Zooplankton inside an Arctic Ocean cold-core eddy: Probable origin and fate

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In September 2004, an extensive survey of a cold-core eddy in the Canada Basin, western Arctic was carried out with high-horizontal-resolution physical and chemical sampling and lower-horizontal-resolution biological sampling. The eddy was located over the continental slope north of the Chukchi Shelf and had a radius of ∼8 km. Its core was centered at a depth of ∼160 m. Water mass characteristics and the presence of copepods from the North Pacific Ocean (Neocalanus flemingeri and Metridia pacifica) demonstrated that the core contained water of Pacific origin. Vertical distributions of zooplankton were associated with the physical structure of the water column. For most taxa, concentrations in the eddy core were elevated compared with those in similar density water in the surrounding Basin. Based on tracer-age estimates and previous observations of eddy formation, the eddy is believed to have been formed during the previous spring/summer from the Chukchi shelfbreak jet. Surprisingly, the eddy also contained elevated abundances of Arctic-origin copepods (Metridia longa and Calanus glacialis). Analysis of a shelf–basin transect occupied in the region in August 2004 showed that these species were present in high abundances in relatively shallow water (50 m) inshore of the shelfbreak due to upwelling of deeper basin water, and copepods, onto the shelf in response to easterly winds. If the formation of the observed eddy occurred during, or shortly after, a period of such winds, upwelled Arctic-origin copepods on the shelf might have been entrained into the feature. Our observations suggest that formation and subsequent migration of such eddies may provide a mechanism for transporting zooplankton from the Chukchi Shelf into the interior Canada Basin. The periodic input of high abundances of zooplankton from productive shelf areas could affect food webs in the less productive basin.