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

<p>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.</p>

<p>Chlorophyll <span class="inline-formula"><i>a</i></span> concentration (Chl <span class="inline-formula"><i>a</i></span>) is a key indicator of phytoplankton biomass. In Lake Tanganyika, Chl <span class="inline-formula"><i>a</i></span> 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 <span class="inline-formula"><i>a</i></span> 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 <span class="inline-formula"><i>a</i></span> dynamics with an unprecedented timespan and spatial coverage.</p>

<p>The analysis reveals distinct seasonal patterns in Chl <span class="inline-formula"><i>a</i></span> concentrations, with shallow regions (depth <span class="inline-formula"><i>&lt;</i></span> 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 <span class="inline-formula"><i>a</i></span> concentrations, each displaying distinct seasonal behaviours that reflect the lake's hydrodynamic cycle. Long-term trends indicate a decline in Chl <span class="inline-formula"><i>a</i></span> concentrations of <span class="inline-formula">−</span>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 <span class="inline-formula"><i>a</i></span> concentrations, mainly observed between November and April, tend to decrease over time at rates between -5 to <span class="inline-formula">−</span>15 % per decade when averaged over entire clusters. In contrast, highest Chl <span class="inline-formula"><i>a</i></span> 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 <span class="inline-formula"><i>a</i></span> distribution, with particularly sharp rises in extreme values.</p>

<p>The findings underscore the complexity of Lake Tanganyika's Chl <span class="inline-formula"><i>a</i></span> 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.</p>