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 sedirnents, resulting in a surface maximum of scalar irradiance (E,) that could be significantly larger that the Incident radiation. In the subsurface, E,, was typically extingu~shed in a quasiexponential manner, with attenuation coefficients (310 nm) ranging from 4 to 21 mm-', depending on sediment type. Ultrav~olet B (at 310 nm) was extingu~shed to 1 "h of the incident between 1.25 and 0 23 mm from the surface W~thin the euphotic zones of these sediments, however, the space-averaged UVB scalar irradiance was very h ~ y h , between 15 and 33'!,, of the incident In natural waters, for example, the same parameter varies between 3 and g':(, 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.