Black Summer fires spawned phytoplankton bloom bigger than Australia

Australia’s 2019/20 bushfires were a crisis on an epic scale. Not only did they burn through an estimated 18 million hectares of land, killing at least 34 people and up to a billion animals, they also had complex and far-reaching after-effects, from browning New Zealand glaciers with soot to carpeting the skies across the southern hemisphere in a haze of smoke.

Now, a new study out today in Nature has found that the 2019/20 bushfires had an unexpected life-giving effect: they produced a phytoplankton bloom larger than the entire country of Australia, in the Southern Ocean between New Zealand and South America.

This bloom was bigger than any ever measured before in the area, according to the international team of researchers, including scientists from the ARC Centre of Excellence for Climate Extremes (CLEX) and the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS).

“The phytoplankton bloom in this region was unprecedented in the 22-year satellite record and lasted for around four months,” says IMAS co-author and CLEX Chief Investigator Peter Strutton.

“What made it more extraordinary is that the part of the season when the bloom appeared is usually the seasonal low point in phytoplankton, but the smoke from the Australian bushfires completely reversed that.”

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More reading: Phytoplankton bloom in the Gulf of Alaska

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But what’s the relationship between smoke from fires and the phytoplankton bloom hundreds of kilometres away?

According to the researchers, the massive aerosol plumes from the bushfires – which reached altitudes of 16 kilometres and altered the winds in the stratosphere – transported the smoke across vast distances. It carried significant concentrations of iron, which is vital for photosynthesis and growth in phytoplankton. These deposits of iron-rich smoke injected three times the usual amount of this metal found in the relatively iron-poor Southern Ocean, resulting in a sudden and prolific bloom.

“The acceleration in phytoplankton growth as the fires took hold in Australia was so quick that it only lagged the blazes by a few weeks, and in some cases just days,” says Jakob Weis, IMAS/CLEX PhD student and contributor to the study.

“This was even as the impact of the smoke was felt in fits and starts rather than appearing as a constant rain of smoke on the ocean. As an example, we found the fires on just one day, January 8 [2020], deposited 25% of the black carbon and iron for the whole of January into that part of the ocean.”

It’s a powerful example of the intricate and unforeseen ways in which events across vastly different ecosystems can interact with one another.

Even more fascinating, these massive phytoplankton blooms are actually an effective carbon sink, making them a climate-balancing counterpoint to increasingly intense bushfires. In fact, fertilising the ocean with iron to stimulate phytoplankton growth has often been proposed as a way to mitigate against climate change.

The researchers estimate that the phytoplankton bloom caused by the 2019/20 fires removed roughly the equivalent amount of carbon from the atmosphere that the bushfires produced. However, permanent sequestration of carbon by phytoplankton is complex, and the scientists could not establish whether that carbon descended into the deep ocean when the bloom was over, or was released back into the atmosphere.

“With increasing risks of bushfires in some areas, and the potential impact on climate, this research shows that we need to turn our attention to the consequences of fires at a global scale,” says Strutton.

“We need a far more comprehensive representation of wildfires in climate models and targeted studies to understand their influence on marine ecosystems. Our capacity to adapt to future climate change depends on it.”