Dynamics of phytoplankton growth and microzooplankton grazing during cross-shelf filament transport in the California Current Ecosystem

In the generally strong environmental gradient that exists across the southern California Current Ecosystem, microzooplankton grazing becomes increasingly more important in regulating the dynamics of phytoplankton toward a steady-state balance with distance from shore. For filaments to be a significant delivery mechanism of cross-shelf export, this regulatory shift needs to break down during offshore transport. In June 2017, we investigated the evolving dynamics of phytoplankton biomass, growth and microzooplankton grazing during filament transport using satellite-tracked drifters to mark discrete water parcels for repeat sampling and as platforms for in situ incubations of dilution experiments. Experimentally determined process rates and observed net changes of the ambient community are compared on a chlorophyll basis for the depth-integrated euphotic zone. Neither phytoplankton growth nor microzooplankton grazing varied substantially with time and distance offshore, suggesting a relatively consistent net rate of phytoplankton growth of 0.2-0.3 d^-1 throughout transport. In contrast, ambient phytoplankton biomass declined at a rapid and accelerating rate during transport, entirely decoupled from what can be explained by microzooplankton grazing. We postulate that strong predatory control by mesozooplankton in the filament keeps microzooplankton in check, ultimately allowing more efficient offshore export of phytoplankton production via zooplankton fecal pellets or direct sinking of aggregates.