Widespread phytoplankton blooms triggered by 2019–2020 Australian wildfires

Abstract

Droughts and climate-change-driven warming are leading to more frequent and intense wildfires [1][2][3] , arguably contributing to the severe 2019-2020 Australian wildfires 4 . The environmental and ecological impacts of the fires include loss of habitats and the emission of substantial amounts of atmospheric aerosols [5][6][7] . Aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals such as nitrogen and iron, respectively [8][9][10] . It has been suggested that the oceanic deposition of wildfire aerosols can relieve nutrient limitations and, consequently, enhance marine productivity 11,12 , but direct observations are lacking. Here we use satellite and autonomous biogeochemical Argo float data to evaluate the effect of 2019-2020 Australian wildfire aerosol deposition on phytoplankton productivity. We find anomalously widespread phytoplankton blooms from December 2019 to March 2020 in the Southern Ocean downwind of Australia. Aerosol samples originating from the Australian wildfires contained a high iron content and atmospheric trajectories show that these aerosols were likely to be transported to the bloom regions, suggesting that the blooms resulted from the fertilization of the iron-limited waters of the Southern Ocean. Climate models project more frequent and severe wildfires in many regions [1][2][3] . A greater appreciation of the links between wildfires, pyrogenic aerosols 13 , nutrient cycling and marine photosynthesis could improve our understanding of the contemporary and glacialinterglacial cycling of atmospheric CO 2 and the global climate system.Human activity is altering the global water and carbon cycles 14 . While the risk of drought associated with climate change varies regionally, warming and drying will increase the risk of more frequent and intense wildfires 1-3 . In turn, wildfires are increasingly viewed as a first-order control on climate. Among other things, wildfires change the Earth's radiative forcing by emitting greenhouse gases and aerosols 15 . The feedbacks between climate and wildfires are complex and often poorly represented in climate models, leading to high uncertainty in future projections.The austral summer of 2019-2020 was one of the most severe wildfire seasons in Australian history. Millions of hectares of vegetation were burned, having ecological, environmental and socioeconomical impacts 5,16 . It is estimated that nearly 3 billion animals may have died or been displaced 17 . According to a study by van der Velde et al. published in this issue of Nature 18 , approximately 715 million tonnes of CO 2 (195 Tg C) were released into the atmosphere during the fire period, exceeding Australia's 2018 anthropogenic CO 2 emissions of 537.4 million tonnes (147 Tg C) 19 .The 2019-2020 Australian wildfires (known in Australia as bushfires) also released an enormous amount of aerosols into the atmosphere 6,7 . Aerosols can influence terrestrial and marine biogeochemistry 20 , via supplying soluble forms of nitrogen 8 , ph...

Southern Ocean Phytoplankton Stimulated by Wildfire Emissions and Sustained by Iron Recycling

“…C phyto anomalies were smaller than chl anomalies, but correspondingly increased with the arrival of the first fire plume in November and December (36% peak anomaly), and returned to climatological conditions by April (Figure 2b). These anomalies agree with in situ chl and b bp anomalies observed by biogeochemical Argo floats profiling the eastern bloom region (Figure S5 and Text S2 in Supporting Information S1; Tang et al, 2021) and are consistent with satellite-detected biological responses to purposeful and natural OIF (Westberry et al, 2013). The anomalies also exceeded anomalies observed after volcanic ash fertilization in the North Pacific (Westberry et al, 2019), likely due to higher soluble iron concentrations in wildfire emissions compared to volcanic ash (Hamilton et al, 2022;Ito et al, 2019;Perron et al, 2021) as well as stronger iron deficiency in the South Pacific compared to the North Pacific (Abadie et al, 2017;Zheng & Sohrin, 2019).…”

“…Between September 2019 and March 2020, the Australian wildfires injected large amounts of iron-bearing aerosols into the atmosphere, which were transported across the southern hemisphere, reaching remote parts of the iron-limited South Pacific within days, where they stimulated widespread chlorophyll anomalies (Tang et al, 2021;Y. Wang et al, 2022).…”

Turbidity and fecal indicator bacteria in recreational marine waters increase following the 2018 Woolsey Fire

“…To our knowledge, only one other study has employed remote sensing and in situ techniques to study the effects of wildfire on coastal water quality 67 . This study illustrates how using both satellite and in situ measurements results in a more complete assessment of coastal water quality.…”

Abundance and Fractional Solubility of Aerosol Iron During Winter at a Coastal City in Northern China: Similarities and Contrasts Between Fine and Coarse Particles

“…Moreover, Fe supply limits or colimits microbial nitrogen fixation and thus also affects primary production over vast tropical nutrient-poor regions (e.g., tropical Pacific, South Atlantic, and Indian Ocean) (Jickells et al, 2014). There are evidence that increased supply of soluble Fe to some oceanic regions would increase primary productivity (Boyd et al, 2007;Tang et al, 2021), having important implications for CO 2 uptake and climate (Jickells et al, 2005). It has been proposed that increase in dust and thus Fe deposition during the last glacial maximum caused increase in carbon export to deep ocean and decrease in atmospheric CO 2 (Martin, 1990).Major external sources of Fe in surface oceans include aerosol deposition, riverine input, continental margins, hydrothermal activities and glacial sediments, and aerosol deposition is a major Fe source for open oceans (Conway & John, 2014;Jickells et al, 2005;Tagliabue et al, 2017).…”