Solar radiation as a primary abiotic factor affecting productivity of seaweeds wasmonitored in the Arctic Kongsfjord on Spitsbergen from 1996 to 1998. The radiation wasmeasured in air and underwater, with special emphasis on the UV-B (ultraviolet B, 280-320nm) radiation, which may increase under conditions of stratospheric ozone depletion. Therecorded irradiances were related to ozone concentrations measured concurrently in theatmosphere above the Kongsfjord with a balloon-carried ozone probe and by TOMS satellite.For comparison, an ozone index (a spectroradiometrically determined irradiance of awavelength dependent on ozone concentration, standardized to a non-affected wavelength) wasused to indicate the total ozone concentration present in the atmosphere. Weather conditionsand, hence, solar irradiance measured at ground level were seldom stable throughout the study.UV-B irradiation was clearly dependent on the actual ozone concentration in the atmospherewith a maximal fluence rate of downward irradiance of 0.27 W m-2 on the ground and amaximal daily fluence (radiation exposure) of 23.3 kJ m-2. To characterize the water body, thelight transmittance, temperature and salinity were monitored at two different locations: (1) at asheltered shallow-water bay and (2) at a wave-exposed, deep-water location within theKongsfjord. During the clearest water conditions in spring, the vertical attenuation coefficient(Kd) for photosynthetically active radiation (PAR) was 0.12 m-1 and for UV-B 0.34 m-1. Inspring, coinciding with low temperatures and clear water conditions, the harmful UV radiationpenetrated deeply into the water column and the threshold irradiance negatively affectingprimary plant productivity was still found at about 5-6 m depth. The water body in spring wascharacterized as a Jerlov coastal water type 1. With increasing temperature in summer, snowlayers and glacier ice melted, resulting in a high discharge of turbid fresh water into the fjord.This caused a stratification in the optical features, the salinity and temperature of the waterbody. During melt-water input, a turbid freshwater layer was formed above the more dense seawater. Under these conditions, light attenuation was stronger than defined for a Jerlov coastalwater type 9. Solar radiation was strongly attenuated in the first few metres of the watercolumn. Consequently, organisms in deeper water are protected against harmful UV-Bradiation. In the surface water, turbidity decreased when rising tide caused an advection ofclearer oceanic water. In the course of the summer season, salinity continuously decreased andwater temperature increased particularly in shallow water regions. The impact of global climatechange on the radiation conditions under water and its effects on primary production ofseaweeds are discussed, since organisms in the eulittoral and upper sublittoral zones areaffected by UV radiation throughout the polar day. In clearer water conditions during spring,this may also apply to organisms inhabiting greater depths.