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

State of Saldanha Bay & Langebaan Lagoon 2011
Benthic macrofauna
7.4.2 Species Diversity Indices
7.4.2.1 Spatial Analysis
Trends in species diversity (represented by the Shannon Weiner Index, H’) for Saldanha Bay and
Langebaan Lagoon in 2010 are presented in Figure 7.9. Small bay generally had the lowest species
diversity of the three areas and Langebaan Lagoon the highest. Spatial patterns of diversity were
patchy throughout the Lagoon and Big Bay. This result is consistent with previous surveys.
The diversity of species in Small Bay is lowest at the Multi-purpose Quay (SB14). This is
mostly likely due to the relatively frequent disturbances experienced at the site which would allow
for few opportunistic species to colonize the area and prevent longer living K-strategist species from
becoming established. The diversity at the remainder of sites in Small Bay was low to moderate.
The area has been subject to permanent modifications to current patterns following the
development of the causeway and ore terminal. This coupled with the range of ongoing activities in
and around Small Bay, is likely to have changed the nature of the environment such that
communities will never recover to their original pre-development state. However; the analysis of
the community composition suggests that “recovery” to a healthy, albeit modified community state,
is possible. The patchy diversity values as well as the spatial variations in community composition
are in indicative of community recovery rates which vary over spatial scales and are dependent on
both the nature of the environment (depth and exposure) and the frequency and type of
anthropogenic disturbance.
The diversity of benthic macrofauna in Big Bay was fairly low throughout, with the exception
of BB26 and BB29 which had comparatively moderate diversities. The analysis of the sediment
characteristics suggested that the central and southern areas of Big Bay were in a relatively healthy
state compared to the rest of the system. It is likely that the communities at these sites are stable
with low levels of environmental disturbance. The intermediate disturbance hypothesis suggests
that at very low frequencies of disturbance most of the community will reach and remain at a climax
state with competitive exclusion reducing the diversity to moderate levels. The sediment analysis
results form 2011 suggest that the northern areas of Big Bay along the Ore Terminal have similar
levels of contamination to that seen in Small Bay. The relatively low species diversity at these sites
suggests that this area of Big Bay has been more recently or frequently disturbed than the central
part of the Bay. Site BB30, which also showed a relatively low diversity, is a very shallow and
exposed site. The communities at this site are thus subject to ongoing disturbance and are not likely
to progress much beyond a pioneer phase with relatively low diversity values. Indeed site BB30 was
dominated by a high abundance of small deposit feeding polychaetes belong to the Spionidae family.
The diversity of benthic macrofauna recorded in 2010 in the Lagoon appeared patchy, with
relatively high levels of diversity at most sites and low levels of diversity at sites LL32 (Kraalbaai) and
LL41 (southern reaches of Lagoon). The Lagoon comprises a system of shallow sand bars and deeper
channels which are subject to strong currents and tidal activities. This ongoing natural disturbance
varies spatially and temporarily depending on sediment dynamics within the lagoon. The high
diversity levels recorded at many of the sites may be a result of intermediate levels of disturbance
which would allow for communities to pass the pioneering phase and increase in diversity but
without reaching a stable state. The areas with low diversity may be a result of high levels of
disturbance selecting for a few opportunistic species.
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