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Long-term spatiotemporal variability of whitings in Lake Geneva from multispectral remote sensing and machine learning
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  • Gaël Many,
  • Nicolas Escoffier,
  • Michele Ferrari,
  • Philippe Jacquet,
  • Daniel Odermatt,
  • Grégoire Mariéthoz,
  • Pascal Perolo,
  • Marie-Elodie Perga
Gaël Many
Université de Lausanne

Corresponding Author:[email protected]

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Nicolas Escoffier
University of Lausanne - Institute of Earth Surface Dynamics
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Michele Ferrari
Université de Lausanne
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Philippe Jacquet
Université de Lausanne
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Daniel Odermatt
Eawag - Swiss Federal Institute of Aquatic Science and Technology
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Grégoire Mariéthoz
University of Lausanne
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Pascal Perolo
Université de Lausanne
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Marie-Elodie Perga
Universiy of Lausanne
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Abstract

Whiting events are massive calcite precipitation events turning hardwater lake waters to a milky turquoise color. The transitory nature of whitings and their variable spatial extent make them poorly captured by traditional monitoring. Herein, we use a multispectral remote sensing approach to describe the spatial and temporal occurrences of whitings in Lake Geneva from 2013 to 2021. Landsat-8, Sentinel-2, and Sentinel-3 sensors are combined and intercalibrated to derive the AreaBGR index and identify whitings using appropriate filters. 95% of the detected whitings are located in the northeastern part of the lake and occur in a highly reproducible environmental setting: a high Rhone River discharge (358.6 +/- 102.1 m3 s-1), air and water temperatures of 21.3 +/- 3.0 °C and 18.0 +/- 1.9 °C respectively, and during the stratified period (thermocline depth of 11.1 +/- 0.6 m). An extended time series of whitings in the last 60 years is reconstructed from a random forest algorithm and analyzed through a Bayesian decomposition for annual and seasonal trends in the number of whiting days. Results show that the annual number of whiting days between 1958 and 2021 does not follow any particular monotonic trend. The inter-annual changes of whiting occurrences significantly correlate to the Western Mediterranean Oscillation Index (WeMOI). Besides, spring whitings have increased since 2000 and significantly follow the Atlantic Multidecadal Oscillation index (AMO). Future climate change in the Mediterranean Sea and the Atlantic Ocean could induce more variable and earlier whiting events in Lake Geneva.