Hydrochemical fluctuations and crustacean community composition in an ephemeral saline lake (Sua Pan, Makgadikgadi Botswana)

Abstract

Fluctuating hydrochemistry, as a result of extreme hydrological regimes, imposes major physiological constraints on the biota of ephemeral saline lakes. While the inverse relationship between salinity and zooplankton species richness is well-known across salinity gradients, few studies have documented closely the response of zooplankton to seasonal changes in salinity. Weekly sampling during two flood seasons at Sua Pan, an intermittent saline lake in central Botswana demonstrated the importance of spatial and temporal salinity gradients for crustacean community composition, associated with a decline in species richness, from 11 to three species. Conductivity ranged between 320 and 125,800 lS cm À1 during seasonal flooding; changing from dominance by HCO À 3 and CO À 3 , Ca 2+ and Mg 2+ , at the beginning of the floods, to NaCl dominated waters as the lake dried out and salinities increased. pH estimates generally ranged between 8.6 and 10, with maximum values recorded during initial flooding. Crustaceans comprised mainly Branchinella spinosa, Moina belli, Lovenula africana and Limnocythere tudoranceai, all of which occurred across a wide range of salinities, while halotolerant freshwater species (Metadiaptomus transvaalensis, Leptestheria striatochonca and the ostracods Plesiocypridopsis aldabrae, Cypridopsis newtoni and a newly identified Potamocypris species) disappeared above conductivities of 1,500 lS cm À1 . A unique crustacean composition in southern Africa was attributed to Sua Pans' rare chemical composition among southern African saline lakes; flood waters on Sua Pan contained a higher proportion of Na + and HCO À 3 , and less K + , Mg 2+ and SO 2þ 4 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 similarities decreased in lakes with higher pH and proportions of Na, and less SO 4 and Mg in their chemical composition. The predominant saline tolerant species on Sua Pan, however, showed a greater similarity to those in saline lakes in southern and East Africa with higher proportions of HCO À 3 and, particularly, Mg 2+ in their chemical composition.

Emergent effects of structural complexity and temperature on predator–prey interactions

“…The greater significance of this is that interactions within ephemeral ponds are in a state of flux owing to the highly dynamic nature of such systems and the frequent and extreme changes in environmental conditions that typically define these habitats (McCulloch et al. , Boven and Brendonck , Bird and Day , Brendonck et al. , O'Neill and Thorp ).…”

From landform to process: Morphology and formation of lake-bed barchan dunes, Makgadikgadi, Botswana

“…Within-basin sediments are consistently composed of greater silt and clay sized fractions and contain a much greater proportion of carbonate-rich and organic material. We interpret this as the mobilisation of sediment deposited under highly seasonal conditions not dissimilar from those found in the basin today, where shallow waters are rich in carbonate ions that precipitate to form sodium and calcium carbonates under high salinities (Shaw et al, 1990;McCulloch et al, 2008) and sustain a seasonal salt-tolerant algal mat growth principally made up of filamental chloroflexus colonies (Shaw et al, 1990). Under freshwater subaqueous conditions, it is likely that carbonate ions would remain in solution and carbonate content would be low as in the case of beach ridge sediments.…”

Pans, Playas and Salt Lakes

“…Regrettably these uses conflict with the inherent value of pans and playas as extreme, unusual and often valuable habitats (e.g. Haukos and Smith, 1994;McCulloch et al, 2008), while development itself is not without difficulties, as the flooding of Salt Lake City in the 1980s shows (Atwood, 1994). Scientifically they have become increasingly important for the elucidation of palaeoenvironmental conditions from their sediments and landforms (e.g.…”