The Smoke Effect: How are wildfires impacting lakes like Tahoe

Three summers ago, the Caldor Fire burned its way up the Western Slope into the Tahoe Basin, scorching 221,835 acres. The fire burned for over two months, cloaking the lake and its surrounding communities in unprecedented smoke. While hundreds fled the area — by choice or mandatory evacuation — scientists took to the water to try and better understand how the smoke and ash were impacting the lake.

With wildfires increasing in frequency and intensity, especially in California, understanding their impact on such a vital resource is key. Ten of the largest California wildfires in recorded history have occurred in the last 20 years with five taking place in 2020 alone, according to the National Integrated Drought Information System.

“Fire is going to be a bigger component of the landscape for the next 100 years, without question, under best case scenarios,” confirms Dr. Steve Sadro, U.C. Davis associate professor of environmental science and policy. “So that means we need to understand how these widespread smoke events are affecting lakes, and we need to understand which lakes might be more or less sensitive to smoke from wildfires… Then if we can come out of this at the other end with an understanding of how we might expect aquatic systems might change; what the implications of those changes are for human health, aesthetics, and water quality; and whether or not we need to worry about some types of systems over others at a regional scale.”

When the Caldor Fire sparked, scientists from the University of Nevada Reno, Utah State University, University of California Davis, Desert Research Institute, and Miami University mobilized to study the different ways smoke and ash might be affecting the aquatic ecosystem of the lake. They measured the effects of the smoke on filtering UV light and photosynthetic wavelengths of light, collected and analyzed the particles of ash being deposited to determine nutrient concentrations, and studied the impacts all of this had on the water quality and growth of algae and aquatic plants.

“The Tahoe study taught us that smoke definitely reduces incoming solar radiation, a substantial amount, and it reduces it enough to affect the growth rates of the algae in the lake,” notes Sadro.

Researchers from all around the basin collected data on water quality, ash nutrient content, algae growth and more.

Because Tahoe is so clear, algae grows lower in the water column than in other lakes so that it gets the right amount of light. With the smoke filtering sunlight, the algae moved up towards the surface. Along with changes in the composition of the different types of algae, the amount of open water algae was the highest annual value recorded — 20% more than the previous record in 2019.

“It looks like the smoke from the wildfire caused some kind of fertilization effect in Tahoe, albeit possibly a short one. It definitely affected water clarity. It definitely changed the community structure of the phytoplankton,” explains Sadro. “The reason we care about that is because it might affect the aesthetics if it results in a short-term decrease in water clarity, but maybe more importantly, if it happens for long enough there are implications for the energy flow in the food web.”

The unanswered question is how far up the food web this could go, adds Sadro. Could it have an impact on the zooplankton, the tiny organisms credited with filtering fine particles from the lake as they eat, aiding in maintaining Tahoe’s famed clarity? Or the native fish that eat the zooplankton?

While residents fled the area, scientists went out on the water to learn everything they could about how the smoke and ash were impacting Tahoe.

“This study helped establish important baseline data for scientists working to understand the holistic impact of wildfire on our ecosystems,” says Caitlin Meyer, chief program officer for the Tahoe Fund, in a joint blog post with the League to Save Lake Tahoe. “We know that wildfire affects forest health and habitat, air quality and communities, but very little is known about what it means for lake clarity and health. These findings lay the groundwork for future research into the topic, and will help Team Tahoe craft more effective ways to protect the lake’s unique, fragile ecology.”

The Tahoe Fund and the League to Save Lake Tahoe helped fund the research.

“Understanding these disturbances at regional scales is becoming particularly important for policymakers as megawildfires become a more frequent occurrence with changing climate and as we try to contextualize the impacts of large-scale disturbances with the finer scale impacting Lake Tahoe’s ecosystem,” concludes the 2021 study by the researchers.

A new paper coming out this summer, co-authored by Sadro, uses remote sensing analysis to put into perspective how many lakes are being impacted by the smoke effect within the broader context of North America. Another examines the effects of temperature and algae production in a dozen different types of lake across California.

“The goal is to really make this about more than a single site because up until this point most of the studies that have been published on the effects of smoke specifically have really been focused on these serendipitous case studies,” adds Sadro. “We are just trying to expand the scope and context of this.”

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