Variations in natural irradiance caused by changing cloud cover were examined as a potential source for the variability of the bio-optical properties in the open ocean. Time series of the scalar irradiance (PAR), beam attenuation coefficient (at 660 nm), stimulated chlorophyll fluorescence, concentration of dissolved oxygen, and water temperature were subject to spectral analysis. These data were acquired in the Sargasso Sea at a depth of 23 m during the Biowatt mooring experiment in 1987. Three cloudy days, on which power spectra of PAR exhibited distinct maxima at periods of about 40 to 100 min, were selected for the analysis. Similar variations occurred in beam attenuation, fluorescence and dissolved oxygen as evidenced by relatively high coherence (>0.5) between PAR and these variables. This is suggested to be related to rapid photoadaptive responses in the phytoplankton community on time scales of minutes. Possible explanations for these responses include changes in fluorescence yield, photosynthetic rate, cellular absorption, cell size and refractive index. No relationship between PAR and the bio-optical parameters was observed when hydrodynamical factors prevailed over time scales similar to those for cloud-induced irradiance fluctuations. The hydrodynamical effects were successfully traced by water temperature variations which, in further studies, may prove useful for separation of phenomena controlling within-day bio-optical variability in the ocean. ¿ American Geophysical Union 1992