Smoke on the Water: How Wildfires Affect Life Underwater

Source Article: Scordo, F., Chandra, S., Suenaga, E. et al. Smoke from regional wildfires alters lake ecology. Sci Rep 11, 10922 (2021). https://doi.org/10.1038/s41598-021-89926-6

Featured Image Caption: Wildfire smoke could have far-reaching consequences for ecology. Image Source: https://pixabay.com/photos/forest-fire-wildfire-smoke-burn-1261752/ Free to use under the Pixabay license
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It’s easy to imagine how a forest might look immediately after a wildfire. It’s harder to imagine how a wildfire could affect life underwater. There are two possible ways that wildfire could affect life in the water. First, remnants of burned plants and soil on the land could flow into a nearby water body when it rains. Second, smoke in the air could block out sunlight and drop small pieces of ash into the water. These effects of burned material or smoke could change the water chemistry, water temperature, or underwater sunlight availability. How might these changes affect lake life?

All food comes from the sun

Phytoplankton and zooplankton play an important role in the food webs of lakes. Phytoplankton perform photosynthesis using sunlight shining through the water to make their food, like plants, but they are too small to see with the naked eye. Zooplankton are tiny organisms that eat phytoplankton and other zooplankton; they serve as food for larger animals, like insects and fish. These creatures all depend on each other for food, and they all ultimately depend on phytoplankton as the original producers of food in lakes using sunlight. So, if phytoplankton are affected by burned material or smoke, their disappearance could, by extension, affect all organisms in the lake.

Do wildfires affect lake ecology?

To investigate how wildfires affect lake ecology, researchers decided to study the water and underwater life of Castle Lake in California, USA. During the 2018 fire season, from late July to early September, smoke hovered over the lake for 87% of the days, which was unusual. By measuring the characteristics of the water and organisms in the lake from 2014 through 2018, they were able to compare four typical years (2014-2017) to this one especially smokey year caused by multiple nearby fires (2018).

They measured water characteristics, such as: concentration of suspended particles in the water and their chemistry, amount of sunlight hitting the lake surface, water transparency, amount of underwater sunlight, and water temperature. They also measured biological characteristics, including: primary productivity (how fast phytoplankton can make food from sunlight), concentration of phytoplankton, mass of zooplankton, and number of fish caught in their usual habitat using deep nets.

Changes in Castle Lake

Their study revealed that the water characteristics of the smokiest year, 2018, differed in many ways from the previous years. In 2018, there were more suspended particles in the water, making it cloudier and less transparent. Additionally, with smoke in the air, less sunlight hit the surface of the lake. Altogether, this resulted in less underwater sunlight, which resulted in colder water. Less of the sun’s radiation that is useful for photosynthesis entered the lake, but also less of the harmful, overly intense, radiation.

As a result, life in the lake in 2018 also differed from previous years. Primary productivity by phytoplankton increased right at the surface, maybe because some of the additional particles in the water came from ash falling in the lake, providing nutrients. This ash in the air above the lake also blocked particularly harmful radiation from the sun, like sunscreen. However, the additional ash particles in the water made it murkier and darker, making photosynthesis much more difficult below the surface. This may explain why, at the surface, primary productivity increased and phytoplankton biomass was typical, but below the surface, both decreased more than usual.

These effects of ash may have driven changes to the rest of the food web. Surprisingly, the total zooplankton biomass did not change. However, on smokey days, no fish could be found in their typical habitat at the bottom of the lake close to shore. Because the surface waters became so productive, there were likely more zooplankton present in the surface waters eating the phytoplankton, which could have attracted the fish toward the surface. If fish were eating lots of these surface zooplankton, they may have kept the total zooplankton biomass from increasing in 2018.

Overall, this study demonstrates that wildfires can have large effects on water characteristics and on aquatic life. These effects can spread through whole food webs, affecting some of the smallest organisms to the largest.

Playing with fire

Wildfires are becoming larger and more common in the western United States, and other regions. This is happening partly due to climate change, and partly due to how dry the area is normally, human activities that start fires, and a history of suppressing fires in some places, leading to a build-up of flammable plant material. On land, wildfires play a large role in nutrient cycling and regeneration of habitats, but we now know that they play a role underwater, too. Smoke from wildfires can change lake water characteristics and dramatically affect lake ecology as a result.

To better understand the consequences of these changes and how food webs recover after fires, further research should investigate how these changes play out across a longer period of time. By learning more about how wildfires affect aquatic life, we can better prepare for changes the future will bring.

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