Spatial variability in nearshore sediment pollution in Lake Tanganyika (East Africa) and implications for fisheries conservation

Abstract

Climate warming coupled with local disturbances within lakes is an accelerating global problem. This issue is acute at Lake Tanganyika (eastern Africa), where warming and overfishing have resulted in declining rates of pelagic fish catches and structural damage to diverse littoral cichlid communities. This deterioration has fueled demand for alternative livelihoods, and thus conversion of shoreline-adjacent forests to agricultural fields and oil palm orchards, which in turn heightens the threat of siltation on nearshore benthic habitats. The spatial variability of sediment pollution is unknown, however, posing a barrier to effective conservation. This paper assesses the spatial patterns of nearshore sediment accumulation within the Tuungane Project co-managed area of Lake Tanganyika in Tanzania. Analysis of lead-210 data show that the mean nearshore mass-based sediment accumulation rate is 0.06 g * cm−2 * year-1 (g cm-2 yr-1) across six sites adjacent to deforested watersheds, double the mean rate (0.03 g cm-2 yr-1) at a comparable but undisturbed control site. Spatial variance among rates is best explained by distance to deltaic point sources and bathymetric gradients. Data documenting carbon flux demonstrate that organic matter burial is higher in surface sediments offshore from deforested watersheds, consistent with onshore land use changes that promote erosion. Knowledge of sediment pollution patterns, coupled with maps of rocky benthic habitats, provide the necessary framework for effective conservation planning of fisheries in the present era of accelerated human interactions with the lake and its watershed. Findings provide a model for improved integrated management practices in large tropical artisanal fisheries in other parts of Africa.