Abstract
Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters1,2,3,4,5. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown1,2,3,4,5. 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 past6,7, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration3,8.
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Acknowledgements
We thank the officers and crews and scientists on board the John P. Tully, El Puma and Kaiyo Maru. We are grateful to S. Toews for shoreside logistical support. The manuscript was improved by comments from K. Currie, R. Frew, C. Hurd, P. Boyd, D. Hutchins and T. Trull. This study was supported by NSERC Canada as part of the C-SOLAS programme, CFCAS, DFO PERD (Canada), the New Zealand PGSF (Ocean Ecosystems), and the Global Environmental Research Fund from the Ministry of Environment, the Fisheries Agency, and the Central Research Institute of Electric Power Industry research funding (Japan).
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Supplementary figure 1: A) An fCO2 contour plot (µatm) of the patch on day 18; B) Algal photosynthetic competence (Fv/Fm, mean mixed-layer values) in the mixed-layer for IN and OUT waters; Supplementary Figure 2: SeaWiFS Ocean Colour images on day 19 and day 24 showing the rapid decline of the bloom; Supplementary Figure 3: Density profiles for IN patch from days 16-25. (PPT 194 kb)
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Boyd, P., Law, C., Wong, C. et al. The decline and fate of an iron-induced subarctic phytoplankton bloom. Nature 428, 549–553 (2004). https://doi.org/10.1038/nature02437
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DOI: https://doi.org/10.1038/nature02437
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