More Than Just Smoke
How wildfires contaminate natural water resources and induce surprising effects in aquatic animals
By now, anyone living in Canada is fully aware that wildfire season is upon us. So far this year, Canada has already had 1,402 wildfires, burning 1,275,957 hectares (ha); most of those fires were in the west. Alberta alone has already recorded over 500 wildfires to date, burning a total of 938,547 hectares (ha) of land. For comparison, by this time last year Alberta had half the number of wildfires it’s had so far this year (251 fires). But the area burnt this year is nearly 2000 times greater than the area burnt by this time last year (472 ha). This trend is not specific to Alberta. Both the frequency and intensity of wildfires has been increasing in Canada and around the world, and human-induced climate change is to blame.
The frequency and intensity of wildfires is affected by the warmer, drier, and windier conditions brought about by climate change. Warm, dry, and windy weather is sometimes referred to as fire weather because when these factors converge, they establish the conditions that are maximally favourable for wildfires to ignite and spread. Those are precisely the conditions that have been occurring in the west and driving the increased frequency and intensity of wildfires. What’s worse is that wildfires themselves release tremendous quantities of carbon into the atmosphere, which in turn drives climate change, leading to even warmer, drier, and windier conditions, and larger, more complex, and more intense wildfires.
Dr. Laura Chasmer, associate professor of geography and environment at the University of Lethbridge, draws a distinction between beneficial fires and devastating fires. Beneficial fires are those fires that have always burned through northern ecosystems. In fact, many ecosystems are dependent on wildfires to function properly. Relatively frequent fires, sometimes burning every 10 or 20 years (but usually more like every 20 to 40 years), are essential for reducing the quantity of standing fuel on the forest floor. They can change the structure of the landscape, opening up new migratory routes for animals to use to repopulate the burned area. They can liberate nutrients that are locked up in trees and other standing biomass, making the soil more nutrient rich to support regrowth and renewal. White and red pines have a thick, fire-resistant bark that can withstand temperatures common to these beneficial fires. Other species, like jack pine, actually need fire for their seeds to germinate. Trees like maple and birch are more vulnerable to these kinds of fires, which is why frequent wildfires favour coniferous (softwood) over deciduous (hardwood) forests.
Most wildfires are small, and half of them are ignited by lightning. According to Dr. Chasmer, the number of wildfires has doubled over the past decade, and they are expected to double again over the next decade (or sooner). Up to 97% of the area burnt by wildfires is caused by only 3% of wildfires. In other words, a relatively small number of large, devastating fires is responsible for most of the damage associated with wildfires. These devastating fires are burning hotter and deeper than the small, beneficial fires. The drier conditions brought about by climate change mean that peatlands, which are typically bog-like and wet, are becoming increasingly dry. Peatlands are important carbon sinks in the boreal region that sequester carbon from the atmosphere over time. Shallow layers represent recent carbon captured from the atmosphere, while deeper layers represent carbon from an earlier time. When hot, devastating wildfires burn into these peatlands, instead of burning just the top few centimetres of peat and liberating a few years worth of carbon to the atmosphere, they’re burning much deeper and liberating hundreds or even thousands of years of carbon all at once.
In addition to accelerating our changing climate through positive feedback, devastating wildfires are also wreaking havoc on our communities. In 2011, a massive wildfire ravaged Slave Lake, Alberta. The fire destroyed or damaged around 80% of the community’s homes, costing insurance companies $700 million. At the time, this was the costliest disaster in Canadian history. Then, in 2016—just five years after the Slave Lake disaster—a fire they called “The Beast” swept through Fort McMurray, another northern Alberta town and the centre of the province’s oil sands district. Some 90,000 people were evacuated from the area, representing the largest evacuation operation in the province’s history. The Fort McMurray fire caused $10 billion in damages, eclipsing the Slave Lake fire, and setting a new bar for the costliest wildfire in the country’s history. In 2021, Lytton, British Columbia, a small fruit growing village in southern B.C. recorded the hottest three consecutive days in Canadian history, at 46.6 ºC (116 ºF), 47.9 ºC (118 ºF), and 49.6 ºC (121 ºF) before a devastating wildfire burnt it into oblivion.
Then there’s the smoke. The smoke from Alberta’s most recent wave of wildfires has caused it to have the worst air quality in the world this week. And this poor air quality is affecting people’s health. Increasing wildfire activity in western North America has led to increasing quantities of toxic particulate matter accumulating in our atmosphere1. Particulate matter smaller than 2.5 microns can lodge deep in lung tissue and cause a range of different respiratory conditions2, such as bronchial inflammation and asthma, some of which can lead to other adverse effects, including cardiac dysfunction and premature death. Small particulate matter associated with wildfire smoke is actually more toxic than other kinds of small particulate matter, probably because of the contaminants it carries3, which can be toxic to human health as well as the health of other species.
One overlooked aspect to wildfires is their contribution to environmental contamination. Of course, there’s the air pollution. The atmospheric contaminants released by wildfires extend far beyond the small particulate matter that lodges in people’s lungs. Wildfire smoke contains a wide range of contaminants, including sulphur and nitrogen dioxide, carbon monoxide, a range of volatile organic molecules, and ozone. But wildfire contaminants can also flush into nearby lakes, rivers, and streams where they can exert their toxic effects. Some contaminants released by wildfires are nutrients4. A small quantity of nutrients released into a system is unlikely to have much of an adverse effect. However, as wildfires become larger and more intense, huge quantities of nutrients are introduced into aquatic ecosystems. The resulting ecological effects are not much different than when raw sewage is released into an aquatic system. The additional nutrients released to the system cause massive algae blooms, which in turn strip the water of oxygen in a process called eutrophication. Water devoid of oxygen cannot support diverse ecological communities.
Wildfires also release other contaminants that can get into surrounding water bodies, such as metals from the soil and various byproducts of combustion5. Most aquatic ecosystems can tolerate contaminant input from small, beneficial forest fires. But as wildfires have grown in frequency and intensity, so too has the quantity of fire-related contaminants entering our freshwater resources. In 2015, researchers from my lab became interested in understanding how these byproducts of combustion affected aquatic animals inhabiting water bodies receiving runoff after a wildfire6.
Studies led by Dr. Patrick Gauthier78 focused on understanding how specific combustion byproducts affected freshwater scuds (Hyalella azteca; planktivorous crustaceans related to shrimp) when they were presented on their own or in combination with other non-combustion contaminants, like metals. In one study he found that the combination of metals and combustion byproducts produced a shockingly powerful toxic effect. The resulting toxicity was hundreds of times worse than what would be expected from the sum of toxic effects exerted by the individual contaminants. This observation led to a series of follow-up investigations in an attempt to understand what was leading to such a strong toxic effect.
In one of the follow-up studies9, Gauthier observed that scuds exposed to combustion byproducts showed behavioural symptoms that mimicked a well known pesticide, malathion. Malathion is a common, organophosphate pesticide used in agriculture, backyard gardens, recreational areas, and in large-scale insect eradication programs, such as those undertaken to control mosquito populations. It is designed to kill by inhibiting normal neuromuscular function in target animals10. The way that malathion inhibits neuromuscular function results in an acute loss of coordination, which impairs the animal’s ability swim. Impaired swimming activity means that they’re not capable of foraging for food efficiently, nor are they able to escape predators.
The effect observed by Gauthier is evident in the two videos below. The first video shows normal scud behaviour. The animals are swimming easily, mostly around the perimeter of the arena. It’s common for prey species to stay close to structures (like the perimeter of the arena) where they remain inconspicuous to potential predators. The second video shows scuds after they’ve been exposed to a specific combustion byproduct. The animals are unable to swim normally owing to uncoordinated movements of their swimming appendages. What’s worse is that they remain in the open part of the arena where they are vulnerable to predators.
The researchers confirmed that the impaired swimming activity was due to the same kind of neuromuscular impairment that results from malathion exposure. In other words, the combustion byproduct was behaving like a pesticide.
In another follow-up study11, graduate student Raegan Plomp produced an extract from burnt plant material and used it to expose scuds. Once again, scuds exposed to the fire extract showed the same uncoordinated behaviour first observed by Gauthier, confirming the results of the original experiment. She also showed that fire extract alone produced the same kind of neuromuscular dysfunction as was observed in the Gauthier experiments. This work showed that contaminants associated with the burnt plant material were responsible for the uncoordinated movements, which increases the scuds’ vulnerability in waters receiving runoff from wildfires.
Those of us that have experienced wildfire smoke firsthand can attest to its potential for negative health effects. Most of the research attention has rightly been placed on understanding how smoke affects humans—particularly the most vulnerable among us, including the young, the old, and those who suffer from pre-existing respiratory conditions. However, very little attention is being paid to the secondary effects of wildfires; such as how they’re negatively affecting aquatic environments. We now know that contaminants associated with wildfires can produce surprisingly severe toxicological effects. Some wildfire contaminants can act like organophosphate pesticides like malathion. As the frequency and intensity of wildfires increase with the progression of climate change, it’s important that we understand how these fires affect our precious freshwater resources; if not for the diminished ecosystem services, then for the diminished quality of our drinking water.
Willmot, T.Y., D.V. Mallia, A. Gannet Hallar, and J.C. Lin. 2022. Wildfire plumes in the Western US are reaching greater heights and injecting more aerosols aloft as wildfire activity intensifies. Nature Scientific Reports. DOI: 10.1038/s41598-022-16607-3.
Liu, J.C. et al. 2017. Wildfire-specific fine particulate matter and risk of hospital admissions in urban and rural counties. Epidemiology. 28: 77-85. DOI: 10.1097/EDE.0000000000000556.
Aguilera, R. T. Corringham, A. Gershunov, and T. Benmarhnia. 2021. Wildfire smoke impacts respiratory health more than fine particles from other sources: observational evidence from Southern California. Nature Communications. DOI: 10.1038/s41467-021-21708-0.
Smith, H.G., G. J. Sheridan, P.N.J. Lane, P. Nyman, and S. Haydon. 2011. Wildfire effects on water quality in forest catchments: A review with implications for water supply. Journal of Hydrology. 396: 170-192. DOI: 10.1016/j.jhydrol.2010.10.043.
These combustion by-products are collectively known as polycyclic aromatic hydrocarbons (PAHs). Some PAHs can be introduced to freshwater systems naturally, such as those released from bitumen seeps. Others are introduced by atmospheric fallout and surface-level runoff from fossil fuel combustion and wildfires.
Gauthier, P.T., W.P. Norwood, E.E. Prepas, and G.G. Pyle. 2014. Metal-PAH mixtures in the aquatic environment: A review of co-toxic mechanisms leading to more-than-additive outcomes. Aquatic Toxicology. 154: 253-269. DOI: 10.1016/j.aquatox.2014.05.026.
Gauthier, P.T., W.P. Norwood, E.E. Prepas, and G.G. Pyle. 2015. Metal–polycyclic aromatic hydrocarbon mixture toxicity in Hyalella azteca. 1. Response surfaces and isoboles to measure non-additive mixture toxicity and ecological risk. Environmental Science and Technology. 49: 11772–11779. DOI: 10.1021/acs.est.5b03231.
Gauthier, P.T., Norwood, W.P., Prepas, E.E., Pyle, G.G. 2015. Metal–polycyclic aromatic hydrocarbon mixture toxicity in Hyalella azteca. 2. Metal accumulation and oxidative stress as interactive co-toxic mechanisms. Environmental Science and Technology. 49. DOI: 10.1021/acs.est.5b03233.
Gauthier, P.T., W.P. Norwood, E.E. Prepas, and G.G. Pyle. 2016. Behavioural alterations from exposure to Cu, phenanthrene, and Cu-phenanthrene mixtures: linking behaviour to acute toxic mechanisms in the aquatic amphipod, Hyalella azteca. Aquatic Toxicology. DOI: 10.1016/j.aquatox.2015.10.019.
For those who may be interested, malathion is an acetylcholinesterase inhibitor. Acetylcholinesterase is an important enzyme involved in neuromuscular coordination. It breaks down the neurotransmitter acetylcholine bound to post-synaptic receptors in the synapse of neuromuscular junctions. The byproducts acetate and choline are reabsorbed by the presynaptic neuron, where new acetylcholine is manufactured in preparation for the next neural firing. When the activity of acetylcholinesterase is inhibited, acetylcholine is not removed from the post-synaptic membrane, preventing the post-synaptic neuron from refiring; i.e., it’s under a constant state of stimulation.
Plomp, R.D., J.L. Klemish, and G.G. Pyle. 2020. The single and combined effects of wildfire runoff and sediment-bound copper on the freshwater amphipod Hyalella azteca. Environmental Toxicology and Chemistry. 39: 1988-1997. DOI: 10.1002/etc.4821.