Effects of UV radiation on freshwater ecosystems

Significant increases in UV‐B radiation have been documented over north and south temperate regions as well as in Antarctica, thus exposing critical freshwater ecosystems to changes in this important environmental variable. A variety of aquatic organisms ranging from bacteria and phytoplankton to insect larvae, zooplankton, and even aquatic vertebrates such as amphibians and fish are vulnerable to damage from levels of solar UV radiation currently observed in nature. The penetration of solar UV radiation into freshwater lakes is controlled primarily by the concentrations of dissolved organic carbon (DOC) in the water. The relationship between DOC and the attenuation depth (1% of surface irradiance) is hyperbolic with an inflection point in the l‐2mg/1 DOC range. This suggests that freshwater ecosystems with DOC concentrations below 1–2mg/1 may experience very rapid increases in UV attenuation depths with small decreases in DOC. Thus both anthropogenic and natural disturbances such as climate change and acid precipitation that alter DOC concentrations in freshwater ecosystems may alter future changes in underwater UV environments in low DOC freshwater ecosystems. In addition, the photo‐oxidative effects of UV radiation on DOC suggest that increases in UV‐B radiation related to stratospheric ozone depletion may create a positive feedback in which UV attenuation depths increase rapidly and substantially in freshwater ecosystems that currently have low DOC concentrations.