Whiting Events in a Large Peri‐Alpine Lake: Evidence of a Catchment‐Scale Process

Whiting events are transient phenomena commonly occurring in hardwater lakes and manifesting as a turquoise coloration of surface waters during massive calcium carbonate precipitation. While biological and physico-chemical drivers of carbonate precipitation are known, their relative contributions in controlling whiting events' timing and spatial extent remain poorly understood. Coupling spatially resolved data obtained for two sampling surveys using multiple analytical techniques and geochemical modeling, this study investigated the mechanisms underlying a whiting event during the early summer of 2019 in Lake Geneva. Satellite observations showed that the phenomenon started during a snowmelt period in the catchment at the Rhône River delta before spreading along the lake's northern shore and covering vast areas of its deeper basin. Authigenic calcite precipitated at the river mouth during mixing of warmer calcite super-saturated lake surface waters with colder snowmelt-diluted, sediment-rich river water containing detrital carbonates as potential nucleation sites. The development of the whiting event depended upon the thermal stratification of the water column and the existence of a physically stable metalimnion, within which a river interflow transported finer particles across the lake. During transport, the whiting plume enriched in authigenic carbonates by settling of coarser detrital particles and additional precipitation likely both on the fine-grained carbonate fraction and through biologically induced mechanisms in the superficial layers of the lake. This study provides novel mechanistic insights on the conditions controlling whiting events in lakes, highlighting a tight coupling of their dynamics with processes acting at the catchment scale.