Possibility of recent changes in vertical distribution and size composition of chlorophyll-a in the western North Pacific region |
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| Authors: | Hiroshi Ishida Yutaka W Watanabe Joji Ishizaka Toshiya Nakano Naoki Nagai Yuji Watanabe Akifumi Shimamoto Nobuhiro Maeda Michimasa Magi |
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| Institution: | (1) The General Environmental Technos Co., Ltd., Azuchimachi, Chuo-ku, Osaka 541-0052, Japan;(2) Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan;(3) Faculty of Fisheries, Nagasaki University, Nagasaki 852-8521, Japan;(4) Global Environment and Marine Department, Japan Meteorological Agency, Tsukuba 305-0052, Japan;(5) Maizuru Marine Observatory, Kyoto 624-0946, Japan;(6) Research Institute of Innovative Technology for the Earth, Kyoto 619-0292, Japan |
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| 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% year−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 year−1 in STA137 and KEA175; and (3) decreasing linear trend of larger-size Chl-a (>3 μm) by 0.1 and 2.5% year−1 in STA137 and KEA175, respectively. Water density (σ
θ
) at 75 m depth had also decreased by 0.006 and 0.05 year−1 in STA137 and KEA175, respectively. The ratio of biogenic opal to biogenic CaCO3 in the sinking flux decreased by 0.015 year−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. |
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| Keywords: | Chlorophyll-a climate change western North Pacific sediment trap phytoplankton size |
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