The paper summarizes the results of the examination, through Utermöhl's microscope, of a large number of water samples from NW Mediterranean (years 1960-1991, 1388 samples) and SE Caribbean (years 1963-65, 1144 samples). Samples were obtained in the photic zone, down to 110 m and cover all the year. The resulting compound diversity is high. It is best expressed by the slope of the logarithms of the numbers of cells of the different species, when graded in order of decreasing abundances. Diversity is a bit higher for the Mediterranean, in accordance with a lower productivity, a lower average cell concentration and a slightly larger average size of the cells. These global expressions of regional diversities is compounded by much lower local diversities, especially low in blooms. Blooms rework genetic materials taken from the general bank and give them back, unchanged or changed, to the same general reserve of biodiversity.