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

Abstract

Extensive research has been conducted at Lake Tanganyika with the aim of understanding its vulnerability to both 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, that stretches ~670 km (3.4–8.9°S), across several bathymetric basins. While strong evidence suggests that lake warming has played a critical role in decreasing fish abundance in Lake Tanganyika, limnological changes associated with warming have not been shown to be a lake- wide phenomenon. Paleolimnological methods can be used to compare the trajectories of environmental change among regions. This study examines southern Lake Tanganyika to provide insight into whether paleo-environmental changes observed in the northern basin are, in fact, representative of lake-wide phenomena, and in the process help to improve fisheries management. Here, we present new paleoecological analyses from two deep-water sediment cores from the southern basin of Lake Tanganyika. Fossil diatom assemblages show a shift in dominance towards lightly-silicified taxa (Nitzschia spp.) after ~200 YBP, with fewer 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. A general decrease in diatom concentration in the late Holocene is probably reflective of a decrease in net primary productivity in response to the inferred decrease in convective mixing. In contrast, the episodic presence of deep benthic invertebrates (ostracodes and molluscs) at these deep sites, which also co-occur with high Mn, indicates infrequent and short-lived pulses of much deeper ventilation of the southern basin than has been recognized previously. The coincident 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. Thus, changes in lake productivity in the southern basin appear to be climate-mediated, but in ways not previously documented in the northern part of the lake.