title: The paleolimnological perspective on the evolution of the Lake Victoria ecosystem
Creators: name: Wienhues, Giulia
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 550 Earth sciences &amp; geology
subject: 570 Life sciences; biology

description: Lake Victoria is a key example of both exceptional biodiversity and increased human pressure leading to ecosystem degradation. Although this lake is known for one of the most diverse and renowned radiations of endemic fish species, many of which are facing extinction, the origin and evolutionary linkage of these species to past climate fluctuations, nutrient cycles, and plankton communities have remained unknown. This thesis aims to shed light on the paleoenvironmental context of aquatic evolution in Lake Victoria over space and time by studying four long sediment records across a depth transect ranging from deep-water to nearshore, shallow-water sites.
The initial study addresses the key question of how Lake Victoria refilled during the Latest Pleistocene and the paleoclimate conditions influencing its re-emergence after a dry period. By reconstructing the hydrological variability of the lake through a detailed analysis of four new sediment cores, the research sheds light on previously debated aspects of its refilling history following the Latest Pleistocene desiccation. The analysis reveals that extensive wetland ecosystems existed before the current lake was established, suggesting rapid refilling driven by changing precipitation. This refined history aligns with regional climate records, highlighting complex past conditions in Eastern Africa.
The second study investigates spatiotemporal changes in primary production within Lake Victoria over the past 17,000 years and their link to environmental factors. To gain a comprehensive understanding of Lake Victoria’s long-term paleolimnological evolution, we employed a multiproxy approach on a set of long sediment cores. The results reveal phases of ecosystem shifts driven by hydroclimate, mixing, and nutrient availability. During the initial wetland phase, green algae and chromophytes dominated. With lake-level rise during the African Humid Period, aquatic productivity increased, leading to cyanobacteria and chromophyte dominance. Strong mixing and high primary production with diatom blooms characterized the early and mid-Holocene.
A collaborative study examines eutrophication and ecological consequences in Mwanza Gulf over the past century. This study of short sediment cores from this southern embayment indicates that the lake is sensitive to human impacts, such as increased nutrients and rising water levels, which can lead to eutrophication and disruption of the food web. The study’s key findings indicate a gradual increase in algal production since the 1920s, reaching its peak in the 1980s, which reflects the growing eutrophication of the lake. Additionally, the study reveals the significant impact of the 1960s lake-level rise on cladocerans, indicating the effects of habitat alteration. Finally, the study found no clear evidence of predation control or grazing pressure on plankton communities.
Collectively, this thesis unravels the past of Lake Victoria’s ecosystem, revealing its interconnected evolution with climate and human development over thousands of years. By integrating advanced methodologies and various biogeochemical markers, it paints a detailed picture of the lake’s changing conditions, highlighting the diverse environments within this large tropical ecosystem, and provides important baseline conditions for interpreting the evolutionary trajectories in Lake Victoria.