Short‐term changes in the biology of a Gulf Stream warm‐core ring: Phytoplankton biomass and productivity1

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Abstract
Temporal changes in chlorophyll a and primary production were followed in a Gulf Stream warm‐core ring as it underwent an interaction with the Gulf Stream and a subsequent wind event. As the physical events altered the structure of the mixed layer, Chl a within the euphotic zone increased from about 10 to 42 mg m−2 while productivity rates increased from 140 to >2,400 mg C m−2 d−1. Carbon‐specific growth rates in Ring 81‐D derived from ATP‐C biomass and 14C productivity rates increased fivefold. Nanoplankton (µm) initially accounted for nearly 90% of the total Chl a biomass in ring surface waters, although there was a subsurface maximum of net‐plankton Chl a near the base of the euphotic zone before the wind event. After the wind event, Chl a was uniformly distributed throughout the mixed layer and net plankton accounted for an increased proportion of total Chl a standing stock and productivity. In the peripheral high velocity region and the surrounding Slope Water, phytoplankton standing stocks and productivity rates were less than the maximum values found at ring center. Our observations indicate that phytoplankton standing stock and productivity within a warm‐core ring can increase rapidly in response to physical processes on the event time scale.

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