UC San Diego

In the California Current System (CCS), mesoscale features such as fronts and filaments horizontally stir planktonic communities, generating heterogeneity in their spatial and temporal distributions, or “patchiness.” Planktonic biomass and community structure have also been found to be nonuniform within the flow features themselves, often in correlation with water-mass properties such as temperature and salinity. But due to high horizontal velocities (50–80 km d^{-1}) along fronts and filaments, the transit times of waters within a flow feature (a few days) are shorter than the timescales required for significant biological community changes (several days to weeks). Given this strong lateral advection, mesoscale flow features must contain distinct plankton patches that propagate along their jets, and these patches must be predominantly structured by processes occurring at their upstream origins. However, few studies have investigated the underlying dynamics and upstream controls that lead to plankton patchiness at mesoscale fronts and filaments, and so this hypothesis remains largely untested.In this dissertation, I used a diverse suite of hydrographic and biological measurements from satellite and field sampling to characterize spatiotemporal patterns in hydrographic and plankton patchiness at (across and along) mesoscale fronts and filaments in the CCS. I also developed and employed a Lagrangian particle backtracking method to investigate the relative contribution of physical and biological mechanisms in structuring the observed patchiness. I found that mesoscale fronts and filaments are highly advective systems that act as conduits of different water masses with distinct planktonic communities. “Plankton patches” are entrained into fronts or filaments, where they can converge with other patches and advect along a jet. Distinct plankton patches, with nonuniform community structure and biomass, are shaped by fluctuations in upstream wind-driven upwelling intensity and source-water nutrient concentrations, as well as biological processes—such as growth, grazing, and iron limitation—that occur along Lagrangian trajectories. A combined Eulerian-Lagrangian approach is needed to appropriately characterize and interpret planktonic patchiness in mesoscale fronts and filaments, which are fundamentally non-steady-state systems. These findings help us to identify the physical-biological dynamics that structure biological productivity and diversity, ecological hotspots, and carbon export in Eastern Boundary Upwelling Systems.