Sua Pan, Makgadikgadi Botswana)
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Hydrobiologia (2008) 596:31–46 DOI 10.1007/s10750-007-9055-8 PRIMARY RESEARCH PAPER Hydrochemical fluctuations and crustacean community composition in an ephemeral saline lake (Sua Pan, Makgadikgadi Botswana) Graham Paul McCulloch Æ Kenneth Irvine Æ Frank D. Eckardt Æ Rob Bryant Received: 21 January 2007 / Revised: 3 June 2007 / Accepted: 16 June 2007 / Published online: 4 August 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Fluctuating hydrochemistry, as a result of the lake dried out and salinities increased. pH extreme hydrological regimes, imposes major phys- estimates generally ranged between 8.6 and 10, with iological constraints on the biota of ephemeral saline maximum values recorded during initial flooding. lakes. While the inverse relationship between salinity Crustaceans comprised mainly Branchinella spinosa, and zooplankton species richness is well-known Moina belli, Lovenula africana and Limnocythere across salinity gradients, few studies have docu- tudoranceai, all of which occurred across a wide mented closely the response of zooplankton to range of salinities, while halotolerant freshwater seasonal changes in salinity. Weekly sampling during species (Metadiaptomus transvaalensis, Leptestheria two flood seasons at Sua Pan, an intermittent saline striatochonca and the ostracods Plesiocypridopsis lake in central Botswana demonstrated the impor- aldabrae, Cypridopsis newtoni and a newly identified tance of spatial and temporal salinity gradients for Potamocypris species) disappeared above conductiv- crustacean community composition, associated with a ities of 1,500 lScmÀ1. A unique crustacean compo- decline in species richness, from 11 to three species. sition in southern Africa was attributed to Sua Pans’ Conductivity ranged between 320 and 125,800 lS rare chemical composition among southern African cmÀ1 during seasonal flooding; changing from dom- saline lakes; flood waters on Sua Pan contained a À À 2+ 2+ + À + inance by HCO3 and CO3 ,Ca and Mg , at the higher proportion of Na and HCO3 , and less K , 2+ 2þ beginning of the floods, to NaCl dominated waters as Mg and SO4 than over 80% of records from salt pans elsewhere in southern African. The freshwater species of crustaceans in Sua Pan were similar to those found in other southern Africa lakes, and these Handling editor: J. M. Melack similarities decreased in lakes with higher pH and G. P. McCulloch (&) Á K. Irvine proportions of Na, and less SO4 and Mg in their Department of Zoology, University of Dublin, Trinity chemical composition. The predominant saline toler- College, Dublin 2, Ireland ant species on Sua Pan, however, showed a greater e-mail: [email protected] similarity to those in saline lakes in southern and East À F. D. Eckardt Africa with higher proportions of HCO3 and, partic- 2+ Department of Environmental Sciences, University of ularly, Mg in their chemical composition. Cape Town, Cape Town, South Africa Keywords Saline lake Á Ionic composition Á R. Bryant Department of Geography, University of Sheffield, Zooplankton Á Makgadikgadi Á Species richness Á Sheffield, UK Salinity 123 32 Hydrobiologia (2008) 596:31–46 Introduction crustacean species richness and a simultaneous change in species composition in Lake Chad were Unpredictable and seasonal flood regimes (rainfall attributed to changes in temperature, oxygen and occurrence and intensity) among shallow and ephem- predation, while salinity was not suggested as a major eral saline lakes contribute high variability to their influencing factor (Saint Jean, 1983). physical and chemical character (Langbein, 1961; Although widely distributed in southern Africa, Hammer, 1986; Comin & Williams, 1994; Meintjes few detailed studies have been done on the hydro- et al., 1994; Williams, 1998). Salinity can vary chemistry and zooplankton communities of the enormously among and during individual flood and region’s shallow and ephemeral saline lakes (Allan- drying periods; through varying degrees of cyclical son et al., 1990; Seaman et al., 1991; Day, 1993). dissolution of surface evaporates during flooding and Ephemeral saline lakes in Namibia and South Africa evaporative concentration during drying phases (Har- range in salinities from 3 to 276 g lÀ1 and are die et al., 1978; Williams, 1998). Evaporative con- generally dominated by Na+ and ClÀ ions, with À 2À centration and sequential mineral precipitation on the HCO3 +CO3 prevalent in lower salinity waters surface and sub-surface layers alter the ionic propor- (Hutchinson et al., 1932; Seaman et al., 1991; Day, tions of vanishing flood waters (Stumm & Morgan, 1993). The aquatic fauna of these lakes are thought 1970; Eugster & Hardie, 1978). Such changes affect depauperate, with no well-defined fauna confined to lake ecology profoundly (Hammer, 1986; Williams, saline pans; appearing rather to comprise mostly 1998). tolerant freshwater forms (Seaman et al., 1991). Day Differences in zooplankton communities have (1990) found that crustacean species richness among been observed among lakes with different ionic small ephemeral pans in Namibia was largely influ- composition (e.g. Hecky & Kilham, 1973; Williams enced by isolation and did not correlate with salinity. et al., 1990; Bos et al., 1996; Derry et al., 2003) and However, in nearly all cases, where the hydrochem- the inverse relationship between zooplankton species istry and associated zooplankton of these saline lakes richness and salinity in saline lakes is well estab- and ephemeral pans have been sampled, data related lished (e.g. LaBarbara & Kilham, 1974; Green, 1986, to single samples and seasonal analysis is scarce 1993; Hammer, 1986, 1993; Williams et al., 1990). (Hutchinson et al., 1932; Allanson et al., 1990; Day, Other, or a combination of other abiotic and biotic 1990, 1993; Seaman et al., 1991). factors, such as predation and competition are also This study reports the hydrological, chemical and likely important for species richness and community biological characteristics of Sua Pan; part of the composition (Williams et al., 1990; Williams, 1998; Makgadikgadi salt pan complex, Botswana, one of Herbst, 2001). However, little is known about the the largest ephemeral lake systems in Africa. Until sequential response of zooplankton communities to our study, the chemical composition of Makgadikg- changing salinity and chemical composition in indi- adi was poorly understood and not included in vidual lakes. Melack (1988) studied changes in the reviews of saline water chemistry in southern Africa biota of Lake Elmenteita during a period of lake (Mepham, 1987; Allanson et al., 1990; Seaman et al., drying and salinity increase and suggested that 1991; Day, 1993). pH values from Sua Pan and small increased salinity directly affected a key macrofaunal pans adjacent to it, taken by the Rhodesian Schools element (Paradiaptomus africanus) and an important Exploration Society in 1957, were 9 and 10 (Eccles, primary producer (Anabaenopsis arnoldii), while D. H., Undated). Brendonck & Riddoch (1997) other biota were affected through altered biological recorded pH of 8.9–9.5 and conductivities of 5.2– interactions. Vareschi & Vareschi (1984), over sev- 21.3 mS cmÀ1 in the north basin of Sua Pan and eral years, not only associated disappearances of some small adjacent pans. Details on the aquatic biota Lovenula africana in Lake Nakuru with increases of of Makgadikgadi is also scant (Seaman et al., 1991). conductivities above 25,000 lS, but also identified Identification of invertebrate taxa collected by the that the availability of Spirulina platensis at the base Rhodesian Schools Exploration Society suggested the of the food chain was important for the zooplankters crustacean species composition in the Makgadikgadi and rotifer population dynamics. During a 2-year was unusual for southern Africa. Subsequent collec- period of drought, a considerable reduction in tions of ostracods (Martens, 1988), copepods (Rayner 123 Hydrobiologia (2008) 596:31–46 33 & Heeg, 1994) and anostracans (Brtek, 1967; Bren- lowest part of the Makgadikgadi complex, covers donck & Riddoch, 1997) have confirmed the presence approximately 3,400 km2, with an extensive catch- of some of the species identified in Eccles’ report. ment of about 27,000 km2, comprising Kalahari Similarities to crustacean communities in East Afri- sediment based calcisols, luvisols, vertisols and can lakes have been attributed to Makgadikgadis’ gleysols underlain by Carboniferous-Jurassic sand- relative proximity to East Africa and a similar stone and basalts (Thomas & Shaw, 1991). Most of chemical composition to East African saline lakes, the soils in the area, particularly those close to the with high HCO3 and CO3 concentrations (Seaman pan, have calcrete layers at shallow or moderate et al., 1991). This article provides the first detailed depths. The pan can be divided into three hydrolog- account of the Makgadikgadis’ crustacean commu- ically separate basins, the north, middle and south nity and its association with variable salinity and basins, separated by two sand spits that protrude from water chemistry. The work involved a detailed and the eastern shore (Fig. 1). During the wet season, five systematic survey over 2 years, and the results were rivers (Nata, Semowane, Mosetse, Lepashe and compared with other saline lakes of southern and east Mosope) flow intermittently onto their respective Africa. This provides an overview of the primary basins. The north basin sump, at 890 m above sea chemical characteristics contributing to the species level, is the deepest