Prochlorococcus and synechococcus cells in the central Atlantic Ocean: Distribution, growth and mortality (grazing) rates

The distribution, primary production, growth and grazing rates of Prochlorococcus and Synechococcus were investigated across the tropical Atlantic ocean during boreal autumn and spring. The picophytoplankton fraction contributed significantly to total phytoplankton community biomass and production (>70% and > 50%, respectively). Prochlorococcus reaching up to 2.5 · 10⁵ cells ml^-1 numerically dominated the picophytoplankton community in the central oligotrophic Atlantic waters while Synechococcus was abundant at the ends of the transects (i.e. in nutrient rich coastal waters). High resolution surface samples showed the meridional patterns of Prochlorococcus and Synechococcus abundance to be highly variable within a few degrees latitude and correlated well with intermediate scale variability (< 500 km) in water mass structure along the transects. Growth rates of Prochlorococcus and Synechococcus in surface waters range from 0.40 to 1.47 d^-1, and 0.22 to 1.58 d^-1, respectively. There was a positive relationship (r=0.77, p<0.05) between growth and grazing rates, and the average differences between growth and grazing rates were -0.17 ± 0.25 d^-1, and 0.15 ± 0.11 d^-1, for Prochlorococcus and Synechococcus, respectively. High grazing rates on Prochlorococcus and Synechococcus indicate that top down control of picophytoplankton abundance is most likely important in Central Atlantic waters.