Fluctuations in the densities of marine phytoplankton species in the open and coastal ocean follow a sequence wherein one or several species may increase in abundance very much without causing a reduction in abundance of other forms. Theoretical calculations of the maximum size of nutrient—depleted zones about the plankton cells show that overlapping of these zones cannot occur at cell densities less than 3 x 108/liter. Since densities rarely exceed 106/liter in the open and coastal ocean, there is no possibility of an abundant species monopolizing the nutrient supply and forcing a less abundant form to extinction. The change from a coccolithophore—dominated plankton in the Sargasso Sea to the diatom—dominated plankton in the coastal water off New York cannot, therefore, be attributed to competition for nutrients by the two types of cells. But in some very shallow estuaries, Great South Bay and Moriches Bay near New York and Salt Pond at Woods Hole, the dominant forms sink considerably more slowly than the coastal and oceanic species and would thus be at an advantage over the latter. Plankton concentrations in the estuaries may come close to or exceed 109/litter, and monopolization of the nutrient supply should aid in the continued dominance of the abundant forms, eventually bringing about the extinction of many of the residual forms. This sequence is considered to be a real possibility in these estuaries and to be the reason for the reduced diversity of species observed there.