Ocean circulation patterns determine in large part the patterns of biological productivity in coastal and oceanic waters. Prediction of the spatial patterns of secondary production in the ocean is particularly difficult since dispersal of zooplankton is a function of both passive transport and active swimming by zooplankton, which may exhibit highly variable behavioral responses to flow. Quantitative estimates of dispersal are particularly difficult to obtain for marine zooplankton; the difficulty of direct observation of dispersal processes is magnified by their small size and numerical abundance and by the vast distances they traverse. The inaccessibility of the open ocean further compounds the problem, since many parts of the ocean are difficult to sample with the spatial intensity and temporal frequency necessary to determine dispersal processes. Unlike biomass and abundance estimates for marine planktonic species, dispersal processes may not be amenable to remote monitoring.
Why should oceanographers be concerned about the patterns of zooplankton dispersal in the ocean? Dispersal is one of the most important processes determining the distribution and abundance of organismal populations. Examples of oceanographic questions include: are there identifiable regions that have sufficient secondary production that they function as source populations for recruitment to other regions?; how much of the export from these source regions is lost, in ecological and evolutionary terms, by being transported to areas where the zooplankton cannot reproduce?; do some planktonic ecosystems have recirculation cells so that--on some time and space scale--the biological production is recycled and a genetically distinct population is maintained?; are planktonic species genetically cohesive or are they partitioned by the formation of geographic races and subspecies? These questions all center on one ecological phenomenon: dispersal.
The fluid regime of the oceans requires us to think differently about the population dynamics of planktonic species than about terrestrial species. First, planktonic populations are far more difficult to delimit--either numerically or spatially--than are many terrestrial species. Second, the planktonic realm may not be characterized by stable communities, but rather by ephemeral assemblages of transient populations. The concept of a population as a genetically cohesive, geographically persistent entity may not be meaningful in this system, where physical processes may intermix individuals of different source regions, and where individuals from the same source can have very different fates. On the other hand, there is evidence of stability in the relative abundances of planktonic species in geographic regions over long periods of time [ Fager and McGowan, 1963; Ashjian and Wishner, 1993b]. Although planktonic individuals may traverse thousands of km and travel in large-scale circulation patterns, community structure may be maintained in highly advective regimes by coherent and persistent mesoscale phenomena (see, e.g., Wiebe et al. [1992]).
For planktonic organisms in the ocean, dispersal by advective transport of individuals may be relatively more important in determining population abundances than in situ reproduction and mortality. In order to understand the nature and causes of population fluctuations in marine planktonic species, ocean circulation patterns and mixing processes should be considered to be primary drivers. Patterns of secondary production in the oceans may be governed primarily by dispersal processes driven by ocean circulation. Our ability to understand and predict these patterns on relevant time and space scales may be dependent on our ability to understand the relationships between circulation, dispersal, and reproduction.
One promising means of tracking and predicting spatial patterns of dispersal is through molecular analysis of gene flow patterns in zooplankton species. Analysis of gene flow involves the assay of genetically variable traits of individuals; the frequencies of the variants in each population may be used to infer patterns of gene flow across the species' geographic range. The practical application of these studies will be through incorporation of population genetic analysis into predictive models of the dynamic functioning of ocean ecosystems, including prediction of the productivity of marine fisheries and assessment of ecosystem responses to global climate change.