Combined Earth observations reveal the sequence of conditions leading to a large algal bloom in Lake Geneva

In this study the ¹⁴C measurements on soil organic matter, DOC and DIC clearly discriminate the source of carbon used by bloom algae. This enabled to identify a heterotrophic alga that consumes DOC, derived from the impounding of soil that has fallen into the lake as a result of landslides, as the main trigger of the bloom.

Freshwater algae exhibit complex dynamics, particularly in meso-oligotrophic lakes with sudden and dramatic increases in algal biomass following long periods of low background concentration. While the fundamental prerequisites for algal blooms, namely light and nutrient availability, are well-known, their specific causation involves an intricate chain of conditions.

Here we examine a recent massive Uroglena bloom in Lake Geneva (Switzerland/France). We show that a certain sequence of meteorological conditions triggered this specific algal bloom event: heavy rainfall promoting excessive organic matter and nutrients loading, followed by wind-induced coastal upwelling, and a prolonged period of warm, calm weather. The combination of satellite remote sensing, in-situ measurements, ad-hoc biogeochemical analyses, and three-dimensional modeling proved invaluable in unraveling the complex dynamics of algal blooms highlighting the substantial role of littoral-pelagic connectivities in large low-nutrient lakes. These findings underscore the advantages of state-of-the-art multidisciplinary approaches for an improved understanding of dynamic systems as a whole.

Reference: Rahaghi A. I., …, Hatté C., …, 2024. Combined Earth observations reveal the sequence of conditions leading to a large algal bloom in Lake Geneva. Communications earth & environment 5, 229 – doi: 10.1038/s43247-024-01351-5