# Investigating the Impact of Wildfires on Lake Water Quality Using Earth Observation Satellites
## 4 Discussion  
The unique datasets from the Lakes_cci and Fire_cci projects, providing global long-term consistent data at high temporal resolution, allowed us to develop one of the first studies into how the water quality of lakes responds to fire impacts via the terrestrial pathway at a global scale. The use of the SPR approach allowed for the identification of lakes that were potentially more prone to be affected in their water quality by fire-derived substances (burned materials and released nutrients) transported via terrestrial pathways in their catchments. We investigated the effects of wildfires on lake water quality using different data analysis approaches to identify parameters that could help in explaining lake responses in terms of Chl-a and turbidity variations. These included the calculation of lake Chl-a and turbidity concentrations and peaks during the study period (2017-2020), as well as calculation of successive peaks in Chl-a and turbidity that developed after a fire occurred  
Figure 11: Time series data of burned area (km\({}^{2}\)), standardized precipitation index (SPI; red and light blue boxes: extremely dry and wet conditions, respectively), water temperature (LSWT; C\({}^{\circ}\)), Chl-a (mg m\({}^{-3}\)), and turbidity (NTU) in Lake Togo (Togo), Lake Mtera (Tanzania), and Lake Hirakud (India). in a lake catchment. In addition, with the purpose of detecting fire-induced short-term changes in lake water quality, we compared pre- and post-fire values of Chl-a and turbidity within six months after a major fire occurrence in their catchment. Time series plots of indicative parameters for fires, catchments, weather (SPI), and lake water quality responses were also generated, with graphical analysis used to investigate the temporal succession of events in the catchment of the selected lakes and to support global findings. Among the large number of hydro-morphological lake characteristics considered in the analysis, we found that lake average depth was a significant factor for determining the Chl-a concentration of maximum peaks, as values differentiated among three lake depth typologies (shallow, medium, and deep), being typically higher in shallower lakes. This result was confirmed by multiple linear regression, since lake depth was a significant negative parameter for predicting maximum Chl-a peaks concentration, followed by lake latitude (+), mean ratio BA/A (+), which is a proxy for the extension of the fire event, and mean SPI (-). The results of our study, finding higher Chl-a in shallower lakes than in deeper lakes, are in agreement with other studies [56; 57] concluding that water transparency and Chl-a concentration in European lakes were greatly related to lake depth, with shallow lakes having higher Chl-a and deep lakes having higher transparency. Shallow lakes typically have higher production per unit area when compared to deep lakes with similar phosphorus and total dissolved matter concentrations, as a result of faster nutrient cycling and better mixing and lighting conditions [58; 59]. For fire-burned materials and soil-released nutrients transported via a terrestrial pathway, especially by increased runoff in a lake catchment after precipitation events, the morphology of the final receptor lake is therefore a determinant for the internal process dynamics and ultimately for lake primary production [8]. It is long acknowledged that lake basin morphology plays a key role in modulating lake water quality, and in particular depth has important consequences on mixing regimes, light, and nutrient availability, affecting biological production and processes within a lake. Early studies on lake eutrophication such as that by Vollenweider [60] developed empirical relationships between lake mean depth and tolerable phosphorus loading, with mean depth also important in representing a general indicator of hydraulic residence time [61].