Introduction
The study of the detrimental effect of UV-B radiation on aquatic organisms has mostly focused on Antarctica but the increase of UV-B radiation in northern latitudes (20% at 310nm, at 55°N) has generated concern about its impact on temperate aquatic ecosystem of the Alpine region. The present study has been carried out in the frame of the EU project MICOR (MIcrobial COmmunity response to UV-B stress in European waters) with the purpose of evaluating the effect of UV-B radiation on natural picoplanktonic populations (size range 0.2-2 µm), both autotrophic (APP) and heterotrophic (HPP). We have used quartz and Mylar D screened quartz tubes to measure APP photosynthetic activity and HPP production under natural and artificially increased UV-B radiation.
This research was addressed to the following questions:

	how strong is the UV-B underwater radiation in latitude around 45°N and how deep does it penetrate ?
	are freshwater pico size populations affected by UV-B under ambient UV-B climate?
	under stressing UV-B conditions are APP more susceptible to photoinhibition than HPP?

The few data available on the effects of UV-B to picocyanobacteria cells show that, as for other algal groups, UV-B inhibits the PSII reaction centre activity, altering the structure of D1/D2 polypeptides and the light-harvesting complex (Rajagopal and Murthy, 1996). Bergeron and Vincent (1997) have found a significant decrease of APP and not of HPP cells in the presence of UV-B, but no data on the activity of these two pico fractions were presented. Contrasting data have demonstrated that ambient UV-B can inhibit bacterial activities as much as 40% (Herndl et al. 1993).