Das Projekt "Einfluss von UV-B auf die Photosynthese des Phytoplanktons in Schweizer Seen von 400 bis 2600 m ue.M." wird vom Umweltbundesamt gefördert und von Universität Zürich, Institut für Pflanzenbiologie, Limnologische Station durchgeführt. A lake system in the northern canton of Tessin (Piora region; Lake Cadagno, 1900 m and Lake Scuro 2600 m a.s.l.) was investigated for the influence of UV-B on the specific primary production rates near the surface (down to 2 m) and compared with Lake Zuerich data. Dissolved organic carbon (DOC) is a key factor in the attenuation of UV radiation (Doc.Ist.ital.Idrobiol. 63,65-70, 1998) and thus influences the UV-B effect on primary production. Phytoplankton in high mountain lakes show decreased photosynthesis with increasing UV-B-radiation only if the UV-B-intensity has passed a threshold value. In contrast, the phytoplankton in Lake Zuerich were suppressed by relatively weak UV-B intensities. Several experiments with natural phytoplankton and with algal cultures, where the photosynthetic efficiency was followed using the PAM-fluorometry technique, answer questions about the time scale of photoinhibition and its recovery period.