State of the Bay Report 2010-Final - Anchor Environmental

and percentage organic nitrogen (PON) between 2008 and 2009 in Big Bay, and the concentrations
of mud and heavy metals were very low within Big Bay. These results together with the lack of
obvious anthropogenic discharges into Big Bay suggest that the changes in species composition
within the Bay are most likely a result of natural perturbations. The sites at which the numbers of
polychaetes increased the most were also the sites where the sensitive sea-pen were found. It is
thus unlikely that the sudden appearance of polychaetes is due to an increase in pollution. The
average W statistic calculated for Big Bay in 2010 was higher than that of 2009. This may be due to
the reduction in the densities of polychaetes which suggests that pollution levels had reduced
between 2009 and 2010. However, this does not correlate with physical data recorded in Big Bay,
which suggests that these fluctuations may be a consequence of natural perturbations and
community interactions.
The W statistics calculated for each of the sites in Big Bay in 1999, 2004, 2008, 2009 and
2010 were compared in Table 8.4. The W statistic at three of the seven sites in Big Bay has reduced
since these sites were first surveyed (in 1999 or 2008). All three sites were positioned near the ore
jetty at the northern end of Big Bay. The disturbance level, as indicated by the W statistic, at these
sites is moderate and it is most likely that the source of the disturbance is primarily anthropogenic.
The W statistic increased at six of the seven sites in Big Bay between 2009 and 2010 indicating that
the proportion of large, long-lived species in the community had increased. This suggests that
disturbance has been reduced and that the benthic community is recovering at these sites.
Average W statistic
Average W statistic
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Year; LS Means
Current effect: F(4, 53)=3.8349, p=.00825
Effective hypothesis decomposition
Vertical bars denote 0.95 confidence intervals
Include condition: V3="SB"$
1999
Year; LS Means
Current effect: F(3, 24)=.33708, p=.79866
Effective 2004hypothesis 2008 decomposition 2009 2010
Vertical bars denote 0.95 confidence intervals
Include condition: Year V3="LL"$
2004 2008 2009 2010
Year
Small Bay
Langebaan Lagoon
Year; LS Means
Current effect: F(4, 34)=1.6740, p=.17873
Effective hypothesis decomposition
Vertical bars denote 0.95 confidence intervals
Include condition: V3="BB"$
1999 2004 2008 2009 2010
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 175
Average W statistic
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
Year
Big Bay
Figure 8.13. Mean W statistic (± 0.95 confidence intervals) for all years sampled in Small Bay, Big Bay and
Langebaan Lagoon (W = 1 indicates undisturbed, W = 0 indicates moderately disturbed, W = -1
indicates grossly disturbed)