Interaction between coastal
processes and lagoonal fauna,
between Walvis Bay and
Liideritzbucht, South West Africa
by
Brian Kensley
Abstract 55
Introduction 55
2 Description of the area. . . . . . . . . . . . . . . . . 55
3 Faunal observations 56
4 Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5 Acknowledgements 60
6 References. . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Aspects of the faunal composition and ecology of a
recently dried-up lagoon in the Conception Bay area
are discussed. The reasons for the demise of the lagoon
are speculated on. and related to similar changes in
coastal structures at several other localities on the
South West African coast.
The relatively uninterrupted coastline of South West
Africa otTers few areas for marine organisms seeking a
sheltered habitat. Where the few sheltered areas do
occur. e.g. Liideritzbucht and Sandvis. a fairly rich
estuarine or lagoonal fauna becomes established. This
fauna is characterised. as with the intertidal rocky
shore fauna. by large populations of a relatively small
number of species. Much of the South West African
coastline is formed by the shifting sands of the Namib
Desert. and the unstable nature of this coastline is
well illustrated at Sandvis. British Admiralty charts
over a period of about 90 years show a progressive
silting up. from what was once a deep water anchorage
for sailing vessels to the present shallow lagoon. with
the position of the sea mouth visibly changing over?
a three-year period of observation (1973 - 1976). Yet
another and more dramatic sign of this changing nature
of the coastline was seen recently at the Conception
Bay area (24.00S.? 14.30E.) south of Sandvis. A con-
siderable salt pan area was observed between the sea
and the dunes. On examination. this pan revealed vast
numbers of dead mollusc shells. representing what was
at an earlier stage a sheltered lagoonal area similar
to Sandvis. Many of the molluscs. and especially the
bivalves had died in their life positions. and this led
to speculation on the faunal constitution of this 'dead'
lagoon. and on the causes for its demise.
The area in which the shells were observed stretches
south-east from the slight embayment of Conception
Bay. At the northern end is a small true lagoonal
area. apparently receiving sea water only at spring
tides. This lagoon is not more than 20 - 30 cm deep
and apart from some polychaete worms. seems to be
uninhabited by macro-organisms. To the north-east the
area is bounded by deflation plains. while to the south
and south-east the pan is covered by a broad peninsula
of continuous dunes. the northern edge of which is
broken up into small barchan dunes and tongues of
sand. The western edge of the area is the beach with
a fairly steep slope which etTectively prevents the
high-tide water from spilling onto the plain of the salt
pan.
Mollusc shells were found over a vast area of the pan.
Different parts of the pan appeared to vary in species
composition and thorough collecting of these areas was
carried out. The identification of the molluscs allowed
a certain degree of speculation on the ecological con-
ditions ruling when the animals were alive. Five roughly
defined areas could be distinguished, which merge
almost imperceptibly with one another.
3.1 The eastern edge of the pan. This is a relatively
narrow area bounded by a one-metre high bank, which
probably represents the remains of a storm beach,
containing as it does agate, jasper, and yellow
chalcedony bearing - quartz, gravel and sand. At the
base of this bank, shells of Marginella capensis,
Nassarius plicatellus and Siphonaria capensis were
abundant, while shells of Crepidula procellana, Turri-
tella capensis and Petricola bicolor were fairly com-
mon. N. plicatellus, T. capensis and M. capensis
inhabit sandy or muddy substrates, while C. porcellana
and S. capensis have a limpet-like habit on rocks.
P. bicolor is a bivalve which bores into soft rocks or
is sometimes found in tightly packed clumps of mytilid
bivalves attached to rocks. This mixture of rock and
sand dwellers may indicate deposition by limited wave
action at the old shore line of a shallow open bay
before the formation of the lagoon. Similarly, the
southern end of the pan, where not covered by dune
sand, would also seem to be an old shore line, with
vast numbers of shells representing a mixture of
habitats. These include Calyptraea trochiformis, Argo-
buccinum argus, Thais cingulata, Fissurella mutabilis,
and Ostrea sp. (all rock dwellers), Bullia digitalis,
Donax serra, Lima rotundata and Mactra glabrata
(usually sandy intertidal-subtidal exposed beach dwel-
lers). Along with these were the shells of characteristi-
cally lagoonal species including Dosinia lupinus, Tapes
corrugata, Bullia laevissima, and Turritella capensis.
3.2 By far the largest area of the pan is dominated
by the bivalve Lutraria lutraria. This mollusc is a
sand or mud dweller which at Sandvis is found at a
depth of about 20 cm below the substrate surface,
in areas which are only exposed at the low tides of
springs. At the Conception salt pan, large areas are
covered with pairs of valves of Lutraria, the valves
of each pair usually touching, and positioned vertically
with the anterior ends held in place in the sand by
a hard crust of salt. (Plate 1). Assuming the species
to live in about 20 cm of sand, and the shells now
being half exposed, about 25 - 30 cm of substrate
has been removed over large areas of the pan, probably
by wind action. (A graphic account of the action of
strong wind in the Conception Bay area is given by
Harger (1914) who comments on the denudation of
the South African subcontinent in general terms.) The
abrasive action of wind-driven sand can be seen on
several of the Lutraria shells which have been worn
away. Lying amongst the vertical pairs of Lutraria
shells (which are seldom denser than 1 or 2/ m2)
are large numbers of other mollusc shells. These include
the sand or mud dwelling bivalves Dosinia lupinus,
Gastrana matadoa, Leporimetis hanleyi and Tapes
corrugata, (Plate 2) all of which are abundant, and
sometimes found with both valves together. These four
bivalve species burrow into the substrate to a shallower
depth than Lutraria and are thus now lying loose
on the pan surface. Shells of the gasteropods Margi-
nella capensis, Turritella capensis, Nassarius plica-
tellus, and Bullia laevissima are also common in the
Lutraria-dominated area. All four are mud-dwellers
and are usually found on the surface or in the upper
three centimetres of the substrate, usually in subtidal
areas.
3.3 More limited areas than the Lutraria flats are
dominated by the shells of Dosinia lupinus often
exposed in pairs in the life position. In these areas
Lutraria is absent. This may be accounted for by
reference to substrate tolerance or preference. It was
noticed at Sandvis that Dosinia was found in sedi-
ments ranging from coarse sand to very fine mud,
while Lutraria tended to be confined to coarse sedi-
ments. In the Dosinia-dominated areas, Tapes corru-
gata, Leporimetis hanleyi and Bullia laevissima shells
occur but are never common.
3.4 In a few isolated areas, the filled-in remains of
drainage channels could be seen, marked by pairs of
Perna perna valves which are locally abundant.
(Plate 3). These are usually the very large shells
characteristic of a sheltered habitat. Populations, living
in an exactly similar situation, have been observed
at Sandvis. Amongst the Perna shells (which in life
are anchored in the sand by the densely matted byssus
threads) the shells of Natica vittata were common.
Many of these still had the calcareous operculum in
position in the shell mouth. Tapes corrugata and
Leporimetis hanleyi were abundant in this area, the
former often showing the countersunk hole on the side
of a valve indicating predation by Natica.
3.5 A small area of low hummocks of salt encrusted
sand was found. On investigation it was found that
these low mounds were formed around the tubes of
the large polychaete worm Diopatra neopolitana.
(Plate 4). The parchment-like material of the actual
tube was still present, surrounded and probably pre-
served by a sheath of hardened mud. The mouths
of these tubes are usually level with the substrate
surface (as observed at Sandvis) and as the mounds
in the area under discussion are about 10 - 15 cm
high, this amount of substrate has been removed by
wind action.

14?
MAP I: Map showing coastal localities.
A glance at the map (Map 1) shows a repetition of
broadly similar geographic conditions at Conception
Bay, Sandvis, and Walvis Bay. The similarities in-
clude a bulge in the coastline, forming to the north
a shallow embayment, the southern end of which ends
in a salt pan. At Walvis Bay the embayment is con-
siderably larger than at the other two localities. In
all three cases the most westerly-extending part of the
coastline is a sandbar, formed under the influence of
the strong northward-flowing longshore currents. Vary-
ing conditions in the general north-flowing current,
localised eddying effe~ts, winds, and tides, will all have
an influence on the form of this sand bar. The sand
bar at Pelican Point, Walvis Bay, has been extending
northward over the past few years, while the whole
character of Sandvis has changed appreciably in the
last 90 years. Hottentots Bay (26.07S., 14.57E.) just
north of Liideritz, presents a similar situation. Kaiser
(1926: 414) explains the silting up of the lagoon south
of Hottentots Bay as a combination of the shifting
of the shore line, and the addition of wind-borne sand.
Here the embayment is formed by a rocky island which
has been connected to the mainland. The southern
lagoon formed here is also becoming a salt pan, and
bivalves dead in situ (including Lutraria sp. and
Solen sp.) are visible. (J. Rogers, Geological Survey,
pel's. comm.)
The coastline between Liideritz and Walvis Bay should
perhaps be viewed as a dynamic system of interaction
between the sea and its currents and tides, and the
land with its wind-borne sand. The same set of con-
ditions has obviously been duplicated at various times
and at different localities on this coast. Thus, lagoonal
areas have been formed through the action of currents
depositing sandbars under the localised influence of
rocky outcrops or islands. These lagoons, which are
colonised by various organisms seeking sheltered water,
may be of temporary duration, and having reached a
certain stage of development, begin to disappear, either
because of silting, or because of a change in the set
of the local currents. The four sites mentioned above,
along this stretch of the coast, may thus be seen as
different stages of the same process. Walvis Bay is still
a well-established lagoon. (That its potential for sup-
porting a considerable animal and plant ecosystem is
not realised, must be seen as an effect of man's
activities, especially those of shipping pollution and
the formation of commercial salt-pans.) Sandvis al-
ready has a documented history of change and is be-
coming shallower. The pan at Hottentots Bay, although
holding some water, is too saline to support marine
organisms, and dead molluscs are already apparent.
The Conception Bay pan is probably older, and has for
some time been dry (with the exception of the tiny
lagoon at the northern end) and has lost a considerable
amount of the original sediments, thus exposing the
remains of large populations of molluscs. Wieneke and
Rust (1973) postulate a direct correlation betwreen
climatic and geomorphic conditions on the coastal
areas of the Namib, and refer to alternate phases f
arid stability and humid activity in relation to coa al
morphology.
Under these dynamic conditions the influence of single
events must not be under-estimated. A single north-west
storm, perhaps coinciding with a spring tide, could
as easily remove an entire established sand bank as it
could deposit a new sandbar. This seemingly small
and localised event could, by closing the mouth of
a lagoon, effectively kill the entire marine population
of such a lagoon. This is perhaps what happened at
Conception Bay, followed by a slower and more steady
build-up of the general beach line to make the clqsure
permanent. This slow build-up of the coast is illustrated,
not only by the position of old storm beaches on the
eastern margin of the pan, but also by the position
of the shipwreck of the EvaI'd Bolin. This metal-hulled
vessel was driven ashore in 1910, and its remains are
now several hundred metres from the present shoreline.
I am grateful to Mr J. Rogers of the Geological Survey,
and Dr A. J. Tankard of the South African Museum,
for reading the MS. and for making useful criticisms.
My thanks are due to the Geological Survey Windhoek,
and the State Museum, Windhoek, for inviting me to
accompany their trip through the Namib to the various
coastal localities, and to Mr H. van del' Westhuizen
of the Anglo-American Corporation Windhoek for
supplying me with useful information and for per-
mission to enter Diamond Area No.2.
HARGER, H. S.
1914: Some features associated with the denudation of the
South African Continent. Trans. Geol. Soc. S. Afr.,
16: xxii-xli.
KAISER, E. .~.
1926: Die Diamantenwiiste Siidwest-Afrikas. Vol. 2. Berlin~
Dietrich Reimer.
WIENEKE, F. and RUST, U.
1973: Variations du niveau marin et phases morphoclima-
tiques dans Ie desert du Namib Central, Afrique du
Sud-Ouest. Finisterra-Reilta port. Geogr., 48-65.
Interaction between coastal
processes and lagoonal fauna,
between Walvis Bay and
Liideritzbucht, South West Africa
by
Brian Kensley
Abstract 55
Introduction 55
2 Description of the area . . . . . . . . . . . . . . . . . 55
3 Faunal observations 56
4 Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5 Acknowledgements 60
6 References. . . . . . . . . . . . . . . . . . . . . . . . . . . 60
t)r~!.l, / /,1 I
MADOQUA, VOL. II, NO. Y,1978(55-60) 55
Aspects of the faunal composition and ecology of a
recently dried-up lagoon in the Conception Bay area
are discussed. The reasons for the demise of the lagoon
are speculated on. and related to similar changes in
coastal structures at several other localities on the
South West African coast.
The relatively uninterrupted coastline of South West
Africa offers few areas for marine organisms seeking a
sheltered habitat. Where the few sheltered areas do
occur, e.g. Liideritzbucht and Sandvis, a fairly rich
estuarine or lagoonal fauna becomes established. This
fauna is characterised, as with the intertidal rocky
shore fauna, by large populations of a relatively small
number of species. Much of the South West African
coastline is formed by the shifting sands of the Namib
Desert, and the unstable nature of this coastline is
well illustrated at Sandvis. British Admiralty charts
over a period of about 90 years show a progressive
silting up, from what was once a deep water anchorage
for sailing vessels to the present shallow lagoon, with
the position of the sea mouth visibly changing over
a three-year period of observation (1973 - 1976). Yet
another and more dramatic sign of this changing nature
of the coastline was seen recently at the Conception
Bay area (24.00S., 14.30E.) south of Sandvis. A con-
siderable salt pan area was observed between the sea
and the dunes. On examination, this pan revealed vast
numbers of dead mollusc shells, representing what was
at an earlier stage a sheltered lagoonal area similar
to Sandvis. Many of the molluscs, and especially the
bivalves had died in their life positions, and this led
to speculation on the faunal constitution of this 'dead'
lagoon, and on the causes for its demise.
The area in which the shells were observed stretches
south-east from the slight embayment of Conception
Bay. At the northern end is a small true lagoonal
area, apparently receiving sea water only at spring
tides. This lagoon is not more than 20 - 30 cm deep
and apart from some polychaete worms, seems to be
uninhabited by macro-organisms. To the north-east the
area is bounded by deflation plains, while to the south
and south-east the pan is covered by a broad peninsula
of continuous dunes, the northern edge of which is
broken up into small barchan dunes and tongues of
sand. The western edge of the area is the beach with
a fairly steep slope which effectively prevents the
high-tide water from spilling onto the plain of the salt
pan.