A modified fiber‐optic light microprobe to measure spherically integrated photosynthetic photon flux density: Characterization of periphyton photosynthesis‐irradiance patterns

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 O 2 microelectrode allowed 250‐ µ m‐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 µ m, and photosynthesis was not saturated up to 1,800 µ mol quanta m −2 s −1 . In a benthic diatom assemblage, light attenuation was greater than in the Ulothrix filaments in the top 250 µ m, photosynthesis approached saturation at 1,000 µ mol quanta m −2 s −1 , and photosynthetic rates were extremely low at the 250‐ and 500‐ µ m depths.