Desiccation of the Sahara since the middle Holocene has eradicated all but a few natural archives recording its transition from a "green Sahara" to the present hyperarid desert. Our continuous 6000-year paleoenvironmental reconstruction from northern Chad shows progressive drying of the regional terrestrial ecosystem in response to weakening insolation forcing of the African monsoon and abrupt hydrological change in the local aquatic ecosystem controlled by site-specific thresholds. Strong reductions in tropical trees and then Sahelian grassland cover allowed large-scale dust mobilization from 4300 calendar years before the present (cal yr B.P.). Today's desert ecosystem and regional wind regime were established around 2700 cal yr B.P. This gradual rather than abrupt termination of the African Humid Period in the eastern Sahara suggests a relatively weak biogeophysical feedback on climate.

We investigated the impact of excess sediment pollution on the biodiversity of Lake Tanganyika, Africa. The lake’s basin is undergoing deforestation at an alarming rate; rapid erosion as a consequence of this deforestation is resulting in the discharge of large volumes of sediment into normally clear‐water littoral and sublittoral environments. We determined species richness patterns among ostracodes, fish, and diatoms for undisturbed, moderately disturbed, and highly disturbed areas to see if there is a consistent pattern of lower species richness in disturbed areas. Ostracodes are significantly less diverse in highly disturbed sites than in less disturbed ones for both soft and hard substrate littoral environments, with reductions in species richness ranging from 40% to 62%. Species richness patterns for profundal ostracodes show smaller differences between low‐ and high disturbance environments (7–32%) that are not statistically significant. Fish census data show a similar pattern to ostracodes, although the sample sizes are too small to be analyzed statistically. For the four water depths where comparative transects were made, species richness was between 35% and 65% lower at high disturbance sites than at low disturbance sites. Diatoms showed only minor and statistically insignificant reductions in species richness between low and high disturbance sites (15–20%). Ostracodes and fish may be more affected by sedimentation because they are mostly endemic and may require clearwater habitats, whereas the benthic diatom species in the lake are largely cosmopolitan and in many cases also occur in turbid affluent rivers (such as the Ruzizi). Fossil and sedimentological data from short cores are required to confirm whether the low diversity observed at high disturbance sites is a consequence of increasing sedimentation rates through time. Our results have implications for establishing and managing underwater reserves in Lake Tanganyika. They suggest that lake margins adjacent to small, erosion‐resistant watersheds will be most easily protected in the event of deforestation. Conversely, lake reserves established adjacent to large and/or easily eroded watersheds should incorporate those adjacent areas as part of the reserve to prevent subsequent sedimentation damage to the reserve ecosystems.

Vertical and latitudinal differences in bacterial community composition (BCC) in Lake Tanganyika were studied during the dry season of 2002 by means of denaturing gradient gel electrophoresis analysis of PCR-amplified 16S RNA fragments. Dominant bands were sequenced and identified as members of the Cyanobacteria, Actinobacteria, Nitrospirae, green nonsulfur bacteria, and Firmicutes divisions and the Gammaand Deltaproteobacteria subdivisions. The BCC in the lake displayed both vertical and latitudinal variation. Vertical changes in BCC were related to the thermal water column stratification, which influences oxygen and nutrient concentrations. Latitudinal variation was related to upwelling of deep water and increased primary production in the south of the lake. The number of bands per sample increased with bacterial production in the epilimnion of the lake, suggesting a positive diversity-productivity relationship.Since the first application of molecular tools to the study of the ecology of aquatic bacteria, bacterial community composition (BCC) has been studied in a wide variety of aquatic ecosystems, ranging from shallow to deep lakes to coastal seas and oceans. Molecular studies of BCC in lakes and rivers have revealed a consistent set of typical freshwater bacteria. Zwart et al. (51) discerned 34 putative phylogenetic clusters which occur in a wide range of freshwater environments. The dominant divisions include the Proteobacteria, Bacteroidetes, Verrucomicrobia, Cyanobacteria, Actinobacteria, and green nonsulfur bacteria. Urbach et al. (46), however, detected unusual bacterial communities in the ultraoligotrophic Crater Lake, in which there was a greater dominance of Gammaproteobacteria. So far, few studies have focused on the large, ancient lakes of the world or on tropical lakes. To our knowledge, no information is available yet on BCC in tropical lakes. Of the large lakes, data are available only for Lake Baikal (1,3,6,40). Lake Tanganyika is the third largest lake by volume and the deepest lake after Lake Baikal. Studies on bacteria in Lake Tanganyika have been limited to monitoring data on bacterial abundance and production (15, 39) or taxonomic studies dealing with specific bacterial groups (9, 10). As this lake is well known for its rich endemic fauna (4), studies on the BCC may be worthwhile.Lake Tanganyika is a meromictic, permanently temperaturestratified lake with strong vertical oxygen concentration gradients. This lake is anoxic below a depth of 100 to 200 m, and it contains the largest volume of anoxic freshwater in the world. The permanent temperature and oxygen gradients may affect the BCC in the lake. In stratified lakes and seas, vertical zonation of BCC has been found along temperature and/or oxygen concentration gradients (21). Several studies have demonstrated a relationship between BCC and oxygen concentration (21) or temperature (42). During the dry season, Lake Tanganyika also displays pronounced latitudinal differences in the depth of the thermocline. The differences in thermocline d...

Diatoms are frequently used as indicators of eutrophication in temperate systems, but little is known about their application to impacted African tropical systems. Five streams located within Gombe Stream National Park and five streams supporting human settlements draining into Lake Tanganyika, East Africa, were investigated for species composition, richness and diversity of epilithic algae. In addition, a trophic diatom index (TDI) developed for monitoring European rivers was applied to these tropical systems. 54 specific and infraspecific diatom taxa representing 20 genera were identified for all sites with Achnanthes s.l., Gomphonema and Navicula s.l. being the most common genera. Species richness varied between 10 and 21 in disturbed streams and 13 and 19 in undisturbed streams. Nutrients were significantly enriched in streams draining the deforested watersheds but indices of diversity and evenness (Shannon H, J and Simpson-Yule D, E) did not show any significant differences between streams in forested and deforested watersheds. Significant differences were observed between pooled data for the TDI between forested and deforested watersheds. Analysis of percent pollution tolerant diatom taxa indicates that organic pollution of streams in deforested watersheds may be contributing to eutrophication. This study shows that African diatoms, cosmopolitan or resembling well-known North American and European taxa, allows for trophic indices tailored to the autecological preferences of species to be applied to new regions, although intensive studies on these African taxa will lead to more accurate results. Measures of species-richness and diversity, historically used to describe the state of an ecosystem, may not be suitable to evaluate streams which are not grossly polluted.

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Multi-proxy analysis of a well-dated 25,000-year (25 ka) lake-sediment sequence from Lake Challa, on the eastern flank of Mt Kilimanjaro, reveal the climatic controls which govern both the lake’s palaeohydrology and the climate-proxy record contained in the mountain’s receding ice cap. The oxygen-isotope record extracted from diatom silica (d18Odiatom) in Lake Challa sediments captured dry conditions during the last glacial period and a wet late-glacial transition to the Holocene interrupted by Younger Dryas drought. Further, it faithfully traced gradual weakening of the southeastern monsoon during the Holocene. Overall, d18Odiatom matches the branched isoprenoid tetraether (BIT) index of rainfall-induced soil run-off, except during 25–22 ka BP and the last 5 ka when insolation forcing due to orbital precession enhanced the northeastern monsoon. This pattern arises because during these two periods, a weakened southeastern monsoon reduced the amount of rainfall during the long rain season and enhanced the opposing effect of evaporation intensity and/or length of the austral winter dry season. Importantly, our lake-based reconstruction of moisture-balance seasonality in equatorial East Africa also helps understand the oxygen-isotope record contained in Mt. Kilimanjaro ice. Negative correlation between ice-core d18O and Lake Challa d18Odiatom implies that temperature, not moisture balance, is the primary climate control on the long-term trend in ice-core d18O

We report 2 yr of monthly data from the Congo River (the second largest river in the world) on dissolved silicon concentrations, biogenic silica particle concentrations, isotopic signatures of dissolved silicon, and biogenic siliceous particle counts. Diatoms predominated in the biogenic silica fraction, especially during low flow; phytoliths and sponge spicules were more abundant than diatoms only during the rainy season, when biogenic silica fluxes are low. Biological processes dominated the seasonal variations of the dissolved d 30 Si ratio that are superimposed on a constant abiotic d 30 Si value of +0.70% 6 0.05% throughout the year. The measured biogenic silica concentration is less than the amount required to explain the monthly variations of dissolved d 30 Si signatures. We use these signatures and a Rayleigh isotopic fractionation model to calculate that 82% 6 7% of the diatoms produced each month would be exported out of the water column, probably through settling in the Malebo Pool or further upstream. The uptake of dissolved silicon by diatoms during low water flow periods could explain the absence of the dilution effect observed for the other major elements. Annual Si export to the estuary is 1.17 3 10 10 mol yr 21 in the form of biogenic silica and 2.23 3 10 11 mol yr 21 in dissolved Si form, with a mean dissolved d 30 Si of +0.96% 6 0.27%. Phytoliths make only a minor contribution to the annual biogenic silica flux, and dissolved Si fluxes predominate over biogenic Si fluxes.