Biomass of the micronekton in Gulf Stream ring 82-B and environs: changes with time

Abstract

Micronekton biomass was sampled in warm-core Gulf Stream ring 82-B (at ages 2, 4 and 6 months), the Slope Water, the Gulf Stream and the Sargasso Sea during three multi-ship cruises. There was no significant diel difference in the 0–1000 m integrated biomass for fishes or invertebrate micronekton in 24 paired day-night tows, and no evidence for diel vertical migration across the 1000 m level. Biomass showed a consistent upward shift at night; although it varied with hydrographic regime and sampling date, median biomass depth shift for invertebrates averaged 184 and 137 m for fishes. For total micronekton, median biomass depth ranged from 550 to 750 m by day and 300 to 600 m by night. Fish biomass was always centered deeper than 250 m, while the center of invertebrate biomass rose above that level in only one of 12 night tows. In April, ring-core biomass for fishes and invertebrates (2.2 and 11.9 cc m −2, respectively) was significantly lower than that for the high velocity region (3.6 and 19.8 cc m −2) or that for the Slope Water (4.7 and 20.8 cc m −2). By June, ring-core standing stocks had doubled while that for the high velocity region had been reduced to half of the April levels. No significant changes in Slope Water biomass occurred over the April–August period. Fluctuations in ring-core micronekton biomass were probably due to both in situ processes and advective exchange. On the average, invertebrate biomass was about 4 times that of fish biomass. Total micronekton was 20–30% of the macrozooplankton biomass in April and June, and 7–16% in August. Vertically, the invertebrate fraction overlapped the macrozooplankton more than did the fish fraction. Greater overlap of micronekton and macrozooplankton occurred at night.