Wildfire smoke threatens lakes, too. How will it impact Lake Tahoe’s future?

Lake Tahoe, California — On a clear day at Lake Tahoe’s beaches, visitors to the cobalt waters of North America’s largest alpine lake don’t see signs of smoke from wildfires raging across much of California in what is shaping up to be one of the worst fire seasons in the state’s history. 

Historically, wildfires haven’t been seen as much of a threat to lakes like Tahoe—after all, fire and water don’t mix. Efforts to "Keep Tahoe Blue," as the ubiquitous bumper stickers here say, have primarily targeted impacts of mass tourism.

But a flurry of new research has begun to highlight the connection between lakes and wildfires—or, more specifically, the smoke that fires produce. Recent studies reveal that California’s lakes—and indeed lakes across all of North America—are increasingly exposed to wildfire smoke at much higher intensities and for longer durations than just a few years ago.

It’s still unclear what this will mean for lakes, but scientists warn that the impacts could be significant. Increased smoke may alter light and temperature conditions in lakes, leading to disruptions in the ecosystem’s food webs and energy flows. Nutrient-rich ash deposited in lakes could stimulate harmful algae growth.

“It could potentially change entire ecosystems,” says Sudeep Chandra, director of the Global Water Center at the University of Nevada, Reno, who has been at the forefront of the research efforts.

He and others point out that lakes are already among the most vulnerable habitats found in nature, with some lakes, like Tahoe, under enormous pressure from tourism and building developments, and many lakes suffering from biodiversity loss, species invasions, and an array of climate change impacts. “Wildfire smoke is adding another major threat to lakes,” Chandra says.

Smoke days

In the U.S., California is one of the states most affected by climate change, and wildfires have grown more frequent and destructive throughout the state. This year alone, fires—including the still-active Park Fire—have scorched four times the average acreage of the past five years.

To examine how climate change impacts lakes, scientists have previously concentrated on issues like reduced snowpacks—the amount of compressed snow that melts and trickles down into streams, rivers and lakes in the spring. Only relatively recently have researchers begun analyzing the impact of wildfire smoke on lakes, using remote sensing and satellite data to quantify the problem.

The findings are striking. For example, some California lakes have suffered smoke exposure for up to two months certain years. And it’s not just lakes in California, or even the western United States, that are affected. According to one recent study in Global Change Biology, almost all of North America’s lakes experienced at least one “smoke day”—where smoke can be seen in the air— per year between 2019 and 2021, and nearly 90 percent of all lakes endured more than 30 such days.

“Smoke has become more pervasive and extreme than in the past, and even lakes in regions that don’t typically experience wildfires are now feeling the effects,” says Mary Jade Farruggia, an ecology graduate student at the University of California, Davis and the lead author of the study.

The fallout of this smoke exposure is yet to be fully understood. But smoke impacts many of the key factors, such as light, temperature, and nutrient deposits, that also drive fundamental lake processes. Thick smoke can block sunlight, which in turn can lead to cooler water, reduced photosynthesis, and slower rates of cellular respiration—meaning aquatic plants and microbes produce energy more slowly.

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Ash falling into lakes adds nutrients like nitrogen and phosphorus to the ecosystem.  Algae feed on these nutrients, and the extra food disrupts normal nutrient cycles and could have ripple effects up the food chain. When it comes to ash being deposited in lakes, impacts may depend on the composition and origin of that ash.

During the 2021 Caldor Fire, large areas around and into the Tahoe basin burned, exposing the lake to days of smoke and ash. Researchers found that smoke closer to the source of that fire, which contained heavier ash, caused less algae growth in Lake Tahoe than finer ash, which actually may contain more key nutrients, that was deposited farther from the fire. “It shows that not all smoke is created equal,” says Chandra.

Blocking ultraviolet light

California’s many mountain lakes, which are typically low in nutrients, may be especially vulnerable to large influxes of nutrient-rich ash. “Any addition can mean a doubling or tripling of the amount of nutrients [in mountain lakes], and that can really shift these fragile ecosystems,” says Janice Brahney, an environmental biogeochemist at Utah State University in Logan.

However, the most important determinant for how a lake may respond to smoke exposure may be its size, according to Adrianne Smits, an aquatic ecosystem ecologist at the University of California, Davis. “Small lakes will be more sensitive and reactive to perturbations of any kind,” she says.

One small mountain lake that has been well studied is Castle Lake, nestled in northern California’s Shasta Mountains. Researchers have monitored the lake across smoke-filled as well as clear years. In 2018, smoke covered the lake for 55 days straight, and lake water temperatures cooled substantially. Moreover, smoke also reduced ultraviolet light, which normally cleans the top of a lake, and increased algae levels in shallow waters.

Castle Lake’s surface only spans 47 acres, compared to Lake Tahoe’s 122,000 acres. This year, the lake’s airshed—meaning the area from which pollutants can reach the lake’s surface—has once again been intermittently covered in smoke, even if much of the smoke from the Park Fire has drifted northeast, away from the Shasta Mountains.

Researchers say changing light conditions in lakes can also impact fish behavior, including feeding for some species. “Trout and salmon are visual predators that need light to hunt,” says Chandra. “If that light is diminished by smoke cover, the fish might have to move into new areas of the lake.”

A big deal

With many of California’s freshwater systems—some of which have been heavily modified by diversions and dams—already under extreme pressures, the threat of  increased wildfire smoke presents an additional stressor.

But with the focus largely on the human health effects of wildfire smoke, raising awareness about this environmental threat could be challenging. “It’s an issue that hasn’t risen to the policy level yet,” says Patrick Wright, who leads the California Wildfire and Forest Resilient Task Force. But, he adds, “the fact that some of our most treasured ecosystems are at risk—obviously it’s a really big deal.”

The key, Wright contends, is to not tackle issues in a vacuum. “We’re constantly reminded of the notion that everything is connected,” he says. “We really need to look at whole watersheds, whole ecosystems, whole forests, and treat everything holistically. The smoke issue adds another level of urgency to that effort.”