Biological and optical measurements were compared during studies in Bedford Basin, Nova Scotia. A principal objective was to evaluate a tethered spectral radiometer buoy, which measured downwelling irradiance at 490 nm and upwelling radiance in wavebands corresponding to the SeaWiFS satellite, along with the fluorescence of chlorophyll (alpha) . Records of upwelling radiance clearly described optical variability in the water. Optical profiles, including diffuse attenuation at 3 wavelengths, detected a subsurface dinoflagellate bloom in August 1993. When the bloom was entrained into surface waters by afternoon winds, the radiometer buoy easily distinguished darker, red water from the background green water. During the study, a red:green radiance ratio and a measure of chlorophyll fluorescence (683 nm: 670 nm radiance ratio) were well correlated with chlorophyll concentration near the surface, whereas green:blue ratios were not. Optical detection of a bloom during its development demonstrates that simple autonomous instruments might be used for detecting some phytoplankton blooms prior to significant environmental impact.