Detecting lake mixing anomalies using Earth Observation
Authors: Calamita Elisa , Brechbühler Michael , Woolway Iestyn , Albergel Clement , Odermatt Daniel 
Published: February 2023

Lakes are responding rapidly to climate change and one of the most tangible responses is the increase in lake surface water temperature. Such an increase can intensify thermal stratification and dampen the intensity of vertical mixing. In turn, surface warming has the potential to alter the mixing regime of lakes, potentially leading to abrupt shifts in ecosystem functioning. Reduced mixing between the surface and bottom waters can indeed decrease the upwelling of essential nutrients from deep water to the lake surface and the oxygen transport in the opposite direction. This can result in a decrease in lake productivity and can increase the risk of anoxia at depth, respectively.Despite the important consequences of such lake mixing anomalies, we lack a systematic overview of their occurrence, mainly due to the lack of systematic data to detect and analyze them worldwide. Remotely sensed lake surface water temperature available from ESA CCI (Climate Change Initiative) and similar sources represent spatial skin temperature gradients, but they do not resolve vertical gradients. They are hence often used to prove the lakes&#8217; long-term warming in terms of spatial average. However, the horizontal gradients of such data could help us better understand the internal processes of lakes and the identification of lake mixing anomalies. Given that seasonal overturning often occurs at different times across the lake, the spatial character of remotely sensed data can reveal important processes in freshwater systems and can help assess the long-term variability in the overturning behaviour of large lakes in the context of climate change. Within our project, we use the spatial component of satellite Earth Observation data to reveal information about lake mixing and mixing anomalies. We apply a thermal front tracking method, a technique much more exploited in oceanography than limnology, to identify mixing anomalies in dimictic lakes worldwide.Our study suggests that the spatially distributed property of Earth Observation can be useful to spot lake mixing anomalies in dimictic lakes worldwide. Thus, we present the first global-scale assessment of lake mixing anomalies occurrence in the last 20 years. Earth Observation data can also be used to calculate how susceptible lakes are to undergo a mixing regime shift. Interestingly, we found that lakes experiencing more mixing anomalies are also those more susceptible to undergoing a mixing regime shift. Moreover, using Earth Observation, we detected mixing anomalies that have already been documented and, more interestingly, we spotted mixing anomalies occurring in unstudied remote lakes. Although further investigations would be needed to specifically assess the impact of climate change on these remote lakes, these cases highlight that remote sensing can be used as a first screening tool to spot lake mixing anomalies worldwide. Thus, Earth Observation and our methodology can be potentially used as an early warning system for lake mixing regime shifts.