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The transport of plankton by highly dynamic (sub)mesoscale currents—often associated with fronts and eddies—shapes the structure of plankton communities on the same time scales as biotic processes, such as growth and predation (days–weeks). The resulting biophysical couplings generate heterogeneities in their finescale distributions (1–10 km), or “patchiness.” Here, we test the hypothesis that cross‐frontal plankton patchiness at a front found 200–250 km offshore in the California Current System was influenced by wind‐driven upwelling conditions upstream of the front. We show that in situ Eulerian measurements (cross‐frontal transects) can be interpreted in a Lagrangian framework by using satellite‐derived current velocities to trace water parcels backward in time to their coastal origins. We find that the majority of the water parcels sampled at this front originated along the central California coast during different episodic wind‐driven upwelling pulses and followed various trajectories before converging temporarily at the front. In response to nutrient injections at the coast, plankton communities transformed during their journeys from the coast to the sampling zone, with a succession of phytoplankton and zooplankton blooms. The cross‐frontal sampling captured the convergence of these distinct water parcels at different points in their biological histories, which resulted in the observed spatial patchiness. Our results suggest that identifying the processes controlling frontal plankton communities requires understanding them in the context of their spatial and temporal histories. In particular, Lagrangian approaches should be more widely applied to understand critical ecological patterns in highly dynamic systems.

Locally enhanced biological production and increased carbon export are persistent features at oceanic density fronts. Studies often assume biological properties are uniform along fronts or hypothesize that along‐ and across‐front gradients reflect physical‐biological processes occurring in the front. However, the residence times of waters in fronts are often shorter than biological response times. Thus, an alternate—often untested—hypothesis is that observed biological patchiness originates upstream of a front. To test these two hypotheses, we explore an eddy‐associated front in the California Current System sampled during two surveys, separated by 3 weeks. Patches of high phytoplankton biomass were found at the northern ends of both surveys, and phytoplankton biomass decreased along the front. While these patches occurred in similar locations, it was unclear whether the same patch was sampled twice, or whether the two patches were different. Using an advection‐reaction framework combined with field and satellite data, we found that variations in along‐front gradients in dissolved oxygen, particle biovolume, and salinity support the conclusion that the two phytoplankton patches were different. They were only coincidentally sampled in similar locations. Backward‐ and forward‐in‐time tracking of water parcels showed that these phytoplankton patches had distinct origins, associated with specific, strong coastal upwelling pulses upstream of the front. Phytoplankton grew in these recently upwelled waters as they advected into and along the frontal system. By considering both local and upstream physical‐biological forcings, this approach enables better characterizations of critical physical and biogeochemical processes that occur at fronts across spatial and temporal scales.