The variability of bio-optical properties of seawater associated with planktonic responses to the daily light cycle was examined. We performed a spectral analysis of a 60-day time series of open ocean mooring measurements of the scalar irradiance (photosynthetically available radiation, PAR), beam attenuation coefficient at 660 nm (c660), stimulated fluorescence, and dissolved oxygen concentration. The measurements were done from April through May 1989 in the North Atlantic south of Iceland, as a part of the Marine Light in the Mixed Layer program. We have shown that the statistical significance of the daily cycles of bio-optical properties of the open ocean varies in time throughout the spring season. The diurnal periodicity of c660 and O2 was especially well pronounced during the development of the phytoplankton bloom in May. The fluorescence signal was dramatically affected by the ambient light intensity. The measurements at depths of 10 and 30 m showed fluorescence rhythms completely out of phase with each other. The comparison between the 10- and 30-m beam attenuation signals suggests that the 30-m signal was more sensitive to within day PAR variability. Further investigations are needed to determine how widespread the daily variations of bio-optical properties in the ocean are, what conditions favor this cycling, and how this variability may impact procedures for estimating the phytoplankton biomass and production. ¿ American Geophysical Union 1992