Effects of UV-B radiation on biomass and composition in marine phytoplankton communities

Biomass and the composition in marine phytoplankton communities in in-door aquaria were studied for seven days. Two of them were exposed to UV-B irradiation (0,64 W*m(-2), 6 h/day) and two served as controls. Each aquarium had a volume of 181 and they were run semicontinuously by replacing half of the volume each day with freshly filtered (GF/C) sea water. The aquaria were located in a thermostated incubation chamber at Kristineberg Marine Research Station on the Swedish west coast. The seed community was sea water from the upper mixed layer of the Gullmar fjord screened through a 118 m screen. Samples for determination of species composition were taken at the start and on four succeeding occasions during the experiment. The total biomass in the communities exposed to UV-B was lower than in the controls, mainly due to a lag phase at the start of the experiment. After this the growth rates were only slightly lower in the UV-B exposed communities. The main difference in the succession of the phytoplankton was a reduction in growth of Bacillariophyceae by over 30%, and an increase in Chrysophyceae in the UV-B exposed communities. Within the Bacillariophyceae, however, large differences in effects were observed between the different genera or species. On an average, the algal cells were larger in the controls than in the UV-B exposed communities, but this was explained by the main changes in succession patterns between different systematic groups. It was not possible to find any clear relation between the size of the algal cells and their sensitivity to UV-B irradiation.