inland salt waters of southern africa

HydrobioIogia 210: 75-91, 1991. W. D. Williams (ed.). Salt Lakes and Salinity. 0 199 1 Kluwer Academic Publishers. Printed in Belgium.

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Inland salt waters of southern Africa

M. T. Seaman ‘, P. J. Ashton’ & W. D. Williams 3 ‘Department of Zoology and Entomology, University of the Orange Free State, Bloemfontein, South Africa; *Division of Water Technology, Council for Scient$c and Industrial Research, Pretoria, South Africa ; 3Department of Zoology, University of Adelaide, Adelaide, Australia

Key words: saline lakes, athalassic, southern Africa, Namibia, Botswana, South Africa

Abstract

Inland salt lakes are widely distributed in southern Africa: they are particularly common in South Africa, but many occur in Namibia and Botswana. All are shallow, and most are ephemeral with salinities that are not very high (mostly < 50 g 1 - ’ ), Fringing zones of halophytes or submerged macrophytes are neither well-developed nor taxonomically diverse. The Cyanobacteria, especially Nodularia spumigena, often dominate the phytoplankton. The fauna of the Makgadikgadi area (northeast Botswana) is diverse and is similar to that of East African salt lakes. The aquatic fauna of salt water south of the Makgadikgadi Basin, on the other hand, is extremely depauperate, has no well-defined assemblage confined to saline waters, and appears mostly to comprise tolerant freshwater forms. Lovenula falcifra and Metadiaptomus transvaalensis (diaptomid copepods), Moina micrura (Cladocera) and Brachionus plicatilis (Rotifera) are frequently encountered zooplankton species, a few species of insects (Anisops sp., beetles, chironomids and ephydrids) are the principal non-planktonic macroinvertebrates. Artemia ‘salina’ is occasionally present, but may be an introduced form. The avifauna, in contrast to the aquatic macroinvertebrate fauna, is rich, with the greater and lesser flamingo often common.

Introduction

Salt lakes occur widely in Africa. Indeed, those in northern Africa were amongst the first such lakes to be investigated comprehensively (Gauthier, 1928 ; Gauthier-Lievre, 193 1; Beadle, 1943). Investigations of those in East Africa quickly followed (Jenkin, 1932, 1936; Beadle, 1932; Worthington & Ricardo, 1936). More recently, salt lakes in these and many other areas of Africa have been the subject of renewed attention; Beadle (1981) has provided a relatively recent review and Burgis & Symoens (1987) compiled a detailed directory (these publications considered both salt and fresh waters).

Salt lakes in southern Africa have attracted considerably less limnological attention. The only attempts to investigate those in South Africa comprehensively are those by Hutchinson, Pickford & Schuurman (1932) who studied a variety of waters (many fresh) largely in the Transvaal, and by Ashton & Schoeman (e.g. 1983,1988) who studied Pretoria salt pan. Several limnologists (e.g. Vareschi, Melack & Kilharn, 1981) clearly believe that this lack of attention accorded southern African salt lakes reflects their low numbers. This belief is erroneous: salt lakes are geographically widespread and numerous in southern Africa. It is not surprising that Hammer (1986) made few references to them, but perhaps

76

it is surprising that they continue to attract little attention from southern African limnologists (cf. Mepham, 1987 ; Allanson, Hart, O’Keefe & Robarts, 1990).

The purpose of the present paper is threefold: First, to draw attention to the large number and geographically wide occurrence of salt lakes in southern Africa; second, to provide an introduction to the somewhat dispersed literature dealing with their various features ; third, to document and discuss the results of our own chemical and biological studies of several salt lakes in southern Africa.

Geographical distribution

For present purposes, southern Africa is taken to include Namibia, Botswana and southern parts of Zimbabwe and Mozambique to the north, and South Africa and contiguous countries to the south. This region more or less corresponds to land south of latitude 18’ S and dealt with by Allanson et al. (1990) as a single entity: ‘southern Africa’. It excludes Madagascar, which does have salt lakes (Moreau, 1987), and also Malawi, in which lies Lake Chilwa, a large, shallow and moderately saline lake; it has been studied by several investigators and most of their results have been summarized by Kalk, McLachlan & Howard-Williams (1979). Cantrell (1988) and Mepham (1987) provide more recent contribu-

NAMIBIA --A

I cw BOTSWANA

-\ I/.,-.1,.

.-.-‘-‘-I ,I,- \

SOUTH AFRICA

Fig. I. Distribution of salt pans in southern Africa. Dots mark positions, but because of scale, dots may represent more than one pan. Based on a variety of sources (see text). Regions of pan concentration (fresh and saline) according to Shaw (1988) are enclosed by broken lines: A, northern Cape region; B, southern Kalahari, C, northwest Zimbabwe (and Makgadikgadi pans);

D, western Orange Free State; E, western Transvaal/northeast Cape; F, eastern Transvaal; G, Mozambique lowveld.

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tions to our knowledge of this lake. Within southern Africa, it may first be noted

that all inland saline lakes are shallow and most contain water only ephemerally. The commonest local names applied to such water-bodies arepans and vleis. There is no definitive distinction between these names, but, according to Burgis & Symoens (1987) - as well as several earlier authors -pans lie in endorheic systems, and vleis include waters associated with river drainage sys-

tems. In South Africa, according to Dr. J. Day (personal communication, 2 1.6.1989), vleis in the southwestern Cape Province are usually temporary or permanent lakelets and/or marshes, or even estuarine lagoons; in the rest of South Africa, however, vleis are nearly always wetlands (mostly with Phragmites reedbeds) on river courses and are never open bodies of water.

Figure 1 (and Table 1) document the location of many inland bodies of salt water. Only athalas-

Table 1. Locality details and major ion chemistry of 53 salt pans in southern Africa. Salinity values as g l- ‘; ions as mg l- ‘.

Name Position Sal. Na K Cl Mg Cl so4 HCO, +

Lat. S Long. E co3

Brak Pan # 1 26” 14’ 28” 18’ 3.0 900 20 81 66 1080 500 380 Brak Pan #3 28” 48’ 25”43’ 3.1 700 4 230 80 850 800 400 Finch Pan, N. 28” 07’ 20” 04’ 3.1 760 50 190 90 1340 630 20 Riet Pan # 1 28”46’ 25 o 50’ 3.1 620 5 190 160 680 900 550 Apex Pan 26” 13’ 28” 20’ 3.1 700 7 270 110 1400 480 180 Groot Witpan 27” 45’ 20” 40’ 3.2 670 9 270 100 940 930 460 Konga Pan 27” 30’ 20” 42’ 3.4 900 16 240 66 1300 625 250 Soetendals Vlei 34” 27’ 20” 12’ 3.4 922 14 148 122 1636 419 178 Tete Pan 27” 10’ 32” 28’ 3.7 796 0 310 181 2094 163 165 Ganna Pan # 26” 48’ 19” 45’ 3.8 740 21 350 180 1600 770 100 Rynfield Pan 26” 08’ 28’ 20’ 3.9 1380 11 3 2 1060 680 780 Brak Pan #2 28” 10’ 25” 50’ 4.1 1280 26 8 66 900 750 1070 Annas Pan, E. 28’ 32’ 25”48’ 4.3 1360 40 19 100 1260 1130 400 Elandsfontein 26” 10’ 28” 13’ 4.3 1500 22 7 3 1080 600 1120 Nyamithi Pan 26” 50’ 32” 28’ 4.4 1400 0 73 290 2284 0 387 Present Pan 28’ 06’ 25’ 26’ 4.9 1550 55 50 110 2100 860 130 Modder Pan 26” 11’ 28’ 27’ 4.9 1700 12 33 32 1600 1040 520 Langklip Pan 28” 14’ 20” 22’ 5.4 1660 9 37 110 970 2100 560 Ratelgat Pan 26” 07’ 18” 40’ 5.5 1260 45 470 170 3000 450 130 Amabele Pan 25” 55’ 18” 59’ 5.9 1400 8 440 140 1200 2500 250 Wolwe Pan 28” 14’ 25” 35’ 6.1 2090 40 5 7 1490 1150 1340 Ferndale Pan 27”41’ 25’ 30’ 6.4 2200 65 20 3 2080 440 1620 Klein Aarpan 27” 22’ 20” 43’ 6.9 1640 13 480 180 1600 2800 180 Riet Pan #2 28” 46’ 25’ 47’ 7.7 2660 13 25 30 2600 2000 400 Koopan North 26” 53’ 20” 38’ 8.1 2400 105 330 170 3500 1490 150 Gemsbok Pan 27” 29’ 20”31’ 8.5 1910 15 230 420 1400 4300 190 Kingswood Pan 27” 30’ 25” 46’ 8.9 3120 90 25 31 3000 1650 1000 Rooikraal Pan 26” 19’ 28”21’ 9.4 3280 39 1 2 1750 1740 0 Toronto Pan 27” 59’ 26” 42’ 10.1 3118 57 157 160 3544 2693 401 Kamfersdam 28” 45’ 24” 40’ 10.3 2890 112 428 204 3750 2830 93 Koopan South 27” 12’ 20” 26’ 11.6 2750 115 900 300 4600 2700 250 Middelpos Pan 26” 55’ 20” 10’ 12.7 3300 75 660 460 6600 1470 140 Nigel Pan 26” 22’ 28”27’ 12.9 4600 73 10 5 4420 2720 1100 Flamingo Pan 27” 59’ 26”41’ 16.4 5642 200 164 192 7453 2024 677 Dealesville Pan 28” 40’ 25”45’ 17.5 7200 3 18 91 9800 300 85 Leeuw Pan 26” 17’ 28” 18’ 18.1 6400 108 10 11 5300 4650 1620

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Table 1. (continued)

Name

Sout Pan Etosha Pan (July ‘71) Seven Springs Pretoria Salt Pan Geduld Pan Teviot Salt Pan Koppieskraal Pan N. Swink Pan Karee Pan Koppieskraal Pan S. Stink Pan Etosha Pan (Sept. ‘71) Rensburg Salt Pan Hosabes Pool (1982) S koppan Hosabes Pool (1984) Britten Pan Florisbad Pan Oranjemund Pan

Position Sal. Na K Cl Mg Cl SO4 HCO, +

Lat. S Long. E co3

28” 58’ 25” 31’ 18.7 4560 145 1500 530 11000 860 70 18” 40’ 16” 40’ 20.9 7820 111 5 3 10400 890 1629 27” 26’ 25” 36’ 21.5 7800 140 45 51 9090 3900 500 25” 24’ 28” 05’ 31.6 11600 5 1 1 10200 300 9516 26” 14’ 28”21’ 34.8 13400 70 1 6 18090 2500 720 31”40’ 25” 35’ 35.0 10890 651 937 499 16350 5520 134 26” 54’ 20” 18’ 35.2 11000 295 670 870 17250 5000 110 28” 29’ 25” 36’ 38.7 13500 320 250 130 14500 9500 500 27” 30’ 25” 35’ 41.8 15200 70 5 3 17100 8500 950 26” 55’ 20” 18’ 48.6 15000 700 760 1400 25500 5000 280 27” 46’ 26” 40’ 51.1 19583 285 997 778 24923 13966 598 18” 40’ 16” 40’ 62.0 22650 300 4 3 29870 3700 5429 28” 55’ 26” 05’ 102.4 36275 109 1119 508 51368 12845 187 23’ 30’ 15” 05’ 115.9 41000 1478 826 1220 55000 16099 238 28” 40’ 26” 05’ 160.1 61000 34 60 1000 91000 7000 24 23’ 30’ 15” 05’ 161.7 59400 2205 58 1732 75000 23000 281 27” 45’ 25”21’ 181.6 63650 2250 370 2000 95000 18250 70 28” 45’ 26” 05’ 197.3 78000 22 67 1900 110000 7300 6 28” 35’ 16” 35’ 302.4 100410 2074 3955 8536 180200 6890 305

sic waters (sensu Bayly, 1967) known to be highly available limnological information on Etosha saline or with waters of salinity > 3 g l- ’ are con- pan. Day & Seely (1988) noted the occurrence of sidered. Coastal lakes and vleis, even if they are highly saline water fed by a spring some distance not in aquatic continuity with the sea and have inland from Walvis Bay (for complete analysis, salinities > 3 g l- i, are not plotted in Fig. 1 if they see Table 1). Further athalassic salt lakes have are essentially thalassic. For the most part they are been observed near the coast at a latitude of 19” S blind or relictual estuarine embayments con- (salinity, 22 g l- ‘) (Dr. J. A. Day, personal com- taining a mixture of relict estuarine and freshwater munication, 1.2.1989). Additionally, we record forms tolerant to elevated salinities. Several have the occurrence of three salt pans in the southeast been intensively studied, e.g. Lake Sibaya on the and one in the southwest (Table 1). None of the eastern coastal plain (see Allanson, 1979), and a saline springs along the Namibian coast and refer- series of adjoining lakes on the southern coastal red to by Watson (1979) is plotted. The paucity of plain known as the Wilderness Lakes (e.g. known localities for Namibian inland salt lakes Davies, 1982; Coetzee, 1980, 1983, 1986). undoubtedly reflects their actual paucity in an Allanson (198 1) has comprehensively reviewed extremely arid region. Namibian pans, fresh and studies of these and other southern African saline, are concentrated in the south-east of the coastal lakes and Mepham (1987) has given an country according to Shaw (1988) (Fig. 1, re- updated review. gion A).

In Namibia, only seven athalassic saline localities can be plotted with confidence. Water bodies on the surface of Etosha pan, in northern Namibia, were sampled by Berry (1972) and found to have salinities of 20.2 and 61.4 g l- I. Mepham (1987) has recently brought together all

Botswana is less arid and numerous ephemeral pans occur widely scattered over an extensive area, particularly in the southern Kalahari (Lancaster, 1978; Goudie & Thomas, 1985) (region B of Fig. 1). Boocock & Van Straten (1962) referred to saline pans as one of three pan

79

types which could be distinguished in the region. Lancaster (1974) also referred to saline pans in the southern Kalahari. Only three are plotted in Fig. 1, and for no pan is detailed limnological information available. There are also some saline pans in northern Botswana (region C in Fig. 1); Eccles (undated) gave some faunistic data for a few associated with the Makgadikgadi Basin. Mepham (1987) also dealt briefly with the Makgadikgadi pans. The Sua Pan, lying in the east of the Makgadikgadi Basin, is some 100 x 45 km in size and said to contain unlimited amounts of evaporite salts, especially sodium chloride, carbonate and sulphate (Shaw, 1988).

Numerous ephemeral water-bodies occur in Zimbabwe, most notably in the north-west (Flint & Bond, 1968 ; Goudie & Thomas, 1985 ; Shaw,

1988; region C, Fig. 1). None is recorded as saline. However, limnological knowledge of pans in this region of southern Africa is very limited (cf. Weir, 1966, 1968, 1969). Likewise, no saline waters have been reported from Mozambique (Mepham, 1987).

It is in South Africa, of all southern African countries, that salt lakes occur most widely and profusely (Figs 1, regions A,D-G, & 2). They attracted early attention from geologists because of the commercial value of their deposits (Pelling, 1925; Ferguson & Juritz, 1938). Almost all are shallow contain water only briefly after rain or, at best in a wet year, for several months. Hugo (1974) and, in part, Noble & Hemens (1978) discussed their distribution in broad terms. They are especially common in the northern Cape,

Fig. 2. Representative southern African saline water-bodies. a, salt pan near Welkom, Orange Free State; b, salt pan south of Kalahari Gemsbok National Park; c, Pretoria Salt Pan; d, artificial salt pond near Bloemfontein.

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Karoo, western Orange Free State and the Trans- vaal. Geldenhuys (1982) has drawn attention to the very large numbers of them in the western part of the Orange Free State; on the basis that four of the six types of pan he distinguished contain saline water, some 7000 occur in the area he considered.

Most South African salt pans lie within the major areas of distribution identified for pans irrespective of salinity (de Bruiyn, 1972; Goudie & Thomas, 1985; Shaw, 1988; Fig. 1, regions A,D-G), but outside these regions, numerous salt pans also occurpassim. The Pretoria salt pan is known from early geological studies (Alford, 1890; Cohen, 1895; Wagner, 1922) as well as more recent (palae0)limnologica.l ones (e.g. Ashton & Schoeman, 1983; Scott, 1988). Some saline water-bodies are associated with the Pongolo River; these are floodplain lakes (especially Nyamithi, Mhlolo and Tete) in which salts are concentrated by evaporation during the dry season (Colvin, 1971; Heeg, Breen, Colvin, Furness & Musil, 1987; Heeg & Breen, 1982). And there are several salt pans scattered throughout the southern Cape region - at least some of which, though near the coast, appear to be athalassic (Harrison, 1962; Hugo, 1974).

Several factors seem to have been important in pan genesis throughout southern Africa (Shaw, 1988). Most authors agree, however, that defla- tion of surfaces by aeolian action has been particularly important (e.g. van Rooyen & Burger, 1973 ; Le Roux, 1978; Goudie & Thomas, 1985 ; Marshall, 1987). Surface erosion by animals’ salt weathering, and the occurrence of suitable terrain lacking integrated drainage are other factors that have been advanced as important in pan genesis. The orientation of many pans along the direction of the prevailing wind and the presence of leeward lunettes is evidence for the importance of winds in pan genesis. The Pretoria salt pan is probably of volcanic origin, and as such the only maar lake in southern Africa, but meteorite impact has been suggested as an alternative origin for the crater it occupies (Fuldah, Gold & Gurney, 1973). The saline lakes on the Pongolo floodplain are of riverine origin.

Given the size of the area in southern Africa where salt lakes occur, it is not surprising that a variety of climates is encountered. In general, however, the climate of the area can be described as semi-arid to arid: Mean annual precipitation (mostly as summer rain) is c 50 cm, mean annual evaporation, > 180 cm. Mean annual air tempera- ture is N 17 “C, with an approximate range from 10 to 25 “C.

Chemical features

Apart from the data for Pretoria Salt Pan given by Ashton & Schoeman (1983), there are no comprehensive chemical analyses for any southern African saline water-body. The few published data on the chemistry of southern African salt lakes, as well as our own chemical data, concern only major ions.

Major ion analyses are available for 53 localities (55 samples) where salinity at the time of sampling exceeded 3 g 1 - i, the arbitrary value demarcating ‘fresh’ from ‘saline’ water (Table 1). Samples comprise 2 from Etosha Pan, Namibia (Berry, 1972), 2 from a spring pool at Hosabes (Day & Seely, 1988), 1 for Nyamithi Pan (Heeg et al., 1978), and 1 for Tete Pan (du Preez, 1967). The remaining analyses in Table 1 are from 49 localities visited by one or other of us. Not included are the comprehensive analyses of Ashton & Schoeman (1983) for Pretoria Salt Pan since these are essentially similar to our own analysis for this locality. Our analyses entailed the use of standard analytical techniques (A.P.H.A., 1975) at either the Division of Water Technology (Pretoria), or the Australian National University (Canberra).

Table 2 provides a summary of patterns of cationic and anionic dominance within selected salinity ranges, and Fig. 3 provides a diagram- matic summary in two ranges of salinity. Amongst cations, Na is invariably dominant, but a variety of patterns occur for other cations; the most common are Ca > Mg > K (in 33 per cent of all samples) and Mg > Ca > K (35 per cent). There are no obvious regional differences amongst

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Table 2. Patterns ofionic dominance in 55 salt pans in southern Africa. Data based on analyses in Table 1 converted to equivalent percentages.

Ion dominances Salinity (g l- ‘) Total no.

3-10 10.1-50 50. I-100 > 100.1 pans

Na>Mg>Ca>K 9 5 Na>Ca>Mg>K 11 6 Na>Mg>K>Ca 3 1 Na>K>Mg>Ca 3 3 Na>K>Ca>Mg 2 3

Cl > SO, > HCO, + CO, 20 16 Cl > HCO, + CO, > SO, 4 2 SO, > Cl > HCO, + CO3 4 0

1 1 0

7 0 0

19 18 6 7 5

44 7 4

cation dominance patterns, but their variety decreases with increasing salinity.

Amongst anions, Cl is usually - but not invariably- dominant (93 per cent), with SO4 some-

Fig. 3. Ternary diagrams illustrating patterns ofionic dominances in 55 salt pans in southern Africa. Plots are based on equivalent percentage of individual ions calculated from Table 1. Numbers in parentheses alongside a few plots indicate the number of pans with that composition. Plots

without numbers are for single pans.

times dominant (7 per cent). The most common pattern of anion dominance is Cl > SO, > HCO, + CO, (80 per cent), with Cl > HCO, + CO, > SO, the next most common pattern (13 per cent). When HCO, is more abundant than SO, in highly saline waters, di- valent cation concentrations are often low (thus, in Etosha Pan and Pretoria Salt Pan, patterns are Na>K>Ca or Mg : Cl>HCO,>SO,). Again, regional differences amongst anionic patterns are not obvious, but all pans in which SO, is dominant occur in the northern Cape/southern Namibia region (Fig. 1, region A, where also occur, however, many Na-Cl pans); and, Etosha Pan apart, all pans in which HCO, is moderately abundant occur in the east (where, likewise, also occur many pans in which HCO, is not abundant). Diversity of anionic patterns of dominance, as for cation patterns, also decreases with increasing salinity; all highly saline pans are dominated by Cl.

Our results (Table 1) are in general accord with the few previously published data on major ion chemistry not recorded in Table 1. Only two inland pans (both in the Transvaal) with salinities greater than 3 g 1 - ’ were studied by Hutchinson et al. (1932). Recalculating their gravimetric data for these pans gave patterns of ionic dominance ofNa+K>Ca>Mg:Cl>HCO,+CO, > SO, for both Blaauwater Pan 1 (salinity, 3.6 g 1-l) and Banagher Pan 3 (salinity, 21.1 g l- ‘). Our results from Pretoria Salt Pan, as indicated,

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also agree with those of Ashton & Schoeman (1983).

Our results also accord with the extensive data given by Hugo (1974) for subsurface brines collected from 190 salt pans in South Africa. His samples were collected from boreholes, wells and furrows when subsurface brine was being pumped for salt production; his data are therefore not directly comparable with those of Table 1. Even so, and not surprizingly, the patterns of ionic dominance in the subsurface brine samples were usually Na > Mg > Ca > K: Cl > SO, > HCO,.

As for global patterns of major ionic dominances, southern African salt lakes appear un- remarkable: worldwide, Na-Cl waters are the most common type (Hammer, 1986). Southern African salt lakes, nevertheless, are rather more heterogeneous in major ion chemistry than those in some regions (e.g. Australia), and less than in others (e.g. Saskatchewan). Of particular note is that southern Africa (with the possible exception of pans in the Makgadikgadi Basin - see below) is quite different from East Africa in its obvious lack of carbonate-dominated saline lakes (Talling & Talling, 1965); even Etosha Pan and Pretoria Salt Pan, although having high concentrations of HCO, + CO,, are still dominated by Cl.

With regard to the origins of the ions in southern African saline waters, at least those in South Africa seem mostly to be of connate origin. The majority of South African salt pans overlie Dwyka and Ecca shales, the characteristic salts of which are chlorides and sulphates of Na, Ca and Mg (Bond, 1946). The composition of underground brines clearly reflects this (Hugo, 1974), as does that of waters in the pans, and it seems reasonable to conclude that most salts in pans are from geological sources. Cyclic contributions (wind-borne marine salt) have also been proposed (Hugo, 1974).

Almost all of the salinities documented in Table 1 relate to single samples from separate pans, and most, it will be noticed, are below 50 g l- ‘. In the absence of seasonal information on these pans, it is not clear therefore whether this reflects a real and natural predominance of low salinity waters in southern Africa. There are, of

course, many man-made highly saline waters from which commercial salt is produced, but perhaps salinities in pans which retain natural seasonal patterns do not become very high and the pans never contain saturated brines. It is perhaps significant that the highest salinity recorded from all the numerous bodies of inland waters in South Africa studied by Hutchinson et al. (1932) was only 29 g 1-l (Banagher Pan 3). In this con- nection, note that modem hydrological views concerning endorheic lakes are that they are far less hydrologically isolated from underlying aquifers than has been assumed until recently. In any event, few subsurface brine samples from South African salt pans are saturated (Hugo, 1974) (though most have salinities considerably above those given in Table 1). Some seasonal fluctuation in salinity in Pretoria Salt Pan was noted by Ashton & Schoeman (1983), but this,apparently, was not excessive. Unpublished data for three salt pans in the Orange Free State and collected on three separate occasions also suggest that annual salinity differences in many salt pans may not be excessive (Table 3). Further studies of seasonal fluctuations in the salinity of salt pans of southern Africa would be rewarding, not least because of biological implications (see below). In their absence, it should not be assumed that as pans dry, salinities increase to high and then saturated values.

Few data on the pH of southern African saline waters are available. Not unexpectedly, given the importance of HC03 + CO,, recorded values for Etosha Pan (9.2) and Pretoria Salt Pan (10.4 at the surface) are amongst the highest recorded (Berry, 1972, Ashton & Schoeman, 1983). Other

Table 3. Salinities in three pans in the Orange Free State sampled on three separate occasions. Data recalculated from Scott, Tausig & Seaman (1980) and original.

Pan

Toronto Flamingo Stink

Salinity (g I- ‘)

Oct. 1978 Feb. 1980 July 1984

3.6 10.2 10.1 7.4 11.4 16.4 7.9 15.1 51.1

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published values are generally lower, but still exceed 7.0. Hutchinson et al. (1932) noted that in the ‘brak’ mud pans of the Transvaal studied by them, pH was over 8.0. The two most saline inland pans they investigated (both with salinities > 3.0 g l- ‘) had pH values of 9.0 and 9.1 on the day of observation. Heeg et al. (1978) recorded 8.4 at Nyamithi mid-pan, Seaman & Kok (1987), 9.1 and 9.0 for two mildly saline pans in the Orange Free State, and Day & Seely (1988), 8.0 and 8.3 at Hosabes Pool. The few values we have are not as high as published values but do exceed 7.0. In underground brine from South African pans, Hugo (1974) recorded values generally > 7.0.

Biological features

Flora. A number of essentially terrestrial halophytic species is known from southern Africa (Good, 1964), and several amphibious or submerged aquatic macrophytes have been associated with southern African inland saline waters. Thus, Geldenhuys (1982) noted that, in four of the six pan types recognized by him in which high salinity was an obvious feature, submerged macrophytes included species of Characeae and Zannichellia, and of more terrestrial plants, a number of halophytic dwarf shrubs such as Salsola aphylla L.f. and Suaedafntcticosa (L.) with the emergent grass Diplachne fusca (L.) particularly characteristic of two pan types. Scirpus robustus and S. spicatus (Cyperaceae) have been recorded as primary colonists of southwest African inland saline are (Chapman, 1975). Rup- pia maritima (L.) is occasionally found as a submerged macrophytes in pans of the Orange Free State (Mitchell & Seaman, 1988). It is not clear which species actually grow in saline conditions of the list of aquatic macrophytes given by Mepham (1987) as associated with the Makgadikgadi pans.

Despite the apparent diversity indicated by these published records, our experience is that chenopods and other terrestrial halophytes are neither diverse nor extensively developed near

southern African saline pans, and that submerged halophytes are typically absent in such pans, even in those of ‘moderate’ salinity (< 20 g 1 - ’ ). In any event, the wide fringing ‘samphire’ zones or dense submerged beds of Ruppia characteristic of many southern Australian salt lakes, for example, are apparently absent from most southern African pans. No obvious halophytes, we note, were recorded by Musil, Grunow & Bornman (1973) in any pans of the Pongolo floodplain (some of which, as previously indicated, become saline during periods of low water).

Diatoms apart, the diversity of algae and Cyanobacteria in southern African salt pans also appears to be depressed, at least on the basis of the limited data available to us. Phytoplankton densities, nevertheless, may certainly reach high values on occasion; Scott et al. (1980) found chlorophyll a concentrations exceeding 400 pg l- ’ in two salt pans of the Orange Free State. The only published data on the phytoplankton of southern African inland salt waters are those by Hutchinson et al. (1932) in two Transvaal pans (with salinities of 3.6 and 21.1 g l- ‘), Scott et al. (1980) in three pans in the Orange Free State (all with salinities < 15.4 g l- ‘), Cholnoky (e.g. 1955, 1966) on the diatoms in a variety of southern African water-bodies including many saline ones, and the series of papers on diatoms in the vicinity of and in the Pretoria Salt Pan (Schoeman & Ashton, 1982a,b, 1983; Schoeman, Archibald & Ashton, 1984).

On the basis of this limited information, it appears that the Cyanobacteria are usually dominant in the phytoplankton, with Nodularia spumigena Mertens particularly characteristic. Other phytoplankton species recorded, diatoms apart, include Pediastrum duplex Meyen, Oedogonium SP., Lepocinclis sp., Anabaena spiroides Lemmerman, A. ?flos-aquae (Lyng.) Brebisson, and Anaebaenopsis sp. Dunaliella salina Teodor, the chlorophycean typical of highly saline lakes worldwide (Borowitzka, 1981), appears not to have been recorded thus far, though perhaps the ‘small green algae of the Chlorella and Chlamydo- monas types’ found by Scott et al. (1980) in Flamingo Pan (Orange Free State) belong to this

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taxon. Diatoms regarded by Cholnoky (1966) as may be remarked, were recorded in the Pretoria indicative of saline conditions are: Amphora acu- Salt Pan by Schoeman & Ashton (1982a); the tiuscula, A. veneta, Cymbella pusilla Grun., Navi- most abundant of the 11 diatom taxa found by cula ammophila, N. halophila (Grun.) Cl., Stauro- them in this locality were Acnanthes exigua, neis wislouchii, and Nitzschia spp. Only two of Anomoeoneis sphaeophora and two species of these taxa (Navicula?halophila, Nitzschia spp.), it Nitzchia (not N. halophila).

Table 4. Fauna in three saline localities in the Makgadikgadi Basin. Data rearranged from Eccles (undated) based on samples collected 1957. + + +, abundant; + + , common; + , rare; - , absent.

Taxon’ Increasing salinity

Makgadikgadi Pan

Baobab’ Pan

Nata Salt Pan

Anostraca: Branchinella ornata Daday B. spinosa (M.-Edw.) -cspWW

~&hXU&lBamind Cladocera:

Daphnia barbata (Weltner) Moina macrocopa (Strauss)

Copepoda: Lover&a africana (Daday) Metadiaptomus transvaalensis Methuen4 Mesocyclops Ieuckarti Claus

Ostracoda: Sclerocypris excerta Sarss

Insecta: Tipulidae (larval) Chironomidae (larval) Ephydridae (larval) Empididae or Dolichopodidae (larval) Culicoides schultzei End.

Anisops (?) sardea Sigara meridionalis Micronecta scutellaris (Stal.) Odonata (nymph) Berosus vitticollis Boh. B. nigriceps (Fabr.) B. sp. Guignotus capensis Reg. G. zanzibarensis Reg.

+++ + +

++

+++ +

+++ +++ ?

+++

- - - - + +++ + + + - - + - -

- +++ -

-

+++

- ++t -

-

-

- -

-

- - - - + - - - - - -

’ Coleoptera identified by J. Balfour-Browne, Chrironomidae by P. Freeman, Hemiptera by G. E. Hutchinson, Culicoides by K. Mayer; H. Gauthier advised on identification of Moina, and J. P. Harding on crustacean identifications.

2 Noted by Eccles (undated, p. 145) as a ‘very saline’ pool, with a pH of 9. There was a ‘pan’ of crystallised salt in the mud under the pool.

3 Specific gravity = 1.09 [- 120x, salinity]; pH = 10. 4 The identity of this taxon has been confirmed by Mrs. N. Rayner (personal communication, 3.7.1989) who was able to obtain

the original material which has been deposited in the British Museum, London. s Described from the original material as a new subspecies by Martens (1988), S. exserta makarikarensis.

85

Only two halophilic bacteria have been recorded; Prior (1978) found a Pseudomonas-like species and Halobacterium halobium in a small, highly saline pan used in the commercial production of salt in the Orange Free State.

Fauna. Apart from our own unpublished information on some pans near Bloemfontein and the Pretoria Salt Pan, other information on the invertebrates of inland saline waters of southern Africa is sparse. The only faunistically comprehensive works are those of Hutchinson et al. (1932) on two Transvaal pans (3.6 and 21.1 g m- i salinity), Scott et al. (1932) on two Transvaal pans (3.6 and 21.1 g 1 - ’ salinity), Scott et al. (1980) on three pans in the Orange Free State (c 15.4 g 1-l salinity), Seaman & Kok (1987) on a few pans (two, mildly saline) also in the Orange Free State, and Eccles (undated) on the Makgadikgadi pan and some associated saline waters of Botswana. The undated and unpublished report by Eccles is of some significance in that it records the greatest fauna1 diversity; the absence of firm chemical data associated with the faunal collections is a matter of considerable regret. Because of the inaccessi- bility of Eccles’ report pertinent details are repro- duced as Table 4. Although faunal identifications were made by known taxonomists, it seems pru- dent to regard Eccles’ list of species as a tentative one. The faunisticahy comprehensive but more general observations of Colvin (1971) and others on the slightly saline pans of the Pongolo floodplain are of little interest in the present con- text.

Additionally, several authors have commented on particular fauna1 elements occurring in southern African saline waters; of note are comments by Paterson, Paterson and van Eeden (1964) on ‘Anopheles gambiae Giles’ (which recent work has shown to comprise a complex of six sibling species), Kok (1974) on Love&a, Dussart (1980) on copepods, De Deckker (198 1) and Martens (1986, 1988, in press) on ostracods, Muspratt & Henning (1983) on culicids, and De Ridder (1987) on rotifers. Some unpublished information on ostracods in southern African saline waters is also available (Dr. K. Martens, personal communication).

Table 5. Summary of macroinvertebrate fauna in twelve athalassic salt pans in South Africa. Original data.

Salinity Number of Taxa (gl-‘1 Pans

<5 2 5-15 2

15-100 4 >lOO 4

Beetles, corixids, ostracods Lovenula falcifra. Metadiaptomus

transvaalensis. Mesocyclops leuckarti. Moina micrura. Homo- cypris conoidea. Kapcypridopsis? sp. nov., Brachionus plicatilis. Anisops sp., Cricotopus sp., Berosus sp., nematodes

No macroinvertebrates Artemia ‘salinn’, ephydrids

Analysis of our own observations (Table 5) and those of others, leads to several comments:

1. The faunal assemblage in the salt pans of the Makgadikgadi Basin appears strikingly different from that in salt pans further south on the continent. It is both more diverse and its components mostly taxonomically different. These differences may reflect chemical differences and/or closer geographical proximity to East African salt lakes. We believe both factors of likely importance. Thus, it seems probable that the waters in the Makgadikgadi Basin, like those of East Africa, contain considerable quantities of HCO, and CO, (it is known that limestones are particularly important in this area (Shaw, 1988), two of the waters from which faunal samples came had pH values of 9 or above, and water from Nata salt pan effervesced on addition of weak acid [ Eccles, undated]. Again, whilst some faunal elements commonly occur elsewhere in southern Africa (especially Metadiaptomus transvaalensis Methuen), faunal similarities appear mostly to be with East Africa. In East African saline lakes, the fauna is often composed of Lovenula africana (Daday) [ G Paradiaptomus africanus, E P. bira- mata], corixids, notonectids and chironomids, with both Branchinella spinosa (M. Edw.) and B. ornata Daday recorded from these lakes (Beadle, 198 1; Livingstone & Melack, 1984). The only known record of L. africana in southern Africa is from the Makgadikgadi basin (Mrs. N. Rayner, personal communication, 21.4.1989).

86

Lovenula falctfera (Loven) and Metadiaptomus meridianus (van Douwe), it may be noted, are the only East African copepods also found in South Africa. Further studies of the ecology of salt lakes in the Makgadikgadi area (Fig. 1, region C) would be of great interest.

2. The invertebrate fauna of southern African inland saline waters south of the Makgadikgadi Basin appears to be extremely depauperate (and, according to Dr. M. Cantrell, personal communication, 19.4.1989, the fauna of waters in Botswana south of the Makgadikgadi basin is also depauperate). There seems to be no well-defined faunal association characteristically confined to saline pans. For the most part, it appears that the fauna essentially comprises elements of the freshwater fauna able to tolerate slightly elevated salinities.

Thus, the commonest copepods of southern African salt pans (up to - 2 1 g 1 - ’ salinity) are Lovenula falctfera and Metadiaptomus transvaalensis: both also occur in a variety of only slightly saline to quite fresh water-bodies. Lovenula falci- fera and L. excellens Kiefer are synonymous according to some authors (Kok, 1974), but others regard them as distinct species. According to Mrs. N. Rayner (personal communication, 21.4.1989), they are distinct species, with L. excellens found only in Lake Chrissie (South Africa). Similarly, the commonest halophilic cla- docerans (especially Moina micrura Kurz, but Daphnia barbata Weltner and D. gibba Methuen as well) also occur in quite fresh waters. None of these is known from waters of salinity > 25 g 1 - ‘.

Likewise, all ostracods regarded as ‘typical’ of southern African saline water-bodies, although occasionally found at salinities as high as 25gl-1, usually occur in much lower salinities, including those generally regarded as indicative of fresh waters (Gomphocythere expansa Sars, Apa- telecypris schultzei Daday, Parastenocypris hodg- soni &I-S, and Sarscypridopsis aculeata Costa). Additionally, many ostracods regarded ‘typically’ as forms of fresh waters also occur in moderately saline waters (e.g. some Sclerocypris species).

Most other elements of the fauna of southern African salt lakes also seem to be halotolerant forms from a freshwater fauna1 assemblage:

Brachionus plicatilis Miiller and the few species of insect occasionally recorded (note especially here the culicids Anopheles (Cellia) listen’ De Meillon and Aedes (Aedimorphus) albocephalus (Theobald), durbanensis (Theobald) and natronius Edwards). Additionally, Homocypris conoidea Sars, an ostracod found by us in Bankfontein pan near Dealesville at a salinity of 3.4 g l- l, is generally regarded as a freshwater species (P. De Deckker, personal communication, 13.4.1989).

3. The only faunal elements in southern Afri- can saline waters south of the Makgadikgadi Basin that are not also found in southern African fresh waters are Artemia ‘salina’ L., Anopheles (Cellia) merus Donitz and Culex (Culex) thalassius Theobald. Perhaps the species of ephydrid recorded from highly saline waters (Table 5) also falls into this category. Of these taxa, we believe that A. ‘salina’(and perhaps the ephydrid species) has been introduced into South Africa by man. Apart from two records of A. ‘salina’ from saline pans near Cape Town (Dr. J. A. Day, personal communication), South African records of ‘A. salina’ are from artificially reconstructed salt pans producing commercial salt. A. ‘salina’ could have been introduced to improve water clarity, as is often the case elsewhere. The two species of culicid involved occur in coastal saline waters, and it is from these, without doubt, that coloniza- tion of inland saline waters has taken place. Thus, A. (Cellia) merus is otherwise confined to a fairly narrow belt along the eastern Transvaal low-veld and Natal coast (Cross & Theron, 1983).

4. The apparently impoverished diversity of the fauna of southern African athalassic saline lakes south of the Makgadikgadi Basin, and the complete absence of any regionally endemic assemblage of species confined to such waters, contrasts strongly with the situation in the only other southern hemisphere continent (Australia) where the fauna of saline waters has been comprehensively surveyed. In Australia, the fauna of such waters is highly diverse, and has a characteristic assemblage of regionally endemic taxa which include many species of ostracods (several endemic genera), centropagid copepods, Daphniopsis spp. (Cladocera), Coxiella spp.

87

(Gastropoda), and Purartemiu spp. (Anostraca) (e.g. De Deckker, 1983)

It was, indeed, the existence of this rich Australian fauna which was an important prompt in our investigation of southern African salt lakes: Given that (1) biogeographic affinities between southern Africa and Australia have been de- monstrated for several elements of the freshwater fauna (e.g. Hutchinson, 1967) and for Tomichia (a gastropod found in southern regions of South Africa where it occurs in fresh and saline thalassic waters) and Coxiella (Davis, 1981; see also Williams & Mellor, in press), and (2) that centropagid copepods had early been reported from South African pans (Hutchinson, Pickford & Schuurman, 1929), it seemed reasonable to sup- pose that an investigation of inland saline waters in southern Africa might elicit at least some re- flection of the Australian situation. That has proved not to be the case. All calanoids in southern African inland waters are diaptomids, and, with the possible exception of phylogenetic relationships between Tomichia and Coxiella, the invertebrate faunas of Australian and southern African saline lakes appear to have no similarities and no biogeographical relationship.

A reasonable explanation for this state of affairs is that previous periods of climatic aridity have caused the extinction of any regionally endemic athalassic fauna that may have existed in southern Africa, and its place has been taken - at least in part - by elements of the local freshwater fauna tolerant to increased salinities, or, in the case of the fauna in the Makgadikgadi pans, by fauna which dispersed from the reasonably proxi- mate saline localities in East Africa and beyond drought affected areas. Of interest in this con- nection are the conclusions of Martens & Coomans (in press) concerning the generic evolution of megalocypridine ostracods in Africa. On the basis of a phylogenetic analysis, they con- cluded that most of the generic evolution of this subfamily of ostracods occurred in East Africa. Of the genera involved, two, Apatelocypris and Sclerocypnk, are abundant in Namibian and South African saline waters.

Whilst the precise nature of past climatic

change in southern Africa is not clear, there is general agreement that at times the climate of the region has been considerably more arid than at present (cf. Meadows, 1988). During these arid periods, no suitable refugial areas for the athalassic saline fauna presumably existed (the occurrence of suitable refugia must have been the case during similar periods of past aridity in Australia). The Drakensberg would have pre-empted survival in the east, and increased aridity in the Namib would have pre-empted survival in the west. Hypogeal retreat can also be ruled out as a survival strategy if for no other reason than that salinities would be too high.

It is of course possible that, except in certain areas (see below) and until recently, southern Africa never had the appropriate concatenation of factors other than climate that could lead to the formation of numerous salt pans (e.g. endorheic drainage patterns, appropriate geomorphological processes). Thus, whilst the Makgadikgadi pans, Etosha Pan, Pretoria Salt Pan and some few other southern African salt pans are remnants of palaeolakes (Grove, 1969; Butzer, Fock, Stuckenrath & Zilch, 1973; Rust, 1984; Cooke & Verstappen, 1984; Shaw & Cook, 1986), perhaps most southern African salt pans are of relatively recent and primarily aeolian origin. Whatever the case, as Scott (1988) in particular emphasizes for the Pretoria Salt Pan in other connections, examination of cores from salt pans occupying the sites of palaeolakes should be of great interest. Ostracod valves and gastropod shells would be of particular value as fossils.

Finally, no discussion of the fauna of southern African saline waters would be complete without some mention of the associated avifauna. A number of authors have discussed this com- ponent, but of special value are the papers of Uys, Broekhuysen, Martin & MacLeod (1963), Sauer & Rocher (1967), Berry (1972), Geldenhuys (1982) and Mepham (1987). Information from these (and other) sources clearly indicates that in contrast to the poverty of the invertebrate fauna, a large number of bird species is found on southern African salt lakes, and some of the species are characteristically associated with such

88

habitats. Characteristically associated species most notably include the greater and lesser flamingo (Phoenicopterus ruber L., P. minor (Geoffroy)), Kittlitz’s sandplover (Charudrius pe- cuarius Temminck), the little stint (Culidris minuta (Leisler)) and the black-winged stilt (Himantopus (L.)). Etosha Pan is the only known mass breeding ground of the greater and lesser flamingoes in southern Africa, and at the Makgadikgadi Pans flocks of flamingoes have been observed ‘so huge that they cover tens of square kilome- ters’(Mepham, 1987: 502). The densities of the avifauna associated with certain southern African salt lakes suggest high primary production rates in these.

Acknowledgements

Many people have added to ideas expressed in this paper. In particular, we would like to acknowledge discussions with, or comments from Prof. Brian Allanson (Knysna), Dr. Chris Appleton (University of Natal, Pietermaritzburg), Dr. Mike Cantrell (University of Botswana), Dr. Jenny Day (University of Cape Town), Dr. D. Kok (University of the Orange Free State), Dr. Mike Kokkinn (South Australian Institute of Technology, Adelaide), Dr. Koen Martens (Koninklijk Belgisch Instituut voor Natuur- wetenschappen, Brussels) [also for the gift of literature], Dr. Steve Mitchell (University of Orange Free State), Dr. Ferdie de Moor (Albany Museum, Grahamstown), Mrs. Nancy Rayner (University of Natal, Pietermaritzburg), and Dr. Kevin Rogers (University of Witwatersrand). Dr. Patrick De Deckker (Australian National Univer- sity) is thanked for identifying ostracod specimens and Mr. Robert Taaffe (University of Adelaide) for identifying insect larvae. The in- direct support of the Foundation for Research Development (CSIR) and its coordinator, Dr. R. D. Walmsley, is also acknowledged. One of us (W.D.W.) acknowledges the support or the Australian Water Research Advisory Council during the period which this paper was written.

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