Chlorophyll distribution and photosynthetic activity in the north and equatorial Pacific Ocean along 155°W

Abstract

The distribution of chlorophylla and photosynthetic characterestics of phytoplankters were investigated along 155°W between 50°N and 15°S during the KH-69-4 cruise of the R. V. Hakuhō Maru (Aug. 12–Nov. 13, 1969). High concentrations of chlorophylla (more than 0.2 mg Chla/m³) were observed above the depths of 150 m at all stations except in 17°N, 5°S and 15°S. North of 20°N, the depths of chlorophyll accumulation shifted from near the surface to 50–100 m with southwards. In the equatorial region, chlorophyll accumulation centered at a depth of about 70 m and ranged vertically between 10 and 150 m. In all cases in the present study area, chlorophyll accumulation occurred within the euphotic zone (above the depth corresponding to 1% of the surface illumination), and except in the subarctic and some equatorial waters, this was usually prevalent in the lower half of the euphotic zone.

The photosynthetic activities (initial slope of P vs I curve) of samples from the depths of chlorophyll accumulation were similar to, or lower than, those of shallow samples from the depths of upper half of the euphotic zone. At the depths of chlorophyll accumulation, calculatedin situ photosynthesis was high in the central Pacific and equatorial waters but low in the subarctic waters.