A fiber-optic light sensor was modified by adding a broadband filter (420–730 nm) to sense photosynthetic photon flux density. A sphere of acrylic paint added to the fiber tip allowed estimation of spherically integrated irradiance. The modified light probe and an O2 microelectrode allowed 250-µm-resolution photosynthesis—irradiance profiles to be determined. In Ulothrix-dominated periphyton, there was greater ability to utilize low light as depth increased from 0 to 750 µm, and photosynthesis was not saturated up to 1,800 µmol quanta m−2 s−1. In a benthic diatom assemblage, light attenuation was greater than in the Ulothrix filaments in the top 250 µm, photosynthesis approached saturation at 1,000 µmol quanta m−2 s−1, and photosynthetic rates were extremely low at the 250- and 500-µm depths.
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Walter K. Dodds (1992) studied this question.