## Appendix A IOP Model  
To carry out RT computations with Hydrolight we need to specify the absorption and scattering coefficients of ocean water along with a phase-scattering function. The total IOPs are built up as a sum of IOPs attributable to the various components of the water body. Thus, the total absorption coefficient is:  
\[a(\lambda)=a_{w}(\lambda)+a_{ph}(\lambda)+a_{CDOM}(\lambda), \tag{1}\]where \(a_{w}\) is the pure ocean water absorption coefficient, \(a_{ph}\) is the particulate-matter absorption coefficient and \(a_{CDOM}\) is the CDOM absorption coefficient. Similarly, the total scattering coefficient is calculated as follows:  
\[b(\lambda)=b_{w}(\lambda)+b_{ph}(\lambda), \tag{10}\]  
where \(b_{w}\) is the pure ocean water scattering coefficient, and \(b_{ph}\) is the particulate-matter scattering coefficient. Additionally, we need to specify a phase-scattering function. An intercomparison of different phase scattering functions was carried out in [PERSON] et al. [10] by comparing the calculated \(K_{d}\) with the measured \(K_{d}\) for different chlorophyll concentrations. Data in this paper show that the physically-based Fournier-Forand (FF) phase function [64] is better in predicting downwelling irradiances than the average Petzold phase function. The FF phase function with a backscatter fraction of 0.003 was used in the computations. This conclusion was confirmed by [PERSON] et al. [44], where comparisons of the calculated diffuse reflectance and measured reflectance were performed. An important feature of the IOP model in the UV region is quantifying the pure water absorption coefficient, \(a_{w}\). At present, there are significant differences between different datasets of \(a_{w}\) in the UV region. A detailed discussion on pure water absorption and comparisons of the different datasets can be found in [PERSON] et al. [44; 61]. In this study, we use interpolation between data given in [PERSON] and [PERSON] [65] and [PERSON] and [PERSON] [66], as suggested by [PERSON] [67]. To reduce a number of input parameters to the IOP model, we assume the so-called Case I water model [43; 68]. According to this model, all ocean water constituents are of biogenic origin and can be related to a single parameter--chlorophyll concentration. A parameterization of the particulate matter absorption coefficient in the UV as a function of chlorophyll concentration is based on in situ data collected in the framework of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) [10]. The particulate matter absorption coefficient is expressed through a chlorophyll-a concentration and a chlorophyll-specific absorption coefficient similar to that used in the visible part of the spectrum:  
\[a_{ph}(\lambda)=A(\lambda)\ \text{Chl}^{1-B(\lambda)}, \tag{11}\]  
where the coefficients \(A(\lambda)\) and \(B(\lambda)\) are tabulated in [10]. The spectral dependence of \(a_{ph}\) and its dependence on chlorophyll concentration are shown in Figure 11 for low chlorophyll concentrations, which are typical for open ocean waters (Case I waters). An important absorber in UV is the colored (or chromophoric) dissolved organic matter (CDOM). The spectral CDOM absorption is parameterized in the conventional form:  
\[a_{CDOM}(\lambda)=a_{CDOM}(\lambda_{0})\exp[-S(\lambda-\lambda_{0})], \tag{10}\]  
where \(\lambda_{0}=440\,\mathrm{nm}\) is the reference wavelength, and \(S\) is the spectral slope. The CDOM spectral slope typically has somewhat higher values in the UV region as compared with the Vis region. We assume \(S=0.017\,\mathrm{nm}^{-1}\), as recommended for the UV [69]. Following [PERSON] and Maritorena [68], the CDOM absorption coefficient at \(440\,\mathrm{nm}\) is parameterized as:  
\[a_{CDOM}(440)=0.2[a_{w}(440)+0.06\,\mathrm{Chl}^{0.65}], \tag{11}\]  
where \(a_{w}=0.0063\,\mathrm{m}^{-1}\) is the pure seawater absorption coefficient [70]. This parameterization assumes that there is some CDOM absorption even in the absence of chlorophyll and models the presence of background CDOM in ocean waters.