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

Carbon assimilation is usually measured at fairly constant light intensitie s. Under natural conditions, however, planktonic algae are moved through th e water column and experience light of fluctuating intensity and spectral c omposition. 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 a ssimilation and exudation of algae incubated in UV-transparent quartz and i n UV-absorbing glass bottles which were moved through different water laver s. Computer-controlled elevators were used to simulate mixing depths betwee n 2 and 14 meters. Compared to the glass bottles, particulate C assimilatio n in the quartz bottles was reduced by 20-30% at mixing depths between 2 an d 10 m. There was no significant difference between both types of incubatio n 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. O ur results indicate serious damage of planktonic algae by UV even under con ditions of vertical mixing if the euphotic zone exceeds the mixing depth. D epression was low for circulation through the whole euphotic zone and may d isappear at even deeper mixing. Our results indicate lower photoinhibition per UV dosage at fluctuating than at constant light intensities. A model pr edicting inhibition as function of weighted irradiance spectra was adapted to describe wavelength dependent photoinhibition occurring at different mix ing depths. The model results agreed very well with the inhibition rates me asured under fluctuating light. These preliminary results are used to discu ss the importance of UV on photosynthesis of planktonic algae in aquatic en vironments of different mixing depths and stabilities of stratification.