Deep oxygen status of Lake Geneva: a zombie lake or a prematurely pronounced death?

Marie-Elodie Perga , Damien Bouffard

Lake Geneva has become an emblematic example of a lake trapped in an apparent "zombie" state, where deep-water hypoxia persists despite decades of declining phosphorus concentrations. The persistence of oxygen depletion has largely been attributed to climate-driven reductions in deep winter mixing, assuming a limited responsiveness to successful nutrient management. The analysis of the 53-year record of dissolved oxygen profiles reveals that hypolimnetic oxygen consumption has declined markedly since 2000, but only in the deepest waters (>275 m), where oxygen depletion is most critical for hypoxia development. A mechanistic oxygen-consumption model indicates that the decline is primarily attributable to reduced sediment oxygen uptake rather than changes in water-column mineralization. In parallel, recurrent deep-water oxygen replenishment occurring below the depth of winter convective mixing points to an additional oxygen source, possibly associated with river underflows. Together, these processes partially offset the documented decline in deep winter mixing caused by climate warming, indicating that phosphorus management has improved the lake's resilience to deoxygenation. These findings remind that long-term oxygen dynamics cannot be inferred from changes in mixing alone and highlight the importance of considering both oxygen consumption and alternative oxygen-supply pathways when assessing lake responses to climate change.