By Garfield Kwan & Christine Parisek

Wildfires have become a hot topic. Although wildfires are a natural part of some ecosystems (e.g. the chaparral biome), megafires (fires that burn >100,000 acres of land) are becoming increasingly common as the climate continues to warm and droughts intensify. As of late, California’s fourth largest wildfire, the 2024 Park fire, charred ~425,000 acres (just over the size of Greater London!) and has threatened the survival of spring-run salmon, potentially pushing them near the brink of extinction. Indeed, the Park fire abutted the Lassen National Park, 68.8% of which already burned in the 2021 Dixie Fire, the largest (~960,000 acres) single-source wildfire in California’s history. But while news reporting on the fires usually halts after the flames have been extinguished, wildfire impacts can be felt for decades to come [1]. Unfortunately, this includes the rivers and streams where salmons and other aquatic organisms spawn and call home – and physiological research on this topic remains nascent.

In a recently published paper conducted at Dr. Nann Fangue’s Fish Conservation lab at the University of California (UC) Davis, Delta Science postdoctoral fellow Dr. Garfield Kwan illustrated for the first time the acute physiological impacts of post-wildfire ash input on Chinook salmon (Oncorhynchus tshawytscha) yearlings [2]. This work was performed in collaboration with Drs. Trystan Sanders and Rod Wilson at the University of Exeter (United Kingdom), and with five UC Davis undergraduate students mentored by Garfield at the Fangue lab: Sammuel Huang, Kristen Kilaghbian, Cameron Sam, (Leo) Junhan Wang, and Kelly Weihrauch.

Here, we will first share the potential impacts of megafire (henceforth wildfire) on both the aquatic ecosystem and the fish, followed by some key findings in the study [2].

After the fires are extinguished, the forest is denuded of vegetation. Now amidst the smoldering remains of what was once a lush, thriving landscape, large pillars of black skeletal tree trunks line the skyline where green leaves and thick branches once covered. An eerie silence pervades, with no rustle of leaves anymore, and broken only by the occasional crackle of dying embers. Rivers and lakes are deprived of shading by overhead vegetation, leading to warmer water temperatures. The air smells heavy as soot and ash fly through the air. The sky is dusky orange as if you are watching an old film reel. At least the sunset is gorgeous. 

As summer transitions to the colder and wetter winter and spring seasons, a new problem emerges (Figure 2). Rainstorms, or spring snowmelts, wash over the denuded and charred landscape, which now include a blend of ash, heavy metals, fire retardants and organic volatile pollutants (such as polycyclic hydrocarbons commonly found in gas and oil). These chemicals are rapidly flushed into aquatic systems across the watershed. The ecological impacts of ash runoff can be felt for up to 15 years following the initial scorching according to a recent review on fires across the world [1]. Unsurprisingly, post-wildfire rainstorms have led to mass fish mortality events (e.g. Klamath River following the McKinney Fire 2022).

One of the lesser understood impacts of ash mixing into freshwater system is the increase in water pH. Briefly, ash input releases anions that bind and reduce H⁺ ions in the water, elevating water pH levels and decreased dissolved CO₂ gas. The rapid alteration of water pH and CO₂ levels immediately disrupts internal physiological conditions of organisms, which if left unmitigated would disrupt internal protein folding and function, impede nitrogenous waste (ammonia/ammonium) excretion, and lead to organ failure and mortality.

On average, aquatic systems experience an increase of 1 pH unit following a typical wildfire. If water pH becomes too alkaline, aquatic organisms will be killed. So how high is too high? The United States Environmental Protection Agency (US EPA) Water Quality Criteria recommends freshwater pH between 6.5 to 9.0 as suitable for aquatic life, and numerous past studies show pH >9 induces problems in physiological regulation for salmonids, especially in the ability to eliminate nitrogenous waste (ammonia/ammonium) and thus maintain normal blood plasma conditions.

Because post-wildfire ash input is known to increase pH levels, aquatic systems that are naturally near or exceed pH 9 would be amongst the most vulnerable habitats. In California, many of these aquatic systems support rare and threatened fishes. For instance, Lahontan cutthroat trout (Oncorhychus clarki henshawi) is a federally threatened species native to Pyramid Lake (NV, USA) where pH values reach 9.4. In another case, the federally threatened green sturgeon (Acipenser medirostris) is found in the Klamath River (CA, USA), which naturally exceeds pH 10 during summers. Both species possess unique adaptations to tolerate the high pH conditions, but other species are less tolerant. Furthermore, these are examples of ecosystems where additional alkalinity due to wildfire runoff could push conditions past the brink.

Ironically, aquatic systems with naturally low alkalinity (low buffering capacity) are also vulnerable to impacts of ash input. This includes the many rainfall and/or snow-melt fed mid- to high-elevation California streams that serve as critical spawning grounds for threatened and endangered salmonids. How might rainstorm events that wash ash into the streams influence a salmon run, or a salmon redd or the alevin during this vulnerable early life stage? Sadly we may continue to learn the answers to these questions in the near future in Deer and Mill Creeks, where stronghold populations of spring-run Chinook salmon are being exposed to impacts from the Park Fire.

Yet research examining organismal response to high pH exposure is scarce. In a previous study, nearly all rainbow trout were able to survive pH 9.5 for 72 hours [3]. In another study, Lahontan cutthroat trout naturally acclimated to pH 9.4 experienced ~50% mortality after 72 hours at pH 10 [4]. In both cases, trout were unable to recover blood pH back to normal levels, suggesting survival is time-limited at elevated pH. Neither study used ash to induce a high pH condition, nor did they concurrently test for warmer impacts typical of post-wildfire conditions. So in the Spring of 2023, we retrieved ash from a local control fire in near Davis (CA, USA), recruited help from an enthusiastic group of UC Davis undergraduates, and secured aquatic space and permission necessary to expose Fall-run Chinook salmon yearlings (Feather River Hatchery) to post-wildfire ash conditions [2].

In an effort to mimic the reported wildfire condition, we had to first test how much ash was needed to increase water pH by 1 unit. The water supplying the Center for Aquatic Biology and Aquaculture at the University of California Davis was relatively high in alkalinity – doubled that of the nearby Lake Berryessa and Putah Creek. Even so, it only took 0.25 g/L – or roughly 2 teaspoons of ash in a gallon of water to change the water pH from ~8.1 to ~9.2. Armed with this data, we were now ready to test how ash impacts salmon physiology.

Ash input spiked blood pH within just 1 hour of exposure, which the fish recovered from over the next 24 hours. Yet not all fish survived: we observed 20-33% mortality within 12 hours of ash exposure. This was surprising since past studies found related salmonids to survive pH 9.4 and pH 10 for at least 24 hours – and up to 72 hours. This means pH disturbance alone is unlikely to be the cause of death.

The culprit proposed in past high pH studies was nitrogenous waste ammonia (NH₃) and ammonium (NH₄⁺). Ammonia and ammonium levels can spike to lethal levels if blood pH isn’t returned to nominal conditions. While the ash exposure did increase nitrogenous waste in salmon blood over the 24-hour period, it was 2-2.5x lower than the plasma of Lahontan cutthroat trout that died and their LC₅₀ level (LC₅₀ = the concentration at which there are 50% mortality).

Ash input is also known to leach heavy metals such as copper (Cu) and chromium (Cr) – elements that could interfere with ion-transport critical to maintaining homeostasis. We measured 25 elements, and none was close to their reported LC₅₀. Ash in and of itself could also cause mortality, but again the reported LC₅₀(30 – 40 g/L) is much higher than that used in this study (0.25 g/L).

So what could have killed the fish? One potential culprit is polycyclic hydrocarbons (PAH), a volatile compound produced during the combustion of organic matters and is harmful to organisms. While we did not measure PAHs in our study, wildfires typically record PAH levels that are much lower than some of the reported LC₅₀ in trout. Instead, the most likely explanation is that the summation of the many stressors (despite being non-lethal on their own) led to mortality. Some people refer to this phenomenon as ‘death by a thousand cuts’. For example, although nearly all rainbow trout were able to survive pH 9.5 for 72 hours, the fish that were surgically implanted with a cannula experienced ~40% mortality [3].

Regardless, the fact that 0.25 g/L ash and an exposure to pH 9.2 for 12 hours could cause 20 – 33% mortality in 1-year-old salmon should be alarming to conservationists. There remains an abundance of urgent questions. How will more vulnerable life stages such as the exhausted spawning salmon returning from the ocean, or the redd and alevin fare? How should we protect sensitive species that are endemic to high pH aquatic systems like Pyramid Lake (NV, USA) or Eagle Lake? What about other low alkalinity environments such as the Amazonian Rainforest? 

At least two things are certain: there will be more wildfires, and we will need increased watershed management solutions before the matter gets worse. 

Garfield Kwan is a fish physiologist specializing in acid-base regulation of aquatic organisms. He was a Delta Science Fellow, and he is currently a Marie Skłodowska-Curie Actions postdoctoral fellow at the University of Exeter (United Kingdom) and a research associate at the University of California Davis (United States). Christine A. Parisek is a postdoctoral scholar at the University of California Davis and a Science Communications Fellow at the Center for Watershed Sciences.

Further Reading

1.        Paul MJ, LeDuc SD, Lassiter MG, Moorhead LC, Noyes PD, Leibowitz SG. 2022 Wildfire Induces Changes in Receiving Waters: A Review With Considerations for Water Quality Management. Water Resour. Res. 58. (doi:10.1029/2021WR030699)

2.        Kwan GT, Sanders T, Huang S, Kilaghbian K, Sam C, Wang J, Weihrauch K, Wilson RW, Fangue NA. In press. Fish Blood Response to Ash-Induced Environmental Alkalinization, and their Implications to Wildfire-Scarred Watersheds. Sci. Total Environ. In Press. (doi:https://doi.org/10.1101/2024.01.05.574400)

3.        Wilkie MP, Wood CM. 1991  Nitrogenous Waste Excretion, Acid-Base Regulation, and lonoregulation in Rainbow Trout ( Oncorhynchus mykiss ) Exposed to Extremely Alkaline Water . Physiol. Zool. 64, 1069–1086. (doi:10.1086/physzool.64.4.30157957)

4.        Wilkie MP, Wright PA, Iwama GK, Wood CM. 1993  The physiological responses of the Lahontan cutthroat trout ( Oncorhynchus clarki henshawi ), a resident of highly alkaline Pyramid Lake (pH 9.4), to challenge at pH 10 . J. Exp. Biol. 175, 173–194. (doi:10.1242/jeb.175.1.173)