Leveraging 20 years of remote sensing to characterize surface phytoplankton seasonality and long-term trends in lake Tanganyika

Lake Tanganyika, the world's second-largest freshwater lake by volume, is a vital resource for millions in East Africa, providing water, food, and economic opportunities while supporting exceptional biodiversity. Chlorophyll a concentration (Chl a) is a key indicator of phytoplankton biomass. In Lake Tanganyika, Chl a is known to display strong spatiotemporal horizontal variability with an exceptionally low annual mean and wide ranges of concentrations compared to other tropical or temperate great lakes. This variability is influenced by the lake's hydrodynamic cycle driven by air temperature and wind seasonality. Phytoplankton biomass is suspected to be decreasing due to a strengthening of water column stratification induced by climate change. However, the particular spatiotemporal variability and trends in phytoplankton biomass have never been examined using a lake-wide, temporally continuous long-term record. This study bridges this gap by analyzing satellite remote sensing-derived Chl a data from the ESA Climate Change Initiative Lakes dataset across the entire surface of Lake Tanganyika over a 20-year period. It offers insight into the Chl a dynamics with an unprecedented timespan and spatial coverage. The analysis reveals distinct seasonal patterns in Chl a concentrations, with shallow regions (depth < 170 m) maintaining higher levels year-round, while deeper areas exhibit pronounced seasonality tightly linked to known wind patterns. To further explore these spatial differences in seasonal dynamics, the study identifies seven clusters of co-varying Chl a concentrations, each displaying distinct seasonal behaviours that reflect the lake's hydrodynamic cycle. Long-term trends indicate a decline in Chl a concentrations of −9 % per decade in deep regions, suggesting decreasing phytoplankton biomass. However, this overall decline is nuanced by monthly patterns. In deep regions, the low Chl a concentrations, mainly observed between November and April, tend to decrease over time at rates between -5 to −15 % per decade when averaged over entire clusters. In contrast, highest Chl a values recorded during the most productive months, from August to October, show increasing trends up to 25 %. Nearly all shallow areas, meanwhile, display year-round increases up to 35 % across the Chl a distribution, with particularly sharp rises in extreme values. The findings underscore the complexity of Lake Tanganyika's Chl a dynamics. The observed trends may have significant consequences for the lake's trophic structure and the communities dependent on its resources. Further research is needed to elucidate the underlying drivers of these changes and to assess their broader ecological and socio-economic impacts.

Affiliations

UCLouvainSST/ELI/ELIE - Environmental Sciences

Citations

APA
Chicago
FWB

Toussaint, F., Alonso, A., & Vanclooster, M. (2026). Leveraging 20 years of remote sensing to characterize surface phytoplankton seasonality and long-term trends in lake Tanganyika. Hydrology and Earth System Sciences, 30(9), 2543-2556. https://doi.org/10.5194/hess-30-2543-2026 (Original work published 2026)