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

State of Saldanha Bay & Langebaan Lagoon 2011
Intertidal invertebrates
Some of the sites experienced also temporal fluctuations in filter feeder abundance.
Unquestionably, the two most prominent filter feeders along the southern west coast are the alien
invasive B. glandula and M. galloprovincialis. A worldwide well known coastal invader, M.
galloprovincialis has been described as the ecologically most important and numerically dominant
marine alien species along the southern African coast (Robinson et al. 2005). It was first recorded in
1979 in Saldanha Bay, and has now a distribution bridging three marine biogeographic provinces,
covering over 2000 km of coastline (Robinson et al. 2005). The rate of increase and abundance of M.
galloprovincialis is generally promoted by exposure to strong wave action (Branch et al. 2008). Along
the west coast of South Africa, M. galloprovincialis dominates the rocky intertidal at the expense of
various competitively inferior indigenous mussel and limpet species (Griffiths et al. 1992, Steffani &
Branch 2003a, b, Branch & Steffani, 2004, Robinson et al. 2007, Branch et al. 2008, 2010b). In
general, its competitive strength and impact on other elements of the fauna increases with wave
exposure (Branch et al. 2008, 2010b). In comparison with the indigenous mussels Choromytilus
meridionalis and Aulacomya ater, M. galloprovincialis has a faster growth rate, greater fecundity, and
superior tolerance to desiccation (van Erkom Schurink & Griffiths 1991, 1993, Hockey & van Erkom
Schurink 1992). This led to an upshore broadening of the width of intertidal mussel beds where this
species has invaded (Hockey & van Erkom Schurink 1992).
The time of arrival of the alien barnacle B. glandula is unknown, but it can be traced back to
at least 1992 (Laird & Griffiths 2008). Similar to Mytilus, it is assumed that is has been introduced to
South Africa in the ballast waters of ships (or attached to their hulls) that arrived in the port of
Saldanha Bay (Griffiths et al. 2011). In 2008, its range extended from Cape Point 400 km northwards
along the West Coast, but it is, at present at least, absent from the South Coast (Laird & Griffiths
2008). It is now the most common barnacle along the cool-temperate west coast (Griffiths et al.
2011). The high densities of intertidal B. glandula suggest that it has significant ecological impacts on
the local biota; for example it is thought that it allows the indigenous periwinkle A. knysnaensis to
extend its range further down the shore by providing increased habitat complexity and shelter from
waves (Griffiths et al. 2011).
Relative changes in percentage cover of the two alien invasives as well as the indigenous
ribbed mussel Aulacomya ater, depict clear spatial and temporal patterns (Figure 8.18). As expected,
both B. glandula and mussel cover is generally sparse at wave-protected shores. At Schaapen East,
however, the barnacle invaded the mid shore in 2010 and had by April 2011 doubled its spread to
cover 20% of the rock (see Figure 8.5). At semi-exposed sites, B. glandula is strongly represented in
the mid shore where it is often the most dominant species, covering for example nearly 80% of the
shore at Iron Ore Terminal. In contrast, the high and low shores of this site are almost barnacle free.
Mussels are also restricted to the mid shore. At Lynch Point both B. glandula and Mytilus are
common in the mid shore, whereby the relative dominance of one species over the other fluctuated
over the years. In the low shore, however, B. glandula is typically rare and Mytilus the dominant
filter feeder. With further increases in wave exposure, B. glandula cover in the mid shore reduces
and Mytilus is the general dominant filter feeder (e.g. Marcus Island).
The general picture thus emerges that B. glandula is most common at mid shores of semiexposed
sites, but rarer at exposed sites and low shores; a similar shore-distribution pattern as
described by Laird & Griffiths (2008). M. galloprovincialis, on the other hand, fares best at waveexposed
sites and lower down the shore (see also Branch et al. 2008, 2010b). The distribution
patterns of the two species suggest thus differences in their preferential habitats but it seems that
there are areas of overlap. For example, at the mid shore of the semi-exposed to exposed site Lynch
Point, mussel and barnacle cover fluctuated strongly, clearly showing that an increase of one taxa
resulted in the decrease of the other. In other words, it could be that at this site, where the degree
of wave action is suitable for both, the barnacle and mussel compete. Many studies of competition
on intertidal rocky shores have shown that the resource most often competed for by sessile
organisms is space and that upper and/or lower vertical distribution boundaries on the shores are
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