Latest Pleistocene and Holocene primary producer communities and hydroclimate in Lake Victoria, eastern Africa

The Lake Victoria ecosystem is emblematic of the catastrophic effects that human activities, particularly cultural eutrophication, can have on freshwater biodiversity. However, little is known about the long-term spatial and temporal pattern of aquatic primary paleo-production (PPaq) and producer communities in Lake Victoria and how these patterns relate to past climate variability, landscape evolution, lake hydrology, mixing regimes, nutrient cycling, and biodiversity dynamics in the past 17 kyr. We use sediments from four well-dated cores along a transect from offshore to nearshore sites, and exploit XRF element scanning and hyperspectral imaging data, TC, TN, bSi, delta C-13 and delta N-15, and sedimentary pigments to investigate paleolimnological variability and change. Our findings demonstrate that changes in PPaq and algal communities during the past 17 kyr were closely related to hydroclimatic changes, lake mixing, and nutrient availability. During the wetland phase (16.7-14.5 cal ka BP), PPaq levels remained generally low, while chromophytes and chlorophytes dominated the algal community. Following the rapid lake level rise (similar to 14.2 cal ka BP) during the early African Humid Period (AHP), PPaq levels steadily increased, accompanied by a shift towards cyanobacteria and chromophytes. During the Holocene, our results suggest repeated short-lived arid intervals (similar to 10.5, similar to 9, 7.8-7.2, similar to 4, and 3.2-3.0 cal ka BP) and two distinct periods of enhanced lake mixing associated with high PPaq and high diatom productivity: the first one between 11 and 9 cal ka BP, which coincided with the maximum of the AHP (high precipitation, high wind, enhanced mixing), and the second, less pronounced one, between 7 and 4 cal ka BP. Between these two periods (i.e. 9-7 cal ka BP) we observe reduced diatom productivity, relatively low PPaq, and high C/N ratios, suggesting conditions with more stable lake stratification, likely associated with reduced wind strength, and some nutrient limitation (N and P). Finally, the drier conditions around the end of the AHP (ca. 4 cal ka BP) and during the late Holocene were associated with decreasing lake mixing and increasing dominance of cyanobacteria. Given our reconstruction of PPaq over the past 17 kyr, we conclude that the levels in the 20th century are unprecedentedly high, consistent with the massive human-mediated impact on the Lake Victoria ecosystem including biodiversity loss.