A fiber-optic irradiance microsensor (cosine collector): application for in situ measurements of absorption coefficients in sediments and microbial mats
The manufacture of a new fiber-optic irradiance microsensor with cosine collecting properties is described. The 70 μm wide light collector, cast on the tip of a tapered optical fiber, consisted of a 50 μm wide flat methacrylate diffuser surrounded by an opaque black coating to prevent light entry from angles > 90°. The collector had a directional sensitivity close to the theoretical cosine response, but the sensitivity was lower than the ideal at larger incident angles due to the inherent optical properties of the interface between the collector and the medium. The irradiance microsensor was used concurrently with fiber-optic microsensors for radiance and scalar irradiance in two cyanobacterial mats: a gelatinous laminated mat of Aphanothece sp. and Phormidium sp. and a compact marine intertidal mat of Microcoleus chthonoplastes. At the surface, the ratio of scalar irradiance to downward irradiance depended on spectral absorption characteristics of the sediment and ranged from 1.2 at 430 nm in the Microcoleus mat to 2.0 at 760 nm in the Aphanothece-Phormidium mat. As the light field became more isotropic with depth, the ratio of scalar irradiance to downward irradiance increased at all wavelengths and a maximum of 3.9 was reached in the gelatinous mat at both 675 and 760 nm. The results stress the importance of measuring the right light parameter when photobiological processes in sediments are investigated and the application of scalar irradiance and irradiance as quantitative measures of available light for photosynthesis parameter is discussed. From the distribution of scalar irradiance and irradiance, in situ absorption coefficients were calculated. Within the upper 3 mm of the gelatinous mat, the vertical attenuation coefficients of scalar irradiance, upward radiance and upward irradiance in the Aphanothece-Phormidium mat all increased due to scattering of the light away from the direction of the incident collimated light. Below 3 mm attenuation coefficients for scalar irradiance, irradiance and radiance in the near-infrared spectrum (NIR) became identical indicating that the light field approached an asymptotic radiance distribution. In the compact Microcoleus mat, a near-asymptotic radiance distribution of NIR was obtained at only 0.4 mm depth.