363
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t
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', Beads
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30 m
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0
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50 m
10
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log Side Scatter
Figure 7. Depth profile showing Synechococcus, prochlorophytes, and larger eukaryotic phytoplankton from a
station in the Sargasso Sea north of Bermuda (35°15'N, 66°33'W), October 1989. Side scatter and red (Chi)
fluorescence are plotted from samples analyzed using a Becton Dickinson FACScan. Uniform fluorescent
beads (0.57 Jlm diameter, Polysciences) were added as internal standards; these are visible as both single
beads and doublet peaks, The FACScan is sensitive enough to detect the prochlorophytes in surface waters
as well as deep in the euphotic zone at most stations, On occasion, when the surface cells have very little
pigment, the Coulter EPICS (modified for high sensitivity) is required to separate the cells completely from
the noise (Olson et ai, 1990b),