State of the Bay Report 2010-Final - Anchor Environmental

is determined by the mussel. The future annual surveys will at least help in determining whether the
contrasting abundance patterns observed for the two species continuous, which would strongly
point towards competitive interaction between them.
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 harbor 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.
9.5 Summary of Results
A total of 82 species/taxa were recorded from the eight sampling stations in Saldanha Bay, of
which 58.5% were invertebrates and the rest seaweeds. The species are generally common to the
South African West Coast, and most species were also recorded in the previous surveys.
The most important factor responsible for community differences among the sites is the
exposure to wave action. There was a general trend of increasing biotic cover with increasing wave
force. Multivariate analysis identifies two distinct groups according to wave exposure: one group
consists of the very sheltered boulder sites Dive School and Jetty, as well as the two sheltered
Schaapen Island sites, whereas all other more exposed shores are contained in a second group.
Sheltered sites were typically dominated by seaweeds and grazers, whereas more exposed sites are
characterized by filter-feeders.
A secondary factor structuring the communities is shore topography. At a higher similarity
level, the boulder shores separate from the flattish rocky platforms. Four groups thus emerge, i) the
two very sheltered Small Bay boulder beaches Dive School and Jetty, ii) the two Schaapen Island
sites, iii) the semi-exposed boulder shore Iron Ore Jetty, and iv) the semi-exposed to exposed shores
at Lynch Point, North Bay and Marcus Island.
Differences between the sheltered sites in Small Bay (e.g. Dive School and Jetty) and the two
sites on Schaapen Island may also be linked the geographic locations of the sites in the overall
Saldanha Bay system with the Schaapen Island sites belonging to a transitional zone between the Bay
and the Lagoon, and the nutrient input through seabird guano that favors algal growth on Schaapen
Island.
Temporal comparison of rocky shore communities shows that the grouping of the sites
according to wave exposure and, at a higher similarity level to topography, is consistent throughout
the years. There is, however, also a certain grouping according to years, which is more evident at
some sites than at others. Generally, these inter-annual differences were minor and mostly related
to temporal changes in ephemeral algae cover. Particularly the two Schaapen Island sites as well as
Marcus Island had greatly increased cover of blue-green algae in the high shore, which might be
linked to nutrient input from terrestrial seabird guano. Ephemeral algae typically show strong
temporal variation in their abundances and dense populations are therefore only temporarily.
Changes in ephemeral algae cover over the years are thus likely to be a natural seasonal and interannual
phenomenon, and there is no reason to assume anthropogenic influences.
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 216