Phytoplankton of the North Central Sargasso Sea, 1950–521

U. S. Weather Ship personnel operating at Lat. 35°N, Long. 48°W obtained approximately weekly collections of water samples for phytoplankton enumeration at five depths between 0 and 150 m for a two-year period beginning in February, 1950. Temperature and Secchi disk measurements and zooplankton collections also were made. The average concentration of phytoplankton was sparse, but it extended through a long vertical column, in agreement with previous observations. There was a diatom increase in April of each year, which appeared to be analogous to the vernal augmentation in temperate waters although of miniature size. The seasonal minimum occurred in autumn, and the population tended to be larger in winter than in summer. Destruction of the seasonal thermocline between December and March presumably increased the rate of nutrient supply at that season, but light and turbulence were critical factors. Hence the development of winter diatom pulses was correlated with the stability of the water column. The species composition was mainly subtropical or ubiquitous, but occasional localized concentrations of temperate-water diatoms suggested eddy transfer from the north. Analysis of previous oceanographic observations in the region by three independent methods indicated an average daily rate of net phytoplankton production of about 0.15 g carbon per m2 of sea surface, or a quarter to a third of the observed rates in highly productive temperate waters.