Effects of UV on carbon assimilation of phytoplankton in a mixed water column

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Carbon assimilation is usually measured at fairly constant light intensities. Under natural conditions, however, planktonic algae are moved through the water column and experience light of fluctuating intensity and spectral composition. They may cope with strong UV for a short residence in the upper water layer. In order to estimate the effects of UV on primary production of phytoplankton under conditions of turbulent mixing, we compared carbon assimilation and exudation of algae incubated in UV-transparent quartz and in UV-absorbing glass bottles which were moved through different water layers. Computer-controlled elevators were used to simulate mixing depths between 2 and 14 meters. Compared to the glass bottles, particulate C assimilation in the quartz bottles was reduced by 20-30 % at mixing depths between 2 and 10 m. There was no significant difference between both types of incubation bottles at a mixing depth of 14 m. Exudation was enhanced by UV near the water surface (mixing depth up to 4 m) but not in the deep-mixed samples. Our results indicate serious damage of planktonic algae by UV even under conditions of vertical mixing if the euphotic zone exceeds the mixing depth. Depression was low for circulation through the whole euphotic zone and may disappear at even deeper mixing. Our results indicate lower photoinhibition per UV dosage at fluctuating than at constant light intensities. A model predicting inhibition as function of weighted irradiance spectra was adapted to describe wavelength dependent photoinhibition occurring at different mixing depths. The model results agreed very well with the inhibition rates measured under fluctuating light. These preliminary results are used to discuss the importance of UV on photosynthesis of planktonic algae in aquatic environments of different mixing depths and stabilities of stratification.