CORDIO Status Report 2000

The status of South African
coral reefs
MICHAEL H. SCHLEYER and LOUIS CELLIERS
Oceanographic Research Institute, P.O. Box 10712, Marine Parade 4056, Durban
The major coral-inhabited reefs in KwaZulu-Natal occur
adjacent to the coast between 26 o 50' S and 27 o 55' S.
They are thus some of the southernmost coral reefs in
the world but are not typical of coral reefs resulting
from biogenic accretion. Corals in South Africa grow
rather as a veneer on late Pleistocene sandstone, which
originates from submerged coastal sand dunes. The
reefs run parallel to the coastline and are confined to the
narrow continental shelf, which is between 2 km and 7
km wide in their vicinity. They can be conveniently
grouped into a northern, central and southern complex,
these being found respectively at Kosi Bay, between
Sodwana Bay and Lake Sibaya, and north of Lake St
Lucia.
The reefs range in depth from 8 m to just over 35 m,
with only a few peaks approaching the surface. The
coastline is straight and exposed and, as the prevailing
northeasterly and southerly to southwesterly winds
blow parallel to the coast, they give rise to substantial
swells. The warm Agulhas Current, which has a mean
peak velocity of 1.4 m•s -1 , generates the sub-tropical conditions
in the area. The mean seasonal sea-surface temperatures
range between 22 o C in winter to 26 o C in summer
(SADCO data, 1960-1995), with the salinity varying
between 35.0‰ and 35.5‰. Recent temperature records
at a fixed station on one of the reefs are presented in Fig.
1. These manifest an upward trend that will be discussed
later. The maximum tidal flux is 2 m during
spring tides, dropping to 1 m during neap tides.
A consequence of the conditions described above is
Temp (°C)
Figure 1. Mean, minimum and maximum sea temperatures at a fixed
long-term monitoring site on the central reef complex at Sodwana
Bay.
substantial water movement on the reefs, both in terms
of a current, usually north to south (Schleyer, unpub.
data), and considerable surge from the swell.
The reefs are all basically the same in structure.
They lack the well-defined zonation of true coral reefs
and conform to the topography of the base substratum.
They thus tend to be flat with relatively few features,
comprising low pinnacles and shallow drop-offs and
gullies. In community structure, they can be separated
into reef top and gully communities. The former are
dominated by soft corals while hard corals attain a
greater abundance in the latter.
Corals are the dominant life form on the reefs and
43 scleractinian (hard coral) genera have been found in
the area as well as one member of the fire coral genus
Millepora. Among the alcyonacean soft corals, 11 genera
have been found, with the family Nephtheidae, com-
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