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

State of Saldanha Bay & Langebaan Lagoon 2011
Intertidal invertebrates
settlement and persistence of benthic organisms, as planktonic larvae are more likely to be retained
on rough surfaces while water movement may wash them off smooth surfaces (Eckman 1990,
Guarnieri et al. 2009). This might explain the low sessile cover found on the very smooth rocks at
North Bay (see Figure 8.12).
Boulder shores also contain greater microhabitat diversity (e.g. upper and lower side of the
boulders) than rocky platforms. Where boulders are large, the tops of these boulders stay immersed
for a significantly longer period than smaller boulders (or flat platforms), with each single boulder
essentially having its own shore height zonation. During low tide, the top layer of boulders provides
the lower layers with shade, thus maintaining lower temperatures and higher moisture content
(Takada 1999). Layers of boulders increase the surface area for attachment of organisms, but may
reduce water movement thus accumulating detritus, which can lead to low oxygen conditions. Large
boulders have been shown to considerably reduce the water flow velocity with invertebrate biomass
decreasing significantly downstream of boulders (Guichard & Bourget 1998). Smaller boulders, on
the other hand, may be unstable as they can turn over in heavy weather, and have often been found
to have a more impoverished community than larger rocks (McGuinness 1987, Londoño-Cruz &
Tokeshi 2007, McClintock et al. 2007). Boulder fields are thus are typically found to differ in their
species assemblages to flatter shores (e.g. Sousa 1979, McGuinness 1984, McQuaid et al. 1985,
McGuiness & Underwood 1986, Takada 1999, Cruz-Motta et al. 2003, Davidson et al. 2004, Hir & Hily
2005).
While shore topography is a likely factor controlling the difference in community structure
between Dive School and Jetty, and the rocky shores on Schaapen Island, it may also be related to
the fact that Schaapen Island lies in the transition zone between Saldanha Bay and Langebaan
Lagoon. The water in the Lagoon is generally warmer with also slightly higher salinities compared to
the Bay. This in turn translates into differences in their biological communities (Day 1959, Robinson
et al. 2007). For example, there is a distinct separation in algal composition between communities
from the Bay and the Lagoon, as the latter harbours a considerable number of South Coast seaweeds
due to its warmer waters (Schils et al. 2001). Perlemoenpunt, located less than 1 km from Schaapen
Island on the western site of the entrance to Langebaan Lagoon is described as the transition area
between the Bay and the Lagoon, but with a marked Lagoon affinity (i.e. high similarity with the
Lagoon sites) in its overall algal composition. Differences in community composition between the
Bay and the Lagoon are also described for zooplankton, and rocky and sandy substrate assemblages
(Day 1959, Grindley 1977, Anchor Environmental Consultants 2006, 2009, 2010, 2011).
8.3.3 Temporal Analysis
Temporal variation in biotic cover, species number, evenness, and species diversity at the
eight rocky shores from 2005 to 2011 are depicted in Figure 8.14 and Figure 8.15. Cover and
population indices at the very sheltered site Dive School varied only slightly, while at the second
sheltered boulder beach Jetty there was a general increase in all population measures until 2010,
decreasing again in 2011. Schaapen East displayed little variation in biotic cover but species number,
evenness and diversity increased peaking in 2009, although reducing thereafter. Schaapen West, in
contrast, experienced a drastic increase in percentage cover until 2010 due to abundant growth of
ephemeral and blue-green algae, but these had almost vanished in 2011. This temporal dominance
of ephemerals has probably led to the considerable decline in evenness and diversity observed from
2005 to 2008, which are since then on the increase again. Biotic cover at Iron Ore Terminal, on the
other, hand changed little with time, but species number and more pronounced evenness and
diversity had a noticeable peak in 2010. Intertidal communities at Lynch Point and North Bay
displayed minor temporal fluctuations in cover and species number, whereas evenness and diversity
at Lynch Point peaked in 2009 and show a decreasing trend at North Bay. The most prominent
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