Phytoplankton Light Absorption Impacted by Photoprotective Carotenoids in a Global Ocean Spectrally‐Resolved Biogeochemistry Model

The chlorophyll‐specific absorption spectrum of phytoplankton aPH∗(λ) $\left[{a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda )\right]$ multiplied with phytoplankton chlorophyll provides the total absorption coefficient of phytoplankton [aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$], a fundamental quantity with significance in many marine biogeochemical (BGC) and environmental processes. Representing accurately the sources of variability of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$ in BGC ocean models is a crucial task. The two main sources of variability in aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda )$ are changes in the pigment composition of the phytoplankton community and the size‐dependent constraints to pigment packaging. Therefore, changes in community structure and physiological state impact aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda )$ and consequently aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$. The objective of this work is to improve estimates of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$ in a BGC model of the global ocean by portraying the variability of aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda )$ driven by the variable content in photoprotective carotenoids (PPCs) in the phytoplankton community. We used a three‐dimensional spectrally‐resolved BGC model to simulate the inherent and apparent optical properties of the global ocean based on its content on optically active constituents. The aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda )$ for each phytoplankton type represented in the model were made variable as a function of the type‐specific content in PPCs. By comparing model‐derived aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$ to satellite retrievals and an extensive field data set of optical and BGC observations, we concluded that photoprotective pigments content impacted significantly the contribution of the aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$ to the total non‐water absorption in the ocean. Pigment‐impacted aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda )$ contributed to reproduce the global variability of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$ as well as the observed bio‐optical relationship between aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$ and chlorophyll. The improved representation of the aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda )$ of the phytoplankton community influenced model simulations in terms of water‐leaving radiances. Plain Language Summary The multifaceted inte