Paleoecological analysis of Holocene sediment cores from the southern basin of Lake Tanganyika: implications for the future of the fishery in one of Africa’s largest lakes

Extensive research has been conducted at Lake Tanganyika to understand its vulnerability to a warming climate and fishing pressure. However, much of this work has been restricted to the more accessible northern basin. Studies from a limited geographic region of the lake are insufficient to explain whole-lake dynamics of the world’s longest lake (~ 670 km N-S). While strong evidence suggests that lake warming has played a critical role in decreasing fish abundance, limnological changes associated with warming have not been investigated lake wide. This study used paleolimnological data from two deep-water sediment cores from southern Lake Tanganyika to determine whether changes observed in the northern and central basins are, in fact, representative of lake-wide phenomena. We infer a decrease in diatom concentration after ~ 3000 cal yrs BP to reflect a decrease in net primary productivity associated with decreasing convective mixing. In contrast, the episodic presence of benthic invertebrates (ostracodes and molluscs) at these deep sites between ~ 1800–500 cal yrs BP, along with high Mn, indicates episodic pulses of much deeper ventilation of the southern basin than has been recognized previously. The presence of periphytic diatom species and benthic invertebrates during periods of strong stratification suggests that the lake bottom was periodically ventilated by descending denser (cooler or more sediment-rich) influent waters along the steep slopes of the coastline. Fish fossil abundance also is correlated with the dominance of heavily silicified diatom taxa, which itself requires stronger wave activity and upwelling of deep, nutrient-rich water. Fossil diatom assemblages show P and Si gradients between the two sites. LT17-TANG17-6A shows a shift in dominance towards lightly silicified taxa (Nitzschia spp.) after ~ 200 cal yrs BP. LT17-TANG17-2A shows a reduction in concentration of mostly the lightly silicified taxa, with few periods having heavily silicified taxa. These results are indicative of a relative reduction in convective lake mixing but are not always coincident with temperature trends, suggesting local windiness may also be important for stratification history. Thus, changes in lake productivity in the southern basin appear to be climate mediated, but in ways not previously documented in the northern and central parts of the lake.