The ratio of respiration to maximal photosynthesis in small diatoms is low so they can survive in weakly stratified water. In contrast, the same ratio for dinoflagellates is three times as high and they need waters that are strongly stratified. Some successional phenomena may be explicable in this way. The great fisheries depend on spring or autumn outbursts in temperate waters and on upwelling areas in regions, equatorward of 40° latitude. It is suggested that they are linked to the small diatoms, in traditional food chains, in weakly stratified waters.