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

State of Saldanha Bay & Langebaan Lagoon 2011
Intertidal invertebrates
partly due to the relative ability of the species to compete for space (see review by Menge & Branch
2001). The varying interannual success in competing for space might in turn be related to varying
success in larval development, settlement and/or recruitment. For most benthic marine organisms,
fluctuations in the arrival of broadly dispersing pelagic larvae are among the most important factors
driving population dynamics (Roughgarden et al. 1988, Menge et al. 1997, Menge & Branch 2001).
Because dispersal and supply of larvae to suitable settlement habitats are highly dependent upon
coastal water movements during larval development, large variability in recruitment can occur over
various spatial and temporal scales and may be greatly influenced by the effects of topography and
season on oceanographic processes. A recent study on settlement and recruitment dynamics of
mussels and barnacles (mostly M. galloprovincialis and B. glandula by virtue of their dominance) in
the southern Benguela upwelling region (Pfaff et al. 2011), found that recruitment of both mussels
and barnacles was strongly seasonal, with peaks in austral summer (November to January) and spring
(August to October), respectively. There was further a strong spatial variation, which was on a
regional scale related to differences in upwelling strength (upwelling centre at headlands versus
downstream bays), and on a local scale due to differences of wave exposure, whereby recruitment
rates were consistently higher in wave-exposed than in protected habitats. Inter-annual variability in
recruitment intensity at a particular site was for both taxa, however, only moderate but still
observable, and may thus result in temporal variability of adult populations. Without more research
and particularly experimental work, however, it cannot be ascertained whether there is indeed
competitive interaction between Mytilus and Balanus, and whether the barnacle’s zonation pattern
is determined by the mussel.
The only indigenous filter feeder of any importance at the study sites was the ribbed mussel
Aulacomya ater. Present only with very low cover at the low shores of most shores, A. ater had at
the low shore of Marcus Island increased in abundance from 2005 to 2009, almost disappeared in
2010 only to return again in 2011 with an average of 17% cover. An earlier study by Robinson and
co-workers (2007) investigated the impacts and implications of the invasion of the intertidal zone at
Marcus Island by Mytilus. A single data set taken in 1980 prior to the invasion was compared to a
survey conducted in 2001, using the same technique as the original sampling. Before the invasion,
dense stands of mussels, primarily Aulacomya ater, were restricted to the low shore, whereas scarce
cover of Choromytilus meridionalis was recorded in the mid and low shore. In 2001, Mytilus had
heavily invaded all zones except the very high shore, and replaced the indigenous mussels in the low
shore. The mid shore, previously a patchy environment, comprising mainly bare rock interspersed
with patches of algae and large limpets, was transformed to a less patchy but structurally more
complex mussel matrix with increased invertebrate densities and species richness. The authors
concluded that the invasion had its greatest impact in the mid-to-low shore, and is clearly displacing
A. ater from the rock surface. Experimental manipulations conducted on the West Coast of South
Africa confirm a negative impact of Mytilus presence on A. ater abundance (Branch et al. 2010b).
Although a direct comparison between the 2001 survey at Marcus Island and the current surveys is
not possible (Robinson et al. 2007 reported density not percentage cover) it seems likely that up until
2008, Mytilus cover had even further increased and A. ater reduced. The strong decline of Mytilus
cover in 2009 may has temporarily released the local mussel from the competitive pressure, but with
the return of the alien mussels, it all but disappeared again. In 2011, however, Mytilus cover had
again drastically declined while A. ater gained in cover. Such short cycles in relative dominance
especially for the relatively slower growing indigenous mussel (van Erkom Schurink & Griffiths 1993)
is somewhat surprising. When undistributed, the Mytilus matrix at Marcus Island’s low shore is very
dense and multilayered (Robinson et al. 2007, pers. obs.). Surveying of the shore is done by nondestructive
methods (see Method section) and any biota hidden in the deepest layer of the tight
matrix cannot be seen without removing the top layer. A. ater can often been found burrowed in the
Mytilus matrix (Griffiths et al. 1992, Steffani & Branch 2003b), and it is thus possible that deep in the
lowest depth of the mussel bed, A. ater is always present, but is only exposed when the top Mytilus
layer is removed by, for example, storm waves that often impact the exposed shore of Marcus Island.
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