Genetic tracers of zooplankton transport in coastal filaments off northern California

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Abstract
Genetic markers were used to describe the transport of the calanoid copepod Metridia pacifica in a cool filament off Point Arena, California, in July 1986. The markers used were genetic variants of enzymes (allozymes); individual copepods were assayed for allozymic variability at nine enzyme‐encoding genes (loci) by polyacrylamide gel electrophoresis. Zooplankton samples were collected in a 300 km by 170 km domain. Maps of surface dynamic height in this domain showed an offshore jet, with strong cross‐shore transport, in the presence of a cool anomaly comprising multiple fronts and filaments. The genetic data sorted the samples into two heterogeneous groups (according to maximum likelihood analysis): one comprising samples near the coast and at the head of the filament, and the other comprising offshore samples. The groups themselves were heterogeneous, revealing smaller‐scale patterns of transport and mixing. The genetic spatial structure of the calanoid copepod, Metridia pacifica, collected in the flow field was used to infer patterns of transport in the filament flow field. Patterns of transport inferred from genetic differences between zooplankton samples were compared with expected particle transport based on models which consider physical (geostrophic advection, Ekman drift, and mixing) transport plus vertical migration behavior. According to both the flow models and the genetic data, transport of zooplankton occurred primarily along paths of geostrophic flow. There was little cross‐jet exchange and almost no exchange between samples in the filament and samples in a cyclonic eddy to the south of the jet. The genetic data further indicated that copepod populations at the coastal source of the filament were genetically heterogeneous. Advection in the filament thus resulted in the juxtaposition of genetically distinct zooplankton populations in the jet axis. The genetic structure of zooplankton in the filament resulted from advection of previous distant populations, not (because of the time scales of the processes) from changes in the genetic character of samples during transport. The presence of multiple fronts and filaments within the cool anomaly prevented homogenization of the plankton assemblages during transport. Thus samples collected less than 10 km apart across the jet axis were genetically different. These findings indicated that plankton populations in complex flow fields may show patchiness in biological, biochemical and/or genetic character at small time/space scales.