## 6 Concluding Remarks  
Modeling phytoplankton cells as two-layered spheres tends to increase the bulk backscattering ratio because heterogeneous spheres are more efficient backscatters. Even if the phytoplankton component has the smallest relative abundance, its impact on \(\widehat{b_{bp}^{a}}\) can be important depending on the hyperbolic slope of the Junge distribution. This is because phytoplankton cells have the highest backscattering cross section. For \(\xi=4\), the relative difference (in absolute value), (\(\Delta\varepsilon\)), between \(\widehat{b_{bp}^{a}}\) for the oligotrophic-like and the reference case, having the same bulk refractive index but composed exclusively of homogeneous spheres, can reach about 58%. When minerals are added, the impact of phytoplankton decreases as the scattering by minerals dominates. Considering different sub-populations of particles with different refractive index implies that the bulk refractive index varies with the value of the hyperbolic slope of the PSD, as the contribution of each scattering component varies. Consequently, the development of models, similar to the one proposed by [PERSON] et al. [5] to derive \(\dot{n_{r}}\) from \(\widehat{b_{bp}^{a}}\) and \(\xi\), but accounting for phytoplankton heterogeneity, is not straightforward. Other aspects of the problem and other parameters such as the adoption of a 2D or 3D model, the proportion between cytoplasm and chloroplast, or the variation of phytoplankton refractive index according to the considered species, would have to be taken into account. One option would be to develop a look-up table approach based on the main parameters driving the \(\widehat{b_{bp}^{a}}\) variability. For that purpose, other simulations would have to be performed to be able to identify the pertinent parameters. In this study, a large set of numerical simulations, as well as a proper methodology have been developed to simulate the particulate scattering properties of a water body in its complexity. We show that a special care should be taken in the integration step size when the particulate scattering coefficients are calculated from the particulate scattering function, especially for relatively small values of the PSD slope. We show that an integration angular step of \(0.05^{o}\) (N\({}_{b}=3600\)) is required to obtain the required accuracy considering the inputs (refractive indices and size range) used in this study. The method is adapted to be used repeatedly to model a very large variety of particulate assemblages. While the present study has been limited to three case studies, additional calculations can be conducted to better represent the variability encountered in oceanic environments in terms of particulate community and its complexity in terms of mixing, morphology, size, and chemical nature. Figure 9: Backscattering cross sections, \(C_{sca}^{bb}\), of the different particle groups. The phytoplankton cells are modeled as a two-layered sphere (80%–20%). For example, in waters with significant presence of specific populations of phytoplankton or under bloom conditions, the Junge-like approximation of PSD is usually unsatisfactory [36; 37; 38; 45]. We also showed that the relative volume of the model chloroplast to the cytoplasm or the presence of third layer to model the cell wall can all affect the backscattering ratio (about 5-25%). It means that changes in the phytoplankton structural heterogeneity can explain partly the natural variability of the backscattering ratio, particularly in bloom conditions. Further work is required, mainly experimental studies, to better characterize the internal structure and chemical composition of viruses, heterotrophic bacteria, biogenic detritus, and particle aggregates. This will allow, as [PERSON] et al. [23] did for phytoplankton, suitable models to be developed to describe properly, in numerical code, the morphological properties of various types of particles to provide more realistic simulations of their optical properties. The research is completed through collaboration of all authors.