Does water effectively shield aquatic organisms from UV exposure?
No. Pure water is quite transparent to UV radiation; a beam of UV-B radiation must travel over one-half kilometre through pure water in order to be completely absorbed. Natural waters do contain UV-absorbing substances, such as dissolved organic matter, that partly shields aquatic organisms from UV-B, but the degree of shielding varies widely from one water body to another. In clear ocean and lake waters ecologically-significant levels of UV-B can penetrate to several tens of meters; in contrast, in turbid rivers and wetlands UV-B may be completely absorbed within the top few decimetres. Most organisms in aquatic ecosystems, such as phytoplankton, live in the illuminated euphotic zone close to the water surface where exposure to UV-B can occur. In particular, UV-B radiation may damage those organisms that live at the surface of the water during their early life stages.
