# Investigating the Impact of Wildfires on Lake Water Quality Using Earth Observation Satellites
## 4 Discussion
Although precipitation generally promoted an increase in lake turbidity, on the other hand direct precipitation on the lake surface might dilute terrestrial inputs, resulting in an overall decrease in turbidity concentration. Lathrop [67] found that larger lakes have a greater capability of diluting the inputs of transported material from burned areas, even when a significant proportion of the lake catchment area was burned (25%). When evaluating fire effects on global lakes via terrestrial pathways after precipitation events, the increased turbidity from post-fire runoff might thus be difficult to detect consistently, due to the concurrent diluting effect of precipitation, especially for lakes of large surface area. The impact that wildfires have on soil properties is intricate and depends on soil characteristics, fire intensity, and duration [68], with soil type and texture having an important role in influencing erosion [69]. A previous study found erodibility was a more important factor than hydrological changes in particulate export from a deforested catchment where regrowth was prevented but the influence of erodibility was only clearly visible after two years, complicating the timescale over which the full range of impacts may be expected [23]. In this study, soil type data were not available at global scale and could not be included in our analysis, thus limiting the interpretation of our findings for water quality responses. Our study gave an indication of the timing of Chl-a and turbidity peaks occurring after a fire: the first detected turbidity peak and maximum Chl-a peak can highlight a succession of events in a lake catchment generated by terrestrial transport of burned materials and possibly of nutrients to the lake, inducing phytoplankton responses detected as Chl-a concentration changes. We found that lakes with a ratio BA/A > 0.25 and a burned area closer to the lake basin (level 0-4) tended to show a faster response to fire in developing a maximum Chl-a peak (90 days), suggesting a promoted and accelerated primary production by nutrient input to the lakes, possibly derived from the release of burned and eroded soils after fire occurrence. The peak timing in our results agree with Raoelison et al. [5], since nutrient concentration increase after a fire was generally significant within a year. In our study, when considering global responses to fire occurrence in the less productive lakes (mean Chl-a < 5 mg m\({}^{-3}\)), they were seen to respond slightly faster in terms of the timing of Chl-a development (days) after a fire, but with lower peaks in concentration than lakes at higher trophic conditions. A faster response in oligotrophic lakes might be related to their natural condition of nutrients limitation, with potential input from fire-induced runoff stimulating prompt phytoplankton growth. Although the maximum Chl-a concentration reached was found to be proportional to lake trophic status (Figure 8b), it should be remembered that a relatively small increase in Chl-a concentration in a low productivity lake could be very ecologically relevant, promoting changes in the nature of the whole ecosystem. Some reasons for observing weaker Chl-a increments in oligotrophic lakes after a fire might be due to light reduction caused by fire-derived burned materials, as these lakes generally have high transparency (excluding humic lakes). These may be related to short-term reductions in light availability, which could inhibit phytoplankton growth during the first vegetative period after a fire, regardless of increased nutrient inputs [70; 71], and oligotrophic clear lakes might be more sensitive to light attenuation caused by burned materials and increased dissolved organic carbon from runoff [26] than already less transparent and more turbid eutrophic lakes.