In the bloom-forming filamentous blue-green algae Anabaena, both carbon dioxide and nitrogen fixation share a dependence on light. During daylight, A. reduces direct competition between these processes for light-generated reductant by optimizing carbon fixation during late morning hours while optimizing nitrogen fixation during afternoon hours.Sequential optimization was examined from both biochemical and ecological perspectives. Biochemically, it is sound to optimize carbon prior to nitrogen fixation, due to the higher sensitivity of the former to afternoon increases in dissolved oxygen levels which commonly occur in blooms. It is also advantageous to first assure adequate supplies of fixed carbon prior to incorporating fixed nitrogen. Ecologically, the sequence represents optimal use of radiant energy. A. is able to shift energy flow from a highly inhibited to a less inhibited process, thereby maintaining a sink for photo-generated reductant. Both A. and a non-nitrogen fixing diatom community show similar carbon fixation efficiencies during morning and midday hours. During afternoon however, A. diverts significant portions of photo-reductant from carbon to nitrogen fixation. This allows A. to optimize carbon fixation while maintaining access to nitrogen during periods of ambient nitrogen shortage. Accordingly, A. blooms usually appear during summer months when both nitrogen starvation and photosynthetically-active radiation inputs are maximal.