Evidence for the grazing hypothesis: Grazing reduces phytoplankton responses of the HNLC ecosystem to iron enrichment in the western subarctic pacific (SEEDS II) |
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| Institution: | (1) Ocean Research Institute, University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan;(2) Department of Aquatic Bioscience, University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan;(3) Tohoku National Fisheries Research Institute, Shiogama, Miyagi 985-0001, Japan;(4) Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819, Japan;(5) Faculty of Fisheries Sciences, Hokkaido University, Sapporo, Hokkaido 060-0813, Japan;(6) National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan;(7) Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan;(8) School of Marine Sciences, University of Maine, Libby Hall, Orono, ME, 04469, U.S.A.;(9) Central Research Institute of Electric Power Industry, Abiko, Chiba 270-1194, Japan;(10) National Institute of Radiological Sciences, Hitachinaka, Ibaraki 311-1202, Japan;(11) Romberg Tiburon Center for Environmental Studies, San Francisco State University, 3152 Paradise Drive, Tiburon, CA, 94920, U.S.A.;(12) Institute of Ocean Science, P.O. Box 6000, Sidney, BC, V8L 4B2, Canada;(13) Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan;(14) Faculty of Urban Environmental Sciences, Tokyo Metropolitan University, Tokyo 192-0397, Japan;(15) Marine Biological Research Institute of Japan, Shinagawa-ku, Tokyo 142-0042, Japan;(16) Department of Biology, University of Laval, Quebec, G1K 7P4, Canada;(17) Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan;(18) Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan;(19) Hokkaido National Fisheries Research Institute, Kushiro, Hokkaido 085-0802, Japan;(20) Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo 102-8471, Japan;(21) The General Environmental Technos Co., Ltd., Chuo-ku, Osaka 541-0052, Japan;(22) School of Medicine, University of Western Ontario, 1151 Richmond Street N, UWO, London, Ontario, N6A 5B7, Canada |
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| Abstract: | A mesoscale iron-enrichment study (SEEDS II) was carried out in the western subarctic Pacific in the summer of 2004. The iron
patch was traced for 26 days, which included observations of the development and the decline of the bloom by mapping with
sulfur hexafluoride. The experiment was conducted at almost the same location and the same season as SEEDS (previous iron-enrichment
experiment). However, the results were very different between SEEDS and SEEDS II. A high accumulation of phytoplankton biomass
(∼18 mg chl m−3) was characteristic of SEEDS. In contrast, in SEEDS II, the surface chlorophyll-a accumulation was lower, 0.8 to 2.48 mg m−3, with no prominent diatom bloom. Photosynthetic competence in terms of F
v/F
m for the total phytoplankton community in the surface waters increased after the iron enrichments and returned to the ambient
level by day 20. These results suggest that the photosynthetic physiology of the phytoplankton assemblage was improved by
the iron enrichments and returned to an iron-stressed condition during the declining phase of the bloom. Pico-phytoplankton
(<2 μm) became dominant in the chlorophyll-a size distribution after the bloom. We observed a nitrate drawdown of 3.8 μM in the patch (day 21), but there was no difference in silicic acid concentration between inside and outside the patch. Mesozooplankton
(copepod) biomass was three to five times higher during the bloom-development phase in SEEDS II than in SEEDS. The copepod
biomass increased exponentially. The grazing rate estimation indicates that the copepod grazing prevented the formation of
an extensive diatom bloom, which was observed in SEEDS, and led to the change to a pico-phytoplankton dominated community
towards the end of the experiment. |
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| Keywords: | Iron-enrichment copepod grazing diatom carbon cycling western subarctic Pacific HNLC |
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