## 3 Results and Discussion
### Case Study 2: Addressing Pigment Variability  
Both the phycoerythrin-containing and peridinin-containing assemblages are modelled here, Figures 7 and 8) with \(D_{eff}\) of 12 \(\upmu\)m, so the simulated optical changes as the assemblage changes from _M. rubra_-dominated to dinoflagellate-dominated are all due to differences in pigmentation, for any given Chl \(a\) concentration. From the log-scale \(R_{rs}\), it is evident that the pigment-related differences in \(R_{rs}\) become larger as biomass increases. In the very high biomass blooms (\(\geq\)30 mg\(\cdot\)m\({}^{-3}\)) typical of the Benguela system, it is known that _M. rubrum_--containing assemblages are identifiable from MERIS satellite imagery [(83)] due to the effects of the diagnostic phycoerythrin peak (at 565 nm) appearing in the 560:520 nm band ratio. An analogous study of the sensitivity of the maximum \(\delta R_{rs}\phi\) signal to non-algal constituents is made at 570 nm for the pigment-driven feature appearing at high biomass (10 mg\(\cdot\)m\({}^{3}\), Figure 9). The sensitivity of pigment-driven differences to non-algal effects is in contrast to the Southern Ocean size example in that it is largely driven by variability in \(b_{map}(\lambda)\). As biomass increases past 10 mg\(\cdot\)m\({}^{3}\), the magnitude of the \(\delta R_{rs}\phi\) grows as \(b_{map}(\lambda)\) increases, showing no impact at all of biomass past 20 mg\(\cdot\)m\({}^{3}\). What this means is that while significant biomass is required to detect pigment changes, past a certain upper biomass limit, the magnitude of the pigment differential signal grows proportionally as \(R_{rs}\) is augmented by non-algal scatter. Figure 6: Modelled \(R_{rs}\) for Stations 12 and 13 (**A**), with EAP eukaryote phytoplankton IOPs, and \(a_{gd}(\lambda)\) and \(b_{map}(\lambda)\) components estimated guided by observations in [(75; 76)], respectively; (**B**) shows \(\delta R_{rs}\phi\) for this large change in Chl \(a\) concentration (1 to 7 mg/m\({}^{3}\)) but a small \(\delta D_{eff}\) of 7–8 \(\upmu\)m. The unit of the colour bar is sr\({}^{-1}\). Note that the results are one order of magnitude less than in the previous example; (**C**) shows the negligible effect on \(R_{rs}\phi\) of a change in \(D_{eff}\) from 7 to 8 \(\upmu\)m at the measured Chl \(a\) concentrations. Absorption-based pigment differences are therefore sensitive to scattering variability unless the biomass is very high, and this is particularly relevant in spectral regions affected by scattering variability due to changes in \(D_{eff}\) (Figure 4). Noting the log-scale Chl \(a\) axis of Figure 9, it can be observed that while the respective magnitudes of the pigment-driven (\(D_{eff}\) = 12 \(\upmu\)m, Figure 9) and size-driven (\(D_{eff}\) from 6 to 16 \(\upmu\)m, Figure 4) signals are comparable at the point that pigment differences appear (i.e., 10 mg\(\cdot\)m\({}^{3}\)), the size-driven feature is more sensitive at lower biomass--detectable at around 2 mg\(\cdot\)m\({}^{3}\) for the given change in \(D_{eff}\). This sensitivity will be affected by the size range in question and also by pigment concentrations, but it can be inferred that, generally, where size changes of this range (or larger) take place together with pigment changes, it is the size change that drives the variability in the water-leaving signal, and changes in the reflectance due to a substantial change in \(D_{eff}\) are observable at lower biomass than those due to pigment changes. Figure 8: \(\delta R_{rs}\) shown for a change from a high biomass _Myrioneta rubra_-dominated assemblage, to a high biomass peridinin (carotenoid)-containing dinoflagellate-dominated assemblage. There is no change in \(D_{eff}\). Figure 7: Benguela-like pigment-based experiment: Modelled \(R_{rs}\) shown for Chl _a_-carotenoid pigmented assemblages (solid lines) and phycoerythrin containing assemblages (dotted lines) for identical Chl \(a\) concentrations, at 0.1, 0.3, 3, 10 and 30 mg\(\cdot\)m\({}^{-3}\).