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Ash from wildfires may fuel the growth of plankton in the ocean


Satellite photo of smoke from the Thomas Fire in California in 2017. Burning areas are outlined in red

NASA/UPI/ Alamy

Ash from wildfires that falls in the ocean can provide nutrients that boost the growth of microorganisms – but the impact on the wider ecosystem is unknown.

It has long been known that ash from the burning of vegetation can fertilise the soil and improve crop growth, while in rivers and lakes, wildfire ash can poison fish and molluscs with toxic metals and cause algal blooms that lead to oxygen depletion.

In 2017, the Thomas Fire in California is estimated to have produced between 0.7 billion and 2.4 billion kilograms of ash, much of which ended up in the Pacific Ocean. To investigate the impact of wildfires on marine microorganisms, Tanika Ladd at Western Washington University and her colleagues collected samples of ash from the area burned by the Thomas fire and mixed it with water from the sea nearby. They let the solution sit for a few days to allow chemicals from the ash to leach into the water, before filtering large particles of ash out.

Analysis of this solution, called ash leachate, revealed significantly higher amounts of dissolved nutrients, including nitrogen, silicic acid and metals, compared to ordinary seawater. “It turned the seawater like a yellow colour,” says Ladd.

The team then combined the ash leachate with even more seawater containing microorganisms, including phytoplankton, to see how they responded.

The amount of particulate organic carbon in the water – a proxy for is the number of microorganisms in a sample – was twice as high in the ash-enriched water compared to normal seawater. This implies that the extra nutrients in the water and enhanced their reproduction of these organisms.

“We didn’t see any toxic effects,” says Ladd. “Most of the stuff in the community appeared to benefit from it.”

If the wildfire led to higher numbers of phytoplankton in the sea, what impact that would have had on larger organisms is unclear, says Ladd. They might have benefited from more food, but some types of phytoplankton are toxic. “When these grow to really high abundances in the water, they can cause disease and sickness in marine mammals and birds,” she says.

“This new study adds important insights into the potential effects of wildfire ash on marine biogeochemical cycles,” says Stefan Doerr at the University of Swansea, UK. “The ash used in the leaching experiments was collected from the ground in the burned area. How representative this ash is in terms of its composition of the ash that is exported in the smoke plume remains unclear.”

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