During mid 2004, abrupt environmental changes including a massive die off of the dominant macrophyte Egeria densa were observed within the wetlands of Rio Cruces, a Ramsar site in southern Chile. One of the hypotheses presented to explain these changes was that increased ultraviolet radiation (UVR) may have been responsible. Since variation in UVR operates at regional scales, it is important to understand if the demise of E. densa also occurs at concurrent spatial and temporal scales. The current results demonstrate that monthly and annual variability of UVB was very stable during 1998 to 2006. Furthermore, in situ vertical profiles sampled across the wetlands and nearby rivers showed that UVR penetrates deeper into waters located outside the wetlands, where healthy stands of E. densa are commonly seen. The massive die off of E. densa within the Rio Cruces wetlands cannot be explained by regional variability in UVR, and it is much more likely that the definitive reason is found locally within the wetlands rather than regionally.