Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

# Simulating the Impact of the Internal Structure of Phytoplankton on the Bulk Particulate Backscattering Ratio using a Two-Layered Spherical Geometry for Cells

Version 1 : Received: 2 August 2018 / Approved: 2 August 2018 / Online: 2 August 2018 (17:14:34 CEST)

A peer-reviewed article of this Preprint also exists.

Duforêt-Gaurier, L.; Dessailly, D.; Moutier, W.; Loisel, H. Assessing the Impact of a Two-Layered Spherical Geometry of Phytoplankton Cells on the Bulk Backscattering Ratio of Marine Particulate Matter. Appl. Sci. 2018, 8, 2689. Duforêt-Gaurier, L.; Dessailly, D.; Moutier, W.; Loisel, H. Assessing the Impact of a Two-Layered Spherical Geometry of Phytoplankton Cells on the Bulk Backscattering Ratio of Marine Particulate Matter. Appl. Sci. 2018, 8, 2689.

Journal reference: Appl. Sci. 2018, 8, 2689
DOI: 10.3390/app8122689

## Abstract

The bulk backscattering ratio ($\tilde{b_{bp}}$) is commonly used as a descriptor of the bulk real refractive index of the particulate assemblage in natural waters. Based on numerical simulations, we analyze the impact of heterogeneity of phytoplankton cells on $\tilde{b_{bp}}$. $\tilde{b_{bp}}$ is modeled considering viruses, heterotrophic bacteria, phytoplankton, detritus, and minerals. Three study cases are defined according to the relative abundance of these different components. Two study cases represent typical situations in open ocean, outside (No-B/No-M) and inside bloom (B/No-M). The third study case is typical of coastal waters with the presence of minerals. Phytoplankton cells are modeled by a two-layered spherical geometry representing a chloroplast surrounding the cytoplasm. The $\tilde{b_{bp}}$ values are higher when heterogeneity is considered because the contribution of coated spheres to backscattering is higher than homogeneous spheres. The impact of heterogeneity is however strongly conditioned by the hyperbolic slope $\xi$ of the particle size distribution. Even if the relative concentration of phytoplankton is small (<1%), $\tilde{b_{bp}}$ increases by about 60% (for $\xi=4.3$ and for the No-B/No-M water body), when the heterogeneity is taken into account, in comparison with a particulate population only composed by homogeneous spheres. As expected, heterogeneity has a much smaller impact (about 5$\%$ for $\xi=4.3$) on $\tilde{b_{bp}}$ when minerals are added.

## Keywords

Ocean optics; backscattering ratio; phytoplankton, coated-sphere model, bulk refractive index, seawater component

## Subject

PHYSICAL SCIENCES, Optics

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