The light field of microbenthic communities: Radiance distribution and microscale optics of sandy coastal sediments

Abstract

The light field in coastal sediments was investigated at a spatial resolution of 0.2–0.5 mm by spectral measurements (450–850 nm) of field radiance and scalar irradiance using fiber-optic microprobes. Depth profiles of field radiance were measured with radiance microprobes at representative angles relative to vertically incident collimated light in rinsed quartz sand and in a coastal sandy sediment colonized by microalgae. Upwelling and downwelling components of irradiance and scalar irradiance were calculated from the radiance distributions. Calculated total scalar irradiance agreed well with the scalar irradiance measured directly by a fiber-optic scalar irradiance microprobe. Close to the sediment surface, the light field was highly anisotropic, dominated by incident collimated light, and the scalar irradiance reached a maximum of 200% of incident scalar irradiance. Below the sediment surface, the light field became diffuse with a forward-biased angular light distribution. A few millimeters into the sediment surface, attenuation coefficients of field radiance, irradiance, and scalar irradiance became identical and independent of depth, indicating that the light field approached an asymptotic radiance distribution. Comparison of light fields in wet and dry quartz sand showed that the lower refractive index of air than of water caused a more forward-biased scattering in wet sand. Light penetration was therefore deeper and surface irradiance reflectance was lower in wet sand than in dry sand. The higher reflectance of dry sand resulted in a higher surface maximum of scalar irradiance. Asymptotic values of average cosines and attenuation coefficients were used to calculate the absorption coefficient in quartz sand and in a coastal sediment with diatoms. Absorption coefficients ranged from 2.5 mm−1 at 450 nm to 1.5 mm−1 at 850 nm in the coastal sediment and from 0.8 to 0.4 mm−1 in wet quartz sand. Both attenuation spectra and absorption spectra of the coastal sediment with diatoms exhibited a Chl a absorption maximum at 675 nm. The light field around microalgae in sediments differs strongly from the incident light field with respect to intensity and spectral composition.