Observations on Dinoflagellate Blooms1

Dinoflagellate blooms were observed in Delaware Bay from 1952 to 1954 and in tidal creeks on Sapelo Island, Georgia, during 1955. In all cases the blooms were concentrated at the surface of the water. The organisms were positively phototactic and swam toward the light at 0.3 to 0.8 cm/sec. Because of their tendency to remain at the surface, the dinoflagellates were concentrated by weak convergences. One of the blooms was associated with unusual rainfall but the others occurred during dry weather. The concentrations of phosphorus found in the blooms could be accounted for on the basis of concentration mechanisms without postulating a special nutrient source, such as river flow. However, river flow may contribute other biologically important substances.