthermore dredging activities can lead to the suspension of fine particles and the
release of contaminants adsorbed to sediments. Maintenance dredging took place at Mossgas quay
and the Multi-Purpose Terminal from the end of 2007 to March/April 2008 with an estimated 50 000
m 3 of seabed material being removed from both terminals in order to deepen the berth. The baywide
increases in the percentage mud, particulate organic carbon and various trace metals suggests
that the principle driver of change was the dredging event which suspended fine sediments and
released contaminants that had accumulated from a variety of sources due to the poor circulation
and reduced wave energy within Small Bay.
The 2009 survey indicated that percentage mud, particulate organic carbon and trace metal
concentrations had reduced since 2008. Thus physical parameters were improving in the Bay. The
benthic macrofauna community also showed signs of recovery. The average biomass of benthic
macrofauna increased, the diversity index (H’) increased, and the W statistic increased. The
community composition had shifted such that the abundance and biomass of Ampelisca sp. had
reduced drastically while polychaetes had become dominant. Interestingly the abundance of
individuals did not increase, while the biomass did (this is reflected by the increased W statistic)
indicating that the proportion of slow growing, larger species in the community had increased,
suggesting that disturbance had decreased.
The assessment of sediment samples collected in 2010 revealed that there had been further
improvements of the physical parameters in Small Bay. The percentage mud, particulate organic
carbon and concentrations of trace metals (with the exception of copper) had reduced at most sites
in the Bay (the most notable exception being the Yacht Club Basin). The benthic macrofauna survey
conducted in 2010 revealed that there had been further recovery of the benthic community in Small
Bay with overall increases in biomass, abundance and diversity. The community composition,
broken down by taxonomic groups, indicated that there had been an increase in the proportion of
gastropods, bivalves and crustaceans and a decrease in the proportion of polychaetes in the
community since 2009. Indeed, the bivalve Tellina gilchristi and the gastropod Nassarius speciosus
were dominant species in 2010 while the abundance and biomass of the mud prawn, Upogebia
capensis, also increased between 2009 and 2010. When assessed at a finer scale, the 2010 survey
revealed that there was considerable spatial variation in the recovery process which is likely to be a
result of different anthropogenic and environmental drivers experienced at different locations
throughout the Bay.
Big Bay
The survey conducted in 2008 revealed that there had been an increase in the percentage
mud throughout Big Bay since 2004, while the overall abundance, biomass and diversity of benthic
macrofauna had decreased at sites throughout the Bay. The average W statistic decreased between
2004 and 2008 indicating that the proportion of fast growing opportunistic species in the community
had increased. It is likely that these changes in the benthic community are related to the dredging
activities that were conducted during 2007 and 2008 at the Multi-Purpose Terminal in Small Bay and
along Langebaan North Beach.
There was no clear trend regarding the percentage mud in the sediments at sites in Big Bay
between 2008 and 2009. Some sites experienced an increase while at others there was a reduction
in the percentage mud. The biomass and abundance of benthic macrofauna increased considerably
between 2008 and 2009. Much of the increase in abundance was attributed to substantial increases
in the number of small-bodied opportunistic polychaetes, while the increased biomass was
dominated by large crustacean species such as the mud prawn Upogebia capensis, several species of
crab as well as the mantis shrimp Pterygosquilla armata capensis. The increased biomass and
abundance results suggested that Big Bay benthic communities were in a state of recovery between
2008 and 2009. Indeed, the total biomass and abundance of benthic macrofauna found within Big
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 185