Quality of UVR exposure for different biological systems along a latitudinal gradient.

The exposure of organisms to ultraviolet radiation (UVR) is characterized by the climatology (annual cycle) and the variance (anomalies) of biologically-weighted irradiances at eight geographical locations in austral South America, from 1995-2002. The net effect of UVR on biological systems is a result of the balance of damage and repair which depends on intensity and duration of irradiance and is modulated by its variability. The emphasis in this study is on day-to-day variability, a time scale of importance to adaptive strategies that counteract UVR damage. The irradiances were weighted with DNA- and phytoplankton photosynthesis-action spectra. Low latitude sites show high average UVR. For all sites, the frequency of days with above average irradiances is higher than below average irradiances. Persistence in anomalies is generally low (<or=0.36 autocorrelation coefficient), but higher for DNA- than phytoplankton photosynthesis-weighted irradiances due to their higher correspondence to stratospheric ozone. Cloudiness and other factors with small wavelength dependence (e.g. aerosols and albedo) are highly correlated with UVR anomalies at low latitudes (24-33 degrees S); ozone correlates higher at high latitudes (42-54.5 degrees S). Our results show that organisms in this region deal with several days of excess radiation and fewer, shorter and more intense periods of lower than average radiation. Relief from UVR stress (or higher frequency of days below the climatology) is more prevalent at high latitudes (54.5 degrees S). Thus, lower latitudes are more stressful to organisms not only because of higher average UVR irradiance but also for the higher frequency of days above the climatology.