Biogeochemical response in surface ocean after wildfire ash deposition: results from minicosms experiment.

Cécile Guieu¹, Joan Llort², Matthieu Bressac¹, Frédéric Gazeau¹, Pierre Urruti¹, Maryline Montanes¹, Emmanuelle Uher¹, Cristina Santin³, Maria Santiso³, Kahina Djaoudi⁴, Elvida Pulido⁴, Eva Ortega-Retuerta⁵, Barbara Marie⁵

Affiliations: (1) Laboratoire d’Océanographie de Villefranche (LOV), France (2) Barcelona Supercomputing Centre (BSC), Spain (3) Instituto Mixto de Investigación en Biodiversidad, Spain (4) Institute Méditerranéen d’Océanologie (MIO), France, (5) Laboratoire d’Océanographie MICrobienne (LOMIC), France

Droughts and climate-change-driven warming are leading to more frequent and intense wildfires worldwide. Modelling studies suggest that aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals. Several observational pieces of evidence have recently shown the boost of marine primary production after the deposition of wildfire aerosols.  Yet very few studies have focused on the dissolution of wildfire ash compounds in seawater and its interaction with marine organic compounds at plankton-relevant scales. These aspects are critical to characterising ash's impact on marine primary production and its potential to influence the biological carbon pump.

In this talk, we will describe a novel, and unprecedented mesocosm experiment focused on these questions. The experiment conducted in July 2022 consisted in supplying ash from Mediterranean wildfires into nine 300L tanks filled with filtered/unfiltered seawater and under controlled light, temperature, and mixing conditions. We acquired water samples during the 14 days of the experiment to monitor the dissolution of ash compounds, the release of (micro)nutrients (dissolved inorganic phosphorus and nitrogen; dissolved iron), the changes in bacterial and primary production, and the sedimentation of ash and organic matter, amongst others. Although analyses are ongoing, the first results show a marked increase in phosphate, a two-staged increase in iron concentration (presumably due to opposed actions between dissolution and scavenging while the particles are sinking) and a very high bacterial productivity. While an important focus will be made on iron following the ash seeding, we will also present the temporal evolution of the biological stocks and fluxes in order to draw a complete picture of the biogeochemical response after wildfire ash deposition. This knowledge will be essential for understanding the role that wildfires play, now and in the near future, on the marine carbon cycle and for integrating this knowledge into Earth System models.