Euphausiid invasion/dispersal in gulf stream cold-core rings

Abstract

Invasion/dispersal patterns in the distribution and abundance of euphausiid species in four 2-8-month- old cold-core rings from the Gulf Stream suggest that several different physical exchange mechanisms are operating. The most important of these appear to be horizontal mixing in the mixed layer and exchange due to movement into or out of the trapped region at depth. A zone of minimum exchange is evident between 150 and 400 m. Changes in vertical distribution and abundance of warm-water species invading the ring environs suggest that only a few species, such as Stylocheiron carinatum, are able to penetrate and take advantage of the changing ring conditions in young- to middle-aged rings (2-8 months). Other near-surface warm-water species penetrate at slower rates regardless of whether they are vertical migrators (Euphausia brevis, E. hemigibba, E. tenera, Thysanopoda aequalis) or non-migrators (S. suhmii, S. abbreviatum). Deeper-living species such as S. afine, S. elongatum, Nematoscelis microps, and N. tenella, show minimal penetration of core waters in these four rings. Cold-water species expatriated in cold-core rings also show a varied response to ring decay, with some species disappearing rapidly-3-4 months (Thysanopoda longicaudata)-and others persisting for substantial periods-0.5-1 year (N. megalops, E. krohnii). Distribution of the latter two species indicates dispersal out beyond the ring core at the surface in the case of E. krohnii and at depths of 400-1 000 m in the case of N. megalops.