The composition of phytoplankton is discussed for stations located in the shelf waters of the southeastern United States, the Gulf Stream, and the northwest Sargasso Sea. Water samples were obtained between December 1964 and May 1968 from transects extending from coastal waters seaward.
Considerable species diversity and seasonal fluctuations were noted. The diatoms predominated in waters over the Continental Shelf to the western boundary of the Gulf Stream; thence their numbers declined rapidly into the Sargasso Sea. The major diatoms were Skeletonema costatum and
Rhizosolenia alata. Coccolithophores and pyrrhophyceans were observed in low concentrations over the shelf, and were most abundant in the Gulf Stream. The variety and numbers of coccolithophores, pyrrhophyceans and silicoflagellates were greatest in the Gulf Stream. Major coccolithophores
in the Gulf Stream were Coccolithus huxleyi, Gephyrocapsa oceanica, Syracosphaera mediterranea, and S. pulchra. A wide variety of pyrrhophyceans are listed for the Gulf Stream. The total concentration of phytoplankton and the numbers of species were less in the Sargasso Sea than
in the shelf waters or in the Gulf Stream. Several species were found unique to the Sargasso Sea, with many having a seasonal appearance. The coccolithophores and pyrrhophyceans were the major components of the phytoplankton of the Sargasso Sea, with relatively small numbers of diatoms present.
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