Possibility of recent changes in vertical distribution and size composition of chlorophyll-a in the western North Pacific region

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 (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 µm) by 0.1 and 2.5% year ‐1 in STA 137 and KEA 175 , respectively. Water density (σ 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 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.