Responses of microzooplankton to in situ iron fertilization in the western subarctic Pacific (SEEDS)

Microzooplankton grazing plays an important role in maintaining the phytoplankton biomass low in HNLC waters where iron depletion limits the phytoplankton growth. When the iron limitation is relieved by natural iron supply or manipulation, phytoplankton biomass increases with a floristic shift because responses of phytoplankton and its grazer to such a perturbation are variable by species. During the first in situ iron fertilization (IF) experiment in the western subarctic Pacific (SEEDS), microzooplankton responses to changes in phytoplankton community structure and biomass were examined. Gross growth rate (GGR) of eukaryotic ultraphytoplankton (EUKU, < ca. 5 μm in size) increased 2 days after the IF (D2) and that of the cyanobacteria Synechococcus spp. (SYN) peaked on D4 and then decreased gradually with time until D13, the end of the experiment. Net growth rate (NGR: GGR – grazing rate) increased after the IF however, the GGRs and the grazing rates on EUKU and SYN balanced well at the end of the experiment as well as observed prior to the IF. chlorophyll a concentration increased after D4 and increased to >16 mg m−3 after D9. Although primary productivity was >1.6 g C m−2 d−1 in the surface mixed layer and GGRs of total phytoplankton were >0.8 d−1 during D9 and D13, the total chlorophyll a concentration was more or less steady after D9. This phytoplankton bloom in response to the IF was dominated by a centric diatom Chaetoceros debilis and most of the other diatom species and nano-phytoplankton increased their biomass. Grazing rates on total phytoplankton decreased with the increase in chlorophyll a concentration after the IF from 0.3 d−1 initially to <0.2 d−1 on D9, and then rapidly increased to 0.70 d−1 on D11. Aloricate ciliates were the most dominant microzooplankton (>10 μm) both in abundance and biomass prior to the IF and gradually decreased with the developing diatom bloom. Heterotrophic dinoflagellates, dominated by Gyrodinium spp., increased rapidly after D11. Both the abundance and biomass of heterotrophic dinoflagellates on D13 were 5 times those prior to the IF, and they were the most dominant micrograzer at the end of the experiment. This indicates that grazing by dinoflagellates prevented further development of the diatom bloom after D9, together with limitations of light and iron bioavailability. The present study showed that micrograzers responded to the floristic shift induced by the IF with a time lag and their grazing was an important controlling factor not only on the pico- and nano-phytoplankton but also on the microphytoplankton including chain-forming diatoms. It also showed that heterotrophic dinoflagellate grazing has an important role in the food-web dynamics and biogeochemical cycle after natural or manipulated perturbations in the HNLC western subarctic Pacific.