We have investigated the in situ destruction rates of marine viral particles as well as the decay rates of infectivity for viral isolates along a ca. 400 km transect from oligotrophic offshore waters to productive coastal waters in the Gulf of Mexico. Light-mediated decay rates of viral infectivity averaged over the solar day ranged from 0.7 to 0.85 h^-1 in surface waters at all stations and decreased with depth in proportion to the attenuation of UV-B (305 nm). The destruction rates of viral particles also decreased with depth, although the rates of particle destruction were only 22 to 61% of infectivity when integrated over the mixed layer. The rates of viral particle destruction indicated that at three of four stations 6 to 12 % of the daily bacterial production would have to be lysed in order to maintain ambient viral concentrations. At the fourth station, where there was a dense bloom of Synechococcus spp. and the mixed layer was shallower, 34 to 52 % of the daily bacterial production would have to be lysed. A comparison of the difference between destruction rates of viral particles and infectivity integrated over the depth of the mixed layer implies that host-mediated repair must have restored infectivity to 39 - 78 % of the sunlight damaged viruses daily. The calculated frequency of contacts between viral particles and bacterial cells that resulted in infection (contact success) ranged from ca. 18 to 34 % in offshore waters, where the frequency of contacts between viruses and bacteria was much lower, to ca. 1.0 % at the most inshore station, where contact rates are much higher. This suggests that in offshore waters bacterial communities are less diverse, and that there is less selection to be resistant to viral infection. This paper provides a framework for balancing viral production, destruction and light-dependent repair in aquatic viral communities.