Seasonal and diurnal variations in ultraviolet-B and ultraviolet-A irradiances at and below the sea surface at Helgoland (North Sea) over a 6-year period

Ultraviolet(UV) radiation at four wavelengths (305, 320, 340 and 380 nm) an d photosynthetically active radiation (PAR) were measured from May 1994 to October 1999 using Biospherical UV radiometers. A surface reference sensor located on the roof of the Marine Station at Helgoland recorded values ever y 5 min, and an equivalent profiling underwater sensor was used for measure ments in the sea at approximately monthly intervals. The ratio of 305-nm ra diation to PAR varied seasonally, with a 14-fold increase from winter to su mmer. A much weaker seasonal trend (ca. 1.5-fold) was apparent in the ratio of 320-nm radiation to PAR, but there was no seasonal trend in the ratios of 340- or 380-nm radiation to PAR. The year-to-year variations in 305-nm r adiation were also much greater relative to PAR than for the other UV wavel engths, but there was no evidence of a change in the 305 nm:PAR ratio over the study period. The ratios of both 305- and 320-nm radiation to PAR incre ased from dawn to midday, but those of 340- and 380-nm radiation were almos t constant through the day, except shortly before sunrise and after sunset when the proportions of 340- and 380-nm radiation increased. Underwater mea surements of PAR and UV suggest that the 1% depth for 305-nm radiation was little more than 1 m, but this estimate is valid only for summer and autumn because, in other seasons, few reliable readings for 305-nm radiation coul d be obtained underwater, and no attenuation coefficient could be calculate d. The 1% depths recorded for the other UV wavelengths in the middle 6 mont hs of the year were 2.0 m for 320 nm, 2.6 m for 340 nm and 4.6 m for 380 nm , compared with 12 m for PAR, but the attenuation of all wavebands increase d sharply in October and remained higher until March. An analysis of the in fluence of sun angle, total column ozone concentration, the proportion of s kylight, and cloud cover on the ratio of UV wavelengths to PAR in surface i rradiance demonstrated that solar angle has a greater influence than ozone concentration on the irradiance at 305 nm, and that the typical occurrence of ozone "holes" in spring may not result in higher UV-B irradiances than o ccur under higher ozone concentrations in summer. The implications of the d ata for attempts to model the biological effects of natural UV radiation on marine organisms are considered.