UC San Diego

We investigated phytoplankton growth and micro- and mesozooplankton grazing patterns in the California Current Ecosystem (CCE) during summer 2021. Two water parcels, followed over a duration of 4-5 days using satellite-tracked drogued drifter for quasi- Lagrangian experimental cycles were investigated for inshore and offshore differences. Phytoplankton growth rates and microzooplankton grazing rates were determined using the two-point dilution method, and daily Bongo tows were deployed for mesozooplankton collection, for biomass and grazing estimates based on gut fluorescence. Instantaneous rates of growth and grazing between the two cycles were µ = 0.45 (± 0.13) d-1 for Cycle 2 (inshore) and 0.60 (± 0.1) d-1 for Cycle 3 (offshore), and microzooplankton grazing rates were 0.36 (± 0.21) d-1 for Cycle 2 and 0.37 (± 0.11) d-1 for Cycle 3. Mesozooplankton contributed much less to grazing for both cycles, grazing 0.05 (± 0.02) d-1 inshore and 0.025 (± 0.006) d-1 offshore, removing 4% and 2% of phytoplankton standing stock, respectively. In both cycles, the net calculated phytoplankton growth was positive, but this was only statistically significant for the offshore region. The dominant grazers within the mesozooplankton community were not consistent between the two regions of the CCE: the three smallest size classes (0.2-2 mm) contributed the most to grazing in Cycle 2, while in Cycle 3, the dominant grazers were the two smallest size classes (0.2-1 mm). Mesozooplankton grazing showed changes related to diel vertical migration. These analyses contribute to our understanding of growth and grazing dynamics in plankton food webs, and to understanding carbon cycling in the CCE.