There is limited information on the effects of ultraviolet (UV) radiation on tropical marine angiosperms. An evaluation of the effects of UV radiation on the tropical seagrass Thalassia testudinum was conducted in Southwestern Puerto Rico. This assessment required the determination of surface and underwater UV levels, vertical attenuation coefficients, and biological UV dose rates. Annual average UV-B (280-320 nm) and UV-A (320-400 nm) daily dose rates of 91 and 1099 kJ m^-2 day^-1 were measured with peak values observed in April- May. The percentage of incident surface irradiance reaching benthic communities at 1.5 m depth ranged from 10 to 15 percent for UV-B and 15 to 35 percent for UV-A. The average attenuation coefficients in offshore waters fluctuated from 0.82 m^-1 at 300 nm to 0.25 m^-1 at 400 nm. A field assessment evaluating the responses of T. testudinum to ambient, partially reduced and excluded UV radiation indicated that ambient levels of UV have significant effects on the plant’s turnover rate, leaf succulence, as well as in the concentration of total chlorophyll, carotenoids and photoprotective pigments. Changes in photosynthetic efficiency and in the photosynthetic and photoprotective pigments in this seagrass were also evaluated by outdoor and laboratory supplementation experiments using UV fluorescent lamps and a xenon arc solar simulator. Lamps daily doses for the three outdoor experiments were 120, 40 and 14 kJ m^-2 day^-1. Both supplementation experiments indicate that increased UV levels reduce the photosynthetic efficiency in T. testudinum leaves. A pilot study evaluating alterations in Guanine DNA adducts by oxidative stress in leaves exposed to UV radiation, was conducted for the first time in plants. Guanosine, 2-deoxyguanosine and O-6-methylguanine were the guanine DNA adducts identified in samples. Results indicate that in T. testudinum, even small increments in UV radiation, resulting from slight depletions of stratospheric ozone, could have significant effects on pigment composition, physiological status, and genetic response.