The Gulf Stream System may be thought of as a conveyor belt, entraining and transporting both water and plankton across the North Atlantic Ocean basin on time scales of months. However, recent studies of the dynamics of the transport process suggest that it may be difficult to stay on the conveyer belt. There is significant loss of water at the edges of the Stream to mixing with adjacent waters [ Song et al., 1994], suggesting that only the core of the Stream is a fluid pathway. There are also large losses from the Stream with ring formation [ Richardson, 1983].
A recent study used molecular population genetic analysis of the copepod, Nannocalanus minor, to answer the question: how genetically cohesive are planktonic species with trans-Atlantic distributions? Molecular analysis indicated that N. minor can be divided into two genetically distinct groups [ Bucklin, 1994; unpublished data]. The two groups are distinguished by 10% sequence divergence in a portion of the mitochondrial 16S rRNA gene (Figure 3), and may constitute distinct species. Individuals of the two groups differ in size and geographic distribution: the larger form is found from the Florida Straits throughout the Gulf Stream until the region of the Grand Banks, when it becomes less abundant. The smaller form is found most abundantly in the Sargasso Sea, and also occurs in the Gulf Stream east of Cape Hatteras. The genetically-defined groups correspond to large and small forms of N. minor that have been previously reported to show small-scale differences in distribution across the Gulf Stream axis [ Ashjian and Wishner, 1993a]. Thus, the Gulf Stream does not homogenize the geographically extensive population of N. minor. How the two groups maintain distinct geographic ranges in this highly advective oceanographic domain is an open question. Interestingly, the western form of N. minor may provide a useful tracer of the complex physical processes at the bifurcation of the Gulf Stream and of the fate of Gulf Stream waters in the eastern North Atlantic.