Vertical migration behavior is found in many harmful algal blooms; however, the corresponding impact on ocean optical properties has not been quantified. A near-monospecific population of the dinoflagellate Karenia brevis was encountered off the west coast of Florida. The community was tracked for 24 hours by following a Lagrangian drifter deployed at the beginning of the experiment. A suite of inherent optical and cellular measurements was made. Over the 24 hour period, the K. brevis population increased during the day with concentrations peaking in the late afternoon (1600 local daylight time) in the upper 2 m of the water column. The increase in K. brevis in surface waters resulted in enhanced reflectance at the sea surface with distinct spectral changes. There was a 22% decrease in the relative amount of the green reflectance due to increased pigment absorption. There was enhanced red (35%) and infrared (75%) light reflectance due to the increased particle backscatter and chlorophyll a fluorescence; however, the relative impact of the fluorescence was relatively small despite high cell numbers due to the significant fluorescence quenching present in K. brevis. The relative change in the blue light reflectance was not as large as the change in green light reflectance, which is surprising given the pigment absorption in the blue wavelengths of light. The increased blue light pigment absorption was offset by a significant decrease in nonalgal particle absorption. The inverse relationship between K. brevis and nonalgal particles was robust. This relationship may reflect low grazing on K. brevis populations due to the neurotoxins associated with this dinoflagellate. The low-grazing pressure may provide the mechanism by which this slow-growing dinoflagellate can achieve high cell numbers in the ocean.