In order to assess the effects of UV radiation(UVR,280-400 nm)on the relative sensitivity,attachment,germination success and seedling growth of the zoospores of the green tide macroalga, Ulva prolifera,we exposed the cells to three different radiation treatments(PAB:280-700 nm,PA:320-700 nm,P:400-700 nm)under solar or artificial radiation.The experimental units were covered with three different cut-off filters to obtain three treatments:PAB,PA and P under solar simulator,respectively.The light intensity was set at 900 μmol photons/(m²?s),while the irradiances of ultraviolet radiation were UV-A:19.9 W/m²,UV-B:0.68 W/m²,respectively.Optimum photosynthetic efficiency(Fv/Fm)of zoospores was measured as variable fluorescence of PSII by using a water pulse amplitude modulation(PAM)fluorometer.After being exposed to light treatments,the samples were then placed in the dark so that the zoospores could be attached to the cover glass slices randomly.After 24 h the settled zoospores were counted by microscope.The randomly settled zoospores on the cover glass slices were placed under different light treatments with cut-off filters.The germination success and growth were measured punctually.The results showed that:1)PA and PAB treatments could significantly inhibit the zoospores optimum photosynthetic efficiency(Fv/Fm)under 900 μmol photons/(m²?s),and the extension of inhibition increased with exposure time.Moreover,the inhibition of PAB treatment was more obvious than those of P and PA treatments.(2)The hysteresis effects of zoospore settlement were observed under shortterm ultraviolet radiation,and the results showed that 24 h settlement rate of PA and PAB treatments after short-term exposure to ultraviolet radiations of 5,10 and 15 min were not significant from control(P).(3)The significant differences among the groups of germination success of settled zoospores were found when the samples were placed under different solar radiations.The relative germination rates to P treatment at 3^rd-day and 6^th-day were:PA:57.0% and 58.5%,PAB:39.1% and 34.6%,respectively.There was no significant difference of germination rate between first 3 d and 6 d,which implied zoospore survival depends on the initial 3 d.(4)The seedling growth were significantly inhibited by PA and PAB treatments.The rates of seedlings that contained more than 4 cells at 5^th-day were P:72.9%; PA:14.3%; PAB:10.1% respectively.The growth inhibition rates were 46.2% and 68.1% by PA and PAB treatments.Overall the data showed that optimum photosynthetic efficiency,adhesion time,germination success and seedling growth of zoospore were significantly inhibited by UVR,and the inhibition of PAB was stronger than that of PA.The results suggested that UVR could play the negative roles on the early development and survival,and might have a major impact on the occurrence and recruitment of U.prolifera populations.