Spectral irradiance in pond water: Influence of water chemistry

Knowing the depth of UV penetration in ponds and the chemical variables tha t control underwater spectral irradiance is a prerequisite to predicting th e influence of UV on amphibians and other pond organisms. The present study found that over 99% of UVB (280-315 nm) radiation was attenuated in the to p 10-20 cm of ponds sampled on the edge of the Canadian Shield near Peterbo rough, Ontario. While the principal attenuating substance was, as in lakes, dissolved organic carbon (DOC), neither DOC nor DOC fluorescence were usef ul predictors of the attenuation coefficients other than the observation th at all values of DOC were high and all attenuation coefficients were also h igh. The lack of a reliable relationship between DOC and attenuation result ed from differences throughout the season in the fraction of the DOC capabl e of absorbing radiation (chromophores) and the fraction capable of fluores cing (fluorophores). Attenuation was higher than predicted from DOC during springtime when amphibians lay their eggs. Absorbance coefficients measured using a spectrophotometer proved to be reliable predictors of both UVB and UVA attenuation coefficients measured in the ponds with a spectro-radiomet er, While DOC provides an effective sun screen against the direct damage of UV radiation, the high attenuation means that the photochemical activity s pread over at least 15 m in the ocean is confined to only a few centimeters in ponds.