TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 63 (No. 6), pp. 983-994, 2007

Evidence for the Grazing Hypothesis: Grazing Reduces Phytoplankton Responses of the HNLC Ecosystem to Iron Enrichment in the Western Subarctic Pacific (SEEDS II)

Atsushi Tsuda1*, Shigenobu Takeda2, Hiroaki Saito3, Jun Nishioka4, Isao Kudo5, Yukihiro Nojiri6, Koji Suzuki7, Mitsuo Uematsu1, Mark L. Wells8, Daisuke Tsumune9, Takeshi Yoshimura9, Tatsuo Aono10, Takafumi Aramaki6, William P. Cochlan11, Maki Hayakawa7, Keiri Imai1, Tomoshi Isada7, Yoko Iwamoto1, William K. Johnson12, Sohiko Kameyama13, Shungo Kato14, Hiroshi Kiyosawa15, Yoshiko Kondo2, Maurice Levasseur16, Ryuji J. Machida1, Ippei Nagao17, Fumiko Nakagawa13, Takahiro Nakanishi10, Seiji Nakatsuka18, Akira Narita1, Yoshifumi Noiri5, Hajime Obata1, Hiroshi Ogawa1, Kenji Oguma1, Tsuneo Ono19, Tomofumi Sakuragi10, Motoki Sasakawa20, Mitsuhide Sato2, Akifumi Shimamoto21, Hyoe Takata5, Charles G. Trick22, Yutaka W. Watanabe5, Chi Shing Wong12 and Naoki Yoshie3

1Ocean Research Institute, University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan
2Department of Aquatic Bioscience, University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan
3Tohoku National Fisheries Research Institute, Shiogama, Miyagi 985-0001, Japan
4Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819, Japan
5Faculty of Fisheries Sciences, Hokkaido University, Sapporo, Hokkaido 060-0813, Japan
6National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan
7Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
8School of Marine Sciences, University of Maine, Libby Hall, Orono, ME 04469, U.S.A.
9Central Research Institute of Electric Power Industry, Abiko, Chiba 270-1194, Japan
10National Institute of Radiological Sciences, Hitachinaka, Ibaraki 311-1202, Japan
11Romberg Tiburon Center for Environmental Studies, San Francisco State University, 3152 Paradise Drive, Tiburon, CA 94920, U.S.A.
12Institute of Ocean Science, P.O. Box 6000, Sidney BC, V8L 4B2, Canada
13Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
14Faculty of Urban Environmental Sciences, Tokyo Metropolitan University, Tokyo 192-0397, Japan
15Marine Biological Research Institute of Japan, Shinagawa-ku, Tokyo 142-0042, Japan
16Department of Biology, University of Laval, Quebec, G1K 7P4, Canada
17Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
18Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
19Hokkaido National Fisheries Research Institute, Kushiro, Hokkaido 085-0802, Japan
20Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo 102-8471, Japan
21The General Environmental Technos Co., Ltd., Chuo-ku, Osaka 541-0052, Japan
22School of Medicine, University of Western Ontario, 1151 Richmond Street N, UWO, London, Ontario, N6A 5B7, Canada

(Received 14 April 2007; in revised form 4 July 2007; accepted 5 July 2007)

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 Fv/Fm 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.

*Corresponding author E-mail: tsuda@ori.u-tokyo.ac.jp

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