# Investigating the Impact of Wildfires on Lake Water Quality Using Earth Observation Satellites
## 1 Introduction  
Climate change, particularly global warming and increasing drought, is leading to an increased frequency and severity of wildfires [1]. As a consequence of global warming, heatwaves are becoming more frequent, more intense, and lasting longer [2]. When high temperatures are combined with dry vegetation and low air humidity, it increases the risk of wildfire outbreak when there is a source of ignition [3]. This combination of conditions is now observed over longer periods of time over much of the world [4]. Among the many impacts of wildfires are those on aquatic ecosystems, both via atmospheric deposition of aerosols and via terrestrial runoff within lake catchments where fires occur. Wildfires can have an impact on river and lake water quality by altering the physical, chemical, and biological characteristics of soils and aquatic ecosystems [5]. Much research has focused on the consequences of wildfires on terrestrial ecosystems and airquality, while until recently the effects of wildfires on water quality have been relatively overlooked [6; 7; 8], while the majority of research concerning the aquatic impacts of wildfires is based on streams and rivers rather than lakes. In this study, we investigated how wildfires might affect lakes through the terrestrial pathway, potentially mediated by weather and catchment characteristics; in particular, we used the Source-Pathway-Receptor (SPR) approach. Environmental scientists first used the SPR conceptual model in the late 1970s to describe the potential flow of contaminants from a source to potential receptors via different pathways [9]. In freshwater systems, lakes are considered receptors or sinks in which material and pollutants tend to accumulate and pathways are processes of transport, dilution, and transformation of pollutants in their catchments. The SPR model has been applied as a simple conceptual model for illustrating systems and processes leading to a specific outcome in several environmental risk assessments [10; 11; 12], such as flood risks and erosion [13; 14; 15]. The SPR approach could hence be used to also show how the transport of burned material from wildfires can follow the terrestrial pathway via lake catchment, promoted by periods of rainfall, to finally reach a receptor lake and ultimately affect its water quality. It is in fact acknowledged that wildfires can have an important impact on the geomorphology and hydrology of river catchments, particularly regarding rainfall post-wildfire events leading to runoff, soil erosion, and transport processes which could result in lake water quality changes [16; 17]. These impacts are a function of time since the fire event, and depend on fire types, lake trophic status, and landscape characteristics [3]. Lake ecological context is indeed important to explain and predict lake responses to fire events, since impacts may be mediated by weather, catchment, and lake typology [5]. Studies in burned river catchments have revealed that soil erosion and increased sediment load can occur, with a magnitude depending on intensity and the frequency of post-fire rainfall and associated flow events [18; 19; 20], with increased runoff volume and velocity of transport to lakes in the catchment [21; 22]. Along with the key role of flow, particularly during storm events, research from a deforested catchment where vegetation regrowth was inhibited revealed a dramatic increase in particulate export owing to increased erodibility after two years, being more than twice as important as hydrological changes. Loss of dissolved nitrogen and phosphorus also increased tenfold [23]. This may be particularly relevant given the increasing frequency of fires that may limit regrowth leading to increased erodibility of soils with important consequences for stream and lake systems. In addition, wildfires have the potential to release nutrients and organic matter stored in soils and vegetation, allowing them to be transferred to aquatic ecosystems [24].