1. NIWA Centre for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, PO Box 56, Dunedin, 9003, New Zealand
2. National Institute of Water and Atmospheric Research, 301 Evans Bay Parade, Greta Point, PO Box 14-901, Kilbirnie, Wellington, New Zealand
3. Fisheries and Oceans Canada, Institute of Ocean Sciences, PO Box 6000, Sidney, British Columbia, V8L 4B2 Canada
4. National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan
5. Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, Japan
6. Department of Biology, University of Laval, Quebec, G1K 7P4, Canada
7. Department of Aquatic Bioscience, University of Tokyo, Bunkyo, Tokyo 113-8657, Japan
8. Ocean Sciences Centre, Memorial University of Newfoundland, St John's, Newfoundland, A1C 5S7, Canada
9. Earth and Ocean Sciences, University of British Columbia, 6270 University Blvd, Vancouver, British Columbia, V6T 1Z4, Canada
10. Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong
11. Department of Chemistry, University of Otago, PO Box 56, Dunedin, 9003, New Zealand
12. Department of Biological Sciences, Bowling Green State University, Bowling Green, Ohio 43403, USA
13. National Institute of Advanced Industrial Science and Technology, 1-3-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8921, Japan
14. Marine Biological Research Institute of Japan, Shinagawa, Tokyo 142-0042, Japan
15. Graduate School of Fisheries Science, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
16. Department of Botany, University of British Columbia, 6270 University Blvd, Vancouver, British Columbia, V6T 1Z4, Canada
17. Dalhousie University, Department of Oceanography, Halifax, Nova Scotia, B3H 4J1 Canada
18. Central Research Institute of Electric Power Industry, Abiko, Chiba 270-1194, Japan
19. Marine Biogeochemistry Laboratory, Ocean Research Institute, The University of Tokyo 1-15-1, Minamidai, Nakano, Tokyo 164-8639, Japan
20. Tohoku National Fisheries Research Institute, Shiogama, Miyagi 985-0001, Japan
21. School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, V8W 3N5, Canada

Correspondence to: Philip W. Boyd¹ Email: p.boyd@niwa.co.nz

Abstract

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters^{1, 2, 3, 4, 5}. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown^{1, 2, 3, 4, 5}. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past^{6, 7}, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration^{3, 8}.

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