## 3 Results and Discussion
### Spectral Shape Sensitivity of EAP Size-Based PFT Detection  
To further test the sensitivity of the EAP model and the causal IOP variability in terms of identifiable changes in spectral shape from a multi-spectral perspective, \(R_{rs}\phi\) ratios for 440:560 nm (blue:green), 560:665 nm (green:red) and 665:710 nm (red:NIR) wavelengths were calculated for a range of \(D_{eff}\) and biomass. These are shown in Figure 11, representing corresponding changes in the \(R_{rs}\phi\) and in the phytoplankton backscattering, for these wavelength pairs. The B:G \(R_{rs}\phi\) ratio shows a strong biomass dependency and a small sensitivity to size at large sizes, for \(0.5\leq\mathrm{Chl}\ a\leq 4.5\,\mathrm{mg/m^{3}}\). The R:NIR ratio shows some sensitivity to larger sizes from about 3 mg/m\({}^{3}\), but this decreases as biomass increases. The G:R ratio shows a significant size-related feature for small sizes (\(\leq 6\ \upmu\)m) from biomass of about 2 mg/m\({}^{3}\) upwards (encircled in Figure 10). This is where a peak in the corresponding b\({}_{b\phi}\) ratio appears, suggesting that the large change in magnitude of the Chl \(a\)-specific backscatter \(b_{b}^{*}\phi\) between small \(D_{eff}\) (Figure 12) is directly responsible for the sensitivity in the \(R_{rs}\phi\) G:R ratio seen in Figure 11.  
Figure 10: Maximum \(\delta R_{rs}\phi\) for \(\delta D_{eff}\) from a starting assemblage with \(D_{eff}\) 2 \(\upmu\)m, as Chl \(a\) varies (**A**). Note that the \(\delta R_{rs}\phi\) occurs at different wavelengths from 500 to 600 nm (**B**), and this shows the maximum signal, so there is no exact wavelength information in (**A**). Using a difference of \(1\times 10^{-3}\) sr\({}^{-1}\) as a threshold for detection by satellite, it can be seen that, while the maximum size change here (2 to 8 \(\upmu\)m) is not detectable with Chl \(a<1\) mg/m\({}^{3}\), by 10 mg/m\({}^{3}\), even a small change in \(D_{eff}\) results in a detectable change in \(R_{rs}\). This is an important finding. There is a marked size dependency in all of the \(b_{b}^{*}\phi\) ratios, with the greatest rate of change somewhere between \(D_{eff}\) 2 and 8 um, but it is only in the case of the G:R ratio that the magnitude of the backscatter is sufficient for this signal to be identifiable in the \(R_{rs}\phi\). Given that the radiometric signal in the blue is greatly reduced by large phytoplankton absorption and \(a_{gd}\), and the red and NIR wavelengths are similarly affected by the absorption of water, it can be concluded that the main driver of the useable PFT signal in the green and red is phytoplankton backscatter. Figure 13 shows the rapid increase in the proportional contribution of phytoplankton to total backscatter at 560 and 665 nm. It is known that, for typical diatom/dinoflagellate assemblages, the 560 nm region is more influenced by backscatter than by absorption. The fact that the magnitude of the total backscatter is much lower at 665 than at 560 nm, together with the strong absorption by water in this region, result in a small useable \(R_{rs}\) signal. A contribution of approximately 40% of phytoplankton to total \(b_{b}\) at 560 nm corresponds with the limits of detectable \(\delta R_{rs}\phi\) (see Figure 2), indicating that this is the proportion at which phytoplankton backscatter starts driving the total water-leaving signal around 560 nm. Consequently, this is the minimum contribution for which some \(\delta D_{eff}\) information may be known. For an oceanic bloom example \(\delta D_{eff}\) from 2-6 um, this threshold contribution is reached at about 2 mg/m\({}^{3}\), while to detect an example \(\delta D_{eff}\) of 10 to 20 um in a eukaryotic succession, extremely high biomass is required. The mid-range biomass sensitivity demonstrated here presents  
Figure 11: \(R_{rs}\phi\) ratios for blue:green, green:red and red:NIR (Near Infra-Red) wavelengths as shown, for Chl \(a\) concentrations of 0.1 to 20 mg/m\({}^{3}\) and \(D_{eff}\) to 40 μm.