Why are there about 5000 species of phytoplankton in the sea?

The relative abundances of phytoplankton taxa conform approximately to a finite geometric series in which there are 20-25 species per decade of ranked abundance. Such series can contain 160-400 species between the commonest (10²²-10²⁶ cells) and the rarest (10¹⁰-10¹⁴ cells). Thus, between 12 and 31 such series are needed to explain the observed diversity, ∼5×10³ species, of marine phytoplankton. The number of series is similar to the number (20-25) of upper-ocean water masses defined by dilution time scale of order 10¹-10² years. Interpretations of this coincidence are discussed.