Ultraviolet-B radiation dose to wood frogs (Rana sylvatica) in northern Minnesota vernal pools

J.H. Olker, L.B. Johnson, R.P. Axler, and C.M. Johnson. Natural Resources Research Institute, University of Minnesota Duluth, Duluth, Minnesota, USA 55811

Widespread amphibian malformations and population declines have led to many investigations and theories of causation, including increases in solar ultraviolet–B (UV–B) (290–320 nm) radiation. Despite conflicting field study reports, recent laboratory–based studies established a dose-response relationship between UV–B and malformations in Ranid species. In exploration of the viability of UV–B as a cause of malformations in wood frog (Rana sylvatica), we measured instantaneous UV–B levels with a Macam radiometer as well as daily and cumulative UV–B dose with a multi–probe UV–B datalogger (UV–B Minder sensor) and polysulfone film in two northern Minnesota vernal pools. UV–B levels were significantly influenced by atmospheric conditions, vegetative cover, and water column properties (e.g. dissolved organic carbon and turbidity). During 2002 the study ponds were exposed to sufficiently high UV–B levels at and near the pond surface for amphibians to be at risk of developing malformations. In addition to the physical characteristics of the atmosphere and pond, actual UV–B doses to amphibian embryos and larvae are most affected by egg mass placement and larval behavior in ponds. The risk to vernal pool species is greater than predicted from previous studies in wetlands in northern Minnesota.