Macrozooplankton biomass in Gulf Stream warm‐core rings: Spatial distribution and temporal changes

Abstract

The vertical and horizontal distribution of macrozooplankton biomass was measured at four time points in warm‐core ring 82B. Samples were also collected in two other warm‐core rings, the Slope Water, nine cold‐core rings, and the Sargasso Sea. Biomass in newly formed warm‐core rings was 1.5–2 times lower than that of the surrounding Slope Water but increased within 3–4 months to levels found in the adjacent waters, a rate 4 times faster than the biomass in cold‐core rings reaches levels found in the Sargasso Sea. A mixing model is used to show that for the period March through June the increase in biomass at the center of ring 82B was largely the result of in situ production. Half of the macrozooplankton biomass in the upper 1000 m of warm‐ and cold‐core rings, the Slope Water, and the Sargasso Sea was usually below 200 m at night and 300 m by day. Biomass shoaled during periods of high concentrations of near‐surface plant biomass associated with spring restratification of the upper ocean. Diel vertical shifts in median biomass depth in the upper 1000 m showed significant regional differences; the order of the values averaged for all cruises was ring 82B = 76 m < Slope Water = 115 m < cold‐core rings = 149 m < Sargasso = 212 m.