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

State of Saldanha Bay & Langebaan Lagoon 2011
Intertidal invertebrates
Mobile animals in the low shore included O. tigrina, which was common at sheltered to semiexposed
sites and Parvulastra exigua, frequently encountered in rock depressions and pools. C.
granatina and Burnupena spp. occurred at all low shores at relatively equal densities, whereas S.
cochlear was restricted to wave swept shores where it lives in patches of dense aggregations.
It is understandable that the vertical gradient of emersion up the shore creates a stress gradient that
has important ecological effects, creating the clear zonation patterns observed on rocky intertidal
shores. Among shores, however, the structure of biotic communities is also affected by a horizontal
gradient of exposure to wave action, from sheltered bays to exposed headlands. Viewing the
distribution of the various functional groups shows obvious differences among the shores with
regard to exposure (Figure 8.12). Very sheltered shores had generally low biotic cover consisting
primarily of grazers and trappers (i.e. the limpet Cymbula granatina), with minor cover of sessile
filter feeders and encrusting algae. The sheltered Schaapen Islands sites were dominated by algae
(encrusting and foliose algae) but with further increase in wave force, filter feeders were clearly the
most important group. At Marcus Island, ephemeral algae were also abundant.
Figure 8.12. Contribution of the functional groups to the biotic cover (%) across the whole rocky shore at
the eight study sites (sorted from left to right according to increasing wave exposure).
Multivariate analysis (i.e. cluster analysis and multi-dimensional scaling) finally confirms the
clear separation of the rocky shores with regard to wave exposure (Figure 8.13). At a 50% similarity
level the sites group into three major groups: Group 1 contains the very sheltered shores Dive School
and Jetty, Group 2 consists of the two sheltered Schaapen Island sites, whereas all other more
exposed sites fall into Group 3. At a higher similarity level of 60%, most of the sites within the groups
separate from each other, displaying a great within-site similarity. Only the three exposed sites
Lynch Point, North Bay and Marcus Island still cluster together (Group 3A), signifying that the
communities at these shores are relatively similar, while the steep Iron Ore Terminal shores splits off.
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