Exploring past primary production and its linkage with hydroclimate changes in Lake Victoria

Lake Victoria, Africa’s largest lake, is located between the eastern and western branches of the East African Rift System, and its immense size but relatively shallow depth distinguish it from other lakes in eastern Africa. The Lake Victoria ecosystem serves as a stark example of the devastating impact human activities and anthropogenic eutrophication can have on freshwater biodiversity. However, despite its global significance as a case study of human-induced biodiversity loss, our understanding of the long-term ecological dynamics of the Lake Victoria ecosystem has remained surprisingly limited. Lake sediments provide valuable insights into the long-term ecological dynamics of Lake Victoria and the complex interplay between hydroclimatic variability, nutrient cycles, and biotic communities, which is crucial for deciphering the trajectories of the lake’s ecosystem evolution under multiple abiotic and biotic stressors in the past and future.

We present in a comprehensive paleolimnological study a detailed reconstruction of the changes in aquatic primary production and phytoplankton communities in Lake Victoria over the past 17 ka, based on novel bio-geochemical analyses of four well-dated sediment cores along a depth transect. Sedimentary pigments and biogenic silica were analyzed to infer aquatic productivity. Changes in sediment composition are supported by X-ray Fluorescence (XRF)-derived element geochemistry, ¹³C and ¹⁵N, and sedimentary phosphorus fraction analyses. In this study, we make use of this exceptionally extensive multivariate dataset to gain a more accurate view on environmental changes in LV from the latest Pleistocene to Holocene times.

Our comprehensive study of Lake Victoria's primary production patterns reveals a profound influence of regional hydroclimate changes on the lake's mixing and nutrient regime, ultimately driving the dynamics of primary producer communities. From its initial wetland phase (16.7 – 14.5 ka), Lake Victoria transitioned to an exorheic, productive system during the wetter Holocene with increased monsoonal activity. This period is marked by two distinct phases of elevated diatom production (11 – 9 ka and 7 – 4 ka). In the late Holocene, drier conditions with reduced water column mixing prevailed, leading to the emergence of a cyanobacteria-dominated phytoplankton community. Overall, these shifts in primary producer dominance underscore the lake's ecosystem sensitivity to hydroclimate variability. Our findings provide invaluable insights into the magnitude and direction of changes in Lake Victoria over time, highlighting the profound impact of natural and anthropogenic factors on the lake's aquatic ecosystem.