Optical properties of benthic photosynthetic communities: Fiber-optic studies of cyanobacterial mats. Limnol. Oceanogr. 33: 99-l 13

A fiber-optic microprobe was used to analyze the spectral light gradients in benthic cyanobacterial mats with 50-pm depth resolution and 10-nm spectral resolution. Microcoleus chthonoplastes mats were collected from hypersaline, coastal ponds at Guerrero Negro, Baja California. Gradients of spectral radiance, L, were measured at different angles through the mats and the spherically in-tegrated scalar irradiance, E,, was calculated. Maximal spectral light attenuation was found at the absorption peaks for the dominant photosynthetic pigments: chlorophyll a at 430 and 670 nm, carotenoids at 450-500 nm, phycocyanin at 620 nm, and bacteriochlorophyll a at 800-900 nm. Scattered light had a marked spectral effect on the scalar irradiance which near the mat surface reached up to 190 % of the incident irradiance. The spherically integrated irradiance thus differed strongly from the incident irradiance both in total intensity and in spectral composition. These basic optical propertics are important for the understanding of photosynthesis and light harvesting in benthic and epiphytic communities. The euphotic zone of benthic microalgal communities has a vertical extension of only a few tenths OF a millimeter to several mil-limeters. In spite of these narrow photic boundaries, benthic microalgae growing on sediment surfaces or as epiphytic films on macrophytes and rocks may contribute sig-nificantly to the primary productivity of shallow lakes and coastal marine environ