179 Journal of Oceanography, Vol. 65, pp. 179 to 186, 2009 Keywords: ⋅ Chlorophyll-a, ⋅ climate change, ⋅ western North Pacific, ⋅ sediment trap, ⋅ phytoplankton size. * Corresponding author. E-mail: ishida_hiroshi@kanso.co.jp Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer Possibility of Recent Changes in Vertical Distribution and Size Composition of Chlorophyll-a in the Western North Pacific Region HIROSHI ISHIDA 1,6 *, YUTAKA W. WATANABE 2 , JOJI ISHIZAKA 3 , TOSHIYA NAKANO 4 , NAOKI NAGAI 5 , YUJI WATANABE 1,6 , AKIFUMI SHIMAMOTO 1,6 , NOBUHIRO MAEDA 1,6 and MICHIMASA MAGI 6 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 (Received 13 February 2008; in revised form 16 July 2008; accepted 29 September 2008) 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 re- vealed 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 (STA 137 ) during 1972 to 1997 and in the Kuroshio Extension area along 175°E (KEA 175 ) 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 STA 137 and KEA 175 ; and (3) decreasing linear trend of larger-size Chl-a (>3 μ μ μm) by 0.1 and 2.5% year –1 in STA 137 and KEA 175 , respectively. Water density ( σ σ σ θ θ θ ) at 75 m depth had also decreased by 0.006 and 0.05 year –1 in STA 137 and KEA 175 , respectively. The ratio of biogenic opal to biogenic CaCO 3 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 deep- ening 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. Bering Sea (Sugimoto and Tadokoro, 1997, 1998). These studies showed that Chl-a during the mid 1960s to 1970s were a few times higher than the preceding and follow- ing decades, corresponding with shifts of Northern Hemi- sphere atmospheric pressure system. They also showed that Chl-a has been decreasing since 1980s due to inten- sification of wind speed and weakening of stratifications. In the western North Pacific, decreasing trends of nutri- ents and Chl-a in the surface along 137°E from 1971 to 2000 was reported, as well as a clear negative correlation between their oscillation and Pacific Decadal Oscillation Index (Watanabe et al ., 2005). These studies focused only on Chl-a as an index of total biomass of phytoplankton. Studies of long-term changes of phytoplankton species composition were lim- ited to a local region in the North Pacific. For example, a decrease of the spring-type diatom community was re- 1. Introduction Recent studies indicate that the ocean circulation has probably been changing due to climate change and/or the global warming effect (e.g., Sarmiento et al., 1998; Ono et al., 2001; Watanabe et al., 2001; Emerson et al., 2001; McPhaden and Zhang, 2002). Evidence of decadal-scale ecosystem change is also accumulating in the North Pa- cific. In the central North Pacific subtropical region, chlo- rophyll-a (Chl-a) in water column has nearly doubled during past three decades (Venrick et al ., 1987; Venrick 1993; Karl et al ., 2001). Changes of Chl-a caused by decadal-scale climate changes have been analyzed in the central and western subarctic Pacific and the eastern