# Investigating the Impact of Wildfires on Lake Water Quality Using Earth Observation Satellites
## 4 Discussion
The inclusion of latitude as a positive influence on the Chl-a maximum peak was surprising as the higher temperatures in the tropics lead to more efficient nutrient cycling and higher primary production [62]; however, concentrations of phosphorous and nitrogen have been reported as around double that of the tropics at mid-temperate regions and this may explain the positive relationship [63]. In addition, in broader terms, other secondary impacts of relevance for lake ecosystems include the reduction in oxygen concentrations which can result not only from an increase in chlorophyll-a levels but are also regulated by dissolved organic matter and lake area [64; 65]. In general, we found weak global relationships between burned areas in catchments and lake Chl-a or turbidity concentrations in our study. Finding strong relationships also proved to be difficult in other studies of wildfire impacts on freshwater systems [5], as findings suggested that the percentage of area burned did not affect the extent to which nutrient concentrations would increase after a fire. In a recent study of 15 lakes in the Superior National Forest in Minnesota [28], no post-wildfire changes were found in Chl-a despite significant increases in total phosphorous and total nitrogen in lakes with burned watersheds. However, in our SMLR analysis, we found that the inclusion of the mean ratio of burned area/burnable area as a secondary factor potentially represented an indication of fire impact on Chl-a peaks (+), likely induced by the input of nutrients to lakes released after a fire. Within a restricted group of lakes affected by important and regular fires, located in Africa, again depth was the most significant factor influencing Chl-a peaks and the burned area closer to the lake (calculated as level 0-4 in our study) was a secondary, although negative (-), weak factor. In this restricted lake group, the water residence time (+) was also a significant positive factor in determining Chl-a maximum peaks. A higher water residence time is generally related to greater primary production, with a longer time available for phytoplankton to utilize nutrients, and the persistence and the entirety of fire effects on lake ecosystems may increase with water residence time [3], with potential consequences on Chl-a increases. The results for lake turbidity responses to fires indicated a potential dependence on lake catchment and local weather conditions, with the precipitation index (SPI) in the SMLR analysis as a significant primary but weak factor for predicting turbidity maximum peak development. Within the restricted group of African lakes, additional catchment characteristics such as lake elevation (\(-\)) and the ratio catchment area/lake area (\(+\)) were significant. For turbidity, the location and the size of the lake catchment and how efficiently burned materials produced by wildfires are transported to a lake are key factors, since increases in turbidity concentration (especially when detected by satellite images) might be related to the load of burned material and sediments to a lake. River sediment yield has been estimated to increase with increases in area burned [66]. The ratio catchment area/lake area was also important for lake turbidity conditions and responses since lakes with larger ratios catchments/lake area tend to be more affected by catchment characteristics and dynamics, with stronger influences on their water chemistry. These lakes would be also more prone to the effects of wildfires, since post-fire hydrological processes in their drainage area, such as increased runoff and transport of burned material via terrestrial pathways, will be more effective in determining lake turbidity conditions and variations. In our dataset for instance, Lake Henrik (South Africa) has a large catchment area (70,460 km\({}^{2}\)), a high ratio catchment area/lake area (239), and very turbid waters (annual mean of 35 NTU), developing major peaks of turbidity concentration (up to a maximum of 202 NTU).