Suspended Sediments in Lake Malawi, Africa: A Reconnaissance Study

In order to investigate the source and distributional pathways of suspended sediment in a tropical rift-valley lake, water-column profiles of light transmission (inversely related to water turbidity) were coupled with filtered water samples to determine total suspended sediment (TSS) concentrations at 28 stations throughout Lake Malawi (Lake Nyasa), East Africa, during January, 1992. TSS concentrations of the water column are typically between 0.1 and 0.5 mg/L, lower than those reported for the lower Laurentian Great Lakes. Elevated levels of turbidity are detected both at stations near the shoreline and offshore at four depths: at or near the surface, at the lake floor, at the chemocline, and in a mid-depth layer that is unique to the stations offshore of the Ruhuhu Delta. The latter had the most turbid water detected on this cruise and is interpreted as an interflow injection of fluvial material related to a 2-day rainstorm. Estimates suggest that this event contributed a significant portion of the annual sediment load to the delta. Other important sources of suspended sediment in Lake Malawi probably include biological productivity at the surface, chemical or biological processes at the chemocline, and resuspension events produced by wind-induced waves and currents and by seiche activity.