DETECTING SPATIAL VARIABILITY IN SEDIMENT ACCUMULATION RATES AT THE TUUNGANE PROJECT CO-MANAGEMENT SITE AT NORTHERN MAHALE (LAKE TANGANYIKA, TANZANIA) USING 210PB

Lake Tanganyika’s fisheries are threatened by climate change, destructive fishing practices, and sediment pollution. The sediment pollution problem is hypothesized to result from onshore land use changes, which have and are converting native rift escarpment woodlands/forest to open fields for agriculture. If accurate, this hypothesis predicts that the steep hillslopes upon which deforestation occurs are subsequently subject to enhanced erosion during the summer monsoon season, leading to the transport of tilled soil to the lake via overland flow, and to increasing sediment loads delivered to deltas by rivers. The temporal and spatial variability associated with this type of sediment pollution is not well known, but SCUBA divers have reported the presence of silt blankets on rocky benthic habitats in parts of Lake Tanganyika’s littoral zone near where onshore deforestation has occurred. In this study, we report on a suite of lake sediment cores collected from the Tuungane Project co-management area at northern Mahale (Tanzania). The northern Mahale site encompasses onshore areas with pristine native vegetation, as well as agricultural lands, setting up a natural experiment to test the hypothesis that onshore land cover changes increase the rate of offshore sedimentation. ²¹⁰Pb age-depth models and sediment accumulation rates were calculated for eight sediment cores from the northern Mahale sub-littoral zone. The results show that linear and mass accumulation rates are higher at sites located offshore of deforested lands in comparison to rates offshore from areas with intact native vegetation. Disparities exist between linear and mass accumulation rates calculated for the same stations, especially in Nkonkwa, Mgambo, Buhingu, and Igalula Bays. In addition, considerable spatial variability in long-term mean sediment accumulation rates exists among the impacted sites; the cause of this variability remains an active area of investigation. These results validate inferences regarding higher sediment accumulation rates beginning in the 1960s, made from single core studies conducted by the Lake Tanganyika Biodiversity Project, and provide guidance for onshore conservation action to help protect nearshore benthic habitats used for fish spawning and rookeries from sediment pollution.