Short-term variability of the underwater light field in the oligotrophic ocean in response to surface waves and clouds

Examples of short-term variations of the underwater light field associated with surface-wave focusing and cloud cover, and occurring at intermediate depths of the euphotic zone in the oligotrophic ocean are discussed. Time series of water pressure, spectral downwelling irradiance (Ed), and spectral upwelling radiance (Lu) at 412, 443, 490, 510, 555, 665, and 683 nm were acquired with moored instruments. The mooring was deployed in the Sargasso Sea near Bermuda in September 1994. The measurements were made at 15 and 35 m once every hour from 6 a.m. to 8 p.m. local time, with a 6 Hz sampling rate over time periods of 2 or 5 min duration. Time series of Ed, Lu, and water pressure were subject to spectral analysis. Under clear sky conditions, the power spectra of Ed at the blue/green wavelengths exhibited distinct maxima due to focusing of sunlight by surface waves. These maxima were located at a frequency higher than the dominant frequency in the power spectrum of water pressure variations. Similar maxima were present in the power spectra of Lu, but the position of the peak was dependent on light wavelength. The coefficient of variation for Ed and Lu depended on light wavelength and increased towards the red wavelengths. This coefficient was much smaller for Lu than for Ed, which reflects the fact that the upwelling light field is less affected by wave focusing than the downwelling light field. At depths of our measurements, the light fluctuations in the red can be attributed to variations in inelastically scattered radiation, that is, the fluorescence of phytoplankton cells and Raman scattering. This is because the fluoresced light and Raman scattered light in the red follow fluctuations that occur at shorter excitation wavelengths. When clouds covered the sky, we observed a decrease of the coefficient of variation for Ed and Lu. In this case, the fluctuations of light were well correlated with variations of water pressure associated with water surface displacement due to the wave motion.