## 3 Results and Discussion
### Case Study 1: Separating the Effects of Biomass from the Effects of \(D_{eff}\) Change  
Figure 3 presents two distinct events which illustrate the interdependency of the size and biomass signals. Modelled \(R_{rs}\) are shown for selected adjacent stations (20 to 21 is marked A; 12 to 13 is marked B) where a nominal threshold of change detectable by satellite is reached in the blue and green spectral regions, in other words, where a change in \(R_{rs}\) would be evident on a satellite image. Given the ambiguity in the causality of the phytoplankton signal, assessing the magnitude of changes to the water-leaving signal as the in-water constituents vary will give an indication of whether there may be enough radiometric signal at TOA to even detect the change. A threshold in situ measurement resolution of \(1\times 10^{-4}\) sr\({}^{-1}\)[80] is taken as an indication of sensitivity to detecting change in \(R_{rs}\) by direct measurement. Given an average estimated uncertainty in satellite \(R_{rs}\) of \(\pm\) 0.6 \(\times\) 10\({}^{-3}\) sr\({}^{-1}\) across the spectrum [81], here a conservative \(1\times 10^{-3}\) sr\({}^{-1}\) is used to indicate a potentially detectable  
change in water-leaving signal from satellite. These thresholds are not definitive and are used purely for the purpose of contextualising the discussion. Both examples in Figure 3 display large changes in \(R_{rs}\), but these are causally distinct: (A) represents a large change in Chl \(a\) concentration _and_ in \(D_{eff}\), while (B) represents a large change in Chl \(a\) concentration but a negligible change in \(D_{eff}\). Figure 1: Relative contribution of phytoplankton to total \(R_{rs}\) (with \(a_{gd}(400)=0.07\cdot[Chla]^{0.75}\), and \(b_{bmap}(550)=0.005\) m\({}^{-1}\)) for increasing biomass with \(D_{eff}=2\) and \(12\) μm. These populations are idealised examples and not intended to represent any observed relationship between Chl \(a\) concentration and \(D_{eff}\). Figure 2: Example proportional phytoplankton to total Inherent Optical Property (IOP) contributions for Case 1 waters, for idealised eukaryote assemblages of 2 and 12 μm. Figure 3: Modelled \(R_{n}\) for stations 20, 21, 12 and 13 of SOSCEX III. The modelled bulk \(R_{n}\) are calculated using Equivalent Algal Populations (EAP) generalised Chl \(a\)-carotonoid refractive indices and measured Chl \(a\) concentrations for the phytoplankton component, and include estimated \(a_{g\mu}(\lambda)\) and \(b_{\rm amp}(\lambda)\) contributions appropriate for this region [75,76]. Stations 20 to 21 (**A**) represent a large change in both Chl \(a\) concentration and in \(D_{eff}\). Stations 12 to 13 (**B**) represent a large change in Chl \(a\) concentration only. The centre panel shows the measured \(D_{eff}\) for the cruise track (starting at the ice shelf on the bottom right and continuing in an anticlockwise direction.) The effective diameter image is courtesy of SANAE 55 Report [74]. Station 20 to 21 therefore represents a significant phytoplankton community shift, as large changes in both \(D_{eff}\) (from 6 to 16 \(\upmu\)m) and Chl \(a\) concentration (from 1 to 11 /m\({}^{3}\)) were recorded. To isolate this change in phytoplankton signal, the differences in \(R_{rs}\phi\) for an assemblage with \(D_{eff}\) = 6 \(\upmu\)m and an assemblage with \(D_{eff}\) = 16 \(\upmu\)m are presented in Figure 4A for the measured range of Chl \(a\) concentration. Note that the large differences of about 2 \(\times\) 10\({}^{-3}\) sr\({}^{-1}\) in \(R_{rs}\) in the blue, observable in Figure 3A, only appears at very low biomass in Figure 4A as this difference in \(R_{rs}\) is almost entirely due to the difference in biomass and not a change in \(D_{eff}\). The spectral location of the most promising size-related signal for PFT retrieval is evidently dependent on biomass, and, at low biomass, it is positioned near 435 nm, while, at higher biomass, it is around 570 nm.