## 3 Results and Discussion
### Measured Light Scattering Properties of Natural Particulate Assemblages
The presented data have been divided into three groups according to the values of POC/SPM as follows: the least organic-dominated (or the most mineral-dominated) data with POC/SPM \(<\) 0.15; the most organic-dominated data with POC/SPM \(>\) 0.3, and the intermediate data with 0.15 \(\leq\) POC/SPM \(\leq\) 0.3. The selected boundary values of POC/SPM for discriminating between the organic-dominated and mineral-dominated groups of data differ from those used in our previous studies [65; 78], but appear to adequately reflect the patterns in the present data. In particular, the most mineral-dominated samples with POC/SPM \(<\) 0.15 form a clear cluster of data points with the highest \(\widetilde{b}_{pp}^{LISST}\) (Figure 12a) and the lowest Chla/SPM (Figure 12b,d). We also note that no data were collected  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **Chla** & **SPM** & **POC/SPM** & \(\mathbf{b_{p}}\) & \(\widetilde{b}_{pp}\) & \(\underline{\beta_{r}(45^{\circ})}\) & \(\mathbf{DoLP_{p,max}}\) & \(\mathbf{\psi_{max}}\) \\
**ID** & **[mg m\({}^{-3}\)]** & **[g m\({}^{-3}\)]** & **[g/g]** & **[m\({}-1\)]** & **[dim]** & \(\widetilde{b}_{r}(135^{\circ})\) & **  
for POC/SPM between 0.15 and 0.2, so we will refer to all data with POC/SPM > 0.2 as highly organic because they all represent highly significant or dominant role of organic particles. The scatter plot for the data of \(\widetilde{b}_{bp}^{USST}\) vs. POC/SPM suggests the presence of a relational trend with significant negative correlation between the variables (the correlation coefficient \(R=-0.73\)). While the \(\widetilde{b}_{bp}^{USST}\) values are clearly highest for POC/SPM < 0.15, the organic-dominated samples have consistently lower \(\widetilde{b}_{bp}^{USST}\). This result is consistent with the notion that mineral-dominated particulate assemblages with relatively high bulk refractive index of particles tend to have higher backscattering ratio \(\widetilde{b}_{bp}\) compared with organic-dominated assemblages with lower refractive index [17]. However, we also note that highly organic samples with POC/SPM > 0.2 show no clear relationship and essentially no correlation between \(\widetilde{b}_{bp}^{USST}\) and POC/SPM (\(R=-0.11\)). This result may be attributable to the effect of other particle characteristics on \(\widetilde{b}_{bp}^{USST}\), such as variations in refractive index of particles associated with changes in the composition of particulate organic matter, variations in particle size distribution, or both. The scatter plot of \(\widetilde{b}_{bp}^{USST}\) vs. Chla/SPM (Figure 12b) provides interesting insight into this question, as this relationship is significantly better compared with \(\widetilde{b}_{bp}^{USST}\) vs. POC/SPM. Whereas the correlation between \(\widetilde{b}_{bp}^{USST}\) and Chla/SPM for all data is strong (\(R=-0.85\)), the subset of data for highly organic samples (POC/SPM > 0.2) has also a relatively high correlation coefficient of \(-0.51\). This is an important result, suggesting that for particulate assemblages with high organic content,  
Figure 12: LISST-VSF measurements of (**a,b**) the particulate backscattering ratio, \(\widetilde{b}_{bp}^{LIST}\), and (**c,d**) the maximum value of the degree of linear polarization of scattered light, \(DoLp_{p,max}^{USST}\), as a function of the POC/SPM or Chla/SPM ratio. The data are divided into three groups defined by the range of POC/SPM as indicated in the legend. the backscattering ratio \(\widetilde{b}_{bp}\) tends to decrease with increasing proportion of phytoplankton in the particulate assemblage. It is likely that the relationship in Figure 12b is largely driven by a decrease in the bulk particle refractive index with increasing proportion of phytoplankton in the particulate assemblage. Because this trend also holds for the subset of highly organic samples, it may indicate that live phytoplankton cells have generally lower refractive index than non-living organic particles.