Utilizing satellite observations and a simple plankton model to predict zooplankton hotspots in the California Current Ecosystem

The California Current Ecosystem (CCE) is a highly dynamic upwelling environment that supports elevated levels of primary, secondary, and fish production. Wind-driven nutrient supply and primary production, computed from satellite data, provide a synoptic view of how phytoplankton production is coupled to upwelling. In contrast, deriving mechanisms underlying the coupling between upwelling dynamics and zooplankton populations, for which observations are relatively scarce, is problematic due to the inherent patchiness of zooplankton. While phytoplankton responds quickly to environmental forcing (e.g., days), zooplankton grows slower. As a consequence, zooplankton populations tend to be spatially decoupled from the nutrient supply process (i.e., upwelling centers), and their concentrations are subject to variability imposed by ocean currents. To better understand mechanisms controlling the formation of zooplankton hotspots (recurrent areas of elevated concentration), we use a satellite-based Lagrangian method where variables from a plankton model, forced by wind-driven nutrient supply, are advected by near-surface currents following upwelling events. Modeled zooplankton distribution reproduces published accounts of euphausiid (krill) hotspots and highlights the importance of the upwelling process and near-surface currents in shaping the mesoscale distribution of zooplankton aggregations. This satellite-based modeling tool is used to analyze the variability and drivers of zooplankton hotspots in the CCE, and to investigate how water masses of different origin and history converge to form predictable biological hotspots. Implications from this modeling study are discussed for understanding changes in top predator distribution (whales and seabirds) and for monitoring pelagic biodiversity in a moving ocean.