Major low levels of Lake Malawi and their implications for speciation rates in cichlid fishes

Abstract

Lake Malawi, the third largest lake in Africa, is several million years old. Lake levels have fluctuated to a considerable extent in the late Pleistocene. Although tectonism may have influenced earlier level changes, the more recent changes have been climatically controlled. Major recessions occurred in the period before 25000 years ago and 10740 ± 130 years ago, with further large falls between 1150 and 1250 A. D. and within the period 1500-1850. The 1500-1850 lake recession-refilling cycle is documented by using a variety of techniques. Sediment cores show an erosional hiatus stretching across the southern area of Lake Malawi down to water depths of at least 121 m. Diatoms sharply decline in abundance and diversity across this break, with Melosira nyassensis dominating in the post-erosion period. During the low stage, exposed littoral sands were reworked into aeolian dune-fields along windward shorelines. Oral histories reflect a group memory of this low period, which is supported by 14 C dated archaeological finds in beach ridges surrounding the lake. Dating by 210 Pb methods show that lacustrine sedimentation had resumed by about 1860. At this time, early explorers, such as Livingstone, were reporting evidence of rising lake levels. Hydrological modelling shows that the lake-level changes indicated are possible in the timespan available. Various permutations of rainfall and timescale are discussed, e. g. a drop of 110 m over 250 years would require rainfall at 50% of modern values. The changes in lake level imply longterm changes in climate; these are highly relevant in the field of drought-risk assessment. The species flock of rocky-shore dwelling Lake Malawi cichlids known as ‘Mbuna’ contains about 200 species in Malawi’s waters. Mitochondrial DNA differentiation shows that the flock as a whole is of extremely recent origin. Almost every rocky outcrop and island has a unique Mbuna fauna, with endemic colour forms and species. As many of these islands and outcrops were dry land within the last 200-300 years, the establishment of the faunas has taken place within that time. The evolution of distinct forms in such a brief timespan is discussed in relation to current ideas on allopatric speciation. The present diversity of the Malawi cichlid-species flock, and particularly the Mbuna, may be readily explained by the rapidity with which small founder populations can diverge from the parent population, as demonstrated by the present chronological evidence on changes in lake levels and by the Mbuna distribution data. The repeated recessions and refillings of the lake have provided numerous opportunities for the establishment of different founder populations and consequently different selection pressures, leading to further bouts of speciation.

Influence of climate change on algal community structure and primary productivity of Lake Malawi (East Africa) from the Last Glacial Maximum to present

“…However, numerous independent proxies all provide firm evidence for a significant fall in lake level in the early Holocene. For example, a lowstand is noted in seismic records (Johnson and Ng'ang'a 1990;Owen et al 1990) and is dated to ,10.7 kyr B.P. (Owen et al 1990).…”

“…For example, a lowstand is noted in seismic records (Johnson and Ng'ang'a 1990;Owen et al 1990) and is dated to ,10.7 kyr B.P. (Owen et al 1990). A study (Brown et al 2000) of redox metals from a sediment core collected from the central basin of Lake Malawi indicates that the water column was oxygenated at a site that is now at a depth of ,300 m. Diatom assemblage records indicate poor preservation and indicate that the lowstand was centered at 10.6 kyr B.P.…”

The oldest fossil cichlids (Teleostei: Perciformes): indication of a 45 million-year-old species flock

“…The ability of cichlids to form species £ocks has been accredited to intrinsic factors of the ¢sh, including behaviour, leading to sympatric speciation or to extrinsic factors, such as the extensive shorelines of the Great Rift Lakes favouring isolation of populations, leading to micro-allopatric speciation (Greenwood 1964;Liem 1973;Fryer 1977;Dominey 1984;Owen et al 1990;Verheyen et al 1996;Seehausen et al 1997). The nature of a small crater lake indicates that the Mahenge cichlids may have speciated sympatrically as has been suggested for other crater lake cichlid £ocks (Schliewen et al 1994).…”

Western Indian Ocean marine and terrestrial records of climate variability: a review and new concepts on land–ocean interactions since AD 1660

“…11a; Holmgren et al 1999;Tyson et al 2002). A lake level record is available from Lake Malawi with annual resolution for the period 1650-1990Owen et al 1990;Johnson et al 2001). A tree ring record from Zimbabwe provides an annually resolved southern Africa summer rainfall reconstruction for November-February for the period 1796-1998 (Therell et al 2006; Fig.…”