A MODIFIED FIBEROPTIC 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 O2 microelectrode allowed 250-mum-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 mum, and photosynthesis was not saturated up to 1,800 Amol quanta m-2 s-1. In a benthic diatom assemblage, light attenuation was greater than in the Ulothrix filaments in the top 250 mum, photosynthesis approached saturation at 1,000 mumol quanta m-2 s-1, and photosynthetic rates were extremely low at the 250- and 500-mum depths.