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

State of Saldanha Bay & Langebaan Lagoon 2011
Intertidal invertebrates
8.3 RESULTS AND DISCUSSION
8.3.1 Species Diversity and Zonation
The survey of the eight rocky shores yielded a total of 84 species/taxa, of which 50 taxa were
invertebrates (59.5%) and 34 (40.5%) algae. The faunal component was represented by 16
species of grazers, 3 trappers, 7 predators and scavengers, 6 anemones, and 18 filter-feeders. The
algal component comprised 22 corticated (foliose) seaweeds, 6 ephemerals, 1 kelp, 4 crustose (or
encrusting) corallines and 1 articulated coralline (it has to be pointed out that this is a gross
underestimation of coralline taxa as most species are not identifiable in the field and are thus
lumped into larger groups).
The overall taxa count has remained relatively constant over the years with most taxa
having also been recorded during one or more of the previous monitoring years (Anchor
Environmental Consultants 2006, 2009, 2010, 2011). Furthermore many of the species are also
listed by other studies conducted in the Saldanha Bay area (e.g. Simons 1977, Schils et al. 2001,
Robinson et al. 2007). The species are generally common to the South African West Coast (e.g.
Day 1974, Branch et al. 2010a), including the two alien invasive species, the Mediterranean
mussel Mytilus galloprovincialis and the acorn barnacle, Balanus glandula. The former was
introduced from Europe sometime in the 1970’s, but is now the dominant west coast mussel,
forming a dense mid- to low shore band in wave-exposed areas (Hockey & van Erkom Schurink
1992). The presence of B. glandula, originating from the Pacific coast of North America, has only
been recognized more recently (Simon-Blecher et al. 2008), but it seems that the species has been
in South Africa since at least the early 1990s and it is now the most abundant intertidal barnacle
along the southern west coast (Laird & Griffiths 2008). The alien’s presence was overlooked for
many years as it was mistaken for the indigenous species Chthamalus dentatus. Apparently as a
result of the invasion by B. glandula, the formerly abundant C. dentatus is now very rare on South
African west coast shores (Laird & Griffiths 2008). At the Saldanha Bay monitoring study sites, the
alien barnacle was first confidently identified in 2008. It is, however, assumed that it had been
present during the baseline survey in 2005 but was confused with the indigenous barnacle.
Consequently, in all analyses involving the 2005 dataset, C. dentatus abundances are converted to
B. glandula.
The composition and distribution of the rocky intertidal biota is strongly influenced by the
prevailing wave exposure at a shore as well as substratum topography. Within a site, however,
shore height is the critical factor as the interface between air and water along with the action of
tides and waves result in a vertical emersion gradient of increasing exposure to air from low shore
to high shore. Clear, well studied, patterns of zonation of flora and fauna thus exist on rocky
shores (Stephenson & Stephenson 1972). The effects of wave action are generally attenuated upshore
and superseded by the uniformly severe desiccation stress experienced high on the shore.
Consequently the high shores were relatively similar among the sites being mostly barren with
few species. At the very sheltered boulder shores Dive School and Jetty, considerable amounts of
sand and gravel had also accumulated amongst the boulders. Typical high shore species,
particularly at the sheltered sites, included the winkle Oxystele variegata and towards the more
exposed sites the small periwinkle Afrolittorina knysnaensis with average densities often
exceeding 100 individuals per 0.5 m 2 (Figure 8.4). The alien B. glandula occurred at almost all high
shores but with very low cover (on average 95% at most high
shores. The exceptions were Schaapen East, which had occasional patches of blue-green algae,
and Marcus Island were a dense low carpet of the ephemeral algae Ulva spp. with occasional tufts
of another ephemeral, Porphyra capensis, covered >70% of the high shore (Figure 8.4).
Ephemerals are opportunistic algae that have short life cycles and are usually the first settlers on
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