Penetration of ultraviolet radiation into shallow water sediments: High exposure for photosynthetic communities

Benthic photosynthetic microorganisms are widespread in shallow-water sediments, microenvironments that are commonly assumed to be virtually opaque to ultraviolet radiation (UVR). We used a newly developed optical microprobe to measure the submillimeter penetration of solar UVR into a variety of these microenvironments. UVR trapping due to strong scattering occurred at the surface of some sediments, resulting in a surface maximum of scalar irradiance (E₀) that could be significantly larger that the incident radiation. In the subsurface, E₀ was typically extinguished in a quasi- exponential manner, with attenuation coefficients (310 nm) ranging from 4 to 21 mm^-1, depending on sediment type. Ultraviolet B (at 310 nm) was extinguished to 1% of the incident between 1.25 and 0.23 mm from the surface. Within the euphotic zones of these sediments, however, the space- averaged UVB scalar irradiance was very high, between 15 and 33% of the incident. In natural waters, for example, the same parameter varies between 3 and 9% of the incident. Thus, in fact, photosynthesis in these environments must develop under strong UV stress, and it must be regarded as potentially labile to the effects of ozone depletion.