Fire Characteristics and Hydrologic Connectivity Influence Short‐Term Responses of North Temperate Lakes to Wildfire

Abstract

Despite increasing wildfires, few studies have investigated seasonal water quality responses to wildfire characteristics (e.g., burn severity) across a large number of lakes. We monitored 30 total lakes (15 burned, 15 control) monthly following the Greenwood Fire in Minnesota, USA, a lake‐rich region with historically prevalent wildfire. We found increases in median concentrations of total nitrogen (68%), total phosphorus (70%), dissolved organic carbon (127%), total suspended solids (71%), and reduced water clarity (48%) and pH (0.45) in burned lakes. Post‐wildfire responses in drainage lakes were often persistent or cumulative throughout the open‐water season, compared to isolated lakes. Total phosphorus (TP) increased linearly with watershed high‐severity burns, and shoreline high‐severity burns explained more variation in TP than lake morphometry and watershed variables. Post‐wildfire chlorophyll‐a responses were nonsignificant and inconsistent, possibly due to light limitation. Our results suggest that increasing wildfires have significant potential to affect water quality of inland lakes.

Investigating the Impact of Wildfires on Lake Water Quality Using Earth Observation Satellites

“…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. Our findings align with recent studies [28] in shallow oligo-mesotrophic lakes, suggesting that clear unproductive lakes could experience post-fire increases in primary productivity, but also that post-fire dynamics of nutrients and light limitation are important factors. Results from studies on ecological assessments of European lakes [56] suggested that light conditions and mineral availability are significant drivers of Chl-a concentrations in oligotrophic lakes.…”

“…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.…”

“…Moreover, the increased water turbidity could reduce light availability for phytoplankton growth, resulting in an overall Chl-a decrease, as observed in our study for different lake typologies. Other studies investigating the effects of wildfires on lakes reported significantly reduced lake water transparency primarily due to increases in dissolved organic carbon [26,28] and in turbidity. Subsequent changes in primary productivity would be mediated by both light availability and nutrient conditions, making the ultimate response of lake water quality to wildfires unpredictable.…”

“…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.…”

Wildfire smoke impacts lake ecosystems

“…Though we are unaware of any studies explicitly examining the role of catchment properties on particle mobilization to lake ecosystems, Brahney et al (2014) found that particulate deposition was more readily mobilized to lake ecosystems in steep, poorly vegetated catchments where up to 30% of the catchment-deposited material made its way to the lake basin. Precipitation and subsequent runoff can redistribute smoke and ash particles to lake ecosystems, which may occur many months post-deposition, particularly if deposition occurs on or beneath snow (McCullough et al, 2023). Further studies on smoke and ash deposition rates and redistribution are needed to understand the time scales for in-lake smoke and ash delivery and the associated physical, chemical, and biological responses.…”