The distribution and abundance of phototrophic ultraplankton in the North Atlantic1,2

We compared the vertical distribution and abundances of procaryotic chroococcoid cyanobacteria ( Synechococcus ) and eucaryotic phototrophic ultraplankton at 50 stations in the North Atlantic. At 34 of these stations, cells were also separated by filter‐size fractionation, permitting comparison of the distribution of size classes with the distribution of pigment types. Both eucaryotic ultraplankton and cyanobacteria have subsurface numerical maxima. In vertically stable water, cyanobacterial abundance peaks at about the 1% light level and that of the eucaryotic ultraplankton at about the 0.5% light level. Cyanobacterial abundance decreases with northerly increasing latitude, and this decrease correlates with decreasing temperature. The eucaryotic assemblage is numerically dominated by the ultraplankton. The numbers of the ultraplankton generally range one order of magnitude less than the cyanobacteria, but they may equal or exceed cyanobacteria at depth and in northerly latitudes.