# Investigating the Impact of Wildfires on Lake Water Quality Using Earth Observation Satellites
## 1 Introduction
Input of burned material is expected to increase water turbidity in lakes and to temporarily decrease water transparency. Following a fire, concentrations of phosphorus and nitrogen, the two most generally limiting nutrients for primary producers in lakes, frequently increase in aquatic systems [25; 26]. Recently, some general patterns and responses of wildfire effects on water quality have emerged [27], including increased concentrations of nutrients, ions, organic material, and a general decrease in water clarity, following post-fire increments in erosion and runoff while changes in chlorophyll-a were less consistent [28]. A recent review of 44 studies worldwide [5] comparing pre- and post-fire water quality field sampling data found that wildfires increased post-fire nutrient export relative to pre-fire levels within a one-year time lag between sampling and fire occurrence. Previous research on the impact of fires on lakes was restricted in terms of lake number, geographical regions, and ecological contexts, with the majority of studies carried out on a limited number of lakes (\(\leq\)15). In this context, there is a need for broad scale research on a large number of lakes, covering many geographical regions and different ecological settings, in order to assess the generalizability of the findings. Remote sensing techniques can be used to achieve these objectives by producing datasets providing global, objective, and consistent information on burned areas, active fires, and water quality products. These datasets are increasingly being used to monitor the state of the Earth's systems [29; 30]. Remote sensing data could help fill the knowledge gap on the long- and short-term effects of wildfires on lake water quality, complementing traditional field sampling data. The aim of this study was to investigate the potential impacts of wildfires on lake water quality at a global scale. To this end, we used satellite products developed by two separate projects on lakes (Lakes_cci) and fire (Fire_cci) managed by the European Space Agency Climate Change Initiative (ESA CCI). The dataset included 2024 lakes allowing for the investigation of a large number of lakes distributed globally and covering a wide range of hydro-morphological and ecological settings combined with burned area data from Fire_cci (FireCCI51; [31]) for a time period of about 20 years. In particular, we aimed at testing whether changes in the satellite-derived chlorophyll-a (Chl-a) concentration and turbidity represented signals of lake water response to fire disturbance, both in terms of the magnitude of peaks and development time following a sequence of fire and precipitation events in their catchments. The SPR approach was adopted because our investigation focused on the effects of wildfires on lakes via the terrestrial pathway, and the approach provided a framework to select lakes whose waters were likely to be most affected by wildfires, given their catchment characteristics and precipitation conditions. A final subset of 106 lakes distributed globally was investigated to evaluate the impact of fire disturbance on lake water quality, through interpreting changes in the satellite products of Chl-a and turbidity.