## 3 Results and Discussion
### Case Study 1: Separating the Effects of Biomass from the Effects of \(D_{eff}\) Change
As this is the phytoplankton-only signal, the question remains to what extent this signal is expressed in the bulk \(R_{rs}\), when the optical impact of the non-algal constituents is also considered. Working with the change in phytoplankton size signal identified at 435 nm (bottom left corner, Figure 4A), \(a_{gd}(\lambda)\) is added at increasing concentrations to simulate a range of bulk \(R_{rs}\) at 435 nm in Figure 4B, and \(b_{pmap}(\lambda)\) is likewise added incrementally at 570 nm in Figure 4C. In these plots, horizontal gradients indicate \(R_{rs}\) sensitivity primarily to the constituent on the \(y\) axis, while vertical gradients indicate that the change in \(R_{rs}\) is driven by the biomass, and is not sensitive to variability on the \(y\) axis. Figure 4B shows that the difference in bulk \(R_{rs}\) for the given \(\delta D_{eff}\) is only detectable at the satellite threshold level (shown in yellow) at low biomass under very low \(a_{gd}(\lambda)\) conditions. As biomass increases, increasing absorption by phytoplankton as well as by additional \(a_{gd}(\lambda)\), reduces the magnitude of the water-leaving signal and renders any \(\delta D_{eff}\) information ambiguous. When additionally considering the brightening effect of \(b_{pmap}(\lambda)\) in the blue (not quantified here), it can readily be perceived that the water-leaving signal is too complex at 435 nm to retrieve useful size information. In Figure 4C, the relationship with \(b_{pmap}(\lambda)\) at 570 nm is more straightforward. Change in \(R_{rs}\) due to \(\delta D_{eff}\) is detectable in the bulk \(R_{rs}\) at the satellite threshold (in red) from about 2.5 mg/m\({}^{3}\) upwards regardless of the \(b_{pmap}(\lambda)\) contribution, at least for Case 1 type conditions. The magnitude of this signal is almost entirely biomass driven. (This is in line with the observation made by [13] that the MODIS  
Figure 4: Southern Ocean stations 20 to 21: \(\delta R_{rs}\phi\) is shown for \(\delta D_{eff}\) of 6 to 16 \(\upmu\)m (**A**). The effect of \(a_{gd}(\lambda)\) at 435 nm is shown in (**B**), and \(b_{pmap}(\lambda)\) at 570 nm in (**C**). The units of the colour bars are sr\({}^{-1}\). wavebands at 531 and 551 nm are good indicators of backscatter anomalies because their magnitude is proportional to the addition or removal of particulate backscattering, and the longer wavelength band at 551 nm is less affected by variability in both \(a_{gd}(\lambda)\) and phytoplankton absorption [10].) It should be appreciated, though, that \(R_{rs}\phi\) in these figures is representing the change in \(R_{rs}\) due to size _at_ a particular biomass (i.e., biomass is constant while assemblage characteristics vary), effectively removing the effects of simultaneous biomass changes. Figure 5 simulates a transition from 6 to 16 \(D_{eff}\) with biomass 1 to 11 mg/m\({}^{3}\), where the intermediate values of both \(D_{eff}\) and Chl \(a\) are simply linearly interpolated. The vertical lines highlight 435 and 570 nm which were identified in Figure 4A as being the spectral regions of greatest size-driven signal. In Figure 5, while biomass and size effects combine to form large changes in \(R_{rs}\phi\) in the blue, it is the smaller signal around 570 nm that contains the most size-driven change as it is not affected by biomass to the same degree. Figure 4B,C show that the signal at 435 nm is sensitive to the effects of variable \(a_{gd}\), while the phytoplankton signal at 570 nm remains robust against variability in the non-algal optical contributions. By contrast, stations 12 to 13 exhibit a large change in \(R_{rs}\)--seen first in Figure 4B; shown again in Figure 6A--with an increase in Chl \(a\) from 0.9 to 7.1 mg/m\({}^{3}\) but only a very small change in \(D_{eff}\) from 7 to 8 \(\upmu\)m. This is likely, given the location in the lee of the South Sandwich Islands, to reflect a diatom bloom associated with island wake effects, due to fertilisation by terrestrial iron [82].