Eastern boundary currents are typically slow-moving and meandering currents that are oceanographically complex and biologically rich. Typical of these currents, the California Current exhibits persistent mesoscale structure which results in significant spatial structuring of biological properties (see, e.g., Haury et al. [1986], Palaez and McGowan [1986], Strub et al. [1991]). Questions about the relative importance of advection and in situ reproduction in determining large-scale patterns of zooplankton abundance in the California Current have been debated for some time [ Chelton et al., 1982]. Since many of the transport events in the California Current are episodic rather than predictable (e.g., wind stress and upwelling intensity; offshore transport via coastal filaments) the movement and mixing of plankton populations in the flow field is difficult to understand by direct observation. Persistent patterns in the dispersal (transport and mixing) and survival of plankton may be inferred from the population genetic structure of the species across its geographic range.
In biogeographic reviews of zooplankton distribution and abundance in the North Pacific Ocean, the open ocean, via the West Wind Drift, has been supposed to constitute a source region for the plankton of the California Current, which are (under this scenario) advected through the Current system and deposited in the inhospitable waters south of the Southern California Eddy [ Chelton et al., 1982]. A. Fleminger (Scripps Institution of Oceanography [dec.]) proposed that there are at least three subspecies of C. pacificus with distinct geographic distributions: C. p. oceanicus occurs in the central North Pacific and West Wind Drift, C. p. californicus is a distinct subspecies restricted to the California Current, and C. p. pacificus occurs in the eastern North Pacific. Molecular analysis has confirmed that C. p. californicus and C. p. oceanicus are genetically distinctive: although the haplotypes differ by only about 1%, there are no haplotypes in common between the two groups [ Bucklin and LaJeunesse, 1994] (Figure 4). The groups are likely to represent different subspecies. Thus, the North Pacific populations of C. pacificus cannot be the source of recruitment and population replenishment for the California Current populations. Presumably there must be local sources of recruitment and retention mechanisms (based on both behavioral adaptations of the plankters and ocean physics) that maintain the abundant C. p. californicus populations of the Current.