Multi-basin depositional framework for moisture-balance reconstruction during the last 1300 years at Lake Bogoria, central Kenya Rift Valley

Multi-proxy analysis of sediment cores from five key locations in hypersaline, alkaline Lake Bogoria (central Kenya Rift Valley) has allowed reconstruction of its history of depositional and hydrological change during the past 1300years. Analyses including organic matter and carbonate content, granulometry, mineralogical composition, charcoal counting and high-resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them. Thesepalaeolimnological data were supplemented with information on present-day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column. A new age model based on Pb-210, Cs-137 and C-14 dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake. An extensive set of chronological tie points between the equivalent high-resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins, enabling interbasin comparison of sedimentation dynamics through time. The resulting reconstruction demonstrates considerable moisture-balance variability through time, reflecting regional hydroclimate dynamics over the past 1300years. Between ca 690 and 950AD, the central and southern basins of Lake Bogoria were reduced to shallow and separated brine pools. In the former, occasional near-complete desiccation triggered massive trona precipitation. Between ca 950 and 1100AD, slightly higher water levels allowed the build-up of high pCO(2) leading to precipitation of nahcolite still under strongly evaporative conditions. Lake Bogoria experienced a pronounced highstand between ca 1100 and 1350AD, only to recede again afterwards. For a substantial part of the time between ca 1350 and 1800AD, the northern basin was probably disconnected from the united central and southern basins. Throughout the last two centuries, lake level has been relatively high compared to the rest of the past millennium. Evidence for increased terrestrial sediment supply in recent decades, due to anthropogenic soil erosion in the wider Bogoria catchment, is a reason for concern about possible adverse impacts on the unique ecosystem of Lake Bogoria

Growth of Clarias gariepinus in Lake Sibaya, South Africa

Pectoral spines from 750 Clarias gariepinus (Pisces:Clariidae) from Lake Sibaya were used for age and growth determinations. The spines have clear rings in the bone matrix. Rings formed on otoliths, vertebrae and opercula were less useful. A close correlation between ages determined from length frequency data and ages obtained by back-calculation of lengths for different ring classes exists. Spine rings are laid down annually, except in the first year. C. gariepinus in Lake Sibaya grow rapidly and remain in good condition to a length of about 500 mm (age 3+ years) but of the larger catfish, few exceed 650 mm (7+ years). Males grow slightly faster than females, and modal sizes are 580 - 590 mm TL, and 540 - 550 mm TL. Over one third of the adult population measures between 500 and 600 mm TL due to the sharp deceleration of growth rate after 500 mm TL. Modal length and growth rate of Sibaya catfish is equal to that of other C. gariepinus populations, but the number and condition of larger individuals is lower, probably because of inadequate food resources in deep water