## 3 Results and Discussion
### Contrasting Examples of Light Field Characteristics and AOPs
In (**e**–**h**) different colors and symbols represent selected light wavelengths as indicated in (**e**). Figure 5: Spectra and vertical profiles of average cosines of underwater light field, \(\overline{\mu}_{d}\) (**a**,**e**), \(\overline{\mu}_{u}\) (**b**,**f**) and \(\overline{\mu}\) (**c**,**g**) as well as irradiance reflectance, \(R\) (**d**,**h**), derived from the irradiance quartet for Guaymas Basin shown in Figure 4. In panels (**a**–**d**) different colors represent depths as indicated in (**a**). The black arrow in (**d**) indicates a spectral feature caused by Raman scattering of water molecules. In (**e**–**h**) different colors and symbols represent light wavelengths as indicated in (**e**). Figure 6: Spectra and vertical profiles of diffuse attenuation coefficients, \(K_{d}\) (**a**,**e**), \(K_{ad}\) (**b**,**f**), \(K_{u}\) (**c**,**g**), and \(K_{out}\) (**d**,**h**), derived from the irradiance quartet for Guaymas Basin in Figure 4. In panels (**a–d**) different colors represent the mid-point depths of the layers within which the \(K\) coefficients were determined as indicated in panel (**a**). Dashed lines represent the spectrum of pure water absorption coefficient, which is a theoretical minimum of \(K\) for a hypothetical case when no inelastic processes and true emission sources are present. In (**e–h**) different colors and symbols represent the light wavelengths as indicated in panel (**e**) and dashed lines represent pure water absorption at respective wavelengths. Figure 7: Vertical profiles of average cosines, irradiance reflectance, and diffuse attenuation coefficients for the station in the Farallon Basin shown in Figure 2b. Data for \(\overline{\mu}_{d}\) panel (**a**); \(\overline{\mu}_{u}\) (**b**); \(\overline{\mu}\) (**c**); \(R\) (**d**); \(K_{d}\) (**e**); and \(K_{u}\) (**f**) are shown for selected light wavelengths as indicated in panel (**a**). The effects of vertically non-uniform IOPs and chlorophyll-_a_ fluorescence were also clearly revealed in the depth profiles of the irradiance quartet at four selected wavelengths (Figure 4e-h). Because the unscattered and elastically scattered light in the red spectral region was efficiently removed within the near-surface layer owing to strong absorption by water molecules, the vertical profiles in the red (650 and 683 nm) were affected by inelastic processes (Raman scattering and chlorophyll-_a_ fluorescence) to much greater extent compared with shorter wavelengths, and hence differed significantly from the profiles in the blue and green (440 and 550 nm). The slope of the profiles in the blue and green showed relatively small variations with depth, which were driven primarily by vertical inhomogeneity in IOPs associated with the strength of absorption and elastic scattering processes. For example, at 440 nm all four irradiances showed steeper profiles (i.e., higher attenuation rate with depth) within the _Chl_max layer, which can be attributed largely to enhanced absorption of blue light by phytoplankton. In contrast, the slope of irradiance profiles in the red varied within the water column to much greater extent compared with shorter wavelengths. For example, in the 650 nm band the profiles of _Ed_ and _Ead_ became gradually less steep with depth, which can be attributed largely to the increasing contribution of Raman scattering. In the 683 nm band where the contribution of chlorophyll-_a_ fluorescence was strongest, _Ed_ and _Ead_ remained nearly constant with increasing depth within the upper portion of the _Chl_max layer (note that no data is available to resolve the depth variations throughout the lower portion of the _Chl_max layer as the next data point at 50 m was near the base of the _Chl_max layer). The effects of inelastic processes were, however, most pronounced for _Eu_ and _Eou_ in the red spectral region. At 683 nm the effect of chlorophyll-_a_ fluorescence on the upwelling light field was so strong that both _Eu_ and _Eou_ increased with depth within the upper portion of the _Chl_max layer.