As part of the E-Flux project, we documented spatial variability and temporal changes in plankton
community structure in a cold-core cyclonic eddy in the lee of the Hawaiian Islands. Cyclone Opal
spanned 200 km in diameter, with sharply uplifted isopycnals (80–100 m relative to surrounding
waters) and a strongly expressed deep chlorophyll a maximum (DCM) in its central core region of 40 km
diameter. Microscopic and flow cytometric analyses of samples from across the eddy revealed dramatic
transitions in phytoplankton community structure, reflecting Opal’s well-developed physical structure.
Upper mixed-layer populations in the eddy resembled those outside the eddy and were dominated by
picophytoplankton. In contrast, the DCM was composed of large chain-forming diatoms dominated by
Chaetoceros and Rhizosolenia spp. Diatoms attained unprecedented levels of biomass (nearly 90 mgCl1
)
in the center of the eddy, accounting for 85% of photosynthetic biomass. Protozoan grazers displayed
two- to three-fold higher biomass levels in the eddy center as well. We also found a distinct and
persistent layer of senescent diatom cells overlying healthy populations, often separated by less than
10 m, indicating that we were sampling a bloom in a state of decline. Time-series sampling over 8 days
showed a successional shift in community structure within the central diatom bloom, from the
unexpected large chain-forming species to smaller forms more typical of the subtropical North Pacific.
The diatom bloom of Cyclone Opal was a unique, and possibly extreme, example of biological response
to physical forcing in the North Pacific subtropical gyre, and its detailed study may therefore help to
improve our predictive understanding of environmental controls on plankton community structure.