Viral lysis of marine microbial phytoplankton has important implications for marine primary production, food web dynamics, and biogeochemical cycling in the oceans. However, biogeographical patterns of viral-induced mortality in phytoplankton remain largely unknown. We investigated the biogeographical distribution of marine viruses and their contribution to phytoplankton mortality along a north-south stratification gradient in the Northeast Atlantic Ocean. The virus composition changed with latitude, and was closely associated with the biogeographical distribution of different phytoplankton groups. Average virus-mediated lysis rates of pico-cyanobacteria groups were lower than of pico- and nano-eukaryotic phytoplankton. Total mortality rates induced by viral lysis were of comparable magnitude as microzooplankton grazing. Total phytoplankton mortality matched the gross growth rates, signifying high turnover rates of marine phytoplankton populations. Moreover, the data reveal a striking large-scale biogeographical trend of reduced viral lysis of phytoplankton at higher latitudes in the North Atlantic. Viral lysis was the dominate loss factor for phytoplankton in the south with an average 35% (3.7 µg photosynthetically fixed C l-1 d-1) being shunted into the dissolved organic carbon pool compared to 25% (2.7 µg C l-1 d-1) being grazed. Our findings suggest that future warming of the ocean surface layer, and subsequent strengthening of vertical stratification and oligotrophic conditions, is likely to increase virus-induced mortality rates of phytoplankton in the temperate waters of the North Atlantic Ocean. Shifts in mode of mortality have the potential to alter ecosystem efficiency and may negatively affect biological carbon export to the deep ocean.