Expressions of climate perturbations in western Ugandan crater lake sediment records during the last 1000 years

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.

Potential anthropogenic regime shifts in three freshwater lakes in Tropical East Asia

“…Persistently low relative abundances of Cyclotella meneghiniana Kützing are also noted in Sampaloc. N. palea and C. meneghiniana have been associated with heavy loadings of organic waste (Mills, Ryves, Anderson, Bryant, & Tyler, ). Furthermore, C. meneghiniana is associated with enrichment of sub‐tropical Lake Taihu (Ai, Bi, & Hu, ; Dong et al., ), and several reservoirs in Brazil, where it is also linked with inputs of wastewater and increased N availability (Costa‐Böddeker et al., ; Fontana et al., ).…”

Sensitivity of East African savannah vegetation to historical moisture-balance variation

“…Nevertheless, systematic differences in the percent abundance of characteristic taxa associated with each vegetation zone help to assess the magnitude of vegetation change which occurred at Lake Chibwera in the last ∼ 250 years. The largest fraction of pollen preserved in the sediments of small-and medium-sized lakes originates from vegetation in their near surroundings (Jacobson and Bradshaw, 1981;Broström et al, 2004;Duffin and Bunting, 2008), and is delivered to the lakes either by wind, precipitation or local surface run-off (Moore et al, 1991). This includes the typically airborne pollen of Poaceae, Euclea, Capparidaceae species, Flueggea virosa, Rhus-type vulgaris and other common savannah taxa, and of some trees in the forest of Kyambura gorge ca.…”

African hydroclimatic variability during the last 2000 years

“…On the western shoulder of the East African plateau, Lake Edward experienced a distinct drying trend over the course of the MCA, with most pronounced drought c. 1050 CE and after 1150 CE, and again ending c. 1200 CE (Russell and Johnson, 2007). This drought is also recorded at several maar crater lakes in western Uganda Mills et al, 2014). At Lake Tanganyika, the MCA seems to have been a period of declining humidity, culminating in drought dated to c. 1150 CE (Alin and Cohen, 2003;Stager et al, 2009).…”

“…In westernmost equatorial East Africa, which is more prominently influenced by moisture sourced in the Atlantic Ocean, lake records from western Uganda (Russell and Johnson, 2007;Mills et al, 2014) and Lake Tanganyika (Cohen et al, 1997;Alin and Cohen, 2003) display a humidity maximum from c. 1200 CE to 1500, followed by sustained dry conditions during most of the main-phase LIA. The high-resolution %Mg record from Lake Edward suggests that regional aridity at that time was comparable to that of climax MCA drought (Russell and Johnson, 2007).…”