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

The chlorophyll-specific absorption spectrum of phytoplankton [a(PH)*(lambda)] multiplied with phytoplankton chlorophyll provides the total absorption coefficient of phytoplankton [aPH(lambda), a fundamental quantity with significance in many marine biogeochemical (BGC) and environmental processes. Representing accurately the sources of variability of a(PH)(lambda) in BGC ocean models is a crucial task. The two main sources of variability in a(PH)*(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 a(PH)*(lambda) and consequently a(PH)(lambda). The objective of this work is to improve estimates of aPH(lambda) in a BGC model of the global ocean by portraying the variability of aPH*(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*(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(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(lambda) to the total non-water absorption in the ocean. Pigment-impacted aPH*(lambda) contributed to reproduce the global variability of aPH(lambda) as well as the observed bio-optical relationship between aPH(lambda) and chlorophyll. The improved representation of the aPH(lambda) of the phytoplankton community influenced model simulations in terms of water-leaving radiances.