Coarse-scale horizontal patchiness and vertical migration of zooplankton in Gulf Stream warm-core ring 82-H

Abstract

A 1-m² MOCNESS with 20 nets was used to make a series of tows in Gulf Stream meander/ring 82-H (September/October 1982) including two 0–100 m undulating “towyos”. One towyo, made at dusk in the core of 82-H (of Sargasso Sea/Gulf Stream origin) permitted study of the effect of diel migration on the spatial variability of copepod and euphausiid species abundance, and species composition in a region of low physical variability. The other towyo taken across a front on the outer edge of 82-H (a mixture of Gulf Stream, Shelf and Slope Water), allowed comparison of spatial variability of the same biological properties in a region of strong physical variability. A sharp transition in euphausiid species composition occurred in the ring core after sunset as dielly migrating euphausiids moved into the surface waters. A similar, but less extreme change took place in copepod species composition because a smaller proportion of these migrated. All copepod migrants also entered surface waters after sunset with species living deeper in the water column during the day arriving in the surface waters later than those living shallower. Enright's (1977, Limnology and Oceanography, 22, 856–872) hypothesis for the metabolic advantages available through diel vertical migration does not account for the observed behavior of the migrating copepods and euphausiids at this time and place. Estimated swimming speeds (typically 50–200 m h⁻¹) of migrating copepods and euphausiids were similar in spite of large differences in body size between the two groups. Variations in species composition were substantially larger at the edge of the ring where species proportions changed radically in concert with changes in water-mass properties. There were also large differences in species composition between the samples from the ring core and the front which equaled those which occurred across the front. Hydrographic differences were stronger than diel changes due to migration for copepods but not for euphausiids. Streamers of surface water which originated within the frontal region and spiraled into the ring core could provide colonizers of many species not present at the time of ring formation.