Extreme Events in Lake Ecosystems

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Freshwater is one of our most valuable resources, providing water for drinking, agriculture, and inland fisheries, as well as supporting important biodiversity. Lakes contain approximately 87% of the liquid surface freshwater on earth, yet the quantity is limited and the quality under pressure from anthropogenic activities, exacerbated by climate change. Some of the most pervasive and concerning consequences of climate change on lakes are the direct and indirect effects of rising lake temperature, which has been reported in recent years as a result of global warming. Parallel to the increase in global air temperatures, studies have also shown an increase in the frequency and severity of extreme events, such as heatwaves, with climate models indicating that they will become more frequent and severe during the 21st century. Studies have also shown the occurrence of heatwaves in the oceans, and these have been demonstrated to have negative impacts on marine ecosystems worldwide (e.g., leading to mass coral bleaching). In comparison, we know very little about how thermal extremes in lakes unfold in time and what the associated impacts are. Understanding the occurrence of thermal extremes in lakes, introduced in this fellowship as 'lake heatwaves', is very important as aquatic ecosystems are strongly impacted by temperature extremes. The damage to lake ecosystems will first likely arise from lake temperatures that are either more extreme, more frequent, or more long-lived (e.g., the thermal tolerance limit of some species being exceeded for a critical duration). Previous studies have even demonstrated that atmospheric heatwaves can result in the development of harmful cyanobacterial blooms in lakes, which rank among some of the main causes of poor water quality issues, and are a serious health threat for cattle, pets, and humans. However, the influence of climate change on lake thermal extremes worldwide has not yet been studied. During this fellowship, I will establish a novel and necessary sub-discipline of lake research, which explores the occurrence and consequences of lake heatwaves, defined for the first time in this project as prolonged periods of anomalously high lake surface temperatures. In this fellowship, I will use globally unique satellite-derived lake surface water temperature data from approximately 1000 lakes from 1991-present to investigate lake heatwaves worldwide. Numerical lake models, driven by state-of-the-art climate model projections from 1861-2099 under three emissions scenarios (low, medium, high), will also be used to investigate how lake heatwaves have changed over the past century and how they will respond to projected future climatic variations. The global drivers of lake heatwaves will also be investigated, including a detailed analysis of the influence of over-lake meteorology, local factors such as lake depth and surface area, as well as the influence of lake ice-cover and water level, both of which are changing rapidly in our warming world. Finally, I will investigate the ecological consequences of lake heatwaves, including a detailed analysis of the influence of lake thermal extremes on the occurrence and severity of algal blooms in lakes and an increase in mass fish die-offs, which have large implications for the ecosystem services and habitat resources upon which many species, including humans, depend.