We measure simultaneously, on sub-minute time scales, the downwelling irradiance spectra and the vertical attenuation spectra for downwelling irradiance of a contained phytoplankton culture while the culture is exposed to the full spectrum of an in situ light treatment. This technique incorporates miniature, fiber optical spectrometers and twin self-contained, underwater photosynthesis apparatus (SUPA). One SUPA serves as the reference, with filtered culture media in the exposure chamber. The other SUPA contains the phytoplankton sample in the exposure chamber. Using the assumptions that irradiance reflectance is small in the SUPA and that upwelling vertical attenuation equals downwelling vertical attenuation in the culture, it is possible to approximate the flux absorbed by the phytoplankton by the product of downwelling irradiance and downwelling vertical attenuation. The concurrent measurements of net carbon uptake and net oxygen production in SUPA, each minute, support calculations of net quantum yield of the phytoplankton in situ. Results from a field study using the red tide organism Gymnodinium breve illustrate the ability to quantify the effects high, fluctuating irradiance exposure near the surface.