TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 65 (No. 2), pp. 179-186, 2009

Possibility of Recent Changes in Vertical Distribution and Size Composition of Chlorophyll-a in the Western North Pacific Region

Hiroshi Ishida1,6*, Yutaka W. Watanabe2, Joji Ishizaka3, Toshiya Nakano4, Naoki Nagai5, Yuji Watanabe1,6, Akifumi Shimamoto1,6, Nobuhiro Maeda1,6 and Michimasa Magi6

1The General Environmental Technos Co., Ltd., Azuchimachi, Chuo-ku, Osaka 541-0052, Japan
2Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
3Faculty of Fisheries, Nagasaki University, Nagasaki 852-8521, Japan
4Global Environment and Marine Department, Japan Meteorological Agency, Tsukuba 305-0052, Japan
5Maizuru Marine Observatory, Kyoto 624-0946, Japan
6Research Institute of Innovative Technology for the Earth, Kyoto 619-0292, Japan

(Received 13 February 2008; in revised form 16 July 2008; accepted 29 September 2008)

Abstract: Our analysis of the last three decades of retrospective data of vertical distributions and size composition of chlorophyll-a (Chl-a) over the western North Pacific has revealed significant changes of three indices related to Chl-a during summer season, as follows: (1) decreasing linear trend of the proportion of Chl-a in surface layer to that of the whole water column by 0.4 and 2.3% yeard-1 in the subtropical area along 137°E (STA137) during 1972 to 1997 and in the Kuroshio Extension area along 175°E (KEA175) during 1990 to 2001; (2) increasing linear trend of the depth of subsurface Chl-a maximum (DCM) by 0.4 and 2.6 m yeard-1 in STA137 and KEA175; and (3) decreasing linear trend of larger-size Chl-a (>3 μm) by 0.1 and 2.5% yeard-1 in STA137 and KEA175, respectively. Water density (σθ) at 75 m depth had also decreased by 0.006 and 0.05 yeard-1 in STA137 and KEA175, respectively. The ratio of biogenic opal to biogenic CaCO3 in the sinking flux decreased by 0.015 yeard-1 in the subtropical region from 1997 to 2005. These findings may indicate that the subsurface chlorophyll maximum is deepening and larger phytoplankton such as diatoms has been decreasing during the past decade, associated with the decreasing density of surface water caused by warming in the western North Pacific, especially in the summer.

*Corresponding author E-mail: ishida_hiroshi@kanso.co.jp

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