PAST ULTRAVIOLET-RADIATION ENVIRONMENTS IN LAKES DERIVED FROM FOSSIL PIGMENTS

Natural levels of ultraviolet (UV) radiation can harm organisms in sha llow aquatic ecosystems in which concentrations of photoprotective dis solved organic carbon are low(1-3). These compounds can be removed as a result of acidic precipitation and climate changes, an effect which may have recently been manifested in up to 200,000 boreal lakes(4,5). Unfortunately, meteorological and biological monitoring studies are us ually too brief to record the magnitudes of past changes in UV radiati on fluxes and their effects. Here we demonstrate that certain fossil p igments in lake sediments can be used to document historical changes i n the UV radiation environment of lakes, These pigments are produced b y benthic algae when exposed to UV radiation and show sedimentary conc entrations that are correlated to the depth of penetration of UV radia tion within lakes. Analysis of fossil profiles from the sediments of t wo mountain lakes suggests that past UV radiation penetration has some times been-at least in these mid-latitude lakes-greater than during th e period of anthropogenic stratospheric ozone depletion.