1 IntroductionDepletion of stratospheric ozone over the Arctic region (Müller et al. 1997; Rex et al. 1997) causes an increase of UV-B radiation (280-315 nm) and could affect algal distribution patterns in the unique marine ecosystem (Wiencke et al. 2000). To date, most of the recent investigations on biological effects of UV radiation on marine ecosystems have concentrated on the Southern Ocean due to the huge "ozone hole" over Antarctica (Holm-Hansen et al. 1993a,b). Increasing ozone depletion over the northern Hemisphere (Blumthaler and Ambach 1990; Wängberg et al. 1996) may also have the potential to change growth conditions for macroalgae and other organisms, especially in Arctic waters. Therefore, in recent years several studies on the distribution and the physiology of several algal species growing in the Arctic environment have been undertaken to investigate the effects of ultraviolet radiation on seaweeds (e.g. Hanelt et al. 1997a; Bischof et al. 1998; Karsten et al. 1998, 1999). In coastal sea wa-ters, UV radiation (UV) and blue light are strongly attenuated (Björn 1993), and this depends largely on the input of dissolved organic matter (DOM) from terrestrial ecosystems. This occurs often during the warmer seasons due to rainfall or melt water from snow layers and glaciers. Moreover, the absolute UV irradiation depends also on the clouds, solar altitude or sun angle which is much lower in the Arctic than at temperate zones. That means that already double of UV-B radiation is impinging on the earth surface in a temperate latitude 20o lower, which needs to be considered for discussing the UV problem in polar regions. However, in comparison to temperate species polar algae are more sensitive to UV.In spring, coinciding with low temperatures and clear water conditions, the harmful UV wavelengths penetrate deeply into the water column in Kongsfjorden (Svalbard; Hanelt et al. 2001). The threshold irradiance with the potential to affect primary plant productivity negatively was still found at about 5-6 m depth. With increasing temperature in summer, snow layers and glacier ice melt, resulting in a high discharge of turbid fresh water into the fjord overlaying the more dense sea water. This causes a stratification in the optical features, salinity and temperature of the water body, strongly attenuating solar radiation in the first meter of the water column. Consequently, organisms in deeper water are more protected against harmful UV-B radiation. Nevertheless, organisms growing in the intertidal are still exposed to high UV-B during low tide and high sun position also in summer. The possible impact of global climate change on the radiation conditions underwater and the effects on primary production of seaweeds can be higher during the spring season, as organisms in the eulittoral and upper sublittoral zone are affected by UV radiation throughout the polar day. In addition, melt water input is only a phenomenon during the warmer summer season. This applies to Arctic shorelines in half-open fjord systems where the water exchange with the clearer oceanic water is retarded. At open coastlines the melt water will be exchanged much faster with oceanic water which will diminish the observed turbidity effects on light penetration.