## 4 Conclusions  
The distinct causal optical effects of variations in phytoplankton biomass, mean assemblage size, pigment-related spectral variability and non-algal constituents are not easily identified, with substantial  
Figure 12: \(b_{b}^{*}\phi\) shown for \(D_{eff}\) 1 to 10 \(\upmu\)m. The largest differences in backscatter across the spectrum occur between 1 and 4 \(\upmu\)m, with the exception of the overlapping of \(b_{b}^{*}\phi\) in the red and NIR. Figure 13: Percentage contribution of phytoplankton to total backscatter (including water, and with nominal \(b_{bmap}(550)=0.005\)), shown for \(D_{eff}\) 1 to 40 \(\upmu\)m and Chl \(a\) from 0.1 to 20 mg/m\({}^{3}\), at 440, 560 and 665 nm. interdependency and spectral ambiguity. Consistent with previous studies [37; 39], it can be seen here that ambiguity is critical in attempting to resolve the phytoplankton community structure signals. The case studies illustrate how most of the \(R_{rs}\) signal that is due to phytoplankton is driven by biomass, an expected result. This concept underpins, after all, the primary missions of most ocean colour sensors: resolving variability in phytoplankton biomass. The study shows that quantitative consideration of the constraints of biomass and the phytoplankton contribution to the total IOP budget is required when addressing the PFT question. The key findings include the assertions that most of the absorption driven phytoplankton signal in \(R_{rs}\) in the blue is too ambiguous to use, and that the most useful PFT signal is caused by spectral backscatter. Furthermore, the ability to assess the PFT signal against non-algal optical contributions is largely driven by biomass and the IOP budget. Overall, spectral scattering properties of natural waters are not well characterised [85; 86], and phytoplankton spectral backscattering characteristics are underexploited in terms of their impact on the water-leaving signal. The importance of better representing the angular and spectrally variable nature of phytoplankton scattering has been established [35], and it is clear that phytoplankton backscatter is at the heart of the PFT question. The size-related PFT signal is driven by phytoplankton scattering, and spectral regions where scattering is at its most sensitive to \(D_{eff}\) show the most potential for PFT detection from the total water-leaving signal. There is most sensitivity to size-related changes in \(b_{bb}\) in the 1-6 \(\upmu\)m size range. Phytoplankton size-related features, most likely driven by phytoplankton absorption variablity, appear in \(R_{rs}\) around 430 nm at low biomass, and scattering-driven size-related features in the 520 to 570 nm spectral region at elevated biomass. The water-leaving signal in the blue spectral region is highly complex and ambiguous, being the result of varied and contrasting effects of absorbing and scattering characteristics of both algal and non-algal in-water constituents. Consequently, the size-related signal appearing in low biomass waters (\(<\)1 mg/m\({}^{3}\)) may be useful only when \(a_{gd}(\lambda)\) and \(b_{bmap}(\lambda)\) are exactly known. Accessory pigment absorption features that persist in \(R_{rs}\) in low biomass suffer from the same vulnerability to uncertainty in the non-algal constituents. Satellite measurement uncertainty and \(a_{gd}(\lambda)\) retrievals may in future be improved (e.g., with the use of radiometry in the UV), but, given current uncertainties, achieving sufficiently accurate satellite estimates of the non-algal optical components is unlikely for this purpose. This finding exposes a vulnerability in historical approaches to phytoplankton identification and quantification based on the features of phytoplankton absorption characteristics in the blue. Satellite PFT methods using this approach all suffer from this shortcoming where assemblage-related variability is secondary to biomass effects, and where phytoplankton relationships with \(a_{gd}(\lambda)\) and \(b_{bmap}(\lambda)\) are not precisely known.