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

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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 142 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna experienced in the Lagoon. The Lagoon is dominated in terms of abundance by polychaetes and crustaceans and in terms of biomass, by crustaceans. The overall biomass in Langebaan Lagoon declined sharply between 1975 and 2004. The reduction in biomass was linked to a reduction in the abundance of many of the taxa present in 1975 (bivalves, polychaete worms, gastropods, echinoderms, and sea-pens). The overall abundance and biomass of macrofauna in Langebaan Lagoon declined sharply again between 2004 and 2008. The 2008 survey also indicated that the proportion of filter feeders had been drastically reduced. The biomass then almost doubled between 2008 and 2009, principally owing to a marked increase in crustaceans. The abundance of macrofauna did not increase proportionately suggesting that larger-bodied crustaceans colonised the lagoon between 2008 and 2009. There were further increases in the abundance and biomass of benthic macrofauna between 2009 and 2010. The increase in the overall biomass in Langebaan Lagoon in 2010 was mainly due to increases in the biomass of polychaetes and echinoderms while the increased abundance of macrofauna was principally attributed to a marked increase in detritivorous crustaceans. The 2011 survey revealed that the abundance of small (low biomass) polychaetes had increased in the Lagoon, while the overall biomass of crustaceans had increased. In addition bivalve communities had increased both in terms of abundance and biomass. The overall biomass measured in 2010 and 2011 exceeded that measured in 1975, however the diversity of taxa has been reduced and crustaceans overwhelmingly dominate the benthic macrofauna biomass. This suggests that the Lagoon may have undergone an ecosystem shift. The 2011 survey results suggest that the Lagoon is in a relatively healthy state given the increases in biomass and abundance and relative stability of functional groups. However, similar to that seen in Saldanha Bay, there had been an increase in the abundance of small polychaetes. The results of the sediment survey in 2011 also revealed system-wide reduction in the mud content and increases in the concentrations of some trace metals. The sediment results coupled with the system wide trends seen in the benthic macrofaunal communities certainly suggest a system wide perturbation, the source or cause of which is unclear. 143 ANCHOR e n v i r o n m en t a l
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