Zooplankton grazing is a major top-down process regulating phytoplankton population dynamics, biomass accumulation, and nutrient cycling in the ocean. Following grazing, part of phytoplankton-derived nitrogen is assimilated into zooplankton biomass and transferred to higher trophic levels or exported through fecal-pellet production and vertical migration. Another fraction may be converted from particulate nitrogen into dissolved nitrogen and retained within the upper ocean for rapid recycling. Grazing therefore influences not only phytoplankton biomass but also the partitioning of primary production between upper-ocean recycling and export to depth. However, conventional dilution experiments generally estimate phytoplankton growth and grazing mortality from measurements made only at the beginning and end of a 24-h incubation. Such daily averages may obscure contrasting processes during the light and dark periods. Moreover, traditional approaches cannot directly resolve the fate of phytoplankton-derived nitrogen during grazing or quantify grazing-mediated transfer of particulate nitrogen to the dissolved nitrogen pool.

To address these limitations, dilution incubations were conducted in the northwestern Pacific, with samples collected at 0, 6, 12, and 24 h. Paired whole-seawater and diluted treatments were analyzed using flow cytometry, chlorophyll-a measurements, and ^15N isotope tracing. Flow cytometry and chlorophyll-a data were used to characterize changes in phytoplankton biomass and to estimate growth and grazing rates over different incubation intervals. Parallel additions of ^15NH₄⁺ and ^15NO₃⁻ were used to trace nitrogen uptake and transformation, and to compare grazing-mediated particulate nitrogen release between phytoplankton supported by regenerated and new nitrogen sources.

Preliminary flow-cytometric results showed that most phytoplankton groups increased during the 0–12 h light period, whereas biomass generally declined during the 12–24 h dark period. Nocturnal losses were particularly pronounced for eukaryotic phytoplankton. These opposing patterns would be partly masked if growth and grazing were evaluated only from the full 0–24 h incubation. By integrating dilution experiments with ^15N labeling, this study further provides a framework for estimating how much phytoplankton-derived particulate nitrogen is returned to the dissolved nitrogen pool following grazing and whether this transfer differs between NH₄⁺- and NO₃⁻-supported phytoplankton. The results will improve understanding of the diel coupling among phytoplankton growth, zooplankton grazing, nitrogen regeneration, and the potential export of primary production in oligotrophic ocean ecosystems.

Keywords: dilution method; phytoplankton; top-down control; zooplankton grazing; ^15N isotope tracing; particulate nitrogen release

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