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Ecology

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Intertidal zonation

Most studies of intertidal seaweed communities on the south coast are general descriptions that focus on zonation patterns. Zonation schemes for seaweeds and animals on south coast rocky shores (Brown & Jarman 1978; Branch & Branch 1981) are based on the pioneering studies of Stephenson (1944) and Stephenson & Stephenson (1972).

Briefly, a typical south coast rocky shore shows a number of fairly distinct zones (see for example, Branch et al. 2010). However, the vertical height and composition of these zones is strongly influenced by a number of factors, especially wave exposure and the presence of sand. The following account is based in the middle of the south coast, in the Port Elizabeth area, and the component species change towards the eastern and western extremes of this coast.

The supralittoral (essentially the spray zone during high tides) is characterized by numerous small Littorinid snails and occasional patches of (usually pale, yellowish) Porphyra, with mats of Bostrychia in crevices and overhangs. Below this the upper eulittoral is characterized by barnacles, limpets, the snail Oxystele and low, scattered patches of various seaweeds including Ulva, Caulacanthus, Splachnidium and sometimes turf-like patches of very small Gelidium pristoides. The mid-eulittoral is often distinguished by a high cover of encrusting coralline algae (especially in hollows), the snail Oxystele tigrina, various limpets (especially Scutellastra longicosta and S. oculus) and sometimes extensive patches of the reef-worm Pomatoleios. A number of seaweeds are found, usually in mixed, low turfs, but Gelidium pristoides may be common to dominant, growing attached to rock, limpet shells, and Pomatoleios tubes. On wave-exposed shores the lower eulittoral is usually dominated by Scutellastra cochlear, with a high cover of crustose corallines, and upright seaweeds limited to crevices ( e.g. G. pristoides, Laurencia flexuosa, Hypnea spicifera). At very exposed sites, or those influenced by sand, this zone is instead often dominated by beds of mussels. The sublittoral fringe may bear a high cover of encrusting corallines and extensive beds of seaweeds, especially species such as Plocamium cornutum, Plocamium corallorhiza, Laurencia flexuosa, Hypnea spicifera, and notably, prominent beds of mixed articulated corallines (Amphiroa, Arthrocardia, Jania). This zone can also bear beds of Pyura, which largely exclude seaweeds.

A general scheme such as this can be greatly modified by moderate sand influence, which excludes many of the animals and tends to produce a lower eulittoral zone/sublittoral fringe completely dominated by a low compact turf of articulated coralline algae that covers either bare rock or a thin layer of encrusting coralline. On wave-sheltered shores zonation is vertically compressed, and some zones (e.g. that of Scutellastra cochlear) may be absent or insignificant.

The sublittoral zone

There are only two studies of subtidal seaweed communities on the south coast: that of Anderson & Stegenga (1989) at Bird Island, near Port Elizabeth, and an unpublished MSc study of shallow subtidal communities at eleven sites between Stilbaai and Mzamba (Wilby 2009).

The shallow sublittoral zone of the south coast (0-3m) is dominated by a mixed community in which more than 50% of the wet biomass is contributed by articulated coralline algae of the genera Amphiroa, Arthocardia and Jania. Significantly, most of the larger fleshy algae belong to genera known for the production of grazer-deterrent chemicals (e.g. Caulerpa, Laurencia and Plocamium). The implication is that grazing has a large effect in structuring these communities, and an investigation of the roles of different suites of herbivores (especially fish) is long overdue.

Diving observations and the study of Anderson & Stegenga (1989) at Bird Island show that below 2-3 m depth, algal biomass generally declines with depth, as might be expected on a coast where the water is often fairly turbid. In our experience the deepest reasonably substantial stands of upright seaweeds are found around the 15-20m mark, and it is only on offshore seamounts and reefs, where the water is usually clear, that substantial beds may be found at even 40-50m depth. Generally, as one dives deeper than a few m below MLWS, algae are progressively replaced by filter feeders (e.g. sponges, ascidians, etc.).

References

Anderson, R.J. & H. Stegenga (1989). Subtidal algal communities at Bird Island, Eastern Cape, South Africa. Botanica Marina 32: 299-311.

Branch, G.M & Branch, M. 1981. The Living Shores of Southern Africa. Cape Town: C. Struik, 272 pp.

Branch, G. M., Griffiths, C. L., Branch, M.L. & L.E. Beckley. 2010. Two Oceans. A Guide to the Marine Life of Southern Africa. Struik Nature, Cape Town, 456 pp.

Brown, A.C. & N.G. Jarman. 1978. Coastal marine habitats. In: Werger, M.J.A. (ed.) Biogeography and Ecology of Southern Africa. Monographs in Biology 31: 1239-1277.

Stephenson, T.A. 1948. The constitution of the intertidal fauna and flora of South Africa, Part 3. Annals of the Natal Museum, 11(2), 207- 324 + 2 plates.

Stephenson, T.A. & Stephenson, A. 1972. Life between tidemarks on Rocky Shores. San Francisco: W.H. Freeman, 425 pp.

Wilby, D. 2009. Shallow Subtidal Seaweed Communities of the Agulhas Marine Province of South Africa. MSc Thesis, Botany Department, University of Cape Town, 97pp.

Citing this publication:

Anderson RJ, Stegenga H, Bolton JJ. 2016. Seaweeds of the South African South Coast.
World Wide Web electronic publication, University of Cape Town, http://southafrseaweeds.uct.ac.za; Accessed on 15 November 2018