Oral Session 8

Phytoplanktonic photoacclimation under clear and cloudy skies

Photoacclimation by phytoplankton is a well-known response to changes in environmental factors such as light or nutrients. The intracellular Chlorophyll to Carbon ratio (Chl: C) increases in low light (Eg) and decreases at high Eg to optimize growth rate, with an observed tenfold variation. Consequently, accurately quantifying growth rates, Net Primary Production (NPP), and carbon export from space requires careful consideration of the dynamics of Chl: C. Current methods for retrieving Chl: C from remote sensing face a challenge: missing data due to cloud cover. On an average day, 72% of the ocean is cloud-covered. Filling these gaps via interpolation is essential for seamless Chl: C (a requirement for NPP products). However, the accuracy of this process hinges on our understanding of phytoplankton behavior under cloudy conditions. Current models approach this problem by assuming no change in photoacclimation state between clear skies and cloudy skies, which can potentially result in a significant underestimation of NPP. Using matchups between ~14,000 BGC-Argo floats and atmospheric MODIS data, we compared the in-situ Chl: C response with in-situ irradiance data under both cloudy and clear skies. We find that phytoplankton photoacclimate in a similar manner regardless of sky conditions. These results hold at global, local, and seasonal scales and account for the timescale of photoacclimation. Our work sheds light on an incorrect assumption in current estimates of NPP leading to a significant bias, and provides a path forward for improving global estimates.

*Charlotte Begouen Demeaux, University of Maine, USA, https://orcid.org/0000-0003-0471-5022

Emmanuel Boss, University of Maine, USA

Michael Behrenfeld, Oregon State University, USA

Jason Graff, Oregon State University, USA

Toby Westberry, Oregon State University, USA

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