Abstract
Abstract. Equatorial East Africa has a complex regional patchwork of climate regimes, sensitive to climate fluctuations over a variety of temporal and spatial scales during the late Holocene. Understanding how these changes are recorded in and interpreted from biological and geochemical proxies in lake sedimentary records remains a key challenge to answering fundamental questions regarding the nature, spatial extent and synchroneity of climatic changes seen in East African palaeo-records. Using a paired lake approach, where neighbouring lakes share the same geology, climate and landscape, it might be expected that the systems will respond similarly to external climate forcing. Sediment cores from two crater lakes in western Uganda spanning the last ~1000 years were examined to assess diatom community responses to late Holocene climate and environmental changes, and to test responses to multiple drivers using redundancy analysis (RDA). These archives provide annual to sub-decadal records of environmental change. Lakes Nyamogusingiri and Kyasanduka appear to operate as independent systems in their recording of a similar hydrological response signal via distinct diatom records. However, whilst their fossil diatom records demonstrate an individualistic, indirect response to external (e.g. climatic) drivers, the inferred lake levels show similar overall trends and reflect the broader patterns observed in Uganda and across East Africa. The lakes appear to be sensitive to large-scale climatic perturbations, with evidence of a dry Medieval Climate Anomaly (MCA; ca. AD 1000–1200). The diatom record from Lake Nyamogusingiri suggests a drying climate during the main phase of the Little Ice Age (LIA) (ca. AD 1600–1800), whereas the diatom response from the shallower Lake Kyasanduka is more complex (with groundwater likely playing a key role), and may be driven more by changes in silica and other nutrients, rather than by lake level. The sensitivity of these two Ugandan lakes to regional climate drivers breaks down in ca. AD 1800, when major changes in the ecosystems appear to be a response to increasing cultural impacts within the lake catchments, although both proxy records appear to respond to the drought recorded across East Africa in the mid-20th century. The data highlight the complexity of diatom community responses to external drivers (climate or cultural), even in neighbouring, shallow freshwater lakes. This research also illustrates the importance of, and the need to move towards, a multi-lake, multi-proxy landscape approach to understanding regional hydrological change which will allow for rigorous testing of climate reconstructions, climate forcing and ecosystem response models.
Highlights
The climate of East Africa has exhibited high inter-decadal variability during the last 2000 years, whilst the short instrumental record is characterised by high magnitude and abrupt climatic events, most apparent in rainfall patterns (Nicholson, 1996, 2000; Nicholson and Yin, 2001; Conway, 2002)
The last 1000 years is a crucial period in East African history, during which time there were major societal transformations and political changes, which have often been linked to fluctuations in climatic conditions (e.g. Taylor et al, 2000; Robertshaw and Taylor, 2000; Verschuren et al, 2000; Robertshaw et al, 2004; Doyle, 2006)
Age models were constructed for both lakes using highresolution 210Pb and 137Cs analyses of the upper sediments and AMS radiocarbon dating of terrestrial macrofossils for the lower core sequences (Table 1)
Summary
The climate of East Africa has exhibited high inter-decadal variability during the last 2000 years, whilst the short instrumental record is characterised by high magnitude and abrupt climatic events, most apparent in rainfall patterns (Nicholson, 1996, 2000; Nicholson and Yin, 2001; Conway, 2002). Palaeoclimatic research in Africa is of immense importance, as it has the means to provide a historical and pre-colonial perspective of past environmental and/or climatic variability (caused by natural and anthropogenic factors). Taylor et al, 2000; Robertshaw and Taylor, 2000; Verschuren et al, 2000; Robertshaw et al, 2004; Doyle, 2006) This time frame is one of the most challenging in which to interpret regional climatic and environmental changes from East African lake sediment records, due to (1) problematic dating of sediments and (2) anthropogenic activity in catchments, especially agricultural (with the development of nucleated, permanent settlements) and the implementation of new technologies (e.g. iron technology and the associated forest clearance for the production of charcoal)
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