Effects of UV-radiation of humic water on primary and secondary production

Bioassays with UV-irradiated surface waters from two humic localities revealed a consistent pattern with phytoplankton production being stimulated at moderate doses (1.1.–5.4 J cm^?2, 312 nm), but with a transition to severe growth inhibition at increasing doses (>10 J cm^?2). Phytoplankton (Selenastrum capricornutum) was inoculated after the irradiation treatment, and the observed growth response gave support to the hypothesis of long-lasting algicidal effects induced by UV-radiation. High UV-doses apparently also liberated nutrients and metals (Al) complexed by humus. Since the applied UV₃₁₂-doses corresponded to mid-summer solar intensity, the results suggest both chemical and ecological implications, and that these effects have a non-linear response on UV-dose. Conversely no effects were detected on dark respiration or during corresponding bioassays with the crustacean zooplankton Daphnia magna.