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

State of Saldanha Bay & Langebaan Lagoon 2011
Intertidal invertebrates
changes were recorded at Marcus Island where both percentage cover and species number steadily
increased since 2005. Evenness and diversity, however, show no consistent trend.
Temporal trends in rocky shore community patterns at the eight study sites are illustrated in
Figure 8.16. Consistent for all years is the clustering according to wave exposure, with the three
same main groups of Dive School and Jetty in Group 1, the Schaapen Island sites in Group 2, and the
semi-exposed to exposed sites in Group 3. A certain inter-annual variability within each site is also
evident, but this is more pronounced at some of the sites than at others. At Iron Ore Terminal, for
example, the replicates from 2005 and 2010 separate from those from 2008 and 2011, while 2009
samples are in between. Similar is apparent for Schaapen West and Marcus Island. The greatest
within-site variability (or patchiness) occurs at the boulder beach Jetty where the replicates per year
often disperse widely.
PERMANOVA tests, conducted for each site over the years, confirm significant differences
with regard to year (p = 0.001 for all tests). Further pair-wise testing reveals that for every site-byyear
combination tested, interannual changes in community composition are significant (note that
for the sake of brevity only combinations involving subsequent years are shown) (Table 8.1).
However, the similarities among the rocky shore communities between the tested years are very
high, especially for the last two years (from 54 to up to 70%). This suggests that for each site
temporal changes in community structure, although statistically significant, are minor.
The SIMPER test reveals which species are responsible for the observed differences in
community structure among the years. Only species contributing >5% to the dissimilarity at any
specific site are listed in Table 8.2. For brevity, only comparisons between 2010 and the current
dataset are presented here. At most of the sites, only one or two species contributed significantly
(>5%) to the differences in community structure between 2010 and 2011, and at Lynch Point and
North Bay, no single species contributed >5%. Most contributing taxa were algae, mainly ephemeral
blue-green algae (Cyanobacteria) that had decreased in abundance at all sites where they previously
were common. This is particularly evident at Schaapen West where the disappearance of blue-green
algae at the high shore contributed ~10% to the temporal dissimilarity. It is well described that bluegreen
algae can cover great areas of open high shore rocks early on in the successional process
temporarily, developing a thin ‘biofilm’ together with other microscopic algae (e.g. diatoms and
spores of macroalgae) (Robles 1982, Cubit 1984, Maneveldt et al. 2009). Ephemeral blue-green algae
may also be indicative of organic pollution (Pinedo et al. 2007). Both Schaapen Island and Marcus
Island are closed to the general public and anthropogenic nutrient input into the high shore is
unlikely, but the islands are important bird resting and breeding sites with a vast abundance of
fertilizing guano. The arrival of blue-green algae at the high shores of Schaapen East, Schaapen West
and Marcus Island were in the analysis of the 2010 survey data identified as the only noteworthy
change in community structure since 2009 (Anchor Environmental Consultants 2011), and it was
suggested that the plentiful nutrient supply from bird guano may have triggered the blue-green algae
growth if washed into the intertidal after heavy rains (Bosman & Hockey 1986, 1988).
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