Using flow cytometry, results have determined that the relative abundance of picophytoplankton (< 2 μ μ m), small nanophytoplankton (2 to 10 μ μ m) and large nanophytoplankton (10-20 μ μ m) are greatly affected by the stratification index of the water column. Cell diversity was greatest in the presence of moderate levels of stratification. If the turbulence is too high, their numbers remain stable or fall, but if there is no turbulence, their numbers do fall. It is postulated that the nutrient-rich boundary layer around each phytoplankton cell is not exhausted, but renewed, by this moderate level of turbulence.