Biological Oceanography

(^) Mesoscale and larger eddies also expatriate organisms from zones of suitable
habitat. Dramatic examples of eddy effects are the cold- and warm-core rings formed
along the Gulf Stream southeast of New England, in the Kuroshio–Oyashio
confluence and along western boundary currents in the southern hemisphere. As the
current jets turn seaward, they meander at the boundaries between cold water
poleward and warm water equatorward, forming loops to both north and south. A loop
to the south of the Gulf Stream can bend enough for the downstream end to join the
upstream end, forming a cyclonic current ring with cold northwest Atlantic slope
water in its core. As the ring moves south into the Sargasso Sea, the warmer, less-
dense ring water spreads toward the center, submerging the cold core. Cold rings can
persist under the Sargasso surface layer for about 1.5 years. At least the larger
zooplankton, such as euphausiids (Wiebe & Boyd 1978; Endo & Wiebe 2007), with
home ranges in the colder waters to the north and east, die out very slowly.
Nematoscelis megalops distributed in slope water from the surface to 600 m, peaking
at ∼300 m, slowly moves down as the rings submerge and dissipate (Fig. 10.13),
showing progressive decline in body condition (carbon content; Boyd et al. 1978),
eventually going extinct in the oldest rings. Euphausia krohni, another cold-water
species, is a strong diel migrator and continues that habit when captured by a cold
core ring, exposing it eventually to the 25°C summer warmth of the Sargasso Sea. It
dies out much more quickly (Endo & Wiebe 2007). It is soon replaced in surface
layers by a suite of subtropical euphausiids (Fig. 10.14). Surely similar displacements,
mortality and replacements occur for the whole plankton community.
Fig. 10.13 Trajectory of Gulf Stream cold-core ring “D” through the Sargasso Sea in
1975, shown with the vertical distributions in the ring at night from multiple-closing-
net tows of the cold-water euphausiid Nematoscelis megalops. Tows were in
Northwest Atlantic slope water (left) and in the ring center at 6 months (August 1975)
and 9 months (November 1975) from ring capture. Columns show two replicate tows
from each date. Most of the population in the ring stayed below the 15°C isotherm
which progressively deepened. No N. megalops were captured when that isotherm
was very deep in June 1976.
(^) (After Wiebe & Boyd 1978.)