Well-developed subsurface chlorophyll maximum near Komahashi No. 2 seamount in the summer of 1991

Water column properties and velocity fields near Komahashi No. 2 Seamount (29°52′N and 133°18′E; 289 m minimum depth) on the Kyushu-Palau Ridge during June 1991 were investigated to describe the formation of a regional subsurface chlorophyll maximum (SCM). Two transects were occupied across the summit of the seamount. Along the first transect, cold (<18°C), nitrate-rich (>3 μM) water was uplifted south of the summit by downstream subsurface currents. Similar water properties were observed above and east of the summit along the second transect. An intensified SCM occurred at depths between 75 and 90 m, just above the uplifted isotherms. The SCM seems to have been enhanced by upward transport of nutrients caused by topography–current interactions over the seamount. Another well-developed SCM was observed above and north of the summit (first transect) and west of the summit (second transect). The second SCM did not coincide with the uplifted isotherms, but was associated with relatively saline water (>34.85), which occurred in a broad subsurface layer extending into the bottom of the euphotic zone. The most saline water (>34.90) sampled occurred at the top of this relatively saline layer. The saline water was clearly distinguished from the surrounding waters based on its temperature–salinity properties, suggesting lateral intrusion of an allochthonous water mass originating to the south. The presence of the SCM of allochthonous origins suggests that biological distributions near seamounts may be affected by processes besides upwelling.