## Methods
### Radiometric Quantities and Apparent Optical Properties  
Underwater radiometric measurements, including the irradiance quartet, \(E_{d}\), \(E_{u}\), \(E_{od}\), and \(E_{ou}\) (for all stations) and upwelling zenith radiance \(L_{u}\) (for a few stations in 2011), were collected using an instrument package equipped with five RAMSES sensors (TriOS Mess- und Datentechnik GmbH, Rastede, Germany). Concurrent in-air downwelling plane irradiance, \(E_{s}(\lambda)\), was also measured with an additional sensor mounted on board the ship. These sensors typically acquired signal over the  
Figure 2: Vertical profiles of water temperature \(T\), density anomaly \(\sigma\), chlorophyll-\(a\) concentration _Chl_, and beam attenuation coefficient of particles at 660 nm \(c_{p}\)(660) measured at two contrasting stations. (**a**) Station in the Guaymas Basin (27.54\({}^{\circ}\) N, 111.64\({}^{\circ}\) W) with lower _Chl_ and \(c_{p}\) and smaller solar zenith angle of -5\({}^{\circ}\) and (**b**) station in the Farallon Basin (25.28\({}^{\circ}\) N, 109.58\({}^{\circ}\) W) with higher _Chl_ and \(c_{p}\) and larger solar zenith angle of -57\({}^{\circ}\). approximate spectral range 320-950 nm (with slight difference among sensors) with \(\sim\)3.3 nm spectral interval across the spectrum. The radiance sensor had a 20\({}^{\circ}\) field of view in air. All five in-water sensors were mounted on a common frame and aligned to the exact same depth level (Figure 3). The in-water package was lowered into water from the sun-exposed side of the vessel at a distance of \(\sim\)15 m from the vessel to minimize ship-induced light field perturbations [80, 81]. Vertical profile measurements were taken at several discrete depths from a few meters below the surface to depths of 50 to 80 m depending on the station. The package was stopped and held every 3-10 m to acquire 5-10 spectral scans at each measurement depth. The integration time of each sensor was automatically adjusted to the light intensity at each measurement depth to ensure an adequate signal to noise ratio. The depth resolution of discrete measurements was highest near the surface and decreased to 10 m below the 20 m depth. Generally, at least one measurement was made near the depth of the _Chl_ maximum as indicated by the chlorophyll-_a_ fluorescence profile from the CTD cast. The raw radiometric data acquired in the field were converted to physical units of spectral irradiance (W m\({}^{-2}\) nm\({}^{-1}\)) and spectral radiance (W m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\)). For the underwater measurements, the correction for immersion effect was made for each sensor with the coefficients provided by the manufacturer, which were verified and revised in the laboratory experiment [82]. All measurements were matched to each other by their time stamp and assigned depth. Each spectrum was filtered using the Savitzky-Golay filter in Matlab R2015b to minimize noise, and the linear interpolation was then applied to obtain the final spectral data with a 1 nm spectral interval. The quality of radiometric measurements was best when the irradiance magnitude was higher than the detection limit of 10\({}^{-6}\) W m\({}^{-2}\) nm\({}^{-1}\). The measurements of _Lu_ below \(\sim\)30-40 m were often not possible because of insufficient sensitivity of the sensor. All measurements with a signal below the detection limit were excluded from the analysis. As a result, at some stations the greatest depth of measurements of upwelling irradiance in the red spectral region was somewhat smaller (by \(\sim\)10-20 m) compared with measurements of upwelling irradiance in the blue-green spectral region or measurements of downwelling irradiances. The underwater irradiance and radiance data that passed quality criteria were adjusted to eliminate the effects of temporal changes in sky conditions during vertical profiling with the underwater system. This was accomplished by appropriate normalization of underwater data acquired at different depths using simultaneously acquired data of _E\({}_{s}\)_.