## 5 Conclusions  
The results obtained by the FC method for the UKCW dataset were combined with models of particle density and of organic carbon and chlorophyll-a intraparticle concentrations to investigate the biogeochemical properties of the particle populations. The success of the resulting SPM, ISM, OSM and (after empirical optimization) POC and ChLA estimations lends further credibility to the PSD and PRID determination capabilities of the FC method, and further supports the usefulness of flow cytometry and of the FC method as a tool to complement other established techniques.  
The fact that a fractal model of particle structure was key to ensure the quality of the SPM, ISM, OSM and refractive index-derived POC and ChLA match-ups suggests that the FC method observes particles as equivalent spheres, and has therefore the potential to be to some extent resilient both to particles which violate Mie-compatible aspect ratios and to the break-up of flocs and aggregates which derives from the flow cytometric measurement technique. Furthermore, this characteristic makes FC method results more readily comparable with those of other more common marine optics instruments, which for the most part observe bulk seawater IOPs and PSDs. It should be also noted that while the dataset was chiefly composed of surface water samples, no particular limitation exists a priori for the application of the FC method in its present form to samples from any water depth.  
The volume scaling models used to calculate POC and ChLA were successfully applied to simple spherical particles instead. This may appear incongruuous at first; however, these models too are revealed to hide proportionalities analogous to those caused by fractal structures, shining interesting new light on the volume scaling coefficients described in marine microbiology literature. We have shown here that the volume scaling models provide a route to better understand the impact of carbon distribution within particles and resulting fractal dimensionality, while the successful optimization of refractive index-based models reinforces previous findings that cell composition in the form of intracellular carbon concentration is well represented by corresponding changes in real refractive index. Finally, the size discrimination offered by FC also allows for a better understanding of the contribution of different size classes to the bulk biogeochemical properties, as it previously did for the IOPs of natural particle populations [2]. Taken in combination, these results represent a significant demonstration of the quality of size and refractive index information that is provided by FC data.  
[PERSON] and [PERSON] conceived and outlined the concept for this study; [PERSON] coordinated work during the HE442 research cruise; [PERSON] and [PERSON] supervised the retrieval of water samples; [PERSON] carried out flow cytometry operations; [PERSON] and [PERSON] carried out SPM, ISM, OSM, POC, and ChLA measurements on the water samples; [PERSON] analyzed the data; [PERSON] provided the Methods paragraphs relative to SPM, ISM, OSM, POC, and ChLA measurements; [PERSON] wrote the paper.  
The HE442 cruise with [PERSON] was conducted under funding by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), grant AWI-HE442. This research was further funded by the Scottish Funding Council (SFC) (grant HR09011) via Marine Alliance for Science and Technology for Scotland (MASTS). This work was originally conceived thanks to work conducted under award of NERC grant NE/H021493/1 to [PERSON] and co-investigators.  
[PERSON] and [PERSON] gratefully acknowledge financial support from the MASTS pooling initiative. The authors wish to thank the captain and the crew of [PERSON] for their support and help during the HE442 research cruise. The authors also duly thank [PERSON] and two anonymous reviewers, who all helped improve this manuscript with their comments and suggestions.  
The authors declare no conflict of interest.