Diatoms in the desert: Plankton community response to a mesoscale eddy in the subtropical North Pacific |
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| Authors: | Susan L Brown Michael R Landry Karen E Selph Eun Jin Yang Yoshimi M Rii RR Bidigare |
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| Institution: | aDepartment of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822, USA;bIntegrative Oceanography Division, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0227, USA;cMarine Environment Research Department, Korea Ocean Research & Development Institute, Ansan P.O. Box 29, Seoul 425-600, South Korea;dHawaii Institute of Marine Biology, Kaneohe, HI 96744, USA |
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| Abstract: | 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 μg C l−1) 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. |
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| Keywords: | Plankton Subtropical North Pacific Eddy Community structure Diatoms |
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