Light regime in an Arctic fiord: a study related to stratospheric ozone depletion as a basis for determination of UV effects on algal growth

Solar radiation as a primary abiotic factor affecting productivity of seawe eds was monitored in the Arctic Kongsfjord on Spitsbergen from 1996 to 1998 . The radiation was measured in air and underwater, with special emphasis o n the UV-B (ultraviolet B, 280-320 nm) radiation, which may increase under conditions of stratospheric ozone depletion. The recorded irradiances were related to ozone concentrations measured concurrently in the atmosphere abo ve the Kongsfjord with a balloon-carried ozone probe and by TOMS satellite. For comparison, an ozone index (a spectroradiometrically determined irradi ance of a wavelength dependent on ozone concentration, standardized to a no n-affected wavelength) was used to indicate the total ozone concentration p resent in the atmosphere. Weather conditions and, hence, solar irradiance m easured at ground level were seldom stable throughout the study. UV-B irrad iation was clearly dependent on the actual ozone concentration in the atmos phere with a maximal fluence rate of downward irradiance of 0.27 W m(-2) on the ground and a maximal daily fluence (radiation exposure) of 23.3 kJ m(- 2) To characterize the water body, the light transmittance, temperature and salinity were monitored at two different locations: (1) at a sheltered sha llow-water bay and (2) at a wave-exposed, deep-water location within the Ko ngsfjord. During the clearest water conditions in spring, the vertical atte nuation coefficient (K-d) for photosynthetically active radiation (PAR) was 0.12 m(-1) and for UV-B 0.34 m(-1). In spring, coinciding with low tempera tures and clear water conditions, the harmful UV radiation penetrated deepl y into the water column and the threshold irradiance negatively affecting p rimary plant productivity was still found at about 5-6 m depth. The water b ody in spring was characterized as a Jerlov coastal water type 1. With incr easing temperature in summer, snow layers and glacier ice melted, resulting in a high discharge of turbid fresh water into the fjord. This caused a st ratification in the optical features, the salinity and temperature of the w ater body. During melt-water input, a turbid freshwater layer was formed ab ove the more dense sea water. Under these conditions, light attenuation was stronger than defined for a Jerlov coastal water type 9. Solar radiation w as strongly attenuated in the first few metres of the water column. Consequ ently, organisms in deeper water are protected against harmful UV-B radiati on. In the surface water, turbidity decreased when rising tide caused an ad vection of clearer oceanic water. In the course of the summer season, salin ity continuously decreased and water temperature increased particularly in shallow water regions. The impact of global climate change on the radiation conditions under water and its effects on primary production of seaweeds a re discussed, since organisms in the eulittoral and upper sublittoral zones are affected by UV radiation throughout the polar day. In clearer water co nditions during spring, this may also apply to organisms inhabiting greater depths.