SPATIAL AND TEMPORAL DISTRIBUTIONS OF PROCHLOROPHYTE PICOPLANKTON IN THE NORTH-ATLANTIC OCEAN

Over the past several years red-fluorescing picoplankton, believed to be prochlorophytes, have been shown to be extremely abundant in the North Atlantic and Pacific Oceans. The dim fluorescence of these tiny cells initially limited studies to the relatively highly pigmented cells near the bottom of the euphotic zone; however, improvements in sensitivity of flow cytometry now enable us to detect the prochlorophytes in surface waters as well. In the Sargasso Sea in May 1988 and May 1989 prochlorophytes were present throughout the upper water column, and we observed the highest concentrations in surface waters within the Gulf Stream. Further south, the prochlorophytes formed subsurface maxima; the median depth of prochlorophyte (but not Synechococcus) populations followed the deepening of the nitracline. Prochlorophytes were not present in waters north of the Gulf Stream in May, although we had observed them there on a previous September cruise. They were present year round near Bermuda, with lowest concentrations in the winter, when Synechococcus was most numerous; the prochorophytes appear to "bloom" later than the Synechococcus, after the onset of seasonal stratification. The latitudinal variations in prochlorophyte and Synechococcus distributions during spring resembled the seasonal pattern near Bermuda. © 1990.