Diatom-based models for inferring past water chemistry in western Ugandan crater lakes

Diatom surface sediment samples and corresponding water chemistry were collected from 56 lakes across a natural conductivity gradient in western Uganda (reflecting a regional climatic gradient of effective moisture) to explore factors controlling diatom distribution. Here we develop a regional training set from these crater lakes to test the hypothesis that this approach, by providing more appropriate and closer analogues, can improve the accuracy of palaeo-conductivity reconstructions, and so environmental inferences in these lake systems compared to larger training sets. We compare this output to models based on larger, but geographically and limnologically diverse training sets, using the European Diatom Database Initiative (EDDI) database. The relationships between water chemistry and diatom distributions were explored using canonical correspondence analysis (CCA) and partial CCA. Variance partitioning indicated that conductivity accounted for a significant and independent portion of this variation. A transfer function was developed for conductivity (r jack 2 = 0.74). Prediction errors, estimated using jack-knifing, are low for the conductivity model (0.256 log10 units). The resulting model was applied to a sedimentary sequence from Lake Kasenda, western Uganda. Comparison of conductivity reconstructions using the Ugandan crater lake training set and the East Africa training set (EDDI) highlighted a number of differences in the optima of key diatom taxa, which lead to differences in reconstructed values and could lead to misinterpretation of the fossil record. This study highlights issues of how far transfer functions based on continental-scale lake datasets such as the EDDI pan-African models should be used and the benefits that may be obtained from regional training sets

Diatom and stable isotope records of late-Holocene lake ontogeny at Indrepollen, Lofoten, NW Norway: a response to glacio-isostasy and Neoglacial cooling

Borg Indrepollen is a coastal lake on the island of Vestvågøy, Lofoten, NW Norway. A sedimentary sequence spanning the last 4500 cal. yr BP was analysed for diatom, C/N and stable isotopes (δ13C and δ15 N). The record provides clear evidence of glacio-isostatic rebound in the Lofoten region. Five distinct lithostratigraphic-geochemical zones (I—V) and four major diatom zones (A—D) were identified. The early record (I—III, A—Biii; 4500—550 cal. yr BP) contains coarse sedimentary material and diatoms indicative of more marine conditions. The correlation between the isotopic data (high δ13C and low C/N) and trends in the biological data (marine/brackish flora) suggest the marine influence on the Indrepollen basin is a controlling factor of the source of organic material. The latter part of the record (IV, C; 550 cal. yr 550—226) indicates a transitional phase from marine-dominated diatom to brackish taxa and is coincident with a section of microlaminations in the core. The youngest zone (V, D; 226 cal. yr BP—present) is indicative of an isolated basin, with the deposition of fine sediments; freshwater diatom taxa dominate the biological record and δ13C are indicative of freshwater conditions. C/N suggests a shift to a terrestrial source of organic matter. The proxies show a transition from full marine conditions, when Indrepollen was a submerged fjord, to more freshwater, lacustrine conditions in the last 200 years. The record of land uplift from Borg Indrepollen mirrors changes in sedimentary records from across Northern Norway and relative sea-level curves for the region

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

Equatorial East Africa has a complex, regional patchwork of climate regimes, with multiple interacting drivers. Recent studies have focussed on large lakes and reveal signals that are smoothed in both space and time, and, whilst useful at a continental scale, are of less relevance when understanding short-term, abrupt or immediate impacts of climate and environmental changes. Smaller-scale studies have highlighted spatial complexity and regional heterogeneity of tropical palaeoenvironments in terms of responses to climatic forcing (e.g. the Little Ice Age [LIA]) and questions remain over the spatial extent and synchroneity of climatic changes seen in East African records. Sediment cores from paired crater lakes in western Uganda were examined to assess ecosystem response to long-term climate and environmental change as well as testing responses to multiple drivers using redundancy analysis. These archives provide annual to sub-decadal records of environmental change. The records from the two lakes demonstrate an individualistic response to external (e.g. climatic) drivers, however, some of the broader patterns observed across East Africa suggest that the lakes are indeed sensitive to climatic perturbations such as a dry Mediaeval Climate Anomaly (MCA; 1000–1200 AD) and a relatively drier climate during the main phase of the LIA (1500–1800 AD); though lake levels in western Uganda do fluctuate. The relationship of Ugandan lakes to regional climate drivers breaks down c. 1800 AD, when major changes in the ecosystems appear to be a response to sediment and nutrient influxes as a result of increasing cultural impacts within the lake catchments. The data highlight the complexity of individual lake response to climate forcing, indicating shifting drivers through time. This research also highlights the importance of using multi-lake studies within a landscape to allow for rigorous testing of climate reconstructions, forcing and ecosystem response

Abrupt onset of carbonate deposition in Lake Kivu during the 1960s: response to recent environmental changes

This study interprets the recent history of Lake Kivu, a tropical lake in the East African Rift Valley. The current gross sedimentation was characterized from a moored sediment trap array deployed over 2 years. The past net sedimentation was investigated with three short cores from two different basins. Diatom assemblages from cores were interpreted as reflecting changes in mixing depth, surface salinity and nutrient availability. The contemporary sediment trap data indicate seasonal variability, governed by diatom blooms during the annual mixing in the dry season, similar to Lakes Malawi and Tanganyika. The ratio of settling fluxes to net sediment accumulation rates implies mineralization rates of 80–90% at the sediment-water interface. The sediment cores revealed an abrupt change ~40 years ago, when carbonate precipitation started. Since the 1960s, deep-water methane concentrations, nutrient fluxes and soil mineral inputs have increased considerably and diatom assemblages have altered. These modifications probably resulted from a combination of three factors, commonly altering lake systems: the introduction of a non-native fish species, eutrophication, and hydrological changes inducing greater upwelling. Both the fish introduction and increased rainfall occurred at the time when the onset of carbonate precipitation was observed, whereas catchment population growth accompanied by intensified land use increased the flux of soil minerals already since the early twentieth century due to more intense erosion

Environmental change over the last millennium recorded in two contrasting crater lakes in western Uganda, eastern Africa (Lakes Kasenda and Wandakara)

The last millennium is a key period for understanding environmental change in eastern Africa, as there is clear evidence of marked fluctuations in climate (effective moisture) that place modern concern with future climate change in a proper context, both in terms of environmental and societal impacts and responses. Here, we compare sediment records from two small, nearby, closed crater lakes in western Uganda (Lake Kasenda and Lake Wandakara), spanning the last 700 (Wandakara) and 1200 years (Kasenda) respectively. Multiproxy analyses of chemical sedimentary parameters (including C/N ratios, δ13C of bulk organic matter and δ13C and δ18O of authigenic carbonates) and biotic remains (diatoms, aquatic macrofossils, chironomids) suggest that Kasenda has been sensitive to climate over much of this period, and has shown substantial fluctuations in conductivity, while Wandakara has a more muted response, likely due to the increasing dominance of human activity as a driver of change within the lake and catchment over the length of our record. Evidence from both records, however, supports the idea that lake levels were low from ∼AD 700–1000 AD, with increasing aridity from AD 1100–1600, and brief wet phases around AD 1000 and 1400. Wetter conditions are recorded in the 1700s, but drought returned by the end of the century and into the early 1800s, becoming wetter again from the mid-1800s. Comparison with other records across eastern Africa suggests that while some events are widespread (e.g. aridity beginning ∼ AD 1100), at other times there is a more complex spatial signature (e.g. in the 1200s to 1300s, and from the 1400s to 1600s). This study highlights the important role of catchment-specific factors (e.g. lakemorphometry, catchment size, and human impact) in modulating the sensitivity of proxies, and lake records, as indicators of environmental change, and potential hazards when regional inference is based on a single site or proxy