SUBSURFACE CHLOROPHYLL MAXIMUM IN THE TROPICAL AND SUBTROPICAL WESTERN PACIFIC-OCEAN - VERTICAL PROFILES OF PHYTOPLANKTON BIOMASS AND ITS RELATIONSHIP WITH CHLOROPHYLL-A AND PARTICULATE ORGANIC-CARBON

Vertical distribution of phytoplankton biomass in terms of carbon content (PC) and its relationship with chlorophyll a and particulate organic carbon (POC) were examined together with phytoplankton growth rates in the tropical and subtropical western Pacific in 1979, where a prominent subsurface chlorophyll maximum (SCM) developed between 65 and 150 m. Fluorescence microscopy combined with image analysis was used for measurement of cell volume which was converted to PC. The SCM coincided consistently with subsurface maximum of PC, and the SCM primarily reflected in situ accumulation of phytoplankton biomass. The PC:chlorophyll a ratio decreased with depth; the ratio was 1.8 times, on average, higher in populations at the SCM compared to those near the surface. This increase in relative cellular chlorophyll a along with depth accentuated the magnitude of the SCM. The PC:POC ratio was substantially lower near the surface, 0.17 on average, and increased sharply around the SCM, with a mean value of 0.53. Thus suspended particles around SCM were richer in phytoplankton than those in the upper layers. A major part of PC was contributed by autotrophic eukaryotes both near the surface and at the SCM, and prokaryotic picoplankton comprised a relatively small proportion (6.3 to 14.9%) of PC. The high phytoplankton biomass around the SCM was suggested to be ascribed to in-situ growth of phytoplankton.