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

State of Saldanha Bay & Langebaan Lagoon 2011
Benthic macrofauna
mussel M. galloprovincialis began establishing dense intertidal beds on two intertidal sand flats close
to the mouth of Langebaan Lagoon (Hanekom and Nel 2002). The mussel beds reached an
estimated biomass of close to eight tons in 1999 raising concerns that the invasion could spread to
the rest of the lagoon and other sandy substrata (Hanekom and Nel 2002). A comparative study
between invaded and non-invaded areas showed a replacement of sandbank species communities
by those typically found in rocky shores where the mussel provided the hard substratum suitable for
their settlement (Robinson and Griffiths 2002). In early 2001, however, the mussels had started to
die off and by mid-2001 only dead shells and anoxic sands remained. The precise causes of the die
off have not been established but siltation and lowered food availability are suggested as possible
reasons behind the declines (Hanekom and Nel 2002). In an effort to prevent the re-settlement of
the mussel South African National Parks began to remove dead mussel shells in late 2001 (Robinson
et al. 2007b). A study looking at the ecological impacts of the invasion and subsequent clearing of
the dead shells was done comparing pristine non-invaded areas, invaded areas that had living
mussel beds, un-cleared areas with no living mussels but a thick remnant mussel shell layer, and
areas cleared of dead mussels (Robinson et al. 2007). The study found that community composition
differed significantly between non-invaded and invaded areas where mussel created a multilayered
complex habitat promoting the colonization of rocky-shore species. This significantly increased
biomass but not species diversity, reflecting a replacement of the natural sandy ecosystem for a
typical rocky-shore system (Robinson et al. 2007). After the die-off and subsequent clearing of the
dead shell remains, some recovery was already evident between non-invaded and cleared areas
after only 5 months. Although no significant differences were found between non-invaded and
cleared areas, the absence of more than 50% of the species from the cleared areas shows that total
recovery had still not been attained. The mussel invasion thus dramatically altered natural
community composition which remained different from non-invaded areas even 5 months after the
clearing, when the study ended. Fortunately this invasion was short lived.
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. These results were possibly linked to the dredging that took place at
the northern end of lagoon as part of the beach erosion mitigation. 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 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
dominated 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.
The average diversity of macrofauna in Langebaan Lagoon was relatively low in 2004, but
increased between 2004 and 2008 and 2008 and 2009 such that the lagoon supported a moderate
level of diversity. The 2010 survey revealed that there had been a very slight decrease in the
diversity of macrofauna. By 2011 the average diversity of benthic macrofaunal communities in the
Lagoon had increased to a fairly high level of diversity. The area is subject to natural disturbance
(strong currents and tidal variation) and under such conditions the diversity of benthic communities
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