This work contributes new knowledge about the distributions of the marine cyanobacterium Prochlorococcus. Specifically, it is the first to compare basin-scale latitudinal transects for the Atlantic and Pacific Oceans; it is the first to show that the ratios of the dominant lineages (ecotypes) vary as a log-linear function with temperature; it is the first to show that despite apparent fitness differences, the dominant ecotypes do not compete to the extinction of others; and is the first to consider temporal disconnects between temperature changes and population shifts.