## 3 Results and Discussion
### Contrasting Examples of Light Field Characteristics and AOPs
In this section we discuss two contrasting scenarios of light fields and AOPs within the euphotic layer, one was located in the Guaymas Basin which was observed in 2011 (Figure 2a) and the other in the Farallon Basin which was observed in 2010 (Figure 2b). Figure 2 compares the vertical profiles of \(T\), \(\sigma\), _Chl_, and \(c_{p}(660)\) at both stations. These two stations have highly contrasting _Chl_ profiles and also exhibit noticeable differences in the \(c_{p}(660)\) profiles. Although both stations had a similar depth of _Chl\({}_{max}\)_ (\(\sim\)30 m) the magnitude and FWHM of the _Chl\({}_{max}\)_ layer were very different. Specifically, the magnitude of _Chl\({}_{max}\)_ in the Farallon Basin (1.3 mg m\({}^{-3}\)) was about twice as high as compared with that observed in the Guaymas Basin (0.64 mg m\({}^{-3}\)). The FWHM was also considerably larger in the Farallon Basin, i.e., 25 m vs. 10 m in the Guaymas Basin. In general, the magnitudes of \(c_{p}(660)\) for these two stations did not differ significantly but the vertical distributions were different. In particular, there was a noticeable local maximum of \(c_{p}(660)\) near the depth of _Chl\({}_{max}\)_ in the Guaymas Basin, and no similar feature was observed in the Farallon Basin. The observed optical differences between the stations could have been caused by differences in the characteristics of suspended particulate matter such as organic and mineral composition. In addition, whereas the measurements were made under clear skies, the solar zenith angle was dramatically different at these two stations, 5.5\({}^{\circ}\) in the Guaymas Basin (the measurement was taken around noon at 12:45 p.m. local time) and 57.1\({}^{\circ}\) in the Farallon Basin (the measurement taken late afternoon at 4:35 p.m.). Thus, the comparison of these two stations provides an illustration of differences in the light field and AOPs, which were affected not only by water optical properties but also solar zenith angle. We will first present the data of irradiance quartet (Figure 4) and AOPs (Figures 5 and 6) for the station in the Guaymas Basin, and then the AOPs for the station in the Farallon Basin (Figure 7). The spectra of irradiances, \(E_{d}\), \(E_{u}\), \(E_{od}\), and \(E_{ou}\), at several discrete depths within the top 70 m of the water column are presented in Figure 4a-d. These results illustrate the well-known general pattern of the vertical change in the irradiance spectra in the upper ocean, including a decrease in the magnitude and narrowing of the spectral distribution with an increase in depth, e.g., [11]. In the investigated waters, all four irradiances exhibited well-pronounced maximum at a wavelength of about 495 nm at the deepest measurement depth of 67 m, but the spectral distributions of downwelling light, \(E_{d}\) and \(E_{od}\), were clearly broader with relatively more light in the green compared with the distributions for the upwelling light, \(E_{u}\) and \(E_{ou}\). A remarkable feature associated with vertically non-uniform fluorescence of chlorophyll-\(a\) was also evident in the irradiance spectra. At a depth of 29 m that coincides with \(z_{Chlmax}\) the spectra of \(E_{d}\) and \(E_{od}\) showed a distinct maximum in the spectral band of maximum fluorescence centered around 683 nm. This maximum was even more pronounced in the spectra of \(E_{u}\) and \(E_{ou}\). This feature weakened greatly for \(E_{d}\) and \(E_{od}\) at depths shallower or deeper than \(z_{Chlmax}\), but was preserved in the spectra of \(E_{u}\) and \(E_{ou}\) throughout the water column, which was expected as long as _Chl_ did not drop to a very low level. Figure 4: Spectra and vertical profiles of the irradiance quartet, \(E_{d}\) (**a**,**e**), \(E_{\alpha d}\) (**b**,**f**), \(E_{u}\) (**c**,**g**), and \(E_{\omega u}\) (**d**,**h**), measured at the station in the Guaymas Basin shown in Figure 2a. In panels (**a**–**d**) different colors represent the measurement depths as indicated in (**a**).