## 3 Results and Discussion
### Contrasting Examples of Light Field Characteristics and AOPs
Although Raman scattering is known to have very important contribution to the upwelling light in the red [9; 60; 64; 71; 88], our data show that the fluorescence effect in the 683 nm band can be dominant in the presence of _Chl_max layer. At 650 nm _Eu_ and _Eou_ remain nearly constant within the upper portion of the _Chl_max layer. This suggests that the fluorescence fingerprint in the upwelling irradiances is still noticeable at this wavelength, which can be attributed to relatively broad emission band of chlorophyll-_a_ fluorescence (FWHM of \(\sim\)20-25 nm) [89; 90]. The vertical variations in the spectral AOPs derived from the irradiance quartet measurements reveal features that were clearly associated with the effects of inelastic processes in the presence of non-uniform profiles of _Chl_ and IOPs (Figures 5 and 6). The spectra and depth profiles of the average cosines, \(\overline{\mu}_{d}\), \(\overline{\mu}_{u}\), and \(\overline{\mu}\), were relatively weakly variable within the blue-green spectral region where the effects of inelastic processes are much weaker compared with the long-wavelength portion of the spectrum (Figure 5a-c,e-g). In the spectral range of 400-580 nm, the values of \(\overline{\mu}_{d}\) and \(\overline{\mu}\) were in the range 0.7-0.9 with higher values observed at shallower depths and related to the high solar elevation in the clear sky during the measurement. Thus, the downwelling light field or the entire light field was characterized by the predominant downward direction of light propagation within 45\({}^{\circ}\) of the vertical. The values of \(\overline{\mu}_{u}\) in the blue-green were around 0.4 suggesting more uniform angular distribution of upwelling light field. Our data of average cosines in the blue-green are in the range of theoretical and experimental values reported in other studies [9; 10; 29; 52; 60; 73]. In the red spectral region, the average cosines exhibited large variations owing to the effects of inelastic processes within the non-uniform water column. We observed a large rapid decrease in \(\overline{\mu}_{d}\) and \(\overline{\mu}\) below the near-surface layer (below 10 m) to about 0.5 and 0.05, respectively, at depths \(\geq\)20 m. For \(\overline{\mu}_{u}\) in the red we observed a variation between about 0.45 and 0.65. The observed values of average cosines in the red differ significantly from those which would have occurred if inelastic processes were absent [9; 73]. In the presence of inelastic processes, the light field in the red approached a more uniform distribution with increasing depth quite rapidly, and could achieve a nearly isotropic distribution within moderately strong _Chl_max layer at relatively shallow depths, such as \(\sim\)30 m in the examined case (Figure 5e-g). Note also that, unlike Raman scattering that leads gradually to a more uniform light field in the red with increasing depth, the vertically non-uniform chlorophyll-_a_ fluorescence caused additional depth variations in the angular distribution of light field within the 683 nm band. In contrast, in the blue-green region where the effects of inelastic processes are much weaker, the light field remained highly directional throughout the examined euphotic layer (Figure 5e-g). Similar to the average cosines, the spectral irradiance reflectance \(R\) (Figure 5d,h) showed relatively little variation with depth in the blue-green spectral region as the \(R\) values ranged between 0.01 and 0.025. In contrast, \(R\) exhibited very large vertical variations at wavelengths longer than about 580 nm. In the red \(R\) varied from about 0.003 at 6 m to 0.8 at \(z_{Chlmax}\) = 29 m, and relatively high values (0.2-0.4) persisted below the \(Chl_{max}\) layer (Figure 5h). Similar observations were previously reported by [PERSON] and [PERSON] [37]. This result was caused by inelastic processes, including a dominant effect of chlorophyll-\(a\) fluorescence that produces a maximum in the \(R\) spectra within the fluorescence emission band centered at 683 nm (Figure 5d).