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

State of Saldanha Bay & Langebaan Lagoon 2011
Intertidal invertebrates
This would explain why firstly Mytilus cover reduced, i.e. removed by wave action, and secondly A.
ater cover is recorded, i.e. only now visible and recordable with non-destructive methods.
Invasive alien species have been identified as one of the major threats to the maintenance of
biodiversity in the marine environment (Carlton & Geller 1993, Carlton 1999, Ruiz et al. 1999, IUCN
2009), particularly in the context of global climate change (Occhipinti-Ambrogi 2007, Occhipinti-
Ambrogi & Galil 2010). To date, 22 confirmed extant marine aliens, plus 18 cryptogenic species, have
been recorded from South African waters, with one additional species found in on-land mariculture
facilities (Griffiths et al. 2009). The true number of introduced species may well exceed these
estimates by several times. The major means of introduction is international shipping, i.e. via ballast
water and as attachment to the hulls of ships, followed by aquaculture (Galil et al. 2008). Saldanha
Bay is a deepwater harbour receiving vessels from all over the world and it thus likely that one of the
greatest perils to the intertidal (and in fact all other) communities in Saldanha Bay is the introduction
of alien species, and their potential to become invasive.
8.4 SUMMARY OF FINDINGS
A total of 84 taxa were recorded from the eight study sites, most of which had also been found in the
previous survey years. 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. The species are generally common to the South African West Coast and many
are listed by other studies conducted in the Saldanha Bay area including the two alien invasive
species, the Mediterranean mussel Mytilus galloprovincialis and the North American acorn barnacle
Balanus glandula.
Within a site, the vertical emersion gradient of increasing exposure to air leads to a clear
zonation of flora and fauna from low shore to high shore. Differences among the rocky shores,
however, are strongly influenced by the prevailing wave exposure at a shore as well as substratum
topography. Very sheltered shores had generally low biotic cover consisting primarily of grazers and
trappers, with minor cover of sessile filter feeders and encrusting algae. With increasing wave force,
filter feeders were clearly the most important group. The two very sheltered boulder beaches in
Small Bay separate from the flat Schaapen Island sites, which may also be related to geographic
location as Schaapen Island lies in a transitional zone between the Bay and the Lagoon, and to the
nutrient input through seabird guano that favours algal growth on Schaapen Island. Similarly, the
steep boulder beach Iron Ore Terminal separates from the other more flattish semi-exposed to
exposed sites.
From the temporal pattern displayed by the rocky shore communities, it is evident that at
none of the sites there is a directional change in community composition that would indicate a
persistent change, such as for example the arrival or loss of a species. Rather the communities show
temporal fluctuations, reflecting the temporary dominance of short-lived ephemeral species and/or
interannual variation in larval supply or recruitment success. In general, rocky shore communities
were relatively stable with only minor changes over the years.
The two most important filter feeders, being also the characteristic species at most shores
and zones, are the aliens M. galloprovincialis and B. glandula. The latter is most abundant at mid
shores of semi-exposed sites, but rarer at exposed sites and low shores. M. galloprovincialis, on the
other hand, fares best at wave-exposed sites and lower down the shore. It is likely that one of the
greatest threats to rocky shore communities in Saldanha Bay is the introduction of alien species via
shipping, and their potential to become invasive.
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