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

State of Saldanha Bay & Langebaan Lagoon 2011
Benthic macrofauna
increase are not clear given that, based on the increases in other taxonomic groups, the ecosystem
did not appear to have been negatively disturbed by anthropogenic or natural perturbations. The
increase in small polychaete species (predominantly detritivores) is likely to be a result of biological
interactions.
Big Bay
Crustaceans and polychaetes have dominated the benthic macrofauna community in Big Bay in
terms of abundance in all surveys conducted since 1999, while crustaceans and tongue worms
(Echiuroidea) have dominated in terms of biomass. The overall biomass and abundance of benthic
macrofauna in Big Bay increased between 1999 and 2004. This is an indication that benthic
environment in Big Bay had been recovering since the dredging events of 1997/8. A dramatic
decrease in both the abundance and biomass of benthic macrofauna in Big Bay was seen between
2004 and 2008. It is likely that this was a response to the dredging events in Small Bay (maintenance
dredging of the Multi Purpose Terminal) in 2007/8 and off north beach at the northern end of
Langebaan Lagoon. Much of the reduction in biomass and abundance could be attributed to the loss
of crustaceans. There was also a dramatic reduction in the density of polychaetes between 2004
and 2008.
There was a substantial increase in the abundance and biomass of benthic macrofauna in Big
Bay between 2008 and 2009. Much of the increase in abundance was attributed to the increase in
polychaetes. This was however not reflected in the biomass, indicating that the community had
become dominated in terms of abundance by small polychaetes. The increase in the overall biomass
of the benthic community between 2008 and 2009 was principally attributed to an increase in
crustacean biomass. The results of the 2010 survey revealed that the abundance of benthic
macrofauna had decreased while the biomass had increased. This indicates that fewer, larger
organisms were dominating, and possibly leading to a reduction in the number of smaller organisms
through predatory or competitive community interactions. This is a typical sign of the succession of
a system following a disturbance. Interestingly, the abundance of polychaetes and crustaceans
increased dramatically between 2010 and 2011. This result was not reflected in the biomass results
indicating that small crustaceans and polychaetes had colonised the Big Bay area by 2011. The
reason for the dramatic increase in the abundance of small polychaetes and crustaceans in Big Bay is
not clear. A similar increase in the abundance of small polychaetes was also seen in Small Bay. It is
likely that a natural Bay-wide fluctuation, possibly based on nutrient availability and productivity,
may have occurred between 2010 and 2011, which supported an increase in the abundance of small
polychaetes. The stability of other taxonomic groups and of the proportions of functional groups
suggests that the system has not been subject to a negative disturbance and remains in a state of
recovery following dredge events
The biomass of the benthic community in Big Bay has been dominated by detritivores in all
years except 2008 when scavengers became dominant. The increased proportion of scavengers was
not reflected in terms of abundance suggesting that few, large scavenging species and many, small
opportunistic detritivores colonised the benthic habitat following dredging. Since 2008, the benthic
community has shifted back to one dominated by detritivores both in terms of abundance and
biomass, indicating that larger detritivores had re-established. Filter feeding organisms are more
abundant and make a greater contribution to the biomass of benthic macrofauna in Big Bay than in
Small Bay.
Langebaan Lagoon
Langebaan Lagoon generally supports a much lower abundance and biomass of benthic macrofauna
than Saldanha Bay. This may be due to the fast water movements and high levels of tidal variation
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