English Summary - Living in Arctic watersThe Arctic ocean and its seasonally changing sea-ice cover influence the heat and energy exchange between cold air masses and warmer ocean currents which finally affects the climate. In these high latitudes scientists from various fields are searching for answers related to global change questions and theirs effects on marine communities. One problem is the ozone-hole and the resulting increase in biologically harmful ultraviolet radiation. The use of divers is essential for many field experiments such as measurements of the underwater radiation climate, collecting of marine organisms and observation and identification of ecological interactions between different species. Many physiological processes of marine macroalgae such as photosynthesis and growth are inhibited by UV radiation. The sensitivity of individual species is generally related to the depth zonation, i.e. eulittoral macroalgae are more resistant than sublittoral plants. The synthesis and accumulation of UV-absorbing substances may prevent photodamage and hence represents an important adaptation mechanism against UV stress. Although UV effects on whole macroalgal communities are not well investigated, losses in productivity and biomass, and consequently decreases in associated animals such as crustaceans and fish can be expected if the UV is further increasing. Benthic marine macroalgae represent suitable bio-indicators for environmental changes in Arctic coastal waters.