Smoke-covered lakes see shifts in biological and energy processes that influence food webs, carbon storage, and more.
By Carolyn Wilke, EOS
Wildfires are on the rise. The smoke they bring darkens the sky and deposits ash.
Ocean research has provided clues about how smoke affects marine ecosystems, but little is known about how it affects freshwater ecosystems like lakes. A new study published in Communications Earth and Environment shows that in some California lakes, smoke can alter physical and biological processes that are key to systems such as nutrient cycling, rates of carbon sequestration, and food web structure.
Both the number of smoky days and the extent of smoke coverage have climbed in recent decades, said Adrianne Smits, an environmental scientist at the University of California, Davis, and coauthor of the new study. “Smoke cover in California is really no longer an ephemeral event,” she said, but “could be thought of more as a seasonal phenomenon.”
“Given that,” she continued, “we need to have a better understanding of how freshwater ecosystems respond to this fundamental change.”
Smoke Cover and Smoke Duration
Smits and her colleagues studied 10 lakes in California and their responses to smoke. First, they assessed how smoke coverage had changed in the Golden State since 2006. Using publicly available data, the team found that the maximum area blanketed by smoke during fire season (June-October) had increased by 70%. Smoke coverage was the greatest in 2018, 2020, and 2023. At some points in 2020, smoke shrouded around 90% of the state’s skies.
In addition to an increase in smoke coverage, the researchers found an increase in the total number of smoke days. Most sites experienced an average of 15 medium-to-high smoke days per year, while in the last years of the study, at least one site experienced more than 60 smoke days.
Pairing information on smoke coverage with data from local weather stations for the three smokiest years revealed that on smoke days, the amount of sunlight reaching the lakes was about 20% less than that on days that weren’t smoky.
Shifting Smoke, Changing Lakes
To explore how smoke cover affected ecosystems, the team gathered data from temperature and dissolved oxygen sensors. These data evaluated the lakes’ metabolism—processes that influence fluxes of energy and matter. Many of the sensors had been deployed previously to study effects of climate change on lakes. “It was a little bit serendipitous that we had sensors out,” Smits said.
Sensor data allowed the team to estimate two key factors in lake metabolism: primary production and respiration. Primary production fixes carbon and produces oxygen through photosynthesis by algae, phytoplankton, and aquatic plants. Meanwhile, respiration by microbes consumes oxygen to break down organic matter.
“These are very important functional characteristics of lakes that have cascading impacts on all sorts of other things.”
“These are very important functional characteristics of lakes that have cascading impacts on all sorts of other things,” said Ana Morales-Williams, a limnologist at the University of Vermont in Burlington who was not involved with the study. An ecosystem’s metabolism affects how much biomass there is to sustain the upper levels of a food web; whether a water body sequesters or emits carbon; and the amount and type of carbon exported downstream to other lakes, rivers, or the ocean. “If we have large-scale shifts in respiration and production,” she said, “we have to think about what that means for larger cycles.”
In their data, the researchers looked for trends in primary production and respiration that correlated with lake characteristics such as size, clarity, and nutrient levels. Many lakes saw changes in primary production on smoke days—increases and decreases. “Nothing really helped us predict what direction that would go in a lake,” Smits said.
Rates of respiration—which consumes oxygen and releases carbon—showed clearer trends.
Low-nutrient lakes with cold, clear water exhibited decreased respiration during smoky conditions. These lakes, several of which are at high altitude, receive relatively high levels of ultraviolet light from the Sun. That light damages DNA and breaks down components of microbial cells. “It takes a lot of energy to basically repair yourself as a cell,” Smits said. By blocking some sunlight, smoke may be lowering the need for energy for microbial repair, leading to less oxygen consumption. For murkier lakes—ones that have more organisms chewing up and releasing carbon—respiration rates stayed about the same or rose slightly with smoke.
People often think of these smaller, clearer lakes as pristine, Smits said, but they’re being affected by wildfire smoke even when they’re far from the source. “There’s concern that some of these clear water lakes are greening. They’re becoming more productive through time, and we don’t fully understand why.”
More research is needed to work out why different types of lakes respond as they do. “This was a study that generated many more questions for me than it answered,” Smits said. Future work could explore the fate of smoke particles, she suggested, which may influence lakes by dosing the water with nitrogen and phosphorous that could fertilize phytoplankton growth.
Climate Change Sentinels
“When we’re seeing these sorts of changes in lakes, it’s really a red flag.”
Smoke is increasing beyond California: Large and intense fires more frequently blaze throughout the United States and Canada, making their effects on lakes a widespread issue, Smits said. Tropical lakes are also experiencing smoke and have received far less research attention than the temperate lakes of North America and Europe, Morales-Williams added.
Not only are lakes important in regulating ecosystem processes, but how they respond to disturbances such as smoke could signal future changes in the environment, Morales-Williams said. “Things that would take much longer to respond in terrestrial systems can happen in lakes very quickly,” she said. “When we’re seeing these sorts of changes in lakes, it’s really a red flag.”
—Carolyn Wilke (@CarolynMWilke), Science Writer
Citation: Wilke, C. (2024), Wildfire smoke affects the function of lake ecosystems, Eos, 105, https://doi.org/10.1029/2024EO240273. Published on 27 June 2024.
