Evidence of human and climate impacts on tropical freshwater crater lakes in western Uganda

Tropical freshwater lakes are critical natural systems of global importance. In western Uganda, crater lakes and their catchments provide vital ecosystem services to some of Earth’s fastest growing and most vulnerable human populations. However, these key services are under increasing threat due to climate change and the impact of human activities. Both climatic and anthropogenic impacts on these fragile ecosystems, such as ecological and biogeochemical functioning, are largely unknown. Previous palaeoecological research carried out in eastern Africa mainly focused on the great lakes, and/or have a focus on the long-term changes in climate. To fill in these gaps this project targets five crater lakes in western Uganda with contrasting catchments (i.e. “natural” vs heavily impacted by human activity) to test the over-arching aim; to distinguish anthropogenic impacts on these five crater lakes in the Bunyaruguru region, western Uganda. Sediment cores (spanning the last c. 50-600 years) were analysed for fossilized pollen and diatoms on a high-resolution. Satellite images (Landsat 7 & 8) covering AD 2000 – 2018 are used as a novel proxy which are applied to the palaeoecological records of vegetation through pollen analysis. The diatom results indicate several lake level changes in all five lakes and changes in diatom composition in the palaeo-records. Most of the detected changes are individualistic per lake, and can be interpreted as local stressors which can be driven by humans. Some changes occur in several lakes around the same time and imply a similar lake level change, which can be interpreted as regional. The pollen results show a change in vegetation in the surrounding area of two lakes. The rapid decrease of Poaceae and increase of Asteraceae at Lake Kabarogi can be an indicator of human activity. The top samples of almost all the sediment cores show a rapid increase in sedimentation rate and/or diatoms living in a non-aquatic environment, with can be associated with human activity. These signals can be used to distinguish (an increasing) human impact from natural drivers. Satellite imagery can still be challenging to use in tropical regions, however, it can be a useful tool for long-term monitoring vegetation and land-use changes, and the affects this might have on the nearby located lakes. Better understanding of past anthropogenic and climatic influences on these ecosystems can act as a guide to their future resilience and sustainability as natural resources providing vital ecosystem services for local populations under increasing environmental change.