Phytoplankton biomass, as chlorophyll
a (Chl
a) and ATP, and carbon productivity rates were determined in three cruises to a warm-core Gulf Stream ring. Ring 82B was formed in late February 1982 with observations made in April to May, June and August. Vertical profiles of Chl
a, ATP and particulate organic carbon (POC) in April to May showed little vertical structure during a period when the ring mixed layer extended from the surface to
>350
m
. Daily productivity rates, Chl
a and ATP biomass estimates of the euphotic zone were similar to those in the source waters, the Sargasso Sea, but lower than those in contiguous Slope Waters. Despite the absence of a stratified surface layer in the ring, the phytoplankton productivity rates, assimilation numbers, and carbon-specific growth rates were relatively high. In June, a pycnocline existed at 25 m across the ring with biomass maxima of Chl
a, ATP, and POC occurring near the seasonal pycnocline. Although the range in productivity in June (0.26 to 0.98 gC m
−2 day
−1) was similar to that in April, the carbon biomass estimated from ATP increased from 1.42 to 4.77 gC m
−2 between the two cruises. The increase was partially attributed to an increased heterotrophic biomass. Carbon-specific specific doubling times in June were, most likely, influenced by the presence of a large heterotrophic ATP component. In August the surface layer of the ring was displaced by intrusions, or overwash, of Slope and Gulf Stream waters. During the 6 month lifespan of 82B total primary production was estimated at 126 gC m
−2, a value similar to productivity estimates for the contiguous Slope Waters
